Parse impl restrictions

compiler/rustc_ast/src/ast.rs

@@ -3542,6 +3542,19 @@ impl VisibilityKind {
     }
 }
 
+#[derive(Clone, Encodable, Decodable, Debug, Walkable)]
+pub struct ImplRestriction {
+    pub kind: RestrictionKind,
+    pub span: Span,
+    pub tokens: Option<LazyAttrTokenStream>,
+}
+
+#[derive(Clone, Encodable, Decodable, Debug, Walkable)]
+pub enum RestrictionKind {
+    Unrestricted,
+    Restricted { path: Box<Path>, id: NodeId, shorthand: bool },
+}
+
 /// Field definition in a struct, variant or union.
 ///
 /// E.g., `bar: usize` as in `struct Foo { bar: usize }`.
@@ -3741,6 +3754,7 @@ pub struct Trait {
     pub constness: Const,
     pub safety: Safety,
     pub is_auto: IsAuto,
+    pub impl_restriction: ImplRestriction,
     pub ident: Ident,
     pub generics: Generics,
     #[visitable(extra = BoundKind::SuperTraits)]

compiler/rustc_ast/src/ast_traits.rs

@@ -8,8 +8,8 @@ use std::marker::PhantomData;
 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, ImplRestriction, Item, NodeId, Param, Pat, PatField, Path,
+    Stmt, StmtKind, Ty, Variant, Visibility, WherePredicate,
 };
 
 /// A trait for AST nodes having an ID.
@@ -97,7 +97,19 @@ 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,
+    Ty,
+    Visibility,
+    ImplRestriction
+);
 impl_has_tokens_none!(
     Arm,
     ExprField,
@@ -242,7 +254,7 @@ 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, Ty, Visibility, ImplRestriction);
 
 impl<T: HasAttrs> HasAttrs for Box<T> {
     const SUPPORTS_CUSTOM_INNER_ATTRS: bool = T::SUPPORTS_CUSTOM_INNER_ATTRS;

compiler/rustc_ast/src/visit.rs

@@ -467,6 +467,7 @@ macro_rules! common_visitor_and_walkers {
             RangeEnd,
             RangeSyntax,
             Recovered,
+            RestrictionKind,
             Safety,
             StaticItem,
             StrLit,
@@ -595,6 +596,7 @@ macro_rules! common_visitor_and_walkers {
                 fn visit_poly_trait_ref(PolyTraitRef);
                 fn visit_precise_capturing_arg(PreciseCapturingArg);
                 fn visit_qself(QSelf);
+                fn visit_impl_restriction(ImplRestriction);
                 fn visit_trait_ref(TraitRef);
                 fn visit_ty_pat(TyPat);
                 fn visit_ty(Ty);
@@ -1115,6 +1117,7 @@ macro_rules! common_visitor_and_walkers {
             pub fn walk_poly_trait_ref(PolyTraitRef);
             pub fn walk_precise_capturing_arg(PreciseCapturingArg);
             pub fn walk_qself(QSelf);
+            pub fn walk_impl_restriction(ImplRestriction);
             pub fn walk_trait_ref(TraitRef);
             pub fn walk_ty_pat(TyPat);
             pub fn walk_ty(Ty);

compiler/rustc_ast_lowering/src/item.rs

@@ -512,6 +512,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 constness,
                 is_auto,
                 safety,
+                // FIXME(impl_restrictions): lower to HIR
+                impl_restriction: _,
                 ident,
                 generics,
                 bounds,

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -576,6 +576,7 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session, features: &Features) {
     gate_all!(coroutines, "coroutine syntax is experimental");
     gate_all!(const_block_items, "const block items are experimental");
     gate_all!(final_associated_functions, "`final` on trait functions is 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/mod.rs

@@ -80,6 +80,10 @@ pub fn vis_to_string(v: &ast::Visibility) -> String {
     State::new().vis_to_string(v)
 }
 
+pub fn impl_restriction_to_string(r: &ast::ImplRestriction) -> String {
+    State::new().impl_restriction_to_string(r)
+}
+
 pub fn meta_list_item_to_string(li: &ast::MetaItemInner) -> String {
     State::new().meta_list_item_to_string(li)
 }

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -1110,6 +1110,10 @@ pub trait PrintState<'a>: std::ops::Deref<Target = pp::Printer> + std::ops::Dere
         Self::to_string(|s| s.print_visibility(v))
     }
 
+    fn impl_restriction_to_string(&self, r: &ast::ImplRestriction) -> String {
+        Self::to_string(|s| s.print_impl_restriction(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

@@ -365,6 +365,7 @@ impl<'a> State<'a> {
                 constness,
                 safety,
                 is_auto,
+                impl_restriction,
                 ident,
                 generics,
                 bounds,
@@ -375,6 +376,7 @@ impl<'a> State<'a> {
                 self.print_constness(*constness);
                 self.print_safety(*safety);
                 self.print_is_auto(*is_auto);
+                self.print_impl_restriction(impl_restriction);
                 self.word_nbsp("trait");
                 self.print_ident(*ident);
                 self.print_generic_params(&generics.params);
@@ -483,6 +485,20 @@ impl<'a> State<'a> {
         }
     }
 
+    pub(crate) fn print_impl_restriction(&mut self, impl_restriction: &ast::ImplRestriction) {
+        match &impl_restriction.kind {
+            ast::RestrictionKind::Restricted { path, shorthand, .. } => {
+                let path = Self::to_string(|s| s.print_path(path, false, 0));
+                if *shorthand {
+                    self.word_nbsp(format!("impl({path})"))
+                } else {
+                    self.word_nbsp(format!("impl(in {path})"))
+                }
+            }
+            ast::RestrictionKind::Unrestricted => {}
+        }
+    }
+
     fn print_defaultness(&mut self, defaultness: ast::Defaultness) {
         if let ast::Defaultness::Default(_) = defaultness {
             self.word_nbsp("default");

compiler/rustc_feature/src/unstable.rs

@@ -518,6 +518,8 @@ declare_features! (
     (unstable, half_open_range_patterns_in_slices, "1.66.0", Some(67264)),
     /// Target features on hexagon.
     (unstable, hexagon_target_feature, "1.27.0", Some(150250)),
+    /// Allows `impl(crate) trait Foo` restrictions.
+    (incomplete, 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_parse/src/errors.rs

@@ -1268,6 +1268,26 @@ pub(crate) struct IncorrectVisibilityRestriction {
     pub inner_str: String,
 }
 
+#[derive(Diagnostic)]
+#[diag("incorrect `impl` restriction")]
+#[help(
+    "some possible `impl` restrictions are:
+    `impl(crate)`: can only be implemented in the current crate
+    `impl(super)`: can only be implemented in the parent module
+    `impl(self)`: can only be implemented in current module
+    `impl(in path::to::module)`: can only be implemented in the specified path"
+)]
+pub(crate) struct IncorrectImplRestriction {
+    #[primary_span]
+    #[suggestion(
+        "help: use `in` to restrict implementations to the path `{$inner_str}`",
+        code = "in {inner_str}",
+        applicability = "machine-applicable"
+    )]
+    pub span: Span,
+    pub inner_str: String,
+}
+
 #[derive(Diagnostic)]
 #[diag("<assignment> ... else {\"{\"} ... {\"}\"} is not allowed")]
 pub(crate) struct AssignmentElseNotAllowed {
@@ -2403,6 +2423,14 @@ pub(crate) struct TraitAliasCannotBeUnsafe {
     pub span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag("trait aliases cannot be `impl`-restricted")]
+pub(crate) struct TraitAliasCannotBeImplRestricted {
+    #[primary_span]
+    #[label("trait aliases cannot be `impl`-restricted")]
+    pub span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag("associated `static` items are not allowed")]
 pub(crate) struct AssociatedStaticItemNotAllowed {

compiler/rustc_parse/src/parser/item.rs

@@ -1025,17 +1025,79 @@ impl<'a> Parser<'a> {
         }
     }
 
-    /// Is this an `(const unsafe? auto?| unsafe auto? | auto) trait` item?
+    /// Is there an `[ impl(in? path) ]? trait` item `dist` tokens ahead?
+    fn is_trait_with_maybe_impl_restriction_in_front(&self, dist: usize) -> bool {
+        // `trait`
+        if self.is_keyword_ahead(dist, &[kw::Trait]) {
+            return true;
+        }
+        // `impl(`
+        if !self.is_keyword_ahead(dist, &[kw::Impl])
+            || !self.look_ahead(dist + 1, |t| t == &token::OpenParen)
+        {
+            return false;
+        }
+        // `crate | super | self) trait`
+        if self.is_keyword_ahead(dist + 2, &[kw::Crate, kw::Super, kw::SelfLower])
+            && self.look_ahead(dist + 3, |t| t == &token::CloseParen)
+            && self.is_keyword_ahead(dist + 4, &[kw::Trait])
+        {
+            return true;
+        }
+        // `impl(in? something) trait`
+        // We catch cases where the `in` keyword is missing to provide a
+        // better error message. This is handled later in
+        // `self.recover_incorrect_impl_restriction`.
+        self.tree_look_ahead(dist + 2, |t| {
+            if let TokenTree::Token(token, _) = t { token.is_keyword(kw::Trait) } else { false }
+        })
+        .unwrap_or(false)
+    }
+
+    /// Is this an `(const unsafe? auto? [ impl(in? path) ]? | unsafe auto? [ impl(in? path) ]? | auto [ impl(in? path) ]? | [ impl(in? path) ]?) trait` item?
     fn check_trait_front_matter(&mut self) -> bool {
-        // auto trait
-        self.check_keyword(exp!(Auto)) && self.is_keyword_ahead(1, &[kw::Trait])
-            // unsafe auto trait
-            || self.check_keyword(exp!(Unsafe)) && self.is_keyword_ahead(1, &[kw::Trait, kw::Auto])
-            || self.check_keyword(exp!(Const)) && ((self.is_keyword_ahead(1, &[kw::Trait]) || self.is_keyword_ahead(1, &[kw::Auto]) && self.is_keyword_ahead(2, &[kw::Trait]))
-                || self.is_keyword_ahead(1, &[kw::Unsafe]) && self.is_keyword_ahead(2, &[kw::Trait, kw::Auto]))
+        // `[ impl(in? path) ]? trait`
+        if self.is_trait_with_maybe_impl_restriction_in_front(0) {
+            return true;
+        }
+        // `auto [ impl(in? path) ]? trait`
+        if self.check_keyword(exp!(Auto)) && self.is_trait_with_maybe_impl_restriction_in_front(1) {
+            return true;
+        }
+        // `unsafe auto? [ impl(in? path) ]? trait`
+        if self.check_keyword(exp!(Unsafe))
+            && (self.is_trait_with_maybe_impl_restriction_in_front(1)
+                || self.is_keyword_ahead(1, &[kw::Auto])
+                    && self.is_trait_with_maybe_impl_restriction_in_front(2))
+        {
+            return true;
+        }
+        // `const` ...
+        if !self.check_keyword(exp!(Const)) {
+            return false;
+        }
+        // `const [ impl(in? path) ]? trait`
+        if self.is_trait_with_maybe_impl_restriction_in_front(1) {
+            return true;
+        }
+        // `const (unsafe | auto) [ impl(in? path) ]? trait`
+        if self.is_keyword_ahead(1, &[kw::Unsafe, kw::Auto])
+            && self.is_trait_with_maybe_impl_restriction_in_front(2)
+        {
+            return true;
+        }
+        // `const unsafe auto [ impl(in? path) ]? trait`
+        self.is_keyword_ahead(1, &[kw::Unsafe])
+            && self.is_keyword_ahead(2, &[kw::Auto])
+            && self.is_trait_with_maybe_impl_restriction_in_front(3)
     }
 
-    /// Parses `unsafe? auto? trait Foo { ... }` or `trait Foo = Bar;`.
+    /// Parses `const? unsafe? auto? [impl(in? path)]? trait Foo { ... }` or `trait Foo = Bar;`.
+    ///
+    /// FIXME(restrictions): The current keyword order follows the grammar specified in RFC 3323.
+    /// However, whether the restriction should be grouped closer to the visibility modifier
+    /// (e.g., `pub impl(crate) const unsafe auto trait`) remains an unresolved design question.
+    /// This ordering must be kept in sync with the logic in `check_trait_front_matter`.
     fn parse_item_trait(&mut self, attrs: &mut AttrVec, lo: Span) -> PResult<'a, ItemKind> {
         let constness = self.parse_constness(Case::Sensitive);
         if let Const::Yes(span) = constness {
@@ -1050,6 +1112,8 @@ impl<'a> Parser<'a> {
             IsAuto::No
         };
 
+        let impl_restriction = self.parse_impl_restriction()?;
+
         self.expect_keyword(exp!(Trait))?;
         let ident = self.parse_ident()?;
         let mut generics = self.parse_generics()?;
@@ -1078,6 +1142,9 @@ impl<'a> Parser<'a> {
             if let Safety::Unsafe(_) = safety {
                 self.dcx().emit_err(errors::TraitAliasCannotBeUnsafe { span: whole_span });
             }
+            if let RestrictionKind::Restricted { .. } = impl_restriction.kind {
+                self.dcx().emit_err(errors::TraitAliasCannotBeImplRestricted { span: whole_span });
+            }
 
             self.psess.gated_spans.gate(sym::trait_alias, whole_span);
 
@@ -1090,6 +1157,7 @@ impl<'a> Parser<'a> {
                 constness,
                 is_auto,
                 safety,
+                impl_restriction,
                 ident,
                 generics,
                 bounds,
@@ -2843,8 +2911,8 @@ impl<'a> Parser<'a> {
                         && !self.is_unsafe_foreign_mod()
                         // Rule out `async gen {` and `async gen move {`
                         && !self.is_async_gen_block()
-                        // Rule out `const unsafe auto` and `const unsafe trait`.
-                        && !self.is_keyword_ahead(2, &[kw::Auto, kw::Trait])
+                        // Rule out `const unsafe auto` and `const unsafe trait` and `const unsafe impl`.
+                        && !self.is_keyword_ahead(2, &[kw::Auto, kw::Trait, kw::Impl])
                     )
                 })
             // `extern ABI fn`