Auto merge of #69023 - Centril:parse_fn, r=petrochenkov

parse: unify function front matter parsing

Part of #68728.

- `const extern fn` feature gating is now done post-expansion such that we do not have conditional compatibilities of function qualifiers *in parsing*.

- The `FnFrontMatter` grammar becomes:
   ```rust
   Extern = "extern" StringLit ;
   FnQual = "const"? "async"? "unsafe"? Extern? ;
   FnFrontMatter = FnQual "fn" ;
   ```

   That is, all item contexts now *syntactically* allow `const async unsafe extern "C" fn` and use semantic restrictions to rule out combinations previously prevented syntactically. The semantic restrictions include in particular:

   - `fn`s in `extern { ... }` can have no qualifiers.
   - `const` and `async` cannot be combined.

- We change `ast::{Unsafety, Spanned<Constness>}>` into `enum ast::{Unsafe, Const} { Yes(Span), No }` respectively. This change in formulation allow us to exclude `Span` in the case of `No`, which facilitates parsing. Moreover, we also add a `Span` to `IsAsync` which is renamed to `Async`. The new `Span`s in `Unsafety` and `Async` are then taken advantage of for better diagnostics. A reason this change was made is to have a more uniform and clear naming scheme.

  The HIR keeps the structures in AST (with those definitions moved into HIR) for now to avoid regressing perf.

r? @petrochenkov

src/librustc/traits/auto_trait.rs

@@ -9,7 +9,6 @@ use crate::ty::fold::TypeFolder;
 use crate::ty::{Region, RegionVid};
 
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
-use syntax::ast;
 
 use std::collections::hash_map::Entry;
 use std::collections::VecDeque;
@@ -350,7 +349,7 @@ impl AutoTraitFinder<'tcx> {
                         already_visited.remove(&pred);
                         self.add_user_pred(
                             &mut user_computed_preds,
-                            ty::Predicate::Trait(pred, ast::Constness::NotConst),
+                            ty::Predicate::Trait(pred, hir::Constness::NotConst),
                         );
                         predicates.push_back(pred);
                     } else {

src/librustc/traits/error_reporting/mod.rs

@@ -695,7 +695,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                             let unit_obligation = Obligation {
                                 predicate: ty::Predicate::Trait(
                                     predicate,
-                                    ast::Constness::NotConst,
+                                    hir::Constness::NotConst,
                                 ),
                                 ..obligation.clone()
                             };

src/librustc/traits/select.rs

@@ -40,19 +40,19 @@ use crate::ty::fast_reject;
 use crate::ty::relate::TypeRelation;
 use crate::ty::subst::{Subst, SubstsRef};
 use crate::ty::{self, ToPolyTraitRef, ToPredicate, Ty, TyCtxt, TypeFoldable, WithConstness};
-use rustc_hir::def_id::DefId;
-
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_hir as hir;
+use rustc_hir::def_id::DefId;
 use rustc_index::bit_set::GrowableBitSet;
 use rustc_span::symbol::sym;
 use rustc_target::spec::abi::Abi;
+use syntax::attr;
+
 use std::cell::{Cell, RefCell};
 use std::cmp;
 use std::fmt::{self, Display};
 use std::iter;
 use std::rc::Rc;
-use syntax::{ast, attr};
 
 pub use rustc::traits::types::select::*;
 
@@ -677,7 +677,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             // if the regions match exactly.
             let cycle = stack.iter().skip(1).take_while(|s| s.depth >= cycle_depth);
             let cycle = cycle.map(|stack| {
-                ty::Predicate::Trait(stack.obligation.predicate, ast::Constness::NotConst)
+                ty::Predicate::Trait(stack.obligation.predicate, hir::Constness::NotConst)
             });
             if self.coinductive_match(cycle) {
                 debug!("evaluate_stack({:?}) --> recursive, coinductive", stack.fresh_trait_ref);

src/librustc/ty/fold.rs

@@ -32,6 +32,7 @@
 //! looking for, and does not need to visit anything else.
 
 use crate::ty::{self, flags::FlagComputation, Binder, Ty, TyCtxt, TypeFlags};
+use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 
 use rustc_data_structures::fx::FxHashSet;
@@ -150,7 +151,7 @@ pub trait TypeFoldable<'tcx>: fmt::Debug + Clone {
     }
 }
 
-impl TypeFoldable<'tcx> for syntax::ast::Constness {
+impl TypeFoldable<'tcx> for hir::Constness {
     fn super_fold_with<F: TypeFolder<'tcx>>(&self, _: &mut F) -> Self {
         *self
     }

src/librustc/ty/mod.rs

@@ -35,15 +35,15 @@ use rustc_data_structures::sync::{self, par_iter, Lrc, ParallelIterator};
 use rustc_hir as hir;
 use rustc_hir::def::{CtorKind, CtorOf, DefKind, Res};
 use rustc_hir::def_id::{CrateNum, DefId, DefIdMap, LocalDefId, CRATE_DEF_INDEX, LOCAL_CRATE};
-use rustc_hir::{GlobMap, Node, TraitMap};
+use rustc_hir::{Constness, GlobMap, Node, TraitMap};
 use rustc_index::vec::{Idx, IndexVec};
 use rustc_macros::HashStable;
 use rustc_serialize::{self, Encodable, Encoder};
 use rustc_span::hygiene::ExpnId;
 use rustc_span::symbol::{kw, sym, Symbol};
 use rustc_span::Span;
 use rustc_target::abi::Align;
-use syntax::ast::{self, Constness, Ident, Name};
+use syntax::ast::{self, Ident, Name};
 use syntax::node_id::{NodeId, NodeMap, NodeSet};
 
 use std::cell::RefCell;

src/librustc/ty/print/pretty.rs

@@ -1818,7 +1818,7 @@ define_print_and_forward_display! {
     ty::Predicate<'tcx> {
         match *self {
             ty::Predicate::Trait(ref data, constness) => {
-                if let ast::Constness::Const = constness {
+                if let hir::Constness::Const = constness {
                     p!(write("const "));
                 }
                 p!(print(data))

src/librustc/ty/structural_impls.rs

@@ -7,6 +7,7 @@ use crate::mir::ProjectionKind;
 use crate::ty::fold::{TypeFoldable, TypeFolder, TypeVisitor};
 use crate::ty::print::{FmtPrinter, Printer};
 use crate::ty::{self, InferConst, Lift, Ty, TyCtxt};
+use rustc_hir as hir;
 use rustc_hir::def::Namespace;
 use rustc_hir::def_id::CRATE_DEF_INDEX;
 use rustc_index::vec::{Idx, IndexVec};
@@ -15,7 +16,6 @@ use smallvec::SmallVec;
 use std::fmt;
 use std::rc::Rc;
 use std::sync::Arc;
-use syntax::ast;
 
 impl fmt::Debug for ty::GenericParamDef {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
@@ -236,7 +236,7 @@ impl fmt::Debug for ty::Predicate<'tcx> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match *self {
             ty::Predicate::Trait(ref a, constness) => {
-                if let ast::Constness::Const = constness {
+                if let hir::Constness::Const = constness {
                     write!(f, "const ")?;
                 }
                 a.fmt(f)

src/librustc_ast_lowering/expr.rs

@@ -106,7 +106,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 ref body,
                 fn_decl_span,
             ) => {
-                if let IsAsync::Async { closure_id, .. } = asyncness {
+                if let Async::Yes { closure_id, .. } = asyncness {
                     self.lower_expr_async_closure(
                         capture_clause,
                         closure_id,

src/librustc_ast_lowering/item.rs

@@ -67,10 +67,12 @@ impl<'a> Visitor<'a> for ItemLowerer<'a, '_, '_> {
             self.lctx.with_parent_item_lifetime_defs(hir_id, |this| {
                 let this = &mut ItemLowerer { lctx: this };
                 if let ItemKind::Impl { constness, ref of_trait, .. } = item.kind {
-                    if constness == Constness::Const {
+                    if let Const::Yes(span) = constness {
                         this.lctx
                             .diagnostic()
-                            .span_err(item.span, "const trait impls are not yet implemented");
+                            .struct_span_err(item.span, "const trait impls are not yet implemented")
+                            .span_label(span, "const because of this")
+                            .emit();
                     }
 
                     this.with_trait_impl_ref(of_trait, |this| visit::walk_item(this, item));
@@ -297,7 +299,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     // `impl Future<Output = T>` here because lower_body
                     // only cares about the input argument patterns in the function
                     // declaration (decl), not the return types.
-                    let asyncness = header.asyncness.node;
+                    let asyncness = header.asyncness;
                     let body_id =
                         this.lower_maybe_async_body(span, &decl, asyncness, body.as_deref());
 
@@ -413,10 +415,10 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     });
 
                 hir::ItemKind::Impl {
-                    unsafety,
+                    unsafety: self.lower_unsafety(unsafety),
                     polarity,
                     defaultness: self.lower_defaultness(defaultness, true /* [1] */),
-                    constness,
+                    constness: self.lower_constness(constness),
                     generics,
                     of_trait: trait_ref,
                     self_ty: lowered_ty,
@@ -430,7 +432,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     .alloc_from_iter(items.iter().map(|item| self.lower_trait_item_ref(item)));
                 hir::ItemKind::Trait(
                     is_auto,
-                    unsafety,
+                    self.lower_unsafety(unsafety),
                     self.lower_generics(generics, ImplTraitContext::disallowed()),
                     bounds,
                     items,
@@ -834,19 +836,16 @@ impl<'hir> LoweringContext<'_, 'hir> {
             }
             AssocItemKind::Fn(ref sig, ref body) => {
                 self.current_item = Some(i.span);
-                let body_id = self.lower_maybe_async_body(
-                    i.span,
-                    &sig.decl,
-                    sig.header.asyncness.node,
-                    body.as_deref(),
-                );
+                let asyncness = sig.header.asyncness;
+                let body_id =
+                    self.lower_maybe_async_body(i.span, &sig.decl, asyncness, body.as_deref());
                 let impl_trait_return_allow = !self.is_in_trait_impl;
                 let (generics, sig) = self.lower_method_sig(
                     &i.generics,
                     sig,
                     impl_item_def_id,
                     impl_trait_return_allow,
-                    sig.header.asyncness.node.opt_return_id(),
+                    asyncness.opt_return_id(),
                 );
 
                 (generics, hir::ImplItemKind::Method(sig, body_id))
@@ -1031,12 +1030,12 @@ impl<'hir> LoweringContext<'_, 'hir> {
         &mut self,
         span: Span,
         decl: &FnDecl,
-        asyncness: IsAsync,
+        asyncness: Async,
         body: Option<&Block>,
     ) -> hir::BodyId {
         let closure_id = match asyncness {
-            IsAsync::Async { closure_id, .. } => closure_id,
-            IsAsync::NotAsync => return self.lower_fn_body_block(span, decl, body),
+            Async::Yes { closure_id, .. } => closure_id,
+            Async::No => return self.lower_fn_body_block(span, decl, body),
         };
 
         self.lower_body(|this| {
@@ -1245,9 +1244,9 @@ impl<'hir> LoweringContext<'_, 'hir> {
 
     fn lower_fn_header(&mut self, h: FnHeader) -> hir::FnHeader {
         hir::FnHeader {
-            unsafety: h.unsafety,
-            asyncness: self.lower_asyncness(h.asyncness.node),
-            constness: h.constness.node,
+            unsafety: self.lower_unsafety(h.unsafety),
+            asyncness: self.lower_asyncness(h.asyncness),
+            constness: self.lower_constness(h.constness),
             abi: self.lower_extern(h.ext),
         }
     }
@@ -1274,10 +1273,24 @@ impl<'hir> LoweringContext<'_, 'hir> {
             .emit();
     }
 
-    fn lower_asyncness(&mut self, a: IsAsync) -> hir::IsAsync {
+    fn lower_asyncness(&mut self, a: Async) -> hir::IsAsync {
         match a {
-            IsAsync::Async { .. } => hir::IsAsync::Async,
-            IsAsync::NotAsync => hir::IsAsync::NotAsync,
+            Async::Yes { .. } => hir::IsAsync::Async,
+            Async::No => hir::IsAsync::NotAsync,
+        }
+    }
+
+    fn lower_constness(&mut self, c: Const) -> hir::Constness {
+        match c {
+            Const::Yes(_) => hir::Constness::Const,
+            Const::No => hir::Constness::NotConst,
+        }
+    }
+
+    pub(super) fn lower_unsafety(&mut self, u: Unsafe) -> hir::Unsafety {
+        match u {
+            Unsafe::Yes(_) => hir::Unsafety::Unsafe,
+            Unsafe::No => hir::Unsafety::Normal,
         }
     }
 

src/librustc_ast_lowering/lib.rs

@@ -1196,7 +1196,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                             &NodeMap::default(),
                             ImplTraitContext::disallowed(),
                         ),
-                        unsafety: f.unsafety,
+                        unsafety: this.lower_unsafety(f.unsafety),
                         abi: this.lower_extern(f.ext),
                         decl: this.lower_fn_decl(&f.decl, None, false, None),
                         param_names: this.lower_fn_params_to_names(&f.decl),