Introduce const Trait (always-const trait bounds)

compiler/rustc_ast/src/ast.rs

@@ -300,9 +300,22 @@ pub enum TraitBoundModifier {
     /// `?Trait`
     Maybe,
 
+    /// `const Trait`
+    Const(Span),
+
     /// `~const Trait`
     MaybeConst(Span),
 
+    /// `const !Trait`
+    //
+    // This parses but will be rejected during AST validation.
+    ConstNegative,
+
+    /// `const ?Trait`
+    //
+    // This parses but will be rejected during AST validation.
+    ConstMaybe,
+
     /// `~const !Trait`
     //
     // This parses but will be rejected during AST validation.
@@ -315,10 +328,11 @@ pub enum TraitBoundModifier {
 }
 
 impl TraitBoundModifier {
-    pub fn to_constness(self) -> Const {
+    pub fn to_constness(self) -> BoundConstness {
         match self {
-            Self::MaybeConst(span) => Const::Yes(span),
-            _ => Const::No,
+            Self::Const(span) => BoundConstness::Always(span),
+            Self::MaybeConst(span) => BoundConstness::Maybe(span),
+            _ => BoundConstness::Never,
         }
     }
 }
@@ -2526,6 +2540,25 @@ pub enum BoundPolarity {
     Maybe(Span),
 }
 
+#[derive(Copy, Clone, PartialEq, Encodable, Decodable, Debug, HashStable_Generic)]
+pub enum BoundConstness {
+    /// `Type: Trait`
+    Never,
+    /// `Type: const Trait`
+    Always(Span),
+    /// `Type: ~const Trait`
+    Maybe(Span),
+}
+
+impl From<Const> for BoundConstness {
+    fn from(constness: Const) -> Self {
+        match constness {
+            Const::Yes(span) => Self::Maybe(span),
+            Const::No => Self::Never,
+        }
+    }
+}
+
 #[derive(Clone, Encodable, Decodable, Debug)]
 pub enum FnRetTy {
     /// Returns type is not specified.

compiler/rustc_ast/src/token.rs

@@ -529,6 +529,8 @@ impl Token {
     }
 
     /// Returns `true` if the token can appear at the start of a generic bound.
+    // FIXME(const_trait_impl): This lacks `Tilde` and `kw::Const` but everythings seems to work
+    // regardless for some reason?
     pub fn can_begin_bound(&self) -> bool {
         self.is_path_start()
             || self.is_lifetime()

compiler/rustc_ast_lowering/src/item.rs

@@ -341,7 +341,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     self.lower_generics(ast_generics, *constness, id, &itctx, |this| {
                         let trait_ref = trait_ref.as_ref().map(|trait_ref| {
                             this.lower_trait_ref(
-                                *constness,
+                                (*constness).into(),
                                 trait_ref,
                                 &ImplTraitContext::Disallowed(ImplTraitPosition::Trait),
                             )

compiler/rustc_ast_lowering/src/lib.rs

@@ -1318,7 +1318,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                         span: t.span,
                     },
                     itctx,
-                    ast::Const::No,
+                    ast::BoundConstness::Never,
                 );
                 let bounds = this.arena.alloc_from_iter([bound]);
                 let lifetime_bound = this.elided_dyn_bound(t.span);
@@ -1426,18 +1426,21 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                             GenericBound::Trait(
                                 ty,
                                 modifier @ (TraitBoundModifier::None
+                                | TraitBoundModifier::Const(_)
                                 | TraitBoundModifier::MaybeConst(_)
                                 | TraitBoundModifier::Negative),
                             ) => {
                                 Some(this.lower_poly_trait_ref(ty, itctx, modifier.to_constness()))
                             }
-                            // `~const ?Bound` will cause an error during AST validation
+                            // `{,~}const ?Bound` will cause an error during AST validation
                             // anyways, so treat it like `?Bound` as compilation proceeds.
                             GenericBound::Trait(
                                 _,
                                 TraitBoundModifier::Maybe
                                 | TraitBoundModifier::MaybeConstMaybe
-                                | TraitBoundModifier::MaybeConstNegative,
+                                | TraitBoundModifier::MaybeConstNegative
+                                | TraitBoundModifier::ConstMaybe
+                                | TraitBoundModifier::ConstNegative,
                             ) => None,
                             GenericBound::Outlives(lifetime) => {
                                 if lifetime_bound.is_none() {
@@ -2176,7 +2179,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
 
     fn lower_trait_ref(
         &mut self,
-        constness: ast::Const,
+        constness: ast::BoundConstness,
         p: &TraitRef,
         itctx: &ImplTraitContext,
     ) -> hir::TraitRef<'hir> {
@@ -2199,7 +2202,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         &mut self,
         p: &PolyTraitRef,
         itctx: &ImplTraitContext,
-        constness: ast::Const,
+        constness: ast::BoundConstness,
     ) -> hir::PolyTraitRef<'hir> {
         let bound_generic_params =
             self.lower_lifetime_binder(p.trait_ref.ref_id, &p.bound_generic_params);
@@ -2323,6 +2326,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     fn lower_trait_bound_modifier(&mut self, f: TraitBoundModifier) -> hir::TraitBoundModifier {
         match f {
             TraitBoundModifier::None => hir::TraitBoundModifier::None,
+            TraitBoundModifier::Const(_) => hir::TraitBoundModifier::Const,
             TraitBoundModifier::MaybeConst(_) => hir::TraitBoundModifier::MaybeConst,
 
             TraitBoundModifier::Negative => {
@@ -2333,12 +2337,13 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 }
             }
 
-            // `MaybeConstMaybe` will cause an error during AST validation, but we need to pick a
+            // `{,Maybe}ConstMaybe` will cause an error during AST validation, but we need to pick a
             // placeholder for compilation to proceed.
-            TraitBoundModifier::MaybeConstMaybe | TraitBoundModifier::Maybe => {
-                hir::TraitBoundModifier::Maybe
-            }
+            TraitBoundModifier::ConstMaybe
+            | TraitBoundModifier::MaybeConstMaybe
+            | TraitBoundModifier::Maybe => hir::TraitBoundModifier::Maybe,
             TraitBoundModifier::MaybeConstNegative => hir::TraitBoundModifier::MaybeConst,
+            TraitBoundModifier::ConstNegative => hir::TraitBoundModifier::Const,
         }
     }
 
@@ -2556,45 +2561,62 @@ struct GenericArgsCtor<'hir> {
 }
 
 impl<'hir> GenericArgsCtor<'hir> {
-    fn push_constness(&mut self, lcx: &mut LoweringContext<'_, 'hir>, constness: ast::Const) {
+    fn push_constness(
+        &mut self,
+        lcx: &mut LoweringContext<'_, 'hir>,
+        constness: ast::BoundConstness,
+    ) {
         if !lcx.tcx.features().effects {
             return;
         }
 
-        // if bound is non-const, don't add host effect param
-        let ast::Const::Yes(span) = constness else { return };
+        let (span, body) = match constness {
+            BoundConstness::Never => return,
+            BoundConstness::Always(span) => {
+                let span = lcx.lower_span(span);
 
-        let span = lcx.lower_span(span);
+                let body = hir::ExprKind::Lit(
+                    lcx.arena.alloc(hir::Lit { node: LitKind::Bool(false), span }),
+                );
 
-        let id = lcx.next_node_id();
-        let hir_id = lcx.next_id();
+                (span, body)
+            }
+            BoundConstness::Maybe(span) => {
+                let span = lcx.lower_span(span);
 
-        let Some(host_param_id) = lcx.host_param_id else {
-            lcx.tcx.sess.span_delayed_bug(
-                span,
-                "no host param id for call in const yet no errors reported",
-            );
-            return;
-        };
+                let Some(host_param_id) = lcx.host_param_id else {
+                    lcx.tcx.sess.span_delayed_bug(
+                        span,
+                        "no host param id for call in const yet no errors reported",
+                    );
+                    return;
+                };
 
-        let body = lcx.lower_body(|lcx| {
-            (&[], {
                 let hir_id = lcx.next_id();
                 let res = Res::Def(DefKind::ConstParam, host_param_id.to_def_id());
-                let expr_kind = hir::ExprKind::Path(hir::QPath::Resolved(
+                let body = hir::ExprKind::Path(hir::QPath::Resolved(
                     None,
                     lcx.arena.alloc(hir::Path {
                         span,
                         res,
-                        segments: arena_vec![lcx; hir::PathSegment::new(Ident {
-                            name: sym::host,
-                            span,
-                        }, hir_id, res)],
+                        segments: arena_vec![
+                            lcx;
+                            hir::PathSegment::new(
+                                Ident { name: sym::host, span },
+                                hir_id,
+                                res
+                            )
+                        ],
                     }),
                 ));
-                lcx.expr(span, expr_kind)
-            })
-        });
+
+                (span, body)
+            }
+        };
+        let body = lcx.lower_body(|lcx| (&[], lcx.expr(span, body)));
+
+        let id = lcx.next_node_id();
+        let hir_id = lcx.next_id();
 
         let def_id = lcx.create_def(
             lcx.current_hir_id_owner.def_id,

compiler/rustc_ast_lowering/src/path.rs

@@ -25,7 +25,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         param_mode: ParamMode,
         itctx: &ImplTraitContext,
         // constness of the impl/bound if this is a trait path
-        constness: Option<ast::Const>,
+        constness: Option<ast::BoundConstness>,
     ) -> hir::QPath<'hir> {
         let qself_position = qself.as_ref().map(|q| q.position);
         let qself = qself.as_ref().map(|q| self.lower_ty(&q.ty, itctx));
@@ -179,7 +179,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         param_mode: ParamMode,
         parenthesized_generic_args: ParenthesizedGenericArgs,
         itctx: &ImplTraitContext,
-        constness: Option<ast::Const>,
+        constness: Option<ast::BoundConstness>,
     ) -> hir::PathSegment<'hir> {
         debug!("path_span: {:?}, lower_path_segment(segment: {:?})", path_span, segment);
         let (mut generic_args, infer_args) = if let Some(generic_args) = segment.args.as_deref() {

compiler/rustc_ast_passes/messages.ftl

@@ -46,6 +46,8 @@ ast_passes_const_and_c_variadic = functions cannot be both `const` and C-variadi
     .const = `const` because of this
     .variadic = C-variadic because of this
 
+ast_passes_const_bound_trait_object = const trait bounds are not allowed in trait object types
+
 ast_passes_const_without_body =
     free constant item without body
     .suggestion = provide a definition for the constant
@@ -181,6 +183,8 @@ ast_passes_match_arm_with_no_body =
 ast_passes_module_nonascii = trying to load file for module `{$name}` with non-ascii identifier name
     .help = consider using the `#[path]` attribute to specify filesystem path
 
+ast_passes_mutually_exclusive_trait_bound_modifiers = `{$left}` and `{$right}` are mutually exclusive
+
 ast_passes_negative_bound_not_supported =
     negative bounds are not supported
 
@@ -195,8 +199,6 @@ ast_passes_nomangle_ascii = `#[no_mangle]` requires ASCII identifier
 ast_passes_obsolete_auto = `impl Trait for .. {"{}"}` is an obsolete syntax
     .help = use `auto trait Trait {"{}"}` instead
 
-ast_passes_optional_const_exclusive = `~const` and `{$modifier}` are mutually exclusive
-
 ast_passes_optional_trait_object = `?Trait` is not permitted in trait object types
 
 ast_passes_optional_trait_supertrait = `?Trait` is not permitted in supertraits

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -1207,6 +1207,9 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                 (BoundKind::TraitObject, TraitBoundModifier::Maybe) => {
                     self.dcx().emit_err(errors::OptionalTraitObject { span: poly.span });
                 }
+                (BoundKind::TraitObject, TraitBoundModifier::Const(_)) => {
+                    self.dcx().emit_err(errors::ConstBoundTraitObject { span: poly.span });
+                }
                 (_, &TraitBoundModifier::MaybeConst(span))
                     if let Some(reason) = &self.disallow_tilde_const =>
                 {
@@ -1235,16 +1238,25 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                     };
                     self.dcx().emit_err(errors::TildeConstDisallowed { span, reason });
                 }
-                (_, TraitBoundModifier::MaybeConstMaybe) => {
-                    self.dcx().emit_err(errors::OptionalConstExclusive {
-                        span: bound.span(),
-                        modifier: "?",
-                    });
-                }
-                (_, TraitBoundModifier::MaybeConstNegative) => {
-                    self.dcx().emit_err(errors::OptionalConstExclusive {
+                (
+                    _,
+                    TraitBoundModifier::ConstMaybe
+                    | TraitBoundModifier::ConstNegative
+                    | TraitBoundModifier::MaybeConstMaybe
+                    | TraitBoundModifier::MaybeConstNegative,
+                ) => {
+                    let (left, right) = match modify {
+                        TraitBoundModifier::ConstMaybe => ("const", "?"),
+                        TraitBoundModifier::ConstNegative => ("const", "!"),
+                        TraitBoundModifier::MaybeConstMaybe => ("~const", "?"),
+                        TraitBoundModifier::MaybeConstNegative => ("~const", "!"),
+                        _ => unreachable!("unexpected trait bound modifier"),
+                    };
+
+                    self.dcx().emit_err(errors::MutuallyExclusiveTraitBoundModifiers {
                         span: bound.span(),
-                        modifier: "!",
+                        left,
+                        right,
                     });
                 }
                 _ => {}

compiler/rustc_ast_passes/src/errors.rs

@@ -540,6 +540,13 @@ pub struct OptionalTraitObject {
     pub span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(ast_passes_const_bound_trait_object)]
+pub struct ConstBoundTraitObject {
+    #[primary_span]
+    pub span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(ast_passes_tilde_const_disallowed)]
 pub struct TildeConstDisallowed {
@@ -580,11 +587,12 @@ pub enum TildeConstReason {
 }
 
 #[derive(Diagnostic)]
-#[diag(ast_passes_optional_const_exclusive)]
-pub struct OptionalConstExclusive {
+#[diag(ast_passes_mutually_exclusive_trait_bound_modifiers)]
+pub struct MutuallyExclusiveTraitBoundModifiers {
     #[primary_span]
     pub span: Span,
-    pub modifier: &'static str,
+    pub left: &'static str,
+    pub right: &'static str,
 }
 
 #[derive(Diagnostic)]

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -1567,6 +1567,17 @@ impl<'a> State<'a> {
                         TraitBoundModifier::Maybe => {
                             self.word("?");
                         }
+                        TraitBoundModifier::Const(_) => {
+                            self.word_space("const");
+                        }
+                        TraitBoundModifier::ConstNegative => {
+                            self.word_space("const");
+                            self.word("!");
+                        }
+                        TraitBoundModifier::ConstMaybe => {
+                            self.word_space("const");
+                            self.word("?");
+                        }
                         TraitBoundModifier::MaybeConst(_) => {
                             self.word_space("~const");
                         }

compiler/rustc_hir/src/hir.rs

@@ -425,6 +425,7 @@ pub enum TraitBoundModifier {
     None,
     Negative,
     Maybe,
+    Const,
     MaybeConst,
 }
 

compiler/rustc_hir_analysis/messages.ftl

@@ -70,7 +70,7 @@ hir_analysis_coercion_between_struct_same_note = expected coercion between the s
 hir_analysis_coercion_between_struct_single_note = expected a single field to be coerced, none found
 
 hir_analysis_const_bound_for_non_const_trait =
-    ~const can only be applied to `#[const_trait]` traits
+    {$modifier} can only be applied to `#[const_trait]` traits
 
 hir_analysis_const_impl_for_non_const_trait =
     const `impl` for trait `{$trait_name}` which is not marked with `#[const_trait]`