Auto merge of #141754 - jhpratt:restrictions-pr1, r=<try>

Introduce `impl` restrictions to AST, lower to `rustc_middle`

RFC approved in [RFC 3323](https://rust-lang.github.io/rfcs/3323-restrictions.html); part of tracking issue #105077 and a [2025H1 project goal](rust-lang/rust-project-goals#257).

This is likely easiest to review commit-by-commit.

First, the `impl` restriction is introduced to the AST, followed by actually parsing it into the AST on trait definitions (the only place it's permitted). Next we add the ability for `rustfmt` to handle it in the most logical manner; unstable features are permitted to choose a sane format without a team decision. After placing it behind a feature gate, the restriction is lowered to `rustc_middle` with identical limitations to visibility. Finally, the diagnostics are partially moved into Fluent to make them a bit more translatable.

This is the groundwork for the restriction and does not actually enforce anything; that is coming in another PR soon.

r? `@Urgau` as discussed

Author: bors

compiler/rustc_ast/src/ast.rs

@@ -32,7 +32,7 @@ use rustc_data_structures::tagged_ptr::Tag;
 use rustc_macros::{Decodable, Encodable, HashStable_Generic};
 pub use rustc_span::AttrId;
 use rustc_span::source_map::{Spanned, respan};
-use rustc_span::{ErrorGuaranteed, Ident, Span, Symbol, kw, sym};
+use rustc_span::{DUMMY_SP, ErrorGuaranteed, Ident, Span, Symbol, kw, sym};
 use thin_vec::{ThinVec, thin_vec};
 
 pub use crate::format::*;
@@ -3316,6 +3316,42 @@ impl VisibilityKind {
     }
 }
 
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct Restriction {
+    pub kind: RestrictionKind,
+    pub span: Span,
+    pub tokens: Option<LazyAttrTokenStream>,
+}
+
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub enum RestrictionKind {
+    Unrestricted,
+    Restricted { path: P<Path>, id: NodeId, shorthand: bool },
+    Implied,
+}
+
+impl Restriction {
+    pub fn unrestricted() -> Self {
+        Restriction { kind: RestrictionKind::Unrestricted, span: DUMMY_SP, tokens: None }
+    }
+
+    pub fn restricted(path: P<Path>, id: NodeId, shorthand: bool) -> Self {
+        Restriction {
+            kind: RestrictionKind::Restricted { path, id, shorthand },
+            span: DUMMY_SP,
+            tokens: None,
+        }
+    }
+
+    pub fn implied() -> Self {
+        Restriction { kind: RestrictionKind::Implied, span: DUMMY_SP, tokens: None }
+    }
+
+    pub fn with_span(self, span: Span) -> Self {
+        Restriction { span, ..self }
+    }
+}
+
 /// Field definition in a struct, variant or union.
 ///
 /// E.g., `bar: usize` as in `struct Foo { bar: usize }`.
@@ -3494,6 +3530,7 @@ impl Default for FnHeader {
 
 #[derive(Clone, Encodable, Decodable, Debug)]
 pub struct Trait {
+    pub impl_restriction: Restriction,
     pub safety: Safety,
     pub is_auto: IsAuto,
     pub ident: Ident,

compiler/rustc_ast/src/ast_traits.rs

@@ -9,8 +9,8 @@ use crate::ptr::P;
 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, WherePredicate,
+    FieldDef, ForeignItem, GenericParam, Item, NodeId, Param, Pat, PatField, Path, Restriction,
+    Stmt, StmtKind, Ty, Variant, Visibility, WherePredicate,
 };
 
 /// A trait for AST nodes having an ID.
@@ -98,7 +98,19 @@ macro_rules! impl_has_tokens_none {
     };
 }
 
-impl_has_tokens!(AssocItem, AttrItem, Block, Expr, ForeignItem, Item, Pat, Path, Ty, Visibility);
+impl_has_tokens!(
+    AssocItem,
+    AttrItem,
+    Block,
+    Expr,
+    ForeignItem,
+    Item,
+    Pat,
+    Path,
+    Restriction,
+    Ty,
+    Visibility
+);
 impl_has_tokens_none!(
     Arm,
     ExprField,
@@ -243,7 +255,7 @@ impl_has_attrs!(
     Variant,
     WherePredicate,
 );
-impl_has_attrs_none!(Attribute, AttrItem, Block, Pat, Path, Ty, Visibility);
+impl_has_attrs_none!(Attribute, AttrItem, Block, Pat, Path, Restriction, Ty, Visibility);
 
 impl<T: HasAttrs> HasAttrs for P<T> {
     const SUPPORTS_CUSTOM_INNER_ATTRS: bool = T::SUPPORTS_CUSTOM_INNER_ATTRS;

compiler/rustc_ast/src/mut_visit.rs

@@ -327,6 +327,10 @@ pub trait MutVisitor: Sized {
         walk_vis(self, vis);
     }
 
+    fn visit_restriction(&mut self, restriction: &mut Restriction) {
+        walk_restriction(self, restriction);
+    }
+
     fn visit_id(&mut self, _id: &mut NodeId) {
         // Do nothing.
     }
@@ -1405,6 +1409,18 @@ fn walk_vis<T: MutVisitor>(vis: &mut T, visibility: &mut Visibility) {
     vis.visit_span(span);
 }
 
+fn walk_restriction<T: MutVisitor>(vis: &mut T, restriction: &mut Restriction) {
+    let Restriction { kind, span, tokens: _ } = restriction;
+    match kind {
+        RestrictionKind::Unrestricted | RestrictionKind::Implied => {}
+        RestrictionKind::Restricted { path, id, shorthand: _ } => {
+            vis.visit_id(id);
+            vis.visit_path(path);
+        }
+    }
+    vis.visit_span(span);
+}
+
 fn walk_capture_by<T: MutVisitor>(vis: &mut T, capture_by: &mut CaptureBy) {
     match capture_by {
         CaptureBy::Ref => {}

compiler/rustc_ast/src/visit.rs

@@ -268,6 +268,9 @@ pub trait Visitor<'ast>: Sized {
     fn visit_vis(&mut self, vis: &'ast Visibility) -> Self::Result {
         walk_vis(self, vis)
     }
+    fn visit_restriction(&mut self, restriction: &'ast Restriction) -> Self::Result {
+        walk_restriction(self, restriction)
+    }
     fn visit_fn_ret_ty(&mut self, ret_ty: &'ast FnRetTy) -> Self::Result {
         walk_fn_ret_ty(self, ret_ty)
     }
@@ -554,7 +557,16 @@ macro_rules! common_visitor_and_walkers {
                             <V as Visitor<$lt>>::Result::output()
                         )?
                     }
-                    ItemKind::Trait(box Trait { safety, is_auto: _, ident, generics, bounds, items }) => {
+                    ItemKind::Trait(box Trait {
+                        impl_restriction,
+                        safety,
+                        is_auto: _,
+                        ident,
+                        generics,
+                        bounds,
+                        items,
+                    }) => {
+                        try_visit!(vis.visit_restriction(impl_restriction));
                         try_visit!(visit_safety(vis, safety));
                         try_visit!(vis.visit_ident(ident));
                         try_visit!(vis.visit_generics(generics));
@@ -1516,6 +1528,20 @@ pub fn walk_vis<'a, V: Visitor<'a>>(visitor: &mut V, vis: &'a Visibility) -> V::
     V::Result::output()
 }
 
+pub fn walk_restriction<'a, V: Visitor<'a>>(
+    visitor: &mut V,
+    restriction: &'a Restriction,
+) -> V::Result {
+    let Restriction { kind, span: _, tokens: _ } = restriction;
+    match kind {
+        RestrictionKind::Unrestricted | RestrictionKind::Implied => {}
+        RestrictionKind::Restricted { path, id, shorthand: _ } => {
+            try_visit!(visitor.visit_path(path, *id));
+        }
+    }
+    V::Result::output()
+}
+
 pub fn walk_attribute<'a, V: Visitor<'a>>(visitor: &mut V, attr: &'a Attribute) -> V::Result {
     let Attribute { kind, id: _, style: _, span: _ } = attr;
     match kind {

compiler/rustc_ast_lowering/src/item.rs

@@ -415,7 +415,15 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     items: new_impl_items,
                 }))
             }
-            ItemKind::Trait(box Trait { is_auto, safety, ident, generics, bounds, items }) => {
+            ItemKind::Trait(box Trait {
+                impl_restriction: _,
+                is_auto,
+                safety,
+                ident,
+                generics,
+                bounds,
+                items,
+            }) => {
                 let ident = self.lower_ident(*ident);
                 let (generics, (safety, items, bounds)) = self.lower_generics(
                     generics,

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -515,6 +515,7 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session, features: &Features) {
     gate_all!(where_clause_attrs, "attributes in `where` clause are unstable");
     gate_all!(super_let, "`super let` is experimental");
     gate_all!(frontmatter, "frontmatters are experimental");
+    gate_all!(impl_restriction, "impl restrictions are experimental");
 
     if !visitor.features.never_patterns() {
         if let Some(spans) = spans.get(&sym::never_patterns) {

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -1071,6 +1071,10 @@ pub trait PrintState<'a>: std::ops::Deref<Target = pp::Printer> + std::ops::Dere
         Self::to_string(|s| s.print_visibility(v))
     }
 
+    fn restriction_to_string(&self, kw: &'static str, r: &ast::Restriction) -> String {
+        Self::to_string(|s| s.print_restriction(kw, r))
+    }
+
     fn block_to_string(&self, blk: &ast::Block) -> String {
         Self::to_string(|s| {
             let (cb, ib) = s.head("");

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

@@ -357,6 +357,7 @@ impl<'a> State<'a> {
                 self.bclose(item.span, empty, cb);
             }
             ast::ItemKind::Trait(box ast::Trait {
+                impl_restriction,
                 safety,
                 is_auto,
                 ident,
@@ -366,6 +367,7 @@ impl<'a> State<'a> {
             }) => {
                 let (cb, ib) = self.head("");
                 self.print_visibility(&item.vis);
+                self.print_restriction("impl", impl_restriction);
                 self.print_safety(*safety);
                 self.print_is_auto(*is_auto);
                 self.word_nbsp("trait");
@@ -473,6 +475,22 @@ impl<'a> State<'a> {
         }
     }
 
+    // FIXME(jhpratt) make `kw` into a const generic once permitted
+    pub(crate) fn print_restriction(&mut self, kw: &'static str, restriction: &ast::Restriction) {
+        match restriction.kind {
+            ast::RestrictionKind::Unrestricted => self.word_nbsp(kw),
+            ast::RestrictionKind::Restricted { ref path, shorthand, id: _ } => {
+                let path = Self::to_string(|s| s.print_path(path, false, 0));
+                if shorthand {
+                    self.word_nbsp(format!("{kw}({path})"))
+                } else {
+                    self.word_nbsp(format!("{kw}(in {path})"))
+                }
+            }
+            ast::RestrictionKind::Implied => {}
+        }
+    }
+
     fn print_defaultness(&mut self, defaultness: ast::Defaultness) {
         if let ast::Defaultness::Default(_) = defaultness {
             self.word_nbsp("default");

compiler/rustc_feature/src/unstable.rs

@@ -534,6 +534,8 @@ declare_features! (
     (unstable, half_open_range_patterns_in_slices, "1.66.0", Some(67264)),
     /// Allows `if let` guard in match arms.
     (unstable, if_let_guard, "1.47.0", Some(51114)),
+    /// Allows `impl(crate) trait Foo` restrictions
+    (unstable, impl_restriction, "CURRENT_RUSTC_VERSION", Some(105077)),
     /// Allows `impl Trait` to be used inside associated types (RFC 2515).
     (unstable, impl_trait_in_assoc_type, "1.70.0", Some(63063)),
     /// Allows `impl Trait` in bindings (`let`).

compiler/rustc_middle/src/hir/map.rs

@@ -1179,14 +1179,15 @@ pub(super) fn crate_hash(tcx: TyCtxt<'_>, _: LocalCrate) -> Svh {
         }
         tcx.sess.opts.dep_tracking_hash(true).hash_stable(&mut hcx, &mut stable_hasher);
         tcx.stable_crate_id(LOCAL_CRATE).hash_stable(&mut hcx, &mut stable_hasher);
-        // Hash visibility information since it does not appear in HIR.
+        // Hash visibility and restriction information since it does not appear in HIR.
         // FIXME: Figure out how to remove `visibilities_for_hashing` by hashing visibilities on
         // the fly in the resolver, storing only their accumulated hash in `ResolverGlobalCtxt`,
         // and combining it with other hashes here.
         resolutions.visibilities_for_hashing.hash_stable(&mut hcx, &mut stable_hasher);
         with_metavar_spans(|mspans| {
             mspans.freeze_and_get_read_spans().hash_stable(&mut hcx, &mut stable_hasher);
         });
+        resolutions.impl_restrictions.hash_stable(&mut hcx, &mut stable_hasher);
         stable_hasher.finish()
     });