diff --git a/compiler/rustc_hir/src/definitions.rs b/compiler/rustc_hir/src/definitions.rs
index 0f77de9fb250f..647d735fb8632 100644
--- a/compiler/rustc_hir/src/definitions.rs
+++ b/compiler/rustc_hir/src/definitions.rs
@@ -87,11 +87,6 @@ impl DefPathTable {
         hash
     }
 
-    /// Used by librustdoc for fake DefIds.
-    pub fn num_def_ids(&self) -> usize {
-        self.index_to_key.len()
-    }
-
     pub fn enumerated_keys_and_path_hashes(
         &self,
     ) -> impl Iterator<Item = (DefIndex, &DefKey, &DefPathHash)> + '_ {
diff --git a/compiler/rustc_metadata/src/rmeta/decoder.rs b/compiler/rustc_metadata/src/rmeta/decoder.rs
index 2ade1bb4f95de..d74fef0045f51 100644
--- a/compiler/rustc_metadata/src/rmeta/decoder.rs
+++ b/compiler/rustc_metadata/src/rmeta/decoder.rs
@@ -1885,10 +1885,6 @@ impl CrateMetadata {
         self.root.hash
     }
 
-    fn num_def_ids(&self) -> usize {
-        self.root.tables.def_keys.size()
-    }
-
     fn local_def_id(&self, index: DefIndex) -> DefId {
         DefId { krate: self.cnum, index }
     }
diff --git a/compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs b/compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs
index bebee9dac3b73..8595a70dd9478 100644
--- a/compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs
+++ b/compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs
@@ -461,10 +461,6 @@ impl CStore {
         self.get_crate_data(def_id.krate).module_expansion(def_id.index, sess)
     }
 
-    pub fn num_def_ids(&self, cnum: CrateNum) -> usize {
-        self.get_crate_data(cnum).num_def_ids()
-    }
-
     pub fn item_attrs(&self, def_id: DefId, sess: &Session) -> Vec<ast::Attribute> {
         self.get_crate_data(def_id.krate).get_item_attrs(def_id.index, sess).collect()
     }
diff --git a/src/librustdoc/clean/auto_trait.rs b/src/librustdoc/clean/auto_trait.rs
index dbdf2e4bbb0f9..3bfc9fea62e90 100644
--- a/src/librustdoc/clean/auto_trait.rs
+++ b/src/librustdoc/clean/auto_trait.rs
@@ -113,7 +113,7 @@ impl<'a, 'tcx> AutoTraitFinder<'a, 'tcx> {
             name: None,
             attrs: Default::default(),
             visibility: Inherited,
-            def_id: self.cx.next_def_id(item_def_id.krate),
+            def_id: FakeDefId::new_fake(item_def_id.krate),
             kind: box ImplItem(Impl {
                 span: Span::dummy(),
                 unsafety: hir::Unsafety::Normal,
diff --git a/src/librustdoc/clean/blanket_impl.rs b/src/librustdoc/clean/blanket_impl.rs
index f5c4034a61de2..68856f82fe94c 100644
--- a/src/librustdoc/clean/blanket_impl.rs
+++ b/src/librustdoc/clean/blanket_impl.rs
@@ -97,7 +97,7 @@ impl<'a, 'tcx> BlanketImplFinder<'a, 'tcx> {
                     name: None,
                     attrs: Default::default(),
                     visibility: Inherited,
-                    def_id: self.cx.next_def_id(impl_def_id.krate),
+                    def_id: FakeDefId::new_fake(item_def_id.krate),
                     kind: box ImplItem(Impl {
                         span: self.cx.tcx.def_span(impl_def_id).clean(self.cx),
                         unsafety: hir::Unsafety::Normal,
diff --git a/src/librustdoc/clean/inline.rs b/src/librustdoc/clean/inline.rs
index 6ad635012b134..5974cd878dd0c 100644
--- a/src/librustdoc/clean/inline.rs
+++ b/src/librustdoc/clean/inline.rs
@@ -15,7 +15,7 @@ use rustc_span::hygiene::MacroKind;
 use rustc_span::symbol::{kw, sym, Symbol};
 use rustc_span::Span;
 
-use crate::clean::{self, Attributes, AttributesExt, GetDefId, ToSource};
+use crate::clean::{self, Attributes, AttributesExt, FakeDefId, GetDefId, ToSource};
 use crate::core::DocContext;
 use crate::formats::item_type::ItemType;
 
@@ -121,7 +121,7 @@ crate fn try_inline(
     };
 
     let (attrs, cfg) = merge_attrs(cx, Some(parent_module), load_attrs(cx, did), attrs_clone);
-    cx.inlined.insert(did);
+    cx.inlined.insert(did.into());
     ret.push(clean::Item::from_def_id_and_attrs_and_parts(
         did,
         Some(name),
@@ -332,7 +332,7 @@ crate fn build_impl(
     attrs: Option<Attrs<'_>>,
     ret: &mut Vec<clean::Item>,
 ) {
-    if !cx.inlined.insert(did) {
+    if !cx.inlined.insert(did.into()) {
         return;
     }
 
@@ -470,7 +470,7 @@ fn build_module(
                 items.push(clean::Item {
                     name: None,
                     attrs: box clean::Attributes::default(),
-                    def_id: cx.next_def_id(did.krate),
+                    def_id: FakeDefId::new_fake(did.krate),
                     visibility: clean::Public,
                     kind: box clean::ImportItem(clean::Import::new_simple(
                         item.ident.name,
diff --git a/src/librustdoc/clean/mod.rs b/src/librustdoc/clean/mod.rs
index 33df9ea3f3e0c..411cfab9f06a0 100644
--- a/src/librustdoc/clean/mod.rs
+++ b/src/librustdoc/clean/mod.rs
@@ -533,7 +533,8 @@ impl Clean<Generics> for hir::Generics<'_> {
                 match param.kind {
                     GenericParamDefKind::Lifetime => unreachable!(),
                     GenericParamDefKind::Type { did, ref bounds, .. } => {
-                        cx.impl_trait_bounds.insert(did.into(), bounds.clone());
+                        cx.impl_trait_bounds
+                            .insert(FakeDefId::new_real(did).into(), bounds.clone());
                     }
                     GenericParamDefKind::Const { .. } => unreachable!(),
                 }
@@ -614,7 +615,7 @@ impl<'a, 'tcx> Clean<Generics> for (&'a ty::Generics, ty::GenericPredicates<'tcx
             .collect::<Vec<GenericParamDef>>();
 
         // param index -> [(DefId of trait, associated type name, type)]
-        let mut impl_trait_proj = FxHashMap::<u32, Vec<(DefId, Symbol, Ty<'tcx>)>>::default();
+        let mut impl_trait_proj = FxHashMap::<u32, Vec<(FakeDefId, Symbol, Ty<'tcx>)>>::default();
 
         let where_predicates = preds
             .predicates
@@ -663,10 +664,11 @@ impl<'a, 'tcx> Clean<Generics> for (&'a ty::Generics, ty::GenericPredicates<'tcx
                         if let Some(((_, trait_did, name), rhs)) =
                             proj.as_ref().and_then(|(lhs, rhs)| Some((lhs.projection()?, rhs)))
                         {
-                            impl_trait_proj
-                                .entry(param_idx)
-                                .or_default()
-                                .push((trait_did, name, rhs));
+                            impl_trait_proj.entry(param_idx).or_default().push((
+                                trait_did.into(),
+                                name,
+                                rhs,
+                            ));
                         }
 
                         return None;
@@ -685,7 +687,13 @@ impl<'a, 'tcx> Clean<Generics> for (&'a ty::Generics, ty::GenericPredicates<'tcx
                 if let Some(proj) = impl_trait_proj.remove(&idx) {
                     for (trait_did, name, rhs) in proj {
                         let rhs = rhs.clean(cx);
-                        simplify::merge_bounds(cx, &mut bounds, trait_did, name, &rhs);
+                        simplify::merge_bounds(
+                            cx,
+                            &mut bounds,
+                            trait_did.expect_real(),
+                            name,
+                            &rhs,
+                        );
                     }
                 }
             } else {
@@ -1175,7 +1183,8 @@ fn clean_qpath(hir_ty: &hir::Ty<'_>, cx: &mut DocContext<'_>) -> Type {
                 if let Some(new_ty) = cx.ty_substs.get(&did).cloned() {
                     return new_ty;
                 }
-                if let Some(bounds) = cx.impl_trait_bounds.remove(&did.into()) {
+                if let Some(bounds) = cx.impl_trait_bounds.remove(&FakeDefId::new_real(did).into())
+                {
                     return ImplTrait(bounds);
                 }
             }
@@ -2006,7 +2015,7 @@ fn clean_extern_crate(
     vec![Item {
         name: Some(name),
         attrs: box attrs.clean(cx),
-        def_id: crate_def_id,
+        def_id: crate_def_id.into(),
         visibility: krate.vis.clean(cx),
         kind: box ExternCrateItem { src: orig_name },
         cfg: attrs.cfg(cx.sess().diagnostic()),
diff --git a/src/librustdoc/clean/types.rs b/src/librustdoc/clean/types.rs
index a556fdba2f3f6..350742e8f2898 100644
--- a/src/librustdoc/clean/types.rs
+++ b/src/librustdoc/clean/types.rs
@@ -1,6 +1,5 @@
-use std::cell::RefCell;
+use std::cell::{Cell, RefCell};
 use std::default::Default;
-use std::fmt;
 use std::hash::{Hash, Hasher};
 use std::iter::FromIterator;
 use std::lazy::SyncOnceCell as OnceCell;
@@ -19,7 +18,7 @@ use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_data_structures::thin_vec::ThinVec;
 use rustc_hir as hir;
 use rustc_hir::def::{CtorKind, DefKind, Res};
-use rustc_hir::def_id::{CrateNum, DefId, DefIndex, CRATE_DEF_INDEX};
+use rustc_hir::def_id::{CrateNum, DefId, DefIndex, LocalDefId, CRATE_DEF_INDEX, LOCAL_CRATE};
 use rustc_hir::lang_items::LangItem;
 use rustc_hir::{BodyId, Mutability};
 use rustc_index::vec::IndexVec;
@@ -49,7 +48,94 @@ use self::ItemKind::*;
 use self::SelfTy::*;
 use self::Type::*;
 
-thread_local!(crate static MAX_DEF_IDX: RefCell<FxHashMap<CrateNum, DefIndex>> = Default::default());
+crate type FakeDefIdSet = FxHashSet<FakeDefId>;
+
+#[derive(Debug, Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Copy)]
+crate enum FakeDefId {
+    Real(DefId),
+    Fake(DefIndex, CrateNum),
+}
+
+impl FakeDefId {
+    #[cfg(parallel_compiler)]
+    crate fn new_fake(crate: CrateNum) -> Self {
+        unimplemented!("")
+    }
+
+    #[cfg(not(parallel_compiler))]
+    crate fn new_fake(krate: CrateNum) -> Self {
+        thread_local!(static FAKE_DEF_ID_COUNTER: Cell<usize> = Cell::new(0));
+        let id = FAKE_DEF_ID_COUNTER.with(|id| {
+            let tmp = id.get();
+            id.set(tmp + 1);
+            tmp
+        });
+        Self::Fake(DefIndex::from(id), krate)
+    }
+
+    crate fn new_real(id: DefId) -> Self {
+        Self::Real(id)
+    }
+
+    #[inline]
+    crate fn is_local(self) -> bool {
+        match self {
+            FakeDefId::Real(id) => id.is_local(),
+            FakeDefId::Fake(_, krate) => krate == LOCAL_CRATE,
+        }
+    }
+
+    #[inline]
+    crate fn as_local(self) -> Option<LocalDefId> {
+        match self {
+            FakeDefId::Real(id) => id.as_local(),
+            FakeDefId::Fake(idx, krate) => {
+                (krate == LOCAL_CRATE).then(|| LocalDefId { local_def_index: idx })
+            }
+        }
+    }
+
+    #[inline]
+    crate fn expect_local(self) -> LocalDefId {
+        self.as_local()
+            .unwrap_or_else(|| panic!("FakeDefId::expect_local: `{:?}` isn't local", self))
+    }
+
+    #[inline]
+    crate fn expect_real(self) -> rustc_hir::def_id::DefId {
+        self.as_real().unwrap_or_else(|| panic!("FakeDefId::expect_real: `{:?}` isn't real", self))
+    }
+
+    #[inline]
+    crate fn as_real(self) -> Option<DefId> {
+        match self {
+            FakeDefId::Real(id) => Some(id),
+            FakeDefId::Fake(_, _) => None,
+        }
+    }
+
+    #[inline]
+    crate fn krate(self) -> CrateNum {
+        match self {
+            FakeDefId::Real(id) => id.krate,
+            FakeDefId::Fake(_, krate) => krate,
+        }
+    }
+
+    #[inline]
+    crate fn index(self) -> Option<DefIndex> {
+        match self {
+            FakeDefId::Real(id) => Some(id.index),
+            FakeDefId::Fake(_, _) => None,
+        }
+    }
+}
+
+impl From<DefId> for FakeDefId {
+    fn from(id: DefId) -> Self {
+        Self::Real(id)
+    }
+}
 
 #[derive(Clone, Debug)]
 crate struct Crate {
@@ -261,7 +347,7 @@ impl ExternalCrate {
 /// Anything with a source location and set of attributes and, optionally, a
 /// name. That is, anything that can be documented. This doesn't correspond
 /// directly to the AST's concept of an item; it's a strict superset.
-#[derive(Clone)]
+#[derive(Clone, Debug)]
 crate struct Item {
     /// The name of this item.
     /// Optional because not every item has a name, e.g. impls.
@@ -271,7 +357,7 @@ crate struct Item {
     /// Information about this item that is specific to what kind of item it is.
     /// E.g., struct vs enum vs function.
     crate kind: Box<ItemKind>,
-    crate def_id: DefId,
+    crate def_id: FakeDefId,
 
     crate cfg: Option<Arc<Cfg>>,
 }
@@ -280,21 +366,6 @@ crate struct Item {
 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
 rustc_data_structures::static_assert_size!(Item, 48);
 
-impl fmt::Debug for Item {
-    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
-        let def_id: &dyn fmt::Debug = if self.is_fake() { &"**FAKE**" } else { &self.def_id };
-
-        fmt.debug_struct("Item")
-            .field("name", &self.name)
-            .field("attrs", &self.attrs)
-            .field("kind", &self.kind)
-            .field("visibility", &self.visibility)
-            .field("def_id", def_id)
-            .field("cfg", &self.cfg)
-            .finish()
-    }
-}
-
 crate fn rustc_span(def_id: DefId, tcx: TyCtxt<'_>) -> Span {
     Span::from_rustc_span(def_id.as_local().map_or_else(
         || tcx.def_span(def_id),
@@ -307,19 +378,19 @@ crate fn rustc_span(def_id: DefId, tcx: TyCtxt<'_>) -> Span {
 
 impl Item {
     crate fn stability<'tcx>(&self, tcx: TyCtxt<'tcx>) -> Option<&'tcx Stability> {
-        if self.is_fake() { None } else { tcx.lookup_stability(self.def_id) }
+        if self.is_fake() { None } else { tcx.lookup_stability(self.def_id.expect_real()) }
     }
 
     crate fn const_stability<'tcx>(&self, tcx: TyCtxt<'tcx>) -> Option<&'tcx ConstStability> {
-        if self.is_fake() { None } else { tcx.lookup_const_stability(self.def_id) }
+        if self.is_fake() { None } else { tcx.lookup_const_stability(self.def_id.expect_real()) }
     }
 
     crate fn deprecation(&self, tcx: TyCtxt<'_>) -> Option<Deprecation> {
-        if self.is_fake() { None } else { tcx.lookup_deprecation(self.def_id) }
+        if self.is_fake() { None } else { tcx.lookup_deprecation(self.def_id.expect_real()) }
     }
 
     crate fn inner_docs(&self, tcx: TyCtxt<'_>) -> bool {
-        if self.is_fake() { false } else { tcx.get_attrs(self.def_id).inner_docs() }
+        if self.is_fake() { false } else { tcx.get_attrs(self.def_id.expect_real()).inner_docs() }
     }
 
     crate fn span(&self, tcx: TyCtxt<'_>) -> Span {
@@ -334,7 +405,7 @@ impl Item {
         } else if self.is_fake() {
             Span::dummy()
         } else {
-            rustc_span(self.def_id, tcx)
+            rustc_span(self.def_id.expect_real(), tcx)
         }
     }
 
@@ -388,7 +459,7 @@ impl Item {
         debug!("name={:?}, def_id={:?}", name, def_id);
 
         Item {
-            def_id,
+            def_id: def_id.into(),
             kind: box kind,
             name,
             attrs,
@@ -412,9 +483,9 @@ impl Item {
             .map_or(&[][..], |v| v.as_slice())
             .iter()
             .filter_map(|ItemLink { link: s, link_text, did, ref fragment }| {
-                match *did {
+                match did {
                     Some(did) => {
-                        if let Some((mut href, ..)) = href(did, cx) {
+                        if let Some((mut href, ..)) = href(did.expect_real(), cx) {
                             if let Some(ref fragment) = *fragment {
                                 href.push('#');
                                 href.push_str(fragment);
@@ -433,8 +504,8 @@ impl Item {
                     None => {
                         let relative_to = &cx.current;
                         if let Some(ref fragment) = *fragment {
-                            let url = match cx.cache().extern_locations.get(&self.def_id.krate) {
-                                Some(ExternalLocation::Local) => {
+                            let url = match cx.cache().extern_locations.get(&self.def_id.krate()) {
+                                Some(&ExternalLocation::Local) => {
                                     if relative_to[0] == "std" {
                                         let depth = relative_to.len() - 1;
                                         "../".repeat(depth)
@@ -473,7 +544,7 @@ impl Item {
     }
 
     crate fn is_crate(&self) -> bool {
-        self.is_mod() && self.def_id.index == CRATE_DEF_INDEX
+        self.is_mod() && self.def_id.as_real().map_or(false, |did| did.index == CRATE_DEF_INDEX)
     }
 
     crate fn is_mod(&self) -> bool {
@@ -586,13 +657,8 @@ impl Item {
         }
     }
 
-    /// See the documentation for [`next_def_id()`].
-    ///
-    /// [`next_def_id()`]: DocContext::next_def_id()
     crate fn is_fake(&self) -> bool {
-        MAX_DEF_IDX.with(|m| {
-            m.borrow().get(&self.def_id.krate).map(|&idx| idx <= self.def_id.index).unwrap_or(false)
-        })
+        matches!(self.def_id, FakeDefId::Fake(_, _))
     }
 }
 
@@ -911,7 +977,7 @@ crate struct ItemLink {
     /// This may not be the same as `link` if there was a disambiguator
     /// in an intra-doc link (e.g. \[`fn@f`\])
     pub(crate) link_text: String,
-    pub(crate) did: Option<DefId>,
+    pub(crate) did: Option<FakeDefId>,
     /// The url fragment to append to the link
     pub(crate) fragment: Option<String>,
 }
@@ -1032,7 +1098,7 @@ impl Attributes {
             }
         }
 
-        let clean_attr = |(attr, parent_module): (&ast::Attribute, _)| {
+        let clean_attr = |(attr, parent_module): (&ast::Attribute, Option<DefId>)| {
             if let Some(value) = attr.doc_str() {
                 trace!("got doc_str={:?}", value);
                 let value = beautify_doc_string(value);
@@ -1652,7 +1718,7 @@ impl Type {
 impl Type {
     fn inner_def_id(&self, cache: Option<&Cache>) -> Option<DefId> {
         let t: PrimitiveType = match *self {
-            ResolvedPath { did, .. } => return Some(did),
+            ResolvedPath { did, .. } => return Some(did.into()),
             Primitive(p) => return cache.and_then(|c| c.primitive_locations.get(&p).cloned()),
             BorrowedRef { type_: box Generic(..), .. } => PrimitiveType::Reference,
             BorrowedRef { ref type_, .. } => return type_.inner_def_id(cache),
diff --git a/src/librustdoc/clean/utils.rs b/src/librustdoc/clean/utils.rs
index 7df8b442e5acc..eda7ebfaab980 100644
--- a/src/librustdoc/clean/utils.rs
+++ b/src/librustdoc/clean/utils.rs
@@ -45,9 +45,9 @@ crate fn krate(cx: &mut DocContext<'_>) -> Crate {
                 // `#[doc(masked)]` to the injected `extern crate` because it's unstable.
                 if it.is_extern_crate()
                     && (it.attrs.has_doc_flag(sym::masked)
-                        || cx.tcx.is_compiler_builtins(it.def_id.krate))
+                        || cx.tcx.is_compiler_builtins(it.def_id.krate()))
                 {
-                    cx.cache.masked_crates.insert(it.def_id.krate);
+                    cx.cache.masked_crates.insert(it.def_id.krate());
                 }
             }
         }
@@ -490,8 +490,6 @@ where
 }
 
 /// Find the nearest parent module of a [`DefId`].
-///
-/// **Panics if the item it belongs to [is fake][Item::is_fake].**
 crate fn find_nearest_parent_module(tcx: TyCtxt<'_>, def_id: DefId) -> Option<DefId> {
     if def_id.is_top_level_module() {
         // The crate root has no parent. Use it as the root instead.
diff --git a/src/librustdoc/core.rs b/src/librustdoc/core.rs
index 212aac0e5b43c..b0d163763b4bd 100644
--- a/src/librustdoc/core.rs
+++ b/src/librustdoc/core.rs
@@ -6,7 +6,7 @@ use rustc_errors::emitter::{Emitter, EmitterWriter};
 use rustc_errors::json::JsonEmitter;
 use rustc_feature::UnstableFeatures;
 use rustc_hir::def::Res;
-use rustc_hir::def_id::{CrateNum, DefId, DefIndex, LocalDefId, LOCAL_CRATE};
+use rustc_hir::def_id::{DefId, LocalDefId, LOCAL_CRATE};
 use rustc_hir::HirId;
 use rustc_hir::{
     intravisit::{self, NestedVisitorMap, Visitor},
@@ -26,13 +26,11 @@ use rustc_span::symbol::sym;
 use rustc_span::Span;
 
 use std::cell::RefCell;
-use std::collections::hash_map::Entry;
 use std::mem;
 use std::rc::Rc;
 
-use crate::clean;
 use crate::clean::inline::build_external_trait;
-use crate::clean::{TraitWithExtraInfo, MAX_DEF_IDX};
+use crate::clean::{self, FakeDefId, TraitWithExtraInfo};
 use crate::config::{Options as RustdocOptions, OutputFormat, RenderOptions};
 use crate::formats::cache::Cache;
 use crate::passes::{self, Condition::*, ConditionalPass};
@@ -66,7 +64,6 @@ crate struct DocContext<'tcx> {
     crate ct_substs: FxHashMap<DefId, clean::Constant>,
     /// Table synthetic type parameter for `impl Trait` in argument position -> bounds
     crate impl_trait_bounds: FxHashMap<ImplTraitParam, Vec<clean::GenericBound>>,
-    crate fake_def_ids: FxHashMap<CrateNum, DefIndex>,
     /// Auto-trait or blanket impls processed so far, as `(self_ty, trait_def_id)`.
     // FIXME(eddyb) make this a `ty::TraitRef<'tcx>` set.
     crate generated_synthetics: FxHashSet<(Ty<'tcx>, DefId)>,
@@ -81,7 +78,7 @@ crate struct DocContext<'tcx> {
     /// This same cache is used throughout rustdoc, including in [`crate::html::render`].
     crate cache: Cache,
     /// Used by [`clean::inline`] to tell if an item has already been inlined.
-    crate inlined: FxHashSet<DefId>,
+    crate inlined: FxHashSet<FakeDefId>,
     /// Used by `calculate_doc_coverage`.
     crate output_format: OutputFormat,
 }
@@ -129,54 +126,14 @@ impl<'tcx> DocContext<'tcx> {
         r
     }
 
-    /// Create a new "fake" [`DefId`].
-    ///
-    /// This is an ugly hack, but it's the simplest way to handle synthetic impls without greatly
-    /// refactoring either rustdoc or [`rustc_middle`]. In particular, allowing new [`DefId`]s
-    /// to be registered after the AST is constructed would require storing the [`DefId`] mapping
-    /// in a [`RefCell`], decreasing the performance for normal compilation for very little gain.
-    ///
-    /// Instead, we construct "fake" [`DefId`]s, which start immediately after the last `DefId`.
-    /// In the [`Debug`] impl for [`clean::Item`], we explicitly check for fake `DefId`s,
-    /// as we'll end up with a panic if we use the `DefId` `Debug` impl for fake `DefId`s.
-    ///
-    /// [`RefCell`]: std::cell::RefCell
-    /// [`Debug`]: std::fmt::Debug
-    /// [`clean::Item`]: crate::clean::types::Item
-    crate fn next_def_id(&mut self, crate_num: CrateNum) -> DefId {
-        let def_index = match self.fake_def_ids.entry(crate_num) {
-            Entry::Vacant(e) => {
-                let num_def_idx = {
-                    let num_def_idx = if crate_num == LOCAL_CRATE {
-                        self.tcx.hir().definitions().def_path_table().num_def_ids()
-                    } else {
-                        self.resolver.borrow_mut().access(|r| r.cstore().num_def_ids(crate_num))
-                    };
-
-                    DefIndex::from_usize(num_def_idx)
-                };
-
-                MAX_DEF_IDX.with(|m| {
-                    m.borrow_mut().insert(crate_num, num_def_idx);
-                });
-                e.insert(num_def_idx)
-            }
-            Entry::Occupied(e) => e.into_mut(),
-        };
-        *def_index = *def_index + 1;
-
-        DefId { krate: crate_num, index: *def_index }
-    }
-
     /// Like `hir().local_def_id_to_hir_id()`, but skips calling it on fake DefIds.
     /// (This avoids a slice-index-out-of-bounds panic.)
-    crate fn as_local_hir_id(tcx: TyCtxt<'_>, def_id: DefId) -> Option<HirId> {
-        if MAX_DEF_IDX.with(|m| {
-            m.borrow().get(&def_id.krate).map(|&idx| idx <= def_id.index).unwrap_or(false)
-        }) {
-            None
-        } else {
-            def_id.as_local().map(|def_id| tcx.hir().local_def_id_to_hir_id(def_id))
+    crate fn as_local_hir_id(tcx: TyCtxt<'_>, def_id: FakeDefId) -> Option<HirId> {
+        match def_id {
+            FakeDefId::Real(real_id) => {
+                real_id.as_local().map(|def_id| tcx.hir().local_def_id_to_hir_id(def_id))
+            }
+            FakeDefId::Fake(_, _) => None,
         }
     }
 }
@@ -419,7 +376,6 @@ crate fn run_global_ctxt(
         lt_substs: Default::default(),
         ct_substs: Default::default(),
         impl_trait_bounds: Default::default(),
-        fake_def_ids: Default::default(),
         generated_synthetics: Default::default(),
         auto_traits: tcx
             .all_traits(LOCAL_CRATE)
@@ -630,12 +586,12 @@ impl<'tcx> Visitor<'tcx> for EmitIgnoredResolutionErrors<'tcx> {
 /// for `impl Trait` in argument position.
 #[derive(Clone, Copy, PartialEq, Eq, Hash, PartialOrd, Ord)]
 crate enum ImplTraitParam {
-    DefId(DefId),
+    DefId(FakeDefId),
     ParamIndex(u32),
 }
 
-impl From<DefId> for ImplTraitParam {
-    fn from(did: DefId) -> Self {
+impl From<FakeDefId> for ImplTraitParam {
+    fn from(did: FakeDefId) -> Self {
         ImplTraitParam::DefId(did)
     }
 }
diff --git a/src/librustdoc/formats/cache.rs b/src/librustdoc/formats/cache.rs
index b6b76a96e7f96..50496f320096c 100644
--- a/src/librustdoc/formats/cache.rs
+++ b/src/librustdoc/formats/cache.rs
@@ -8,7 +8,7 @@ use rustc_middle::middle::privacy::AccessLevels;
 use rustc_middle::ty::TyCtxt;
 use rustc_span::symbol::sym;
 
-use crate::clean::{self, GetDefId};
+use crate::clean::{self, FakeDefId, GetDefId};
 use crate::fold::DocFolder;
 use crate::formats::item_type::ItemType;
 use crate::formats::Impl;
@@ -67,7 +67,7 @@ crate struct Cache {
     /// When rendering traits, it's often useful to be able to list all
     /// implementors of the trait, and this mapping is exactly, that: a mapping
     /// of trait ids to the list of known implementors of the trait
-    crate implementors: FxHashMap<DefId, Vec<Impl>>,
+    crate implementors: FxHashMap<FakeDefId, Vec<Impl>>,
 
     /// Cache of where external crate documentation can be found.
     crate extern_locations: FxHashMap<CrateNum, ExternalLocation>,
@@ -122,7 +122,7 @@ crate struct Cache {
     /// All intra-doc links resolved so far.
     ///
     /// Links are indexed by the DefId of the item they document.
-    crate intra_doc_links: BTreeMap<DefId, Vec<clean::ItemLink>>,
+    crate intra_doc_links: BTreeMap<FakeDefId, Vec<clean::ItemLink>>,
 }
 
 /// This struct is used to wrap the `cache` and `tcx` in order to run `DocFolder`.
@@ -205,7 +205,7 @@ impl<'a, 'tcx> DocFolder for CacheBuilder<'a, 'tcx> {
         // If the impl is from a masked crate or references something from a
         // masked crate then remove it completely.
         if let clean::ImplItem(ref i) = *item.kind {
-            if self.cache.masked_crates.contains(&item.def_id.krate)
+            if self.cache.masked_crates.contains(&item.def_id.krate())
                 || i.trait_.def_id().map_or(false, |d| self.cache.masked_crates.contains(&d.krate))
                 || i.for_.def_id().map_or(false, |d| self.cache.masked_crates.contains(&d.krate))
             {
@@ -216,9 +216,11 @@ impl<'a, 'tcx> DocFolder for CacheBuilder<'a, 'tcx> {
         // Propagate a trait method's documentation to all implementors of the
         // trait.
         if let clean::TraitItem(ref t) = *item.kind {
-            self.cache.traits.entry(item.def_id).or_insert_with(|| clean::TraitWithExtraInfo {
-                trait_: t.clone(),
-                is_notable: item.attrs.has_doc_flag(sym::notable_trait),
+            self.cache.traits.entry(item.def_id.expect_real()).or_insert_with(|| {
+                clean::TraitWithExtraInfo {
+                    trait_: t.clone(),
+                    is_notable: item.attrs.has_doc_flag(sym::notable_trait),
+                }
             });
         }
 
@@ -228,7 +230,7 @@ impl<'a, 'tcx> DocFolder for CacheBuilder<'a, 'tcx> {
                 if i.blanket_impl.is_none() {
                     self.cache
                         .implementors
-                        .entry(did)
+                        .entry(did.into())
                         .or_default()
                         .push(Impl { impl_item: item.clone() });
                 }
@@ -289,7 +291,7 @@ impl<'a, 'tcx> DocFolder for CacheBuilder<'a, 'tcx> {
                     // A crate has a module at its root, containing all items,
                     // which should not be indexed. The crate-item itself is
                     // inserted later on when serializing the search-index.
-                    if item.def_id.index != CRATE_DEF_INDEX {
+                    if item.def_id.index().map_or(false, |idx| idx != CRATE_DEF_INDEX) {
                         self.cache.search_index.push(IndexItem {
                             ty: item.type_(),
                             name: s.to_string(),
@@ -297,7 +299,7 @@ impl<'a, 'tcx> DocFolder for CacheBuilder<'a, 'tcx> {
                             desc: item
                                 .doc_value()
                                 .map_or_else(String::new, |x| short_markdown_summary(&x.as_str())),
-                            parent,
+                            parent: parent.map(FakeDefId::new_real),
                             parent_idx: None,
                             search_type: get_index_search_type(&item, &self.empty_cache, self.tcx),
                             aliases: item.attrs.get_doc_aliases(),
@@ -346,17 +348,20 @@ impl<'a, 'tcx> DocFolder for CacheBuilder<'a, 'tcx> {
                     // `public_items` map, so we can skip inserting into the
                     // paths map if there was already an entry present and we're
                     // not a public item.
-                    if !self.cache.paths.contains_key(&item.def_id)
-                        || self.cache.access_levels.is_public(item.def_id)
+                    if !self.cache.paths.contains_key(&item.def_id.expect_real())
+                        || self.cache.access_levels.is_public(item.def_id.expect_real())
                     {
-                        self.cache
-                            .paths
-                            .insert(item.def_id, (self.cache.stack.clone(), item.type_()));
+                        self.cache.paths.insert(
+                            item.def_id.expect_real(),
+                            (self.cache.stack.clone(), item.type_()),
+                        );
                     }
                 }
             }
             clean::PrimitiveItem(..) => {
-                self.cache.paths.insert(item.def_id, (self.cache.stack.clone(), item.type_()));
+                self.cache
+                    .paths
+                    .insert(item.def_id.expect_real(), (self.cache.stack.clone(), item.type_()));
             }
 
             clean::ExternCrateItem { .. }
@@ -386,7 +391,7 @@ impl<'a, 'tcx> DocFolder for CacheBuilder<'a, 'tcx> {
             | clean::StructItem(..)
             | clean::UnionItem(..)
             | clean::VariantItem(..) => {
-                self.cache.parent_stack.push(item.def_id);
+                self.cache.parent_stack.push(item.def_id.expect_real());
                 self.cache.parent_is_trait_impl = false;
                 true
             }
diff --git a/src/librustdoc/formats/mod.rs b/src/librustdoc/formats/mod.rs
index 1ce6572bbed04..6060b0560cf3c 100644
--- a/src/librustdoc/formats/mod.rs
+++ b/src/librustdoc/formats/mod.rs
@@ -2,9 +2,9 @@ crate mod cache;
 crate mod item_type;
 crate mod renderer;
 
-crate use renderer::{run_format, FormatRenderer};
+use rustc_hir::def_id::DefId;
 
-use rustc_span::def_id::DefId;
+crate use renderer::{run_format, FormatRenderer};
 
 use crate::clean;
 use crate::clean::types::GetDefId;
diff --git a/src/librustdoc/html/format.rs b/src/librustdoc/html/format.rs
index f211a5acf5ea5..fa57c9bda74da 100644
--- a/src/librustdoc/html/format.rs
+++ b/src/librustdoc/html/format.rs
@@ -12,11 +12,14 @@ use std::iter;
 use rustc_data_structures::captures::Captures;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_hir as hir;
+use rustc_hir::def_id::DefId;
 use rustc_middle::ty::TyCtxt;
-use rustc_span::def_id::{DefId, CRATE_DEF_INDEX};
+use rustc_span::def_id::CRATE_DEF_INDEX;
 use rustc_target::spec::abi::Abi;
 
-use crate::clean::{self, utils::find_nearest_parent_module, ExternalCrate, PrimitiveType};
+use crate::clean::{
+    self, utils::find_nearest_parent_module, ExternalCrate, FakeDefId, PrimitiveType,
+};
 use crate::formats::item_type::ItemType;
 use crate::html::escape::Escape;
 use crate::html::render::cache::ExternalLocation;
@@ -637,7 +640,7 @@ crate fn anchor<'a, 'cx: 'a>(
     text: &'a str,
     cx: &'cx Context<'_>,
 ) -> impl fmt::Display + 'a {
-    let parts = href(did, cx);
+    let parts = href(did.into(), cx);
     display_fn(move |f| {
         if let Some((url, short_ty, fqp)) = parts {
             write!(
@@ -865,7 +868,7 @@ fn fmt_type<'cx>(
                 //        everything comes in as a fully resolved QPath (hard to
                 //        look at).
                 box clean::ResolvedPath { did, ref param_names, .. } => {
-                    match href(did, cx) {
+                    match href(did.into(), cx) {
                         Some((ref url, _, ref path)) if !f.alternate() => {
                             write!(
                                 f,
@@ -1143,7 +1146,7 @@ impl clean::FnDecl {
 impl clean::Visibility {
     crate fn print_with_space<'a, 'tcx: 'a>(
         self,
-        item_did: DefId,
+        item_did: FakeDefId,
         cx: &'a Context<'tcx>,
     ) -> impl fmt::Display + 'a + Captures<'tcx> {
         let to_print = match self {
@@ -1153,7 +1156,7 @@ impl clean::Visibility {
                 // FIXME(camelid): This may not work correctly if `item_did` is a module.
                 //                 However, rustdoc currently never displays a module's
                 //                 visibility, so it shouldn't matter.
-                let parent_module = find_nearest_parent_module(cx.tcx(), item_did);
+                let parent_module = find_nearest_parent_module(cx.tcx(), item_did.expect_real());
 
                 if vis_did.index == CRATE_DEF_INDEX {
                     "pub(crate) ".to_owned()
diff --git a/src/librustdoc/html/render/cache.rs b/src/librustdoc/html/render/cache.rs
index 27a8065afb6d2..57520a1a1fb46 100644
--- a/src/librustdoc/html/render/cache.rs
+++ b/src/librustdoc/html/render/cache.rs
@@ -7,7 +7,7 @@ use serde::ser::{Serialize, SerializeStruct, Serializer};
 
 use crate::clean;
 use crate::clean::types::{
-    FnDecl, FnRetTy, GenericBound, Generics, GetDefId, Type, WherePredicate,
+    FakeDefId, FnDecl, FnRetTy, GenericBound, Generics, GetDefId, Type, WherePredicate,
 };
 use crate::formats::cache::Cache;
 use crate::formats::item_type::ItemType;
@@ -39,7 +39,7 @@ crate fn build_index<'tcx>(krate: &clean::Crate, cache: &mut Cache, tcx: TyCtxt<
                 name: item.name.unwrap().to_string(),
                 path: fqp[..fqp.len() - 1].join("::"),
                 desc: item.doc_value().map_or_else(String::new, |s| short_markdown_summary(&s)),
-                parent: Some(did),
+                parent: Some(did.into()),
                 parent_idx: None,
                 search_type: get_index_search_type(&item, cache, tcx),
                 aliases: item.attrs.get_doc_aliases(),
@@ -82,7 +82,7 @@ crate fn build_index<'tcx>(krate: &clean::Crate, cache: &mut Cache, tcx: TyCtxt<
                 defid_to_pathid.insert(defid, pathid);
                 lastpathid += 1;
 
-                if let Some(&(ref fqp, short)) = paths.get(&defid) {
+                if let Some(&(ref fqp, short)) = paths.get(&defid.expect_real()) {
                     crate_paths.push((short, fqp.last().unwrap().clone()));
                     Some(pathid)
                 } else {
@@ -214,7 +214,7 @@ crate fn get_index_search_type<'tcx>(
 
 fn get_index_type(clean_type: &clean::Type, cache: &Cache) -> RenderType {
     RenderType {
-        ty: clean_type.def_id_full(cache),
+        ty: clean_type.def_id_full(cache).map(FakeDefId::new_real),
         idx: None,
         name: get_index_type_name(clean_type, true).map(|s| s.as_str().to_ascii_lowercase()),
         generics: get_generics(clean_type, cache),
@@ -256,7 +256,7 @@ fn get_generics(clean_type: &clean::Type, cache: &Cache) -> Option<Vec<Generic>>
             .filter_map(|t| {
                 get_index_type_name(t, false).map(|name| Generic {
                     name: name.as_str().to_ascii_lowercase(),
-                    defid: t.def_id_full(cache),
+                    defid: t.def_id_full(cache).map(FakeDefId::new_real),
                     idx: None,
                 })
             })
diff --git a/src/librustdoc/html/render/context.rs b/src/librustdoc/html/render/context.rs
index 293c0a40fa799..2f3f87215c3a1 100644
--- a/src/librustdoc/html/render/context.rs
+++ b/src/librustdoc/html/render/context.rs
@@ -18,7 +18,8 @@ use super::print_item::{full_path, item_path, print_item};
 use super::write_shared::write_shared;
 use super::{print_sidebar, settings, AllTypes, NameDoc, StylePath, BASIC_KEYWORDS};
 
-use crate::clean::{self, ExternalCrate};
+use crate::clean;
+use crate::clean::ExternalCrate;
 use crate::config::RenderOptions;
 use crate::docfs::{DocFS, PathError};
 use crate::error::Error;
@@ -218,7 +219,7 @@ impl<'tcx> Context<'tcx> {
                 &self.shared.style_files,
             )
         } else {
-            if let Some(&(ref names, ty)) = self.cache.paths.get(&it.def_id) {
+            if let Some(&(ref names, ty)) = self.cache.paths.get(&it.def_id.expect_real()) {
                 let mut path = String::new();
                 for name in &names[..names.len() - 1] {
                     path.push_str(name);
diff --git a/src/librustdoc/html/render/mod.rs b/src/librustdoc/html/render/mod.rs
index 1271b54debd16..ea57831c0e5de 100644
--- a/src/librustdoc/html/render/mod.rs
+++ b/src/librustdoc/html/render/mod.rs
@@ -54,7 +54,7 @@ use rustc_span::symbol::{kw, sym, Symbol};
 use serde::ser::SerializeSeq;
 use serde::{Serialize, Serializer};
 
-use crate::clean::{self, GetDefId, RenderedLink, SelfTy};
+use crate::clean::{self, FakeDefId, GetDefId, RenderedLink, SelfTy};
 use crate::docfs::PathError;
 use crate::error::Error;
 use crate::formats::cache::Cache;
@@ -87,7 +87,7 @@ crate struct IndexItem {
     crate name: String,
     crate path: String,
     crate desc: String,
-    crate parent: Option<DefId>,
+    crate parent: Option<FakeDefId>,
     crate parent_idx: Option<usize>,
     crate search_type: Option<IndexItemFunctionType>,
     crate aliases: Box<[String]>,
@@ -96,7 +96,7 @@ crate struct IndexItem {
 /// A type used for the search index.
 #[derive(Debug)]
 crate struct RenderType {
-    ty: Option<DefId>,
+    ty: Option<FakeDefId>,
     idx: Option<usize>,
     name: Option<String>,
     generics: Option<Vec<Generic>>,
@@ -128,7 +128,7 @@ impl Serialize for RenderType {
 #[derive(Debug)]
 crate struct Generic {
     name: String,
-    defid: Option<DefId>,
+    defid: Option<FakeDefId>,
     idx: Option<usize>,
 }
 
@@ -709,7 +709,7 @@ fn render_impls(
         .map(|i| {
             let did = i.trait_did_full(cache).unwrap();
             let provided_trait_methods = i.inner_impl().provided_trait_methods(tcx);
-            let assoc_link = AssocItemLink::GotoSource(did, &provided_trait_methods);
+            let assoc_link = AssocItemLink::GotoSource(did.into(), &provided_trait_methods);
             let mut buffer = if w.is_for_html() { Buffer::html() } else { Buffer::new() };
             render_impl(
                 &mut buffer,
@@ -747,7 +747,7 @@ fn naive_assoc_href(it: &clean::Item, link: AssocItemLink<'_>, cx: &Context<'_>)
         AssocItemLink::Anchor(Some(ref id)) => format!("#{}", id),
         AssocItemLink::Anchor(None) => anchor,
         AssocItemLink::GotoSource(did, _) => {
-            href(did, cx).map(|p| format!("{}{}", p.0, anchor)).unwrap_or(anchor)
+            href(did.expect_real(), cx).map(|p| format!("{}{}", p.0, anchor)).unwrap_or(anchor)
         }
     }
 }
@@ -855,7 +855,7 @@ fn render_assoc_item(
                     ItemType::TyMethod
                 };
 
-                href(did, cx)
+                href(did.expect_real(), cx)
                     .map(|p| format!("{}#{}.{}", p.0, ty, name))
                     .unwrap_or_else(|| format!("#{}.{}", ty, name))
             }
@@ -979,7 +979,7 @@ fn render_attributes_in_code(w: &mut Buffer, it: &clean::Item) {
 #[derive(Copy, Clone)]
 enum AssocItemLink<'a> {
     Anchor(Option<&'a str>),
-    GotoSource(DefId, &'a FxHashSet<Symbol>),
+    GotoSource(FakeDefId, &'a FxHashSet<Symbol>),
 }
 
 impl<'a> AssocItemLink<'a> {
@@ -1217,7 +1217,7 @@ fn notable_traits_decl(decl: &clean::FnDecl, cx: &Context<'_>) -> String {
                                 it,
                                 &[],
                                 Some(&tydef.type_),
-                                AssocItemLink::GotoSource(t_did, &FxHashSet::default()),
+                                AssocItemLink::GotoSource(t_did.into(), &FxHashSet::default()),
                                 "",
                                 cx,
                             );
@@ -1476,7 +1476,7 @@ fn render_impl(
             }
             let did = i.trait_.as_ref().unwrap().def_id_full(cx.cache()).unwrap();
             let provided_methods = i.provided_trait_methods(cx.tcx());
-            let assoc_link = AssocItemLink::GotoSource(did, &provided_methods);
+            let assoc_link = AssocItemLink::GotoSource(did.into(), &provided_methods);
 
             doc_impl_item(
                 boring,
@@ -1810,7 +1810,8 @@ fn small_url_encode(s: String) -> String {
 }
 
 fn sidebar_assoc_items(cx: &Context<'_>, out: &mut Buffer, it: &clean::Item) {
-    if let Some(v) = cx.cache.impls.get(&it.def_id) {
+    let did = it.def_id.expect_real();
+    if let Some(v) = cx.cache.impls.get(&did) {
         let mut used_links = FxHashSet::default();
         let cache = cx.cache();
 
diff --git a/src/librustdoc/html/render/print_item.rs b/src/librustdoc/html/render/print_item.rs
index 1bf726dd31a46..70b5458ece894 100644
--- a/src/librustdoc/html/render/print_item.rs
+++ b/src/librustdoc/html/render/print_item.rs
@@ -270,14 +270,18 @@ fn item_module(w: &mut Buffer, cx: &Context<'_>, item: &clean::Item, items: &[cl
                         w,
                         "<tr><td><code>{}extern crate {} as {};",
                         myitem.visibility.print_with_space(myitem.def_id, cx),
-                        anchor(myitem.def_id, &*src.as_str(), cx),
+                        anchor(myitem.def_id.expect_real(), &*src.as_str(), cx),
                         myitem.name.as_ref().unwrap(),
                     ),
                     None => write!(
                         w,
                         "<tr><td><code>{}extern crate {};",
                         myitem.visibility.print_with_space(myitem.def_id, cx),
-                        anchor(myitem.def_id, &*myitem.name.as_ref().unwrap().as_str(), cx),
+                        anchor(
+                            myitem.def_id.expect_real(),
+                            &*myitem.name.as_ref().unwrap().as_str(),
+                            cx
+                        ),
                     ),
                 }
                 w.write_str("</code></td></tr>");
@@ -290,7 +294,7 @@ fn item_module(w: &mut Buffer, cx: &Context<'_>, item: &clean::Item, items: &[cl
 
                     // Just need an item with the correct def_id and attrs
                     let import_item = clean::Item {
-                        def_id: import_def_id,
+                        def_id: import_def_id.into(),
                         attrs: import_attrs,
                         cfg: ast_attrs.cfg(cx.tcx().sess.diagnostic()),
                         ..myitem.clone()
@@ -629,7 +633,7 @@ fn item_trait(w: &mut Buffer, cx: &Context<'_>, it: &clean::Item, t: &clean::Tra
     }
 
     // If there are methods directly on this trait object, render them here.
-    render_assoc_items(w, cx, it, it.def_id, AssocItemRender::All);
+    render_assoc_items(w, cx, it, it.def_id.expect_real(), AssocItemRender::All);
 
     if let Some(implementors) = cx.cache.implementors.get(&it.def_id) {
         // The DefId is for the first Type found with that name. The bool is
@@ -752,7 +756,7 @@ fn item_trait(w: &mut Buffer, cx: &Context<'_>, it: &clean::Item, t: &clean::Tra
         path = if it.def_id.is_local() {
             cx.current.join("/")
         } else {
-            let (ref path, _) = cx.cache.external_paths[&it.def_id];
+            let (ref path, _) = cx.cache.external_paths[&it.def_id.expect_real()];
             path[..path.len() - 1].join("/")
         },
         ty = it.type_(),
@@ -778,7 +782,7 @@ fn item_trait_alias(w: &mut Buffer, cx: &Context<'_>, it: &clean::Item, t: &clea
     // won't be visible anywhere in the docs. It would be nice to also show
     // associated items from the aliased type (see discussion in #32077), but
     // we need #14072 to make sense of the generics.
-    render_assoc_items(w, cx, it, it.def_id, AssocItemRender::All)
+    render_assoc_items(w, cx, it, it.def_id.expect_real(), AssocItemRender::All)
 }
 
 fn item_opaque_ty(w: &mut Buffer, cx: &Context<'_>, it: &clean::Item, t: &clean::OpaqueTy) {
@@ -799,7 +803,7 @@ fn item_opaque_ty(w: &mut Buffer, cx: &Context<'_>, it: &clean::Item, t: &clean:
     // won't be visible anywhere in the docs. It would be nice to also show
     // associated items from the aliased type (see discussion in #32077), but
     // we need #14072 to make sense of the generics.
-    render_assoc_items(w, cx, it, it.def_id, AssocItemRender::All)
+    render_assoc_items(w, cx, it, it.def_id.expect_real(), AssocItemRender::All)
 }
 
 fn item_typedef(w: &mut Buffer, cx: &Context<'_>, it: &clean::Item, t: &clean::Typedef) {
@@ -820,7 +824,7 @@ fn item_typedef(w: &mut Buffer, cx: &Context<'_>, it: &clean::Item, t: &clean::T
     // won't be visible anywhere in the docs. It would be nice to also show
     // associated items from the aliased type (see discussion in #32077), but
     // we need #14072 to make sense of the generics.
-    render_assoc_items(w, cx, it, it.def_id, AssocItemRender::All)
+    render_assoc_items(w, cx, it, it.def_id.expect_real(), AssocItemRender::All)
 }
 
 fn item_union(w: &mut Buffer, cx: &Context<'_>, it: &clean::Item, s: &clean::Union) {
@@ -866,7 +870,7 @@ fn item_union(w: &mut Buffer, cx: &Context<'_>, it: &clean::Item, s: &clean::Uni
             document(w, cx, field, Some(it));
         }
     }
-    render_assoc_items(w, cx, it, it.def_id, AssocItemRender::All)
+    render_assoc_items(w, cx, it, it.def_id.expect_real(), AssocItemRender::All)
 }
 
 fn item_enum(w: &mut Buffer, cx: &Context<'_>, it: &clean::Item, e: &clean::Enum) {
@@ -1000,7 +1004,7 @@ fn item_enum(w: &mut Buffer, cx: &Context<'_>, it: &clean::Item, e: &clean::Enum
             render_stability_since(w, variant, it, cx.tcx());
         }
     }
-    render_assoc_items(w, cx, it, it.def_id, AssocItemRender::All)
+    render_assoc_items(w, cx, it, it.def_id.expect_real(), AssocItemRender::All)
 }
 
 fn item_macro(w: &mut Buffer, cx: &Context<'_>, it: &clean::Item, t: &clean::Macro) {
@@ -1049,7 +1053,7 @@ fn item_proc_macro(w: &mut Buffer, cx: &Context<'_>, it: &clean::Item, m: &clean
 
 fn item_primitive(w: &mut Buffer, cx: &Context<'_>, it: &clean::Item) {
     document(w, cx, it, None);
-    render_assoc_items(w, cx, it, it.def_id, AssocItemRender::All)
+    render_assoc_items(w, cx, it, it.def_id.expect_real(), AssocItemRender::All)
 }
 
 fn item_constant(w: &mut Buffer, cx: &Context<'_>, it: &clean::Item, c: &clean::Constant) {
@@ -1137,7 +1141,7 @@ fn item_struct(w: &mut Buffer, cx: &Context<'_>, it: &clean::Item, s: &clean::St
             }
         }
     }
-    render_assoc_items(w, cx, it, it.def_id, AssocItemRender::All)
+    render_assoc_items(w, cx, it, it.def_id.expect_real(), AssocItemRender::All)
 }
 
 fn item_static(w: &mut Buffer, cx: &Context<'_>, it: &clean::Item, s: &clean::Static) {
@@ -1166,7 +1170,7 @@ fn item_foreign_type(w: &mut Buffer, cx: &Context<'_>, it: &clean::Item) {
 
     document(w, cx, it, None);
 
-    render_assoc_items(w, cx, it, it.def_id, AssocItemRender::All)
+    render_assoc_items(w, cx, it, it.def_id.expect_real(), AssocItemRender::All)
 }
 
 fn item_keyword(w: &mut Buffer, cx: &Context<'_>, it: &clean::Item) {
diff --git a/src/librustdoc/html/render/write_shared.rs b/src/librustdoc/html/render/write_shared.rs
index 8e10c696df05d..c493801d9907f 100644
--- a/src/librustdoc/html/render/write_shared.rs
+++ b/src/librustdoc/html/render/write_shared.rs
@@ -464,6 +464,8 @@ pub(super) fn write_shared(
     // Update the list of all implementors for traits
     let dst = cx.dst.join("implementors");
     for (&did, imps) in &cx.cache.implementors {
+        let did = did.expect_real();
+
         // Private modules can leak through to this phase of rustdoc, which
         // could contain implementations for otherwise private types. In some
         // rare cases we could find an implementation for an item which wasn't
@@ -496,7 +498,7 @@ pub(super) fn write_shared(
                 //
                 // If the implementation is from another crate then that crate
                 // should add it.
-                if imp.impl_item.def_id.krate == did.krate || !imp.impl_item.def_id.is_local() {
+                if imp.impl_item.def_id.krate() == did.krate || !imp.impl_item.def_id.is_local() {
                     None
                 } else {
                     Some(Implementor {
diff --git a/src/librustdoc/json/conversions.rs b/src/librustdoc/json/conversions.rs
index 8ca6342462fc4..e3f1c6b1e2dc4 100644
--- a/src/librustdoc/json/conversions.rs
+++ b/src/librustdoc/json/conversions.rs
@@ -9,13 +9,14 @@ use std::convert::From;
 use rustc_ast::ast;
 use rustc_hir::def::CtorKind;
 use rustc_middle::ty::TyCtxt;
-use rustc_span::def_id::{DefId, CRATE_DEF_INDEX};
+use rustc_span::def_id::CRATE_DEF_INDEX;
 use rustc_span::Pos;
 
 use rustdoc_json_types::*;
 
 use crate::clean;
 use crate::clean::utils::print_const_expr;
+use crate::clean::FakeDefId;
 use crate::formats::item_type::ItemType;
 use crate::json::JsonRenderer;
 use std::collections::HashSet;
@@ -48,7 +49,7 @@ impl JsonRenderer<'_> {
         };
         Some(Item {
             id: from_def_id(def_id),
-            crate_id: def_id.krate.as_u32(),
+            crate_id: def_id.krate().as_u32(),
             name: name.map(|sym| sym.to_string()),
             span: self.convert_span(span),
             visibility: self.convert_visibility(visibility),
@@ -87,7 +88,7 @@ impl JsonRenderer<'_> {
             Inherited => Visibility::Default,
             Restricted(did) if did.index == CRATE_DEF_INDEX => Visibility::Crate,
             Restricted(did) => Visibility::Restricted {
-                parent: from_def_id(did),
+                parent: from_def_id(did.into()),
                 path: self.tcx.def_path(did).to_string_no_crate_verbose(),
             },
         }
@@ -171,8 +172,13 @@ impl FromWithTcx<clean::TypeBindingKind> for TypeBindingKind {
     }
 }
 
-crate fn from_def_id(did: DefId) -> Id {
-    Id(format!("{}:{}", did.krate.as_u32(), u32::from(did.index)))
+crate fn from_def_id(did: FakeDefId) -> Id {
+    match did {
+        FakeDefId::Real(did) => Id(format!("{}:{}", did.krate.as_u32(), u32::from(did.index))),
+        // We need to differentiate real and fake ids, because the indices might overlap for fake
+        // and real DefId's, which would cause two different Id's treated as they were the same.
+        FakeDefId::Fake(idx, krate) => Id(format!("F{}:{}", krate.as_u32(), u32::from(idx))),
+    }
 }
 
 fn from_clean_item(item: clean::Item, tcx: TyCtxt<'_>) -> ItemEnum {
@@ -368,7 +374,7 @@ impl FromWithTcx<clean::Type> for Type {
         match ty {
             ResolvedPath { path, param_names, did, is_generic: _ } => Type::ResolvedPath {
                 name: path.whole_name(),
-                id: from_def_id(did),
+                id: from_def_id(did.into()),
                 args: path.segments.last().map(|args| Box::new(args.clone().args.into_tcx(tcx))),
                 param_names: param_names
                     .map(|v| v.into_iter().map(|x| x.into_tcx(tcx)).collect())
@@ -540,13 +546,13 @@ impl FromWithTcx<clean::Import> for Import {
             Simple(s) => Import {
                 source: import.source.path.whole_name(),
                 name: s.to_string(),
-                id: import.source.did.map(from_def_id),
+                id: import.source.did.map(FakeDefId::from).map(from_def_id),
                 glob: false,
             },
             Glob => Import {
                 source: import.source.path.whole_name(),
                 name: import.source.path.last_name().to_string(),
-                id: import.source.did.map(from_def_id),
+                id: import.source.did.map(FakeDefId::from).map(from_def_id),
                 glob: true,
             },
         }
diff --git a/src/librustdoc/json/mod.rs b/src/librustdoc/json/mod.rs
index ae4d1be3ec2b8..d56acad60c00f 100644
--- a/src/librustdoc/json/mod.rs
+++ b/src/librustdoc/json/mod.rs
@@ -17,7 +17,8 @@ use rustc_session::Session;
 
 use rustdoc_json_types as types;
 
-use crate::clean::{self, ExternalCrate};
+use crate::clean;
+use crate::clean::{ExternalCrate, FakeDefId};
 use crate::config::RenderOptions;
 use crate::error::Error;
 use crate::formats::cache::Cache;
@@ -41,7 +42,7 @@ impl JsonRenderer<'tcx> {
         self.tcx.sess
     }
 
-    fn get_trait_implementors(&mut self, id: rustc_span::def_id::DefId) -> Vec<types::Id> {
+    fn get_trait_implementors(&mut self, id: FakeDefId) -> Vec<types::Id> {
         Rc::clone(&self.cache)
             .implementors
             .get(&id)
@@ -58,10 +59,10 @@ impl JsonRenderer<'tcx> {
             .unwrap_or_default()
     }
 
-    fn get_impls(&mut self, id: rustc_span::def_id::DefId) -> Vec<types::Id> {
+    fn get_impls(&mut self, id: FakeDefId) -> Vec<types::Id> {
         Rc::clone(&self.cache)
             .impls
-            .get(&id)
+            .get(&id.expect_real())
             .map(|impls| {
                 impls
                     .iter()
@@ -89,9 +90,9 @@ impl JsonRenderer<'tcx> {
                     let trait_item = &trait_item.trait_;
                     trait_item.items.clone().into_iter().for_each(|i| self.item(i).unwrap());
                     Some((
-                        from_def_id(id),
+                        from_def_id(id.into()),
                         types::Item {
-                            id: from_def_id(id),
+                            id: from_def_id(id.into()),
                             crate_id: id.krate.as_u32(),
                             name: self
                                 .cache
@@ -205,7 +206,7 @@ impl<'tcx> FormatRenderer<'tcx> for JsonRenderer<'tcx> {
                 .chain(self.cache.external_paths.clone().into_iter())
                 .map(|(k, (path, kind))| {
                     (
-                        from_def_id(k),
+                        from_def_id(k.into()),
                         types::ItemSummary {
                             crate_id: k.krate.as_u32(),
                             path,
diff --git a/src/librustdoc/passes/check_code_block_syntax.rs b/src/librustdoc/passes/check_code_block_syntax.rs
index 68a66806e0476..8d07cde51880c 100644
--- a/src/librustdoc/passes/check_code_block_syntax.rs
+++ b/src/librustdoc/passes/check_code_block_syntax.rs
@@ -111,7 +111,11 @@ impl<'a, 'tcx> DocFolder for SyntaxChecker<'a, 'tcx> {
     fn fold_item(&mut self, item: clean::Item) -> Option<clean::Item> {
         if let Some(dox) = &item.attrs.collapsed_doc_value() {
             let sp = item.attr_span(self.cx.tcx);
-            let extra = crate::html::markdown::ExtraInfo::new_did(self.cx.tcx, item.def_id, sp);
+            let extra = crate::html::markdown::ExtraInfo::new_did(
+                self.cx.tcx,
+                item.def_id.expect_real(),
+                sp,
+            );
             for code_block in markdown::rust_code_blocks(&dox, &extra) {
                 self.check_rust_syntax(&item, &dox, code_block);
             }
diff --git a/src/librustdoc/passes/collect_intra_doc_links.rs b/src/librustdoc/passes/collect_intra_doc_links.rs
index f1064756fdde7..c5fb54e52c7c4 100644
--- a/src/librustdoc/passes/collect_intra_doc_links.rs
+++ b/src/librustdoc/passes/collect_intra_doc_links.rs
@@ -30,7 +30,9 @@ use std::convert::{TryFrom, TryInto};
 use std::mem;
 use std::ops::Range;
 
-use crate::clean::{self, utils::find_nearest_parent_module, Crate, Item, ItemLink, PrimitiveType};
+use crate::clean::{
+    self, utils::find_nearest_parent_module, Crate, FakeDefId, Item, ItemLink, PrimitiveType,
+};
 use crate::core::DocContext;
 use crate::fold::DocFolder;
 use crate::html::markdown::{markdown_links, MarkdownLink};
@@ -246,7 +248,7 @@ enum AnchorFailure {
 
 #[derive(Clone, Debug, Hash, PartialEq, Eq)]
 struct ResolutionInfo {
-    module_id: DefId,
+    module_id: FakeDefId,
     dis: Option<Disambiguator>,
     path_str: String,
     extra_fragment: Option<String>,
@@ -272,7 +274,7 @@ struct LinkCollector<'a, 'tcx> {
     ///
     /// The last module will be used if the parent scope of the current item is
     /// unknown.
-    mod_ids: Vec<DefId>,
+    mod_ids: Vec<FakeDefId>,
     /// This is used to store the kind of associated items,
     /// because `clean` and the disambiguator code expect them to be different.
     /// See the code for associated items on inherent impls for details.
@@ -296,7 +298,7 @@ impl<'a, 'tcx> LinkCollector<'a, 'tcx> {
     ) -> Result<(Res, Option<String>), ErrorKind<'path>> {
         let tcx = self.cx.tcx;
         let no_res = || ResolutionFailure::NotResolved {
-            module_id,
+            module_id: module_id.into(),
             partial_res: None,
             unresolved: path_str.into(),
         };
@@ -524,7 +526,7 @@ impl<'a, 'tcx> LinkCollector<'a, 'tcx> {
                     // but the disambiguator logic expects the associated item.
                     // Store the kind in a side channel so that only the disambiguator logic looks at it.
                     if let Some((kind, id)) = side_channel {
-                        self.kind_side_channel.set(Some((kind, id)));
+                        self.kind_side_channel.set(Some((kind, id.into())));
                     }
                     Ok((res, Some(fragment)))
                 };
@@ -795,7 +797,7 @@ impl<'a, 'tcx> DocFolder for LinkCollector<'a, 'tcx> {
         let parent_node = if item.is_fake() {
             None
         } else {
-            find_nearest_parent_module(self.cx.tcx, item.def_id)
+            find_nearest_parent_module(self.cx.tcx, item.def_id.expect_real())
         };
 
         if parent_node.is_some() {
@@ -807,31 +809,34 @@ impl<'a, 'tcx> DocFolder for LinkCollector<'a, 'tcx> {
         let self_id = if item.is_fake() {
             None
         // Checking if the item is a field in an enum variant
-        } else if (matches!(self.cx.tcx.def_kind(item.def_id), DefKind::Field)
+        } else if (matches!(self.cx.tcx.def_kind(item.def_id.expect_real()), DefKind::Field)
             && matches!(
-                self.cx.tcx.def_kind(self.cx.tcx.parent(item.def_id).unwrap()),
+                self.cx.tcx.def_kind(self.cx.tcx.parent(item.def_id.expect_real()).unwrap()),
                 DefKind::Variant
             ))
         {
-            self.cx.tcx.parent(item.def_id).and_then(|item_id| self.cx.tcx.parent(item_id))
+            self.cx
+                .tcx
+                .parent(item.def_id.expect_real())
+                .and_then(|item_id| self.cx.tcx.parent(item_id))
         } else if matches!(
-            self.cx.tcx.def_kind(item.def_id),
+            self.cx.tcx.def_kind(item.def_id.expect_real()),
             DefKind::AssocConst
                 | DefKind::AssocFn
                 | DefKind::AssocTy
                 | DefKind::Variant
                 | DefKind::Field
         ) {
-            self.cx.tcx.parent(item.def_id)
+            self.cx.tcx.parent(item.def_id.expect_real())
         // HACK(jynelson): `clean` marks associated types as `TypedefItem`, not as `AssocTypeItem`.
         // Fixing this breaks `fn render_deref_methods`.
         // As a workaround, see if the parent of the item is an `impl`; if so this must be an associated item,
         // regardless of what rustdoc wants to call it.
-        } else if let Some(parent) = self.cx.tcx.parent(item.def_id) {
+        } else if let Some(parent) = self.cx.tcx.parent(item.def_id.expect_real()) {
             let parent_kind = self.cx.tcx.def_kind(parent);
-            Some(if parent_kind == DefKind::Impl { parent } else { item.def_id })
+            Some(if parent_kind == DefKind::Impl { parent } else { item.def_id.expect_real() })
         } else {
-            Some(item.def_id)
+            Some(item.def_id.expect_real())
         };
 
         // FIXME(jynelson): this shouldn't go through stringification, rustdoc should just use the DefId directly
@@ -869,7 +874,7 @@ impl<'a, 'tcx> DocFolder for LinkCollector<'a, 'tcx> {
             let (krate, parent_node) = if let Some(id) = parent_module {
                 (id.krate, Some(id))
             } else {
-                (item.def_id.krate, parent_node)
+                (item.def_id.krate(), parent_node)
             };
             // NOTE: if there are links that start in one crate and end in another, this will not resolve them.
             // This is a degenerate case and it's not supported by rustdoc.
@@ -1065,8 +1070,11 @@ impl LinkCollector<'_, '_> {
         // we've already pushed this node onto the resolution stack but
         // for outer comments we explicitly try and resolve against the
         // parent_node first.
-        let base_node =
-            if item.is_mod() && inner_docs { self.mod_ids.last().copied() } else { parent_node };
+        let base_node = if item.is_mod() && inner_docs {
+            self.mod_ids.last().copied()
+        } else {
+            parent_node.map(|id| FakeDefId::new_real(id))
+        };
 
         let mut module_id = if let Some(id) = base_node {
             id
@@ -1111,7 +1119,7 @@ impl LinkCollector<'_, '_> {
                 resolved_self = format!("self::{}", &path_str["crate::".len()..]);
                 path_str = &resolved_self;
             }
-            module_id = DefId { krate, index: CRATE_DEF_INDEX };
+            module_id = FakeDefId::new_real(DefId { krate, index: CRATE_DEF_INDEX });
         }
 
         let (mut res, mut fragment) = self.resolve_with_disambiguator_cached(
@@ -1172,8 +1180,8 @@ impl LinkCollector<'_, '_> {
             report_diagnostic(self.cx.tcx, BROKEN_INTRA_DOC_LINKS, &msg, &diag_info, callback);
         };
 
-        let verify = |kind: DefKind, id: DefId| {
-            let (kind, id) = self.kind_side_channel.take().unwrap_or((kind, id));
+        let verify = |kind: DefKind, id: FakeDefId| {
+            let (kind, id) = self.kind_side_channel.take().unwrap_or((kind, id.expect_real()));
             debug!("intra-doc link to {} resolved to {:?} (id: {:?})", path_str, res, id);
 
             // Disallow e.g. linking to enums with `struct@`
@@ -1227,7 +1235,7 @@ impl LinkCollector<'_, '_> {
                     // doesn't allow statements like `use str::trim;`, making this a (hopefully)
                     // valid omission. See https://github.com/rust-lang/rust/pull/80660#discussion_r551585677
                     // for discussion on the matter.
-                    verify(kind, id)?;
+                    verify(kind, id.into())?;
 
                     // FIXME: it would be nice to check that the feature gate was enabled in the original crate, not just ignore it altogether.
                     // However I'm not sure how to check that across crates.
@@ -1265,9 +1273,9 @@ impl LinkCollector<'_, '_> {
                 Some(ItemLink { link: ori_link.link, link_text, did: None, fragment })
             }
             Res::Def(kind, id) => {
-                verify(kind, id)?;
+                verify(kind, id.into())?;
                 let id = clean::register_res(self.cx, rustc_hir::def::Res::Def(kind, id));
-                Some(ItemLink { link: ori_link.link, link_text, did: Some(id), fragment })
+                Some(ItemLink { link: ori_link.link, link_text, did: Some(id.into()), fragment })
             }
         }
     }
@@ -1333,7 +1341,7 @@ impl LinkCollector<'_, '_> {
 
         match disambiguator.map(Disambiguator::ns) {
             Some(expected_ns @ (ValueNS | TypeNS)) => {
-                match self.resolve(path_str, expected_ns, base_node, extra_fragment) {
+                match self.resolve(path_str, expected_ns, base_node.expect_real(), extra_fragment) {
                     Ok(res) => Some(res),
                     Err(ErrorKind::Resolve(box mut kind)) => {
                         // We only looked in one namespace. Try to give a better error if possible.
@@ -1342,9 +1350,12 @@ impl LinkCollector<'_, '_> {
                             // FIXME: really it should be `resolution_failure` that does this, not `resolve_with_disambiguator`
                             // See https://github.com/rust-lang/rust/pull/76955#discussion_r493953382 for a good approach
                             for &new_ns in &[other_ns, MacroNS] {
-                                if let Some(res) =
-                                    self.check_full_res(new_ns, path_str, base_node, extra_fragment)
-                                {
+                                if let Some(res) = self.check_full_res(
+                                    new_ns,
+                                    path_str,
+                                    base_node.expect_real(),
+                                    extra_fragment,
+                                ) {
                                     kind = ResolutionFailure::WrongNamespace { res, expected_ns };
                                     break;
                                 }
@@ -1366,9 +1377,14 @@ impl LinkCollector<'_, '_> {
                 // Try everything!
                 let mut candidates = PerNS {
                     macro_ns: self
-                        .resolve_macro(path_str, base_node)
+                        .resolve_macro(path_str, base_node.expect_real())
                         .map(|res| (res, extra_fragment.clone())),
-                    type_ns: match self.resolve(path_str, TypeNS, base_node, extra_fragment) {
+                    type_ns: match self.resolve(
+                        path_str,
+                        TypeNS,
+                        base_node.expect_real(),
+                        extra_fragment,
+                    ) {
                         Ok(res) => {
                             debug!("got res in TypeNS: {:?}", res);
                             Ok(res)
@@ -1379,7 +1395,12 @@ impl LinkCollector<'_, '_> {
                         }
                         Err(ErrorKind::Resolve(box kind)) => Err(kind),
                     },
-                    value_ns: match self.resolve(path_str, ValueNS, base_node, extra_fragment) {
+                    value_ns: match self.resolve(
+                        path_str,
+                        ValueNS,
+                        base_node.expect_real(),
+                        extra_fragment,
+                    ) {
                         Ok(res) => Ok(res),
                         Err(ErrorKind::AnchorFailure(msg)) => {
                             anchor_failure(self.cx, diag, msg);
@@ -1421,7 +1442,7 @@ impl LinkCollector<'_, '_> {
                 }
 
                 if len == 1 {
-                    Some(candidates.into_iter().filter_map(|res| res.ok()).next().unwrap())
+                    Some(candidates.into_iter().find_map(|res| res.ok()).unwrap())
                 } else if len == 2 && is_derive_trait_collision(&candidates) {
                     Some(candidates.type_ns.unwrap())
                 } else {
@@ -1435,14 +1456,17 @@ impl LinkCollector<'_, '_> {
                 }
             }
             Some(MacroNS) => {
-                match self.resolve_macro(path_str, base_node) {
+                match self.resolve_macro(path_str, base_node.expect_real()) {
                     Ok(res) => Some((res, extra_fragment.clone())),
                     Err(mut kind) => {
                         // `resolve_macro` only looks in the macro namespace. Try to give a better error if possible.
                         for &ns in &[TypeNS, ValueNS] {
-                            if let Some(res) =
-                                self.check_full_res(ns, path_str, base_node, extra_fragment)
-                            {
+                            if let Some(res) = self.check_full_res(
+                                ns,
+                                path_str,
+                                base_node.expect_real(),
+                                extra_fragment,
+                            ) {
                                 kind =
                                     ResolutionFailure::WrongNamespace { res, expected_ns: MacroNS };
                                 break;
@@ -1795,7 +1819,7 @@ fn resolution_failure(
                         name = start;
                         for &ns in &[TypeNS, ValueNS, MacroNS] {
                             if let Some(res) =
-                                collector.check_full_res(ns, &start, module_id, &None)
+                                collector.check_full_res(ns, &start, module_id.into(), &None)
                             {
                                 debug!("found partial_res={:?}", res);
                                 *partial_res = Some(res);
diff --git a/src/librustdoc/passes/collect_trait_impls.rs b/src/librustdoc/passes/collect_trait_impls.rs
index 7b0b2f28fdfff..90b797da24915 100644
--- a/src/librustdoc/passes/collect_trait_impls.rs
+++ b/src/librustdoc/passes/collect_trait_impls.rs
@@ -46,7 +46,7 @@ crate fn collect_trait_impls(krate: Crate, cx: &mut DocContext<'_>) -> Crate {
 
                 // FIXME(eddyb) is this `doc(hidden)` check needed?
                 if !cx.tcx.get_attrs(def_id).lists(sym::doc).has_word(sym::hidden) {
-                    let impls = get_auto_trait_and_blanket_impls(cx, def_id);
+                    let impls = get_auto_trait_and_blanket_impls(cx, def_id.into());
                     new_items.extend(impls.filter(|i| cx.inlined.insert(i.def_id)));
                 }
             });
@@ -117,8 +117,8 @@ crate fn collect_trait_impls(krate: Crate, cx: &mut DocContext<'_>) -> Crate {
                     // Avoid infinite cycles
                     return;
                 }
-                cleaner.items.insert(target_did);
-                add_deref_target(map, cleaner, &target_did);
+                cleaner.items.insert(target_did.into());
+                add_deref_target(map, cleaner, &target_did.into());
             }
         }
     }
@@ -126,7 +126,7 @@ crate fn collect_trait_impls(krate: Crate, cx: &mut DocContext<'_>) -> Crate {
         // Since only the `DefId` portion of the `Type` instances is known to be same for both the
         // `Deref` target type and the impl for type positions, this map of types is keyed by
         // `DefId` and for convenience uses a special cleaner that accepts `DefId`s directly.
-        if cleaner.keep_impl_with_def_id(type_did) {
+        if cleaner.keep_impl_with_def_id(&FakeDefId::new_real(*type_did)) {
             add_deref_target(&type_did_to_deref_target, &mut cleaner, type_did);
         }
     }
@@ -163,8 +163,10 @@ impl<'a, 'tcx> DocFolder for SyntheticImplCollector<'a, 'tcx> {
     fn fold_item(&mut self, i: Item) -> Option<Item> {
         if i.is_struct() || i.is_enum() || i.is_union() {
             // FIXME(eddyb) is this `doc(hidden)` check needed?
-            if !self.cx.tcx.get_attrs(i.def_id).lists(sym::doc).has_word(sym::hidden) {
-                self.impls.extend(get_auto_trait_and_blanket_impls(self.cx, i.def_id));
+            if !self.cx.tcx.get_attrs(i.def_id.expect_real()).lists(sym::doc).has_word(sym::hidden)
+            {
+                self.impls
+                    .extend(get_auto_trait_and_blanket_impls(self.cx, i.def_id.expect_real()));
             }
         }
 
@@ -174,7 +176,7 @@ impl<'a, 'tcx> DocFolder for SyntheticImplCollector<'a, 'tcx> {
 
 #[derive(Default)]
 struct ItemCollector {
-    items: FxHashSet<DefId>,
+    items: FxHashSet<FakeDefId>,
 }
 
 impl ItemCollector {
@@ -193,7 +195,7 @@ impl DocFolder for ItemCollector {
 
 struct BadImplStripper {
     prims: FxHashSet<PrimitiveType>,
-    items: FxHashSet<DefId>,
+    items: FxHashSet<FakeDefId>,
 }
 
 impl BadImplStripper {
@@ -204,13 +206,13 @@ impl BadImplStripper {
         } else if let Some(prim) = ty.primitive_type() {
             self.prims.contains(&prim)
         } else if let Some(did) = ty.def_id() {
-            self.keep_impl_with_def_id(&did)
+            self.keep_impl_with_def_id(&did.into())
         } else {
             false
         }
     }
 
-    fn keep_impl_with_def_id(&self, did: &DefId) -> bool {
+    fn keep_impl_with_def_id(&self, did: &FakeDefId) -> bool {
         self.items.contains(did)
     }
 }
diff --git a/src/librustdoc/passes/doc_test_lints.rs b/src/librustdoc/passes/doc_test_lints.rs
index c8d2263d81d54..7362cfcb71b7e 100644
--- a/src/librustdoc/passes/doc_test_lints.rs
+++ b/src/librustdoc/passes/doc_test_lints.rs
@@ -53,7 +53,7 @@ impl crate::doctest::Tester for Tests {
 }
 
 crate fn should_have_doc_example(cx: &DocContext<'_>, item: &clean::Item) -> bool {
-    if !cx.cache.access_levels.is_public(item.def_id)
+    if !cx.cache.access_levels.is_public(item.def_id.expect_real())
         || matches!(
             *item.kind,
             clean::StructFieldItem(_)
@@ -105,7 +105,8 @@ crate fn look_for_tests<'tcx>(cx: &DocContext<'tcx>, dox: &str, item: &Item) {
                 |lint| lint.build("missing code example in this documentation").emit(),
             );
         }
-    } else if tests.found_tests > 0 && !cx.cache.access_levels.is_public(item.def_id) {
+    } else if tests.found_tests > 0 && !cx.cache.access_levels.is_public(item.def_id.expect_real())
+    {
         cx.tcx.struct_span_lint_hir(
             crate::lint::PRIVATE_DOC_TESTS,
             hir_id,
diff --git a/src/librustdoc/passes/strip_hidden.rs b/src/librustdoc/passes/strip_hidden.rs
index 54c4ed22f1c4b..e5910d081a5d0 100644
--- a/src/librustdoc/passes/strip_hidden.rs
+++ b/src/librustdoc/passes/strip_hidden.rs
@@ -1,9 +1,8 @@
-use rustc_hir::def_id::DefIdSet;
 use rustc_span::symbol::sym;
 use std::mem;
 
 use crate::clean;
-use crate::clean::{Item, NestedAttributesExt};
+use crate::clean::{FakeDefIdSet, Item, NestedAttributesExt};
 use crate::core::DocContext;
 use crate::fold::{DocFolder, StripItem};
 use crate::passes::{ImplStripper, Pass};
@@ -16,7 +15,7 @@ crate const STRIP_HIDDEN: Pass = Pass {
 
 /// Strip items marked `#[doc(hidden)]`
 crate fn strip_hidden(krate: clean::Crate, _: &mut DocContext<'_>) -> clean::Crate {
-    let mut retained = DefIdSet::default();
+    let mut retained = FakeDefIdSet::default();
 
     // strip all #[doc(hidden)] items
     let krate = {
@@ -30,7 +29,7 @@ crate fn strip_hidden(krate: clean::Crate, _: &mut DocContext<'_>) -> clean::Cra
 }
 
 struct Stripper<'a> {
-    retained: &'a mut DefIdSet,
+    retained: &'a mut FakeDefIdSet,
     update_retained: bool,
 }
 
diff --git a/src/librustdoc/passes/strip_private.rs b/src/librustdoc/passes/strip_private.rs
index fc8bbc9715029..18abeb607a17f 100644
--- a/src/librustdoc/passes/strip_private.rs
+++ b/src/librustdoc/passes/strip_private.rs
@@ -1,6 +1,4 @@
-use rustc_hir::def_id::DefIdSet;
-
-use crate::clean;
+use crate::clean::{self, FakeDefIdSet};
 use crate::core::DocContext;
 use crate::fold::DocFolder;
 use crate::passes::{ImplStripper, ImportStripper, Pass, Stripper};
@@ -16,7 +14,7 @@ crate const STRIP_PRIVATE: Pass = Pass {
 /// crate, specified by the `xcrate` flag.
 crate fn strip_private(mut krate: clean::Crate, cx: &mut DocContext<'_>) -> clean::Crate {
     // This stripper collects all *retained* nodes.
-    let mut retained = DefIdSet::default();
+    let mut retained = FakeDefIdSet::default();
 
     // strip all private items
     {
diff --git a/src/librustdoc/passes/stripper.rs b/src/librustdoc/passes/stripper.rs
index 44d54563f27a2..87399256292a8 100644
--- a/src/librustdoc/passes/stripper.rs
+++ b/src/librustdoc/passes/stripper.rs
@@ -1,12 +1,12 @@
-use rustc_hir::def_id::{DefId, DefIdSet};
+use rustc_hir::def_id::DefId;
 use rustc_middle::middle::privacy::AccessLevels;
 use std::mem;
 
-use crate::clean::{self, GetDefId, Item};
+use crate::clean::{self, FakeDefIdSet, GetDefId, Item};
 use crate::fold::{DocFolder, StripItem};
 
 crate struct Stripper<'a> {
-    crate retained: &'a mut DefIdSet,
+    crate retained: &'a mut FakeDefIdSet,
     crate access_levels: &'a AccessLevels<DefId>,
     crate update_retained: bool,
 }
@@ -42,7 +42,7 @@ impl<'a> DocFolder for Stripper<'a> {
             | clean::TraitAliasItem(..)
             | clean::ForeignTypeItem => {
                 if i.def_id.is_local() {
-                    if !self.access_levels.is_exported(i.def_id) {
+                    if !self.access_levels.is_exported(i.def_id.expect_real()) {
                         debug!("Stripper: stripping {:?} {:?}", i.type_(), i.name);
                         return None;
                     }
@@ -116,7 +116,7 @@ impl<'a> DocFolder for Stripper<'a> {
 
 /// This stripper discards all impls which reference stripped items
 crate struct ImplStripper<'a> {
-    crate retained: &'a DefIdSet,
+    crate retained: &'a FakeDefIdSet,
 }
 
 impl<'a> DocFolder for ImplStripper<'a> {
@@ -127,13 +127,14 @@ impl<'a> DocFolder for ImplStripper<'a> {
                 return None;
             }
             if let Some(did) = imp.for_.def_id() {
-                if did.is_local() && !imp.for_.is_generic() && !self.retained.contains(&did) {
+                if did.is_local() && !imp.for_.is_generic() && !self.retained.contains(&did.into())
+                {
                     debug!("ImplStripper: impl item for stripped type; removing");
                     return None;
                 }
             }
             if let Some(did) = imp.trait_.def_id() {
-                if did.is_local() && !self.retained.contains(&did) {
+                if did.is_local() && !self.retained.contains(&did.into()) {
                     debug!("ImplStripper: impl item for stripped trait; removing");
                     return None;
                 }
@@ -141,7 +142,7 @@ impl<'a> DocFolder for ImplStripper<'a> {
             if let Some(generics) = imp.trait_.as_ref().and_then(|t| t.generics()) {
                 for typaram in generics {
                     if let Some(did) = typaram.def_id() {
-                        if did.is_local() && !self.retained.contains(&did) {
+                        if did.is_local() && !self.retained.contains(&did.into()) {
                             debug!(
                                 "ImplStripper: stripped item in trait's generics; removing impl"
                             );
diff --git a/src/librustdoc/visit_ast.rs b/src/librustdoc/visit_ast.rs
index ab9a112380ec4..191d8d5a2ea3b 100644
--- a/src/librustdoc/visit_ast.rs
+++ b/src/librustdoc/visit_ast.rs
@@ -191,7 +191,7 @@ impl<'a, 'tcx> RustdocVisitor<'a, 'tcx> {
                     } else {
                         // All items need to be handled here in case someone wishes to link
                         // to them with intra-doc links
-                        self.cx.cache.access_levels.map.insert(did, AccessLevel::Public);
+                        self.cx.cache.access_levels.map.insert(did.into(), AccessLevel::Public);
                     }
                 }
             }
@@ -203,7 +203,7 @@ impl<'a, 'tcx> RustdocVisitor<'a, 'tcx> {
             None => return false,
         };
 
-        let is_private = !self.cx.cache.access_levels.is_public(res_did);
+        let is_private = !self.cx.cache.access_levels.is_public(res_did.into());
         let is_hidden = inherits_doc_hidden(self.cx.tcx, res_hir_id);
 
         // Only inline if requested or if the item would otherwise be stripped.