Implment #[cfg] in where clauses

compiler/rustc_ast/src/ast.rs

@@ -429,6 +429,7 @@ impl Default for WhereClause {
 /// A single predicate in a where-clause.
 #[derive(Clone, Encodable, Decodable, Debug)]
 pub struct WherePredicate {
+    pub attrs: AttrVec,
     pub kind: WherePredicateKind,
     pub id: NodeId,
     pub span: Span,

compiler/rustc_ast/src/ast_traits.rs

@@ -11,7 +11,7 @@ use crate::tokenstream::LazyAttrTokenStream;
 use crate::{
     Arm, AssocItem, AttrItem, AttrKind, AttrVec, Attribute, Block, Crate, Expr, ExprField,
     FieldDef, ForeignItem, GenericParam, Item, NodeId, Param, Pat, PatField, Path, Stmt, StmtKind,
-    Ty, Variant, Visibility,
+    Ty, Variant, Visibility, WherePredicate,
 };
 
 /// A utility trait to reduce boilerplate.
@@ -79,6 +79,7 @@ impl_has_node_id!(
     Stmt,
     Ty,
     Variant,
+    WherePredicate,
 );
 
 impl<T: AstDeref<Target: HasNodeId>> HasNodeId for T {
@@ -127,7 +128,16 @@ macro_rules! impl_has_tokens_none {
 }
 
 impl_has_tokens!(AssocItem, AttrItem, Block, Expr, ForeignItem, Item, Pat, Path, Ty, Visibility);
-impl_has_tokens_none!(Arm, ExprField, FieldDef, GenericParam, Param, PatField, Variant);
+impl_has_tokens_none!(
+    Arm,
+    ExprField,
+    FieldDef,
+    GenericParam,
+    Param,
+    PatField,
+    Variant,
+    WherePredicate
+);
 
 impl<T: AstDeref<Target: HasTokens>> HasTokens for T {
     fn tokens(&self) -> Option<&LazyAttrTokenStream> {
@@ -289,6 +299,7 @@ impl_has_attrs!(
     Param,
     PatField,
     Variant,
+    WherePredicate,
 );
 impl_has_attrs_none!(Attribute, AttrItem, Block, Pat, Path, Ty, Visibility);
 

compiler/rustc_ast/src/mut_visit.rs

@@ -331,8 +331,11 @@ pub trait MutVisitor: Sized {
         walk_where_clause(self, where_clause);
     }
 
-    fn visit_where_predicate(&mut self, where_predicate: &mut WherePredicate) {
-        walk_where_predicate(self, where_predicate)
+    fn flat_map_where_predicate(
+        &mut self,
+        where_predicate: WherePredicate,
+    ) -> SmallVec<[WherePredicate; 1]> {
+        walk_flat_map_where_predicate(self, where_predicate)
     }
 
     fn visit_where_predicate_kind(&mut self, kind: &mut WherePredicateKind) {
@@ -1065,15 +1068,20 @@ fn walk_ty_alias_where_clauses<T: MutVisitor>(vis: &mut T, tawcs: &mut TyAliasWh
 
 fn walk_where_clause<T: MutVisitor>(vis: &mut T, wc: &mut WhereClause) {
     let WhereClause { has_where_token: _, predicates, span } = wc;
-    visit_thin_vec(predicates, |predicate| vis.visit_where_predicate(predicate));
+    predicates.flat_map_in_place(|predicate| vis.flat_map_where_predicate(predicate));
     vis.visit_span(span);
 }
 
-pub fn walk_where_predicate<T: MutVisitor>(vis: &mut T, pred: &mut WherePredicate) {
-    let WherePredicate { kind, id, span } = pred;
+pub fn walk_flat_map_where_predicate<T: MutVisitor>(
+    vis: &mut T,
+    mut pred: WherePredicate,
+) -> SmallVec<[WherePredicate; 1]> {
+    let WherePredicate { attrs, kind, id, span } = &mut pred;
     vis.visit_id(id);
+    visit_attrs(vis, attrs);
     vis.visit_where_predicate_kind(kind);
     vis.visit_span(span);
+    smallvec![pred]
 }
 
 pub fn walk_where_predicate_kind<T: MutVisitor>(vis: &mut T, kind: &mut WherePredicateKind) {

compiler/rustc_ast/src/visit.rs

@@ -797,7 +797,8 @@ pub fn walk_where_predicate<'a, V: Visitor<'a>>(
     visitor: &mut V,
     predicate: &'a WherePredicate,
 ) -> V::Result {
-    let WherePredicate { kind, id: _, span: _ } = predicate;
+    let WherePredicate { attrs, kind, id: _, span: _ } = predicate;
+    walk_list!(visitor, visit_attribute, attrs);
     visitor.visit_where_predicate_kind(kind)
 }
 

compiler/rustc_ast_lowering/src/item.rs

@@ -1579,6 +1579,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
     fn lower_where_predicate(&mut self, pred: &WherePredicate) -> hir::WherePredicate<'hir> {
         let hir_id = self.lower_node_id(pred.id);
         let span = self.lower_span(pred.span);
+        self.lower_attrs(hir_id, &pred.attrs);
         let kind = self.arena.alloc(match &pred.kind {
             WherePredicateKind::BoundPredicate(WhereBoundPredicate {
                 bound_generic_params,

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -558,6 +558,7 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session, features: &Features) {
     gate_all!(return_type_notation, "return type notation is experimental");
     gate_all!(pin_ergonomics, "pinned reference syntax is experimental");
     gate_all!(unsafe_fields, "`unsafe` fields are experimental");
+    gate_all!(cfg_attribute_in_where, "`#[cfg]` attribute in `where` clause is unstable");
 
     if !visitor.features.never_patterns() {
         if let Some(spans) = spans.get(&sym::never_patterns) {

compiler/rustc_ast_pretty/src/pprust/state/item.rs

@@ -726,7 +726,8 @@ impl<'a> State<'a> {
     }
 
     pub fn print_where_predicate(&mut self, predicate: &ast::WherePredicate) {
-        let ast::WherePredicate { kind, id: _, span: _ } = predicate;
+        let ast::WherePredicate { attrs, kind, id: _, span: _ } = predicate;
+        self.print_outer_attributes(attrs);
         match kind {
             ast::WherePredicateKind::BoundPredicate(where_bound_predicate) => {
                 self.print_where_bound_predicate(where_bound_predicate);

compiler/rustc_builtin_macros/src/deriving/coerce_pointee.rs

@@ -295,13 +295,15 @@ pub(crate) fn expand_deriving_coerce_pointee(
                 to_ty: &s_ty,
                 rewritten: false,
             };
-            let mut predicate = ast::WherePredicate {
-                kind: ast::WherePredicateKind::BoundPredicate(bound.clone()),
-                span: predicate.span,
-                id: ast::DUMMY_NODE_ID,
-            };
-            substitution.visit_where_predicate(&mut predicate);
+            let mut kind = ast::WherePredicateKind::BoundPredicate(bound.clone());
+            substitution.visit_where_predicate_kind(&mut kind);
             if substitution.rewritten {
+                let predicate = ast::WherePredicate {
+                    attrs: predicate.attrs.clone(),
+                    kind,
+                    span: predicate.span,
+                    id: ast::DUMMY_NODE_ID,
+                };
                 impl_generics.where_clause.predicates.push(predicate);
             }
         }
@@ -383,8 +385,8 @@ impl<'a> ast::mut_visit::MutVisitor for TypeSubstitution<'a> {
         }
     }
 
-    fn visit_where_predicate(&mut self, where_predicate: &mut ast::WherePredicate) {
-        match &mut where_predicate.kind {
+    fn visit_where_predicate_kind(&mut self, kind: &mut ast::WherePredicateKind) {
+        match kind {
             rustc_ast::WherePredicateKind::BoundPredicate(bound) => {
                 bound
                     .bound_generic_params

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

@@ -691,6 +691,7 @@ impl<'a> TraitDef<'a> {
         // and similarly for where clauses
         where_clause.predicates.extend(generics.where_clause.predicates.iter().map(|clause| {
             ast::WherePredicate {
+                attrs: clause.attrs.clone(),
                 kind: clause.kind.clone(),
                 id: ast::DUMMY_NODE_ID,
                 span: clause.span.with_ctxt(ctxt),
@@ -748,8 +749,12 @@ impl<'a> TraitDef<'a> {
                         };
 
                         let kind = ast::WherePredicateKind::BoundPredicate(predicate);
-                        let predicate =
-                            ast::WherePredicate { kind, id: ast::DUMMY_NODE_ID, span: self.span };
+                        let predicate = ast::WherePredicate {
+                            attrs: ThinVec::new(),
+                            kind,
+                            id: ast::DUMMY_NODE_ID,
+                            span: self.span,
+                        };
                         where_clause.predicates.push(predicate);
                     }
                 }

compiler/rustc_expand/src/base.rs

@@ -53,6 +53,7 @@ pub enum Annotatable {
     FieldDef(ast::FieldDef),
     Variant(ast::Variant),
     Crate(ast::Crate),
+    WherePredicate(ast::WherePredicate),
 }
 
 impl Annotatable {
@@ -71,6 +72,7 @@ impl Annotatable {
             Annotatable::FieldDef(sf) => sf.span,
             Annotatable::Variant(v) => v.span,
             Annotatable::Crate(c) => c.spans.inner_span,
+            Annotatable::WherePredicate(wp) => wp.span,
         }
     }
 
@@ -89,6 +91,7 @@ impl Annotatable {
             Annotatable::FieldDef(sf) => sf.visit_attrs(f),
             Annotatable::Variant(v) => v.visit_attrs(f),
             Annotatable::Crate(c) => c.visit_attrs(f),
+            Annotatable::WherePredicate(wp) => wp.visit_attrs(f),
         }
     }
 
@@ -107,6 +110,7 @@ impl Annotatable {
             Annotatable::FieldDef(sf) => visitor.visit_field_def(sf),
             Annotatable::Variant(v) => visitor.visit_variant(v),
             Annotatable::Crate(c) => visitor.visit_crate(c),
+            Annotatable::WherePredicate(wp) => visitor.visit_where_predicate(wp),
         }
     }
 
@@ -127,7 +131,8 @@ impl Annotatable {
             | Annotatable::Param(..)
             | Annotatable::FieldDef(..)
             | Annotatable::Variant(..)
-            | Annotatable::Crate(..) => panic!("unexpected annotatable"),
+            | Annotatable::Crate(..)
+            | Annotatable::WherePredicate(..) => panic!("unexpected annotatable"),
         }
     }
 
@@ -228,6 +233,13 @@ impl Annotatable {
             _ => panic!("expected krate"),
         }
     }
+
+    pub fn expect_where_predicate(self) -> ast::WherePredicate {
+        match self {
+            Annotatable::WherePredicate(wp) => wp,
+            _ => panic!("expected where predicate"),
+        }
+    }
 }
 
 /// Result of an expansion that may need to be retried.
@@ -449,6 +461,10 @@ pub trait MacResult {
         // Fn-like macros cannot produce a crate.
         unreachable!()
     }
+
+    fn make_where_predicates(self: Box<Self>) -> Option<SmallVec<[ast::WherePredicate; 1]>> {
+        None
+    }
 }
 
 macro_rules! make_MacEager {

compiler/rustc_expand/src/expand.rs

@@ -229,6 +229,12 @@ ast_fragments! {
         "variant"; many fn flat_map_variant; fn visit_variant(); fn make_variants;
     }
     Crate(ast::Crate) { "crate"; one fn visit_crate; fn visit_crate; fn make_crate; }
+    WherePredicate(SmallVec<[ast::WherePredicate; 1]>) {
+        "where predicate";
+        many fn flat_map_where_predicate;
+        fn visit_where_predicate();
+        fn make_where_predicates;
+     }
 }
 
 pub enum SupportsMacroExpansion {
@@ -260,7 +266,8 @@ impl AstFragmentKind {
             | AstFragmentKind::GenericParams
             | AstFragmentKind::Params
             | AstFragmentKind::FieldDefs
-            | AstFragmentKind::Variants => SupportsMacroExpansion::No,
+            | AstFragmentKind::Variants
+            | AstFragmentKind::WherePredicate => SupportsMacroExpansion::No,
         }
     }
 
@@ -318,6 +325,9 @@ impl AstFragmentKind {
             AstFragmentKind::Crate => {
                 AstFragment::Crate(items.next().expect("expected exactly one crate").expect_crate())
             }
+            AstFragmentKind::WherePredicate => AstFragment::WherePredicate(
+                items.map(Annotatable::expect_where_predicate).collect(),
+            ),
             AstFragmentKind::Pat | AstFragmentKind::Ty => {
                 panic!("patterns and types aren't annotatable")
             }
@@ -866,7 +876,8 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
             | Annotatable::GenericParam(..)
             | Annotatable::Param(..)
             | Annotatable::FieldDef(..)
-            | Annotatable::Variant(..) => panic!("unexpected annotatable"),
+            | Annotatable::Variant(..)
+            | Annotatable::WherePredicate(..) => panic!("unexpected annotatable"),
         };
         if self.cx.ecfg.features.proc_macro_hygiene() {
             return;
@@ -1003,7 +1014,8 @@ pub fn parse_ast_fragment<'a>(
         | AstFragmentKind::GenericParams
         | AstFragmentKind::Params
         | AstFragmentKind::FieldDefs
-        | AstFragmentKind::Variants => panic!("unexpected AST fragment kind"),
+        | AstFragmentKind::Variants
+        | AstFragmentKind::WherePredicate => panic!("unexpected AST fragment kind"),
     })
 }
 
@@ -1587,6 +1599,19 @@ impl InvocationCollectorNode for ast::Crate {
     }
 }
 
+impl InvocationCollectorNode for ast::WherePredicate {
+    const KIND: AstFragmentKind = AstFragmentKind::WherePredicate;
+    fn to_annotatable(self) -> Annotatable {
+        Annotatable::WherePredicate(self)
+    }
+    fn fragment_to_output(fragment: AstFragment) -> Self::OutputTy {
+        fragment.make_where_predicates()
+    }
+    fn walk_flat_map<V: MutVisitor>(self, visitor: &mut V) -> Self::OutputTy {
+        walk_flat_map_where_predicate(visitor, self)
+    }
+}
+
 impl InvocationCollectorNode for P<ast::Ty> {
     type OutputTy = P<ast::Ty>;
     const KIND: AstFragmentKind = AstFragmentKind::Ty;
@@ -2179,6 +2204,13 @@ impl<'a, 'b> MutVisitor for InvocationCollector<'a, 'b> {
         self.visit_node(node)
     }
 
+    fn flat_map_where_predicate(
+        &mut self,
+        node: ast::WherePredicate,
+    ) -> SmallVec<[ast::WherePredicate; 1]> {
+        self.flat_map_node(node)
+    }
+
     fn visit_expr(&mut self, node: &mut P<ast::Expr>) {
         // FIXME: Feature gating is performed inconsistently between `Expr` and `OptExpr`.
         if let Some(attr) = node.attrs.first() {

compiler/rustc_expand/src/placeholders.rs

@@ -188,6 +188,18 @@ pub(crate) fn placeholder(
             vis,
             is_placeholder: true,
         }]),
+        AstFragmentKind::WherePredicate => {
+            AstFragment::WherePredicate(smallvec![ast::WherePredicate {
+                attrs: Default::default(),
+                id,
+                span,
+                kind: ast::WherePredicateKind::BoundPredicate(ast::WhereBoundPredicate {
+                    bound_generic_params: Default::default(),
+                    bounded_ty: ty(),
+                    bounds: Default::default(),
+                }),
+            }])
+        }
     }
 }
 

compiler/rustc_feature/src/unstable.rs

@@ -396,6 +396,8 @@ declare_features! (
     (unstable, async_for_loop, "1.77.0", Some(118898)),
     /// Allows using C-variadics.
     (unstable, c_variadic, "1.34.0", Some(44930)),
+    /// Allows the use of `#[cfg]` in `where` clauses.
+    (unstable, cfg_attribute_in_where, "CURRENT_RUSTC_VERSION", Some(115590)),
     /// Allows the use of `#[cfg(<true/false>)]`.
     (unstable, cfg_boolean_literals, "1.83.0", Some(131204)),
     /// Allows the use of `#[cfg(overflow_checks)` to check if integer overflow behaviour.

compiler/rustc_hir/src/target.rs

@@ -56,6 +56,7 @@ pub enum Target {
     Param,
     PatField,
     ExprField,
+    WherePredicate,
 }
 
 impl Display for Target {
@@ -96,7 +97,8 @@ impl Target {
             | Target::MacroDef
             | Target::Param
             | Target::PatField
-            | Target::ExprField => false,
+            | Target::ExprField
+            | Target::WherePredicate => false,
         }
     }
 
@@ -217,6 +219,7 @@ impl Target {
             Target::Param => "function param",
             Target::PatField => "pattern field",
             Target::ExprField => "struct field",
+            Target::WherePredicate => "where predicate",
         }
     }
 }

compiler/rustc_hir_pretty/src/lib.rs

@@ -2161,6 +2161,7 @@ impl<'a> State<'a> {
     }
 
     fn print_where_predicate(&mut self, predicate: &hir::WherePredicate<'_>) {
+        self.print_outer_attributes(self.attrs(predicate.hir_id));
         match *predicate.kind {
             hir::WherePredicateKind::BoundPredicate(hir::WhereBoundPredicate {
                 bound_generic_params,