Rollup of 7 pull requests

.git-blame-ignore-revs

@@ -1,3 +1,5 @@
+# Use `git config blame.ignorerevsfile .git-blame-ignore-revs` to make `git blame` ignore the following commits.
+
 # format the world
 a06baa56b95674fc626b3c3fd680d6a65357fe60
 # format libcore

compiler/rustc_ast_lowering/locales/en-US.ftl

@@ -19,6 +19,9 @@ ast_lowering_remove_parentheses = remove these parentheses
 ast_lowering_misplaced_impl_trait =
     `impl Trait` only allowed in function and inherent method return types, not in {$position}
 
+ast_lowering_misplaced_assoc_ty_binding =
+    associated type bounds are only allowed in where clauses and function signatures, not in {$position}
+
 ast_lowering_rustc_box_attribute_error =
     #[rustc_box] requires precisely one argument and no other attributes are allowed
 

compiler/rustc_ast_lowering/src/errors.rs

@@ -79,6 +79,14 @@ pub struct MisplacedImplTrait<'a> {
     pub position: DiagnosticArgFromDisplay<'a>,
 }
 
+#[derive(Diagnostic)]
+#[diag(ast_lowering_misplaced_assoc_ty_binding)]
+pub struct MisplacedAssocTyBinding<'a> {
+    #[primary_span]
+    pub span: Span,
+    pub position: DiagnosticArgFromDisplay<'a>,
+}
+
 #[derive(Diagnostic, Clone, Copy)]
 #[diag(ast_lowering_rustc_box_attribute_error)]
 pub struct RustcBoxAttributeError {

compiler/rustc_ast_lowering/src/lib.rs

@@ -288,31 +288,31 @@ enum ImplTraitPosition {
 impl std::fmt::Display for ImplTraitPosition {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         let name = match self {
-            ImplTraitPosition::Path => "path",
-            ImplTraitPosition::Variable => "variable binding",
-            ImplTraitPosition::Trait => "trait",
-            ImplTraitPosition::AsyncBlock => "async block",
-            ImplTraitPosition::Bound => "bound",
-            ImplTraitPosition::Generic => "generic",
-            ImplTraitPosition::ExternFnParam => "`extern fn` param",
-            ImplTraitPosition::ClosureParam => "closure param",
-            ImplTraitPosition::PointerParam => "`fn` pointer param",
-            ImplTraitPosition::FnTraitParam => "`Fn` trait param",
-            ImplTraitPosition::TraitParam => "trait method param",
-            ImplTraitPosition::ImplParam => "`impl` method param",
-            ImplTraitPosition::ExternFnReturn => "`extern fn` return",
-            ImplTraitPosition::ClosureReturn => "closure return",
-            ImplTraitPosition::PointerReturn => "`fn` pointer return",
-            ImplTraitPosition::FnTraitReturn => "`Fn` trait return",
-            ImplTraitPosition::TraitReturn => "trait method return",
-            ImplTraitPosition::ImplReturn => "`impl` method return",
-            ImplTraitPosition::GenericDefault => "generic parameter default",
-            ImplTraitPosition::ConstTy => "const type",
-            ImplTraitPosition::StaticTy => "static type",
-            ImplTraitPosition::AssocTy => "associated type",
-            ImplTraitPosition::FieldTy => "field type",
-            ImplTraitPosition::Cast => "cast type",
-            ImplTraitPosition::ImplSelf => "impl header",
+            ImplTraitPosition::Path => "paths",
+            ImplTraitPosition::Variable => "variable bindings",
+            ImplTraitPosition::Trait => "traits",
+            ImplTraitPosition::AsyncBlock => "async blocks",
+            ImplTraitPosition::Bound => "bounds",
+            ImplTraitPosition::Generic => "generics",
+            ImplTraitPosition::ExternFnParam => "`extern fn` params",
+            ImplTraitPosition::ClosureParam => "closure params",
+            ImplTraitPosition::PointerParam => "`fn` pointer params",
+            ImplTraitPosition::FnTraitParam => "`Fn` trait params",
+            ImplTraitPosition::TraitParam => "trait method params",
+            ImplTraitPosition::ImplParam => "`impl` method params",
+            ImplTraitPosition::ExternFnReturn => "`extern fn` return types",
+            ImplTraitPosition::ClosureReturn => "closure return types",
+            ImplTraitPosition::PointerReturn => "`fn` pointer return types",
+            ImplTraitPosition::FnTraitReturn => "`Fn` trait return types",
+            ImplTraitPosition::TraitReturn => "trait method return types",
+            ImplTraitPosition::ImplReturn => "`impl` method return types",
+            ImplTraitPosition::GenericDefault => "generic parameter defaults",
+            ImplTraitPosition::ConstTy => "const types",
+            ImplTraitPosition::StaticTy => "static types",
+            ImplTraitPosition::AssocTy => "associated types",
+            ImplTraitPosition::FieldTy => "field types",
+            ImplTraitPosition::Cast => "cast types",
+            ImplTraitPosition::ImplSelf => "impl headers",
         };
 
         write!(f, "{name}")
@@ -1002,8 +1002,6 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         } else {
             self.arena.alloc(hir::GenericArgs::none())
         };
-        let itctx_tait = &ImplTraitContext::TypeAliasesOpaqueTy;
-
         let kind = match &constraint.kind {
             AssocConstraintKind::Equality { term } => {
                 let term = match term {
@@ -1013,8 +1011,14 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 hir::TypeBindingKind::Equality { term }
             }
             AssocConstraintKind::Bound { bounds } => {
+                enum DesugarKind<'a> {
+                    ImplTrait,
+                    Error(&'a ImplTraitPosition),
+                    Bound,
+                }
+
                 // Piggy-back on the `impl Trait` context to figure out the correct behavior.
-                let (desugar_to_impl_trait, itctx) = match itctx {
+                let desugar_kind = match itctx {
                     // We are in the return position:
                     //
                     //     fn foo() -> impl Iterator<Item: Debug>
@@ -1023,7 +1027,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     //
                     //     fn foo() -> impl Iterator<Item = impl Debug>
                     ImplTraitContext::ReturnPositionOpaqueTy { .. }
-                    | ImplTraitContext::TypeAliasesOpaqueTy { .. } => (true, itctx),
+                    | ImplTraitContext::TypeAliasesOpaqueTy { .. } => DesugarKind::ImplTrait,
 
                     // We are in the argument position, but within a dyn type:
                     //
@@ -1032,15 +1036,11 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     // so desugar to
                     //
                     //     fn foo(x: dyn Iterator<Item = impl Debug>)
-                    ImplTraitContext::Universal if self.is_in_dyn_type => (true, itctx),
+                    ImplTraitContext::Universal if self.is_in_dyn_type => DesugarKind::ImplTrait,
 
-                    // In `type Foo = dyn Iterator<Item: Debug>` we desugar to
-                    // `type Foo = dyn Iterator<Item = impl Debug>` but we have to override the
-                    // "impl trait context" to permit `impl Debug` in this position (it desugars
-                    // then to an opaque type).
-                    //
-                    // FIXME: this is only needed until `impl Trait` is allowed in type aliases.
-                    ImplTraitContext::Disallowed(_) if self.is_in_dyn_type => (true, itctx_tait),
+                    ImplTraitContext::Disallowed(position) if self.is_in_dyn_type => {
+                        DesugarKind::Error(position)
+                    }
 
                     // We are in the parameter position, but not within a dyn type:
                     //
@@ -1049,35 +1049,46 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     // so we leave it as is and this gets expanded in astconv to a bound like
                     // `<T as Iterator>::Item: Debug` where `T` is the type parameter for the
                     // `impl Iterator`.
-                    _ => (false, itctx),
+                    _ => DesugarKind::Bound,
                 };
 
-                if desugar_to_impl_trait {
-                    // Desugar `AssocTy: Bounds` into `AssocTy = impl Bounds`. We do this by
-                    // constructing the HIR for `impl bounds...` and then lowering that.
-
-                    let impl_trait_node_id = self.next_node_id();
-
-                    self.with_dyn_type_scope(false, |this| {
-                        let node_id = this.next_node_id();
-                        let ty = this.lower_ty(
-                            &Ty {
-                                id: node_id,
-                                kind: TyKind::ImplTrait(impl_trait_node_id, bounds.clone()),
-                                span: this.lower_span(constraint.span),
-                                tokens: None,
-                            },
-                            itctx,
-                        );
+                match desugar_kind {
+                    DesugarKind::ImplTrait => {
+                        // Desugar `AssocTy: Bounds` into `AssocTy = impl Bounds`. We do this by
+                        // constructing the HIR for `impl bounds...` and then lowering that.
 
-                        hir::TypeBindingKind::Equality { term: ty.into() }
-                    })
-                } else {
-                    // Desugar `AssocTy: Bounds` into a type binding where the
-                    // later desugars into a trait predicate.
-                    let bounds = self.lower_param_bounds(bounds, itctx);
+                        let impl_trait_node_id = self.next_node_id();
 
-                    hir::TypeBindingKind::Constraint { bounds }
+                        self.with_dyn_type_scope(false, |this| {
+                            let node_id = this.next_node_id();
+                            let ty = this.lower_ty(
+                                &Ty {
+                                    id: node_id,
+                                    kind: TyKind::ImplTrait(impl_trait_node_id, bounds.clone()),
+                                    span: this.lower_span(constraint.span),
+                                    tokens: None,
+                                },
+                                itctx,
+                            );
+
+                            hir::TypeBindingKind::Equality { term: ty.into() }
+                        })
+                    }
+                    DesugarKind::Bound => {
+                        // Desugar `AssocTy: Bounds` into a type binding where the
+                        // later desugars into a trait predicate.
+                        let bounds = self.lower_param_bounds(bounds, itctx);
+
+                        hir::TypeBindingKind::Constraint { bounds }
+                    }
+                    DesugarKind::Error(position) => {
+                        self.tcx.sess.emit_err(errors::MisplacedAssocTyBinding {
+                            span: constraint.span,
+                            position: DiagnosticArgFromDisplay(position),
+                        });
+                        let err_ty = &*self.arena.alloc(self.ty(constraint.span, hir::TyKind::Err));
+                        hir::TypeBindingKind::Equality { term: err_ty.into() }
+                    }
                 }
             }
         };

compiler/rustc_ast_passes/locales/en-US.ftl

@@ -11,9 +11,6 @@ ast_passes_forbidden_let_stable =
 ast_passes_deprecated_where_clause_location =
     where clause not allowed here
 
-ast_passes_forbidden_assoc_constraint =
-    associated type bounds are not allowed within structs, enums, or unions
-
 ast_passes_keyword_lifetime =
     lifetimes cannot use keyword names
 

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -71,10 +71,6 @@ struct AstValidator<'a> {
     /// or `Foo::Bar<impl Trait>`
     is_impl_trait_banned: bool,
 
-    /// Used to ban associated type bounds (i.e., `Type<AssocType: Bounds>`) in
-    /// certain positions.
-    is_assoc_ty_bound_banned: bool,
-
     /// See [ForbiddenLetReason]
     forbidden_let_reason: Option<ForbiddenLetReason>,
 
@@ -180,30 +176,12 @@ impl<'a> AstValidator<'a> {
         }
     }
 
-    fn with_banned_assoc_ty_bound(&mut self, f: impl FnOnce(&mut Self)) {
-        let old = mem::replace(&mut self.is_assoc_ty_bound_banned, true);
-        f(self);
-        self.is_assoc_ty_bound_banned = old;
-    }
-
     fn with_impl_trait(&mut self, outer: Option<Span>, f: impl FnOnce(&mut Self)) {
         let old = mem::replace(&mut self.outer_impl_trait, outer);
         f(self);
         self.outer_impl_trait = old;
     }
 
-    fn visit_assoc_constraint_from_generic_args(&mut self, constraint: &'a AssocConstraint) {
-        match constraint.kind {
-            AssocConstraintKind::Equality { .. } => {}
-            AssocConstraintKind::Bound { .. } => {
-                if self.is_assoc_ty_bound_banned {
-                    self.session.emit_err(ForbiddenAssocConstraint { span: constraint.span });
-                }
-            }
-        }
-        self.visit_assoc_constraint(constraint);
-    }
-
     // Mirrors `visit::walk_ty`, but tracks relevant state.
     fn walk_ty(&mut self, t: &'a Ty) {
         match &t.kind {
@@ -1248,7 +1226,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                         // are allowed to contain nested `impl Trait`.
                         AngleBracketedArg::Constraint(constraint) => {
                             self.with_impl_trait(None, |this| {
-                                this.visit_assoc_constraint_from_generic_args(constraint);
+                                this.visit_assoc_constraint(constraint);
                             });
                         }
                     }
@@ -1373,14 +1351,6 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
         visit::walk_param_bound(self, bound)
     }
 
-    fn visit_variant_data(&mut self, s: &'a VariantData) {
-        self.with_banned_assoc_ty_bound(|this| visit::walk_struct_def(this, s))
-    }
-
-    fn visit_enum_def(&mut self, enum_definition: &'a EnumDef) {
-        self.with_banned_assoc_ty_bound(|this| visit::walk_enum_def(this, enum_definition))
-    }
-
     fn visit_fn(&mut self, fk: FnKind<'a>, span: Span, id: NodeId) {
         // Only associated `fn`s can have `self` parameters.
         let self_semantic = match fk.ctxt() {
@@ -1709,7 +1679,6 @@ pub fn check_crate(session: &Session, krate: &Crate, lints: &mut LintBuffer) ->
         outer_impl_trait: None,
         disallow_tilde_const: None,
         is_impl_trait_banned: false,
-        is_assoc_ty_bound_banned: false,
         forbidden_let_reason: Some(ForbiddenLetReason::GenericForbidden),
         lint_buffer: lints,
     };

compiler/rustc_ast_passes/src/errors.rs

@@ -23,13 +23,6 @@ pub struct ForbiddenLetStable {
     pub span: Span,
 }
 
-#[derive(Diagnostic)]
-#[diag(ast_passes_forbidden_assoc_constraint)]
-pub struct ForbiddenAssocConstraint {
-    #[primary_span]
-    pub span: Span,
-}
-
 #[derive(Diagnostic)]
 #[diag(ast_passes_keyword_lifetime)]
 pub struct KeywordLifetime {

compiler/rustc_borrowck/src/lib.rs

@@ -1,6 +1,7 @@
 //! This query borrow-checks the MIR to (further) ensure it is not broken.
 
 #![allow(rustc::potential_query_instability)]
+#![feature(associated_type_bounds)]
 #![feature(box_patterns)]
 #![feature(let_chains)]
 #![feature(min_specialization)]

compiler/rustc_borrowck/src/type_check/canonical.rs

@@ -117,7 +117,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
 
     pub(super) fn prove_predicates(
         &mut self,
-        predicates: impl IntoIterator<Item = impl ToPredicate<'tcx> + std::fmt::Debug>,
+        predicates: impl IntoIterator<Item: ToPredicate<'tcx> + std::fmt::Debug>,
         locations: Locations,
         category: ConstraintCategory<'tcx>,
     ) {

compiler/rustc_hir_analysis/locales/en-US.ftl

@@ -127,5 +127,8 @@ hir_analysis_auto_deref_reached_recursion_limit = reached the recursion limit wh
     .label = deref recursion limit reached
     .help = consider increasing the recursion limit by adding a `#![recursion_limit = "{$suggested_limit}"]` attribute to your crate (`{$crate_name}`)
 
+hir_analysis_where_clause_on_main = `main` function is not allowed to have a `where` clause
+    .label = `main` cannot have a `where` clause
+
 hir_analysis_track_caller_on_main = `main` function is not allowed to be `#[track_caller]`
     .label = `main` function is not allowed to be `#[track_caller]`

compiler/rustc_hir_analysis/src/errors.rs

@@ -316,6 +316,15 @@ pub struct AutoDerefReachedRecursionLimit<'a> {
     pub crate_name: Symbol,
 }
 
+#[derive(Diagnostic)]
+#[diag(hir_analysis_where_clause_on_main, code = "E0646")]
+pub(crate) struct WhereClauseOnMain {
+    #[primary_span]
+    pub span: Span,
+    #[label]
+    pub generics_span: Option<Span>,
+}
+
 #[derive(Diagnostic)]
 #[diag(hir_analysis_track_caller_on_main)]
 pub(crate) struct TrackCallerOnMain {

compiler/rustc_hir_analysis/src/lib.rs

@@ -271,16 +271,10 @@ fn check_main_fn_ty(tcx: TyCtxt<'_>, main_def_id: DefId) {
     } else if !main_fn_predicates.predicates.is_empty() {
         // generics may bring in implicit predicates, so we skip this check if generics is present.
         let generics_where_clauses_span = main_fn_where_clauses_span(tcx, main_def_id);
-        let mut diag = struct_span_err!(
-            tcx.sess,
-            generics_where_clauses_span.unwrap_or(main_span),
-            E0646,
-            "`main` function is not allowed to have a `where` clause"
-        );
-        if let Some(generics_where_clauses_span) = generics_where_clauses_span {
-            diag.span_label(generics_where_clauses_span, "`main` cannot have a `where` clause");
-        }
-        diag.emit();
+        tcx.sess.emit_err(errors::WhereClauseOnMain {
+            span: generics_where_clauses_span.unwrap_or(main_span),
+            generics_span: generics_where_clauses_span,
+        });
         error = true;
     }
 

compiler/rustc_infer/src/infer/combine.rs

@@ -478,10 +478,7 @@ impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
         self.obligations.extend(obligations.into_iter());
     }
 
-    pub fn register_predicates(
-        &mut self,
-        obligations: impl IntoIterator<Item = impl ToPredicate<'tcx>>,
-    ) {
+    pub fn register_predicates(&mut self, obligations: impl IntoIterator<Item: ToPredicate<'tcx>>) {
         self.obligations.extend(obligations.into_iter().map(|to_pred| {
             Obligation::new(self.infcx.tcx, self.trace.cause.clone(), self.param_env, to_pred)
         }))
@@ -814,10 +811,7 @@ pub trait ObligationEmittingRelation<'tcx>: TypeRelation<'tcx> {
     /// Register predicates that must hold in order for this relation to hold. Uses
     /// a default obligation cause, [`ObligationEmittingRelation::register_obligations`] should
     /// be used if control over the obligaton causes is required.
-    fn register_predicates(
-        &mut self,
-        obligations: impl IntoIterator<Item = impl ToPredicate<'tcx>>,
-    );
+    fn register_predicates(&mut self, obligations: impl IntoIterator<Item: ToPredicate<'tcx>>);
 
     /// Register an obligation that both constants must be equal to each other.
     ///