Auto merge of #121053 - clubby789:derive-skip, r=<try>

Implement `#[skip]` for builtin derives

Implement #121050

Still needs some work but here's an initial working implementation to get feedback on the strategy.

compiler/rustc_builtin_macros/messages.ftl

@@ -110,6 +110,12 @@ builtin_macros_derive_path_args_list = traits in `#[derive(...)]` don't accept a
 builtin_macros_derive_path_args_value = traits in `#[derive(...)]` don't accept values
     .suggestion = remove the value
 
+builtin_macros_derive_skip_bad_argument = incorrect usage of the `#[skip]` attribute
+    .note = the `#[skip]` attribute accepts an optional list of traits
+    .help = try using `#[skip]` or `#[skip(Trait)]`
+
+builtin_macros_derive_skip_unsupported = the `#[skip]` attribute does not support this trait
+
 builtin_macros_env_not_defined = environment variable `{$var}` not defined at compile time
     .cargo = Cargo sets build script variables at run time. Use `std::env::var({$var_expr})` instead
     .custom = use `std::env::var({$var_expr})` to read the variable at run time

compiler/rustc_builtin_macros/src/deriving/cmp/eq.rs

@@ -38,9 +38,7 @@ pub fn expand_deriving_eq(
                 cx.attr_nested_word(sym::coverage, sym::off, span)
             ],
             fieldless_variants_strategy: FieldlessVariantsStrategy::Unify,
-            combine_substructure: combine_substructure(Box::new(|a, b, c| {
-                cs_total_eq_assert(a, b, c)
-            })),
+            combine_substructure: combine_substructure(Box::new(cs_total_eq_assert)),
         }],
         associated_types: Vec::new(),
         is_const,

compiler/rustc_builtin_macros/src/deriving/cmp/ord.rs

@@ -30,7 +30,7 @@ pub fn expand_deriving_ord(
             ret_ty: Path(path_std!(cmp::Ordering)),
             attributes: thin_vec![cx.attr_word(sym::inline, span)],
             fieldless_variants_strategy: FieldlessVariantsStrategy::Unify,
-            combine_substructure: combine_substructure(Box::new(|a, b, c| cs_cmp(a, b, c))),
+            combine_substructure: combine_substructure(Box::new(cs_cmp)),
         }],
         associated_types: Vec::new(),
         is_const,
@@ -58,6 +58,7 @@ pub fn cs_cmp(cx: &ExtCtxt<'_>, span: Span, substr: &Substructure<'_>) -> BlockO
         cx,
         span,
         substr,
+        sym::Ord,
         |cx, fold| match fold {
             CsFold::Single(field) => {
                 let [other_expr] = &field.other_selflike_exprs[..] else {

compiler/rustc_builtin_macros/src/deriving/cmp/partial_eq.rs

@@ -23,6 +23,7 @@ pub fn expand_deriving_partial_eq(
             cx,
             span,
             substr,
+            sym::PartialEq,
             |cx, fold| match fold {
                 CsFold::Single(field) => {
                     let [other_expr] = &field.other_selflike_exprs[..] else {
@@ -98,7 +99,7 @@ pub fn expand_deriving_partial_eq(
         ret_ty: Path(path_local!(bool)),
         attributes: thin_vec![cx.attr_word(sym::inline, span)],
         fieldless_variants_strategy: FieldlessVariantsStrategy::Unify,
-        combine_substructure: combine_substructure(Box::new(|a, b, c| cs_eq(a, b, c))),
+        combine_substructure: combine_substructure(Box::new(cs_eq)),
     }];
 
     let trait_def = TraitDef {

compiler/rustc_builtin_macros/src/deriving/cmp/partial_ord.rs

@@ -92,6 +92,7 @@ fn cs_partial_cmp(
         cx,
         span,
         substr,
+        sym::PartialOrd,
         |cx, fold| match fold {
             CsFold::Single(field) => {
                 let [other_expr] = &field.other_selflike_exprs[..] else {

compiler/rustc_builtin_macros/src/deriving/debug.rs

@@ -35,9 +35,7 @@ pub fn expand_deriving_debug(
             attributes: thin_vec![cx.attr_word(sym::inline, span)],
             fieldless_variants_strategy:
                 FieldlessVariantsStrategy::SpecializeIfAllVariantsFieldless,
-            combine_substructure: combine_substructure(Box::new(|a, b, c| {
-                show_substructure(a, b, c)
-            })),
+            combine_substructure: combine_substructure(Box::new(show_substructure)),
         }],
         associated_types: Vec::new(),
         is_const,
@@ -90,6 +88,10 @@ fn show_substructure(cx: &ExtCtxt<'_>, span: Span, substr: &Substructure<'_>) ->
             cx.expr_addr_of(field.span, field.self_expr.clone())
         }
     }
+    let fields = fields
+        .iter()
+        .filter(|fi| !fi.skipped_derives.is_skipped(sym::Debug))
+        .collect::<ThinVec<_>>();
 
     if fields.is_empty() {
         // Special case for no fields.

compiler/rustc_builtin_macros/src/deriving/generic/mod.rs

@@ -188,6 +188,7 @@ use rustc_expand::base::{Annotatable, ExtCtxt};
 use rustc_session::lint::builtin::BYTE_SLICE_IN_PACKED_STRUCT_WITH_DERIVE;
 use rustc_span::symbol::{kw, sym, Ident, Symbol};
 use rustc_span::{Span, DUMMY_SP};
+use smallvec::SmallVec;
 use std::cell::RefCell;
 use std::iter;
 use std::ops::Not;
@@ -263,6 +264,7 @@ pub enum FieldlessVariantsStrategy {
 }
 
 /// All the data about the data structure/method being derived upon.
+#[derive(Debug)]
 pub struct Substructure<'a> {
     /// ident of self
     pub type_ident: Ident,
@@ -273,6 +275,7 @@ pub struct Substructure<'a> {
 }
 
 /// Summary of the relevant parts of a struct/enum field.
+#[derive(Debug)]
 pub struct FieldInfo {
     pub span: Span,
     /// None for tuple structs/normal enum variants, Some for normal
@@ -284,15 +287,47 @@ pub struct FieldInfo {
     /// The expressions corresponding to references to this field in
     /// the other selflike arguments.
     pub other_selflike_exprs: Vec<P<Expr>>,
+    /// The derives for which this field should be ignored
+    pub skipped_derives: SkippedDerives,
 }
 
-#[derive(Copy, Clone)]
+/// Derives for which this field should be ignored
+#[derive(Debug)]
+pub enum SkippedDerives {
+    /// No `#[skip]`
+    None,
+    /// `#[skip(Trait, Names)]`
+    List(SmallVec<[Symbol; 1]>),
+    /// `#[skip]` with no arguments
+    All,
+}
+
+impl SkippedDerives {
+    pub fn add(&mut self, derive: Symbol) {
+        match self {
+            Self::None => *self = Self::List(SmallVec::from([derive])),
+            Self::List(idents) => idents.push(derive),
+            Self::All => (),
+        }
+    }
+
+    pub fn is_skipped(&self, derive: Symbol) -> bool {
+        match self {
+            Self::None => false,
+            Self::List(idents) => idents.contains(&derive),
+            Self::All => true,
+        }
+    }
+}
+
+#[derive(Copy, Clone, Debug)]
 pub enum IsTuple {
     No,
     Yes,
 }
 
 /// Fields for a static method
+#[derive(Debug)]
 pub enum StaticFields {
     /// Tuple and unit structs/enum variants like this.
     Unnamed(Vec<Span>, IsTuple),
@@ -301,6 +336,7 @@ pub enum StaticFields {
 }
 
 /// A summary of the possible sets of fields.
+#[derive(Debug)]
 pub enum SubstructureFields<'a> {
     /// A non-static method where `Self` is a struct.
     Struct(&'a ast::VariantData, Vec<FieldInfo>),
@@ -1215,7 +1251,13 @@ impl<'a> MethodDef<'a> {
 
             let self_expr = discr_exprs.remove(0);
             let other_selflike_exprs = discr_exprs;
-            let discr_field = FieldInfo { span, name: None, self_expr, other_selflike_exprs };
+            let discr_field = FieldInfo {
+                span,
+                name: None,
+                self_expr,
+                other_selflike_exprs,
+                skipped_derives: SkippedDerives::None,
+            };
 
             let discr_let_stmts: ThinVec<_> = iter::zip(&discr_idents, &selflike_args)
                 .map(|(&ident, selflike_arg)| {
@@ -1518,7 +1560,12 @@ impl<'a> TraitDef<'a> {
             .collect()
     }
 
-    fn create_fields<F>(&self, struct_def: &'a VariantData, mk_exprs: F) -> Vec<FieldInfo>
+    fn create_fields<F>(
+        &self,
+        cx: &ExtCtxt<'_>,
+        struct_def: &'a VariantData,
+        mk_exprs: F,
+    ) -> Vec<FieldInfo>
     where
         F: Fn(usize, &ast::FieldDef, Span) -> Vec<P<ast::Expr>>,
     {
@@ -1533,11 +1580,76 @@ impl<'a> TraitDef<'a> {
                 let mut exprs: Vec<_> = mk_exprs(i, struct_field, sp);
                 let self_expr = exprs.remove(0);
                 let other_selflike_exprs = exprs;
+                let mut skipped_derives = SkippedDerives::None;
+                let skip_enabled = cx.ecfg.features.derive_skip
+                    || struct_field.span.allows_unstable(sym::derive_skip);
+                for attr in attr::filter_by_name(&struct_field.attrs, sym::skip) {
+                    if !skip_enabled {
+                        rustc_session::parse::feature_err(
+                            &cx.sess,
+                            sym::derive_skip,
+                            attr.span,
+                            "the `#[skip]` attribute is experimental",
+                        )
+                        .emit();
+                    }
+                    let Some(skip_attr) = ast::Attribute::meta_kind(attr) else {
+                        unreachable!()
+                    };
+
+                    // FIXME: better errors
+                    match skip_attr {
+                        ast::MetaItemKind::Word => {
+                            skipped_derives = SkippedDerives::All;
+                            break;
+                        }
+                        ast::MetaItemKind::List(items) => {
+                            for item in items {
+                                let span = item.span();
+                                let ast::NestedMetaItem::MetaItem(ast::MetaItem {
+                                    path,
+                                    kind: ast::MetaItemKind::Word,
+                                    ..
+                                }) = item
+                                else {
+                                    cx.dcx().emit_err(errors::DeriveSkipBadArgument {
+                                        span,
+                                    });
+                                    continue;
+                                };
+                                let name = path.segments[0].ident;
+                                const SUPPORTED_TRAITS: [Symbol; 5] = [
+                                    sym::PartialEq,
+                                    sym::PartialOrd,
+                                    sym::Ord,
+                                    sym::Hash,
+                                    sym::Debug,
+                                ];
+                                if SUPPORTED_TRAITS.contains(&name.name) {
+                                    skipped_derives.add(path.segments[0].ident.name)
+                                } else {
+                                    let traits = SUPPORTED_TRAITS.iter().map(|s| format!("`{s}`")).collect::<Vec<_>>().join(", ");
+                                    cx.psess().buffer_lint_with_diagnostic(
+                                        rustc_session::lint::builtin::UNSUPPORTED_DERIVE_SKIP,
+                                        span,
+                                        cx.current_expansion.lint_node_id,
+                                        crate::fluent_generated::builtin_macros_derive_skip_unsupported,
+                                        rustc_session::lint::BuiltinLintDiag::DeriveSkipUnsupported { traits },
+                                    )
+                                }
+                            }
+                        }
+                        ast::MetaItemKind::NameValue(lit) => {
+                            cx.dcx().emit_err(errors::DeriveSkipBadArgument { span: lit.span });
+                        }
+                    }
+                }
                 FieldInfo {
                     span: sp.with_ctxt(self.span.ctxt()),
                     name: struct_field.ident,
                     self_expr,
                     other_selflike_exprs,
+                    skipped_derives,
                 }
             })
             .collect()
@@ -1553,7 +1665,7 @@ impl<'a> TraitDef<'a> {
         struct_def: &'a VariantData,
         prefixes: &[String],
     ) -> Vec<FieldInfo> {
-        self.create_fields(struct_def, |i, _struct_field, sp| {
+        self.create_fields(cx, struct_def, |i, _struct_field, sp| {
             prefixes
                 .iter()
                 .map(|prefix| {
@@ -1571,7 +1683,7 @@ impl<'a> TraitDef<'a> {
         struct_def: &'a VariantData,
         is_packed: bool,
     ) -> Vec<FieldInfo> {
-        self.create_fields(struct_def, |i, struct_field, sp| {
+        self.create_fields(cx, struct_def, |i, struct_field, sp| {
             selflike_args
                 .iter()
                 .map(|selflike_arg| {
@@ -1667,13 +1779,19 @@ pub fn cs_fold<F>(
     cx: &ExtCtxt<'_>,
     trait_span: Span,
     substructure: &Substructure<'_>,
+    trait_name: Symbol,
     mut f: F,
 ) -> P<Expr>
 where
     F: FnMut(&ExtCtxt<'_>, CsFold<'_>) -> P<Expr>,
 {
     match substructure.fields {
         EnumMatching(.., all_fields) | Struct(_, all_fields) => {
+            let all_fields = all_fields
+                .iter()
+                .filter(|fi| !fi.skipped_derives.is_skipped(trait_name))
+                .collect::<Vec<&FieldInfo>>();
+
             if all_fields.is_empty() {
                 return f(cx, CsFold::Fieldless);
             }
@@ -1686,7 +1804,7 @@ where
 
             let base_expr = f(cx, CsFold::Single(base_field));
 
-            let op = |old, field: &FieldInfo| {
+            let op = |old, field: &&FieldInfo| {
                 let new = f(cx, CsFold::Single(field));
                 f(cx, CsFold::Combine(field.span, old, new))
             };

compiler/rustc_builtin_macros/src/deriving/hash.rs

@@ -35,9 +35,7 @@ pub fn expand_deriving_hash(
             ret_ty: Unit,
             attributes: thin_vec![cx.attr_word(sym::inline, span)],
             fieldless_variants_strategy: FieldlessVariantsStrategy::Unify,
-            combine_substructure: combine_substructure(Box::new(|a, b, c| {
-                hash_substructure(a, b, c)
-            })),
+            combine_substructure: combine_substructure(Box::new(hash_substructure)),
         }],
         associated_types: Vec::new(),
         is_const,
@@ -62,8 +60,11 @@ fn hash_substructure(cx: &ExtCtxt<'_>, trait_span: Span, substr: &Substructure<'
 
     let (stmts, match_expr) = match substr.fields {
         Struct(_, fields) | EnumMatching(.., fields) => {
-            let stmts =
-                fields.iter().map(|field| call_hash(field.span, field.self_expr.clone())).collect();
+            let stmts = fields
+                .iter()
+                .filter(|fi| !fi.skipped_derives.is_skipped(sym::Hash))
+                .map(|field| call_hash(field.span, field.self_expr.clone()))
+                .collect();
             (stmts, None)
         }
         EnumDiscr(discr_field, match_expr) => {

compiler/rustc_builtin_macros/src/errors.rs

@@ -295,6 +295,15 @@ pub(crate) struct DerivePathArgsValue {
     pub(crate) span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(builtin_macros_derive_skip_bad_argument)]
+pub(crate) struct DeriveSkipBadArgument {
+    #[note]
+    #[help]
+    #[primary_span]
+    pub(crate) span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(builtin_macros_no_default_variant)]
 #[help]

compiler/rustc_feature/src/unstable.rs

@@ -442,6 +442,8 @@ declare_features! (
     (unstable, deprecated_suggestion, "1.61.0", Some(94785)),
     /// Allows deref patterns.
     (incomplete, deref_patterns, "CURRENT_RUSTC_VERSION", Some(87121)),
+    /// Allows using the `#[skip]` attribute in derives
+    (unstable, derive_skip, "CURRENT_RUSTC_VERSION", Some(121050)),
     /// Controls errors in trait implementations.
     (unstable, do_not_recommend, "1.67.0", Some(51992)),
     /// Tells rustdoc to automatically generate `#[doc(cfg(...))]`.

compiler/rustc_lint/src/context/diagnostics.rs

@@ -347,5 +347,8 @@ pub(super) fn builtin(sess: &Session, diagnostic: BuiltinLintDiag, diag: &mut Di
                 "reduce the glob import's visibility or increase visibility of imported items",
             );
         }
+        BuiltinLintDiag::DeriveSkipUnsupported { traits } => {
+            diag.help(format!("the supported traits are {traits}"));
+        }
     }
 }