Remove support for associated_type_bound nested in dyn types

compiler/rustc_ast_lowering/messages.ftl

@@ -8,6 +8,10 @@ ast_lowering_arbitrary_expression_in_pattern =
 
 ast_lowering_argument = argument
 
+ast_lowering_assoc_ty_binding_in_dyn =
+    associated type bounds are not allowed in `dyn` types
+    .suggestion = use `impl Trait` to introduce a type instead
+
 ast_lowering_assoc_ty_parentheses =
     parenthesized generic arguments cannot be used in associated type constraints
 
@@ -100,9 +104,6 @@ ast_lowering_match_arm_with_no_body =
     `match` arm with no body
     .suggestion = add a body after the pattern
 
-ast_lowering_misplaced_assoc_ty_binding =
-    associated type bounds are only allowed in where clauses and function signatures, not in {$position}
-
 ast_lowering_misplaced_double_dot =
     `..` patterns are not allowed here
     .note = only allowed in tuple, tuple struct, and slice patterns

compiler/rustc_ast_lowering/src/errors.rs

@@ -94,11 +94,12 @@ pub struct MisplacedImplTrait<'a> {
 }
 
 #[derive(Diagnostic)]
-#[diag(ast_lowering_misplaced_assoc_ty_binding)]
-pub struct MisplacedAssocTyBinding<'a> {
+#[diag(ast_lowering_assoc_ty_binding_in_dyn)]
+pub struct MisplacedAssocTyBinding {
     #[primary_span]
     pub span: Span,
-    pub position: DiagnosticArgFromDisplay<'a>,
+    #[suggestion(code = " = impl", applicability = "maybe-incorrect", style = "verbose")]
+    pub suggestion: Option<Span>,
 }
 
 #[derive(Diagnostic, Clone, Copy)]

compiler/rustc_ast_lowering/src/lib.rs

@@ -197,7 +197,6 @@ trait ResolverAstLoweringExt {
     fn get_label_res(&self, id: NodeId) -> Option<NodeId>;
     fn get_lifetime_res(&self, id: NodeId) -> Option<LifetimeRes>;
     fn take_extra_lifetime_params(&mut self, id: NodeId) -> Vec<(Ident, NodeId, LifetimeRes)>;
-    fn remap_extra_lifetime_params(&mut self, from: NodeId, to: NodeId);
 }
 
 impl ResolverAstLoweringExt for ResolverAstLowering {
@@ -256,11 +255,6 @@ impl ResolverAstLoweringExt for ResolverAstLowering {
     fn take_extra_lifetime_params(&mut self, id: NodeId) -> Vec<(Ident, NodeId, LifetimeRes)> {
         self.extra_lifetime_params_map.remove(&id).unwrap_or_default()
     }
-
-    fn remap_extra_lifetime_params(&mut self, from: NodeId, to: NodeId) {
-        let lifetimes = self.extra_lifetime_params_map.remove(&from).unwrap_or_default();
-        self.extra_lifetime_params_map.insert(to, lifetimes);
-    }
 }
 
 /// Context of `impl Trait` in code, which determines whether it is allowed in an HIR subtree,
@@ -1084,88 +1078,38 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 hir::TypeBindingKind::Equality { term }
             }
             AssocConstraintKind::Bound { bounds } => {
-                enum DesugarKind {
-                    ImplTrait,
-                    Error(ImplTraitPosition),
-                    Bound,
-                }
-
-                // Piggy-back on the `impl Trait` context to figure out the correct behavior.
-                let desugar_kind = match itctx {
-                    // in an argument, RPIT, or TAIT, if we are within a dyn type:
-                    //
-                    //     fn foo(x: dyn Iterator<Item: Debug>)
-                    //
-                    // then desugar to:
-                    //
-                    //     fn foo(x: dyn Iterator<Item = impl Debug>)
-                    //
-                    // This is because dyn traits must have all of their associated types specified.
-                    ImplTraitContext::ReturnPositionOpaqueTy { .. }
-                    | ImplTraitContext::TypeAliasesOpaqueTy { .. }
-                    | ImplTraitContext::Universal
-                        if self.is_in_dyn_type =>
-                    {
-                        DesugarKind::ImplTrait
-                    }
-
-                    ImplTraitContext::Disallowed(position) if self.is_in_dyn_type => {
-                        DesugarKind::Error(position)
-                    }
-
-                    // We are in the parameter position, but not within a dyn type:
-                    //
-                    //     fn foo(x: impl Iterator<Item: Debug>)
-                    //
-                    // 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`.
-                    _ => DesugarKind::Bound,
-                };
-
-                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.
-
-                        let impl_trait_node_id = self.next_node_id();
-                        // Shift `impl Trait` lifetime captures from the associated type bound's
-                        // node id to the opaque node id, so that the opaque can actually use
-                        // these lifetime bounds.
-                        self.resolver
-                            .remap_extra_lifetime_params(constraint.id, impl_trait_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,
-                            );
+                // Disallow ATB in dyn types
+                if self.is_in_dyn_type {
+                    let suggestion = match itctx {
+                        ImplTraitContext::ReturnPositionOpaqueTy { .. }
+                        | ImplTraitContext::TypeAliasesOpaqueTy { .. }
+                        | ImplTraitContext::Universal => {
+                            let bound_end_span = constraint
+                                .gen_args
+                                .as_ref()
+                                .map_or(constraint.ident.span, |args| args.span());
+                            if bound_end_span.eq_ctxt(constraint.span) {
+                                Some(self.tcx.sess.source_map().next_point(bound_end_span))
+                            } else {
+                                None
+                            }
+                        }
+                        _ => None,
+                    };
 
-                            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);
+                    let guar = self.dcx().emit_err(errors::MisplacedAssocTyBinding {
+                        span: constraint.span,
+                        suggestion,
+                    });
+                    let err_ty =
+                        &*self.arena.alloc(self.ty(constraint.span, hir::TyKind::Err(guar)));
+                    hir::TypeBindingKind::Equality { term: err_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);
 
-                        hir::TypeBindingKind::Constraint { bounds }
-                    }
-                    DesugarKind::Error(position) => {
-                        let guar = self.dcx().emit_err(errors::MisplacedAssocTyBinding {
-                            span: constraint.span,
-                            position: DiagnosticArgFromDisplay(&position),
-                        });
-                        let err_ty =
-                            &*self.arena.alloc(self.ty(constraint.span, hir::TyKind::Err(guar)));
-                        hir::TypeBindingKind::Equality { term: err_ty.into() }
-                    }
+                    hir::TypeBindingKind::Constraint { bounds }
                 }
             }
         };

tests/ui/associated-type-bounds/assoc-type-eq-with-dyn-atb-fail.rs

@@ -28,9 +28,9 @@ impl Bar for AssocNoCopy {
 
 impl Thing for AssocNoCopy {
     type Out = Box<dyn Bar<Assoc: Copy>>;
+    //~^ ERROR associated type bounds are not allowed in `dyn` types
 
     fn func() -> Self::Out {
-        //~^ ERROR the trait bound `String: Copy` is not satisfied
         Box::new(AssocNoCopy)
     }
 }

tests/ui/associated-type-bounds/assoc-type-eq-with-dyn-atb-fail.stderr

@@ -1,9 +1,13 @@
-error[E0277]: the trait bound `String: Copy` is not satisfied
-  --> $DIR/assoc-type-eq-with-dyn-atb-fail.rs:32:18
+error: associated type bounds are not allowed in `dyn` types
+  --> $DIR/assoc-type-eq-with-dyn-atb-fail.rs:30:28
    |
-LL |     fn func() -> Self::Out {
-   |                  ^^^^^^^^^ the trait `Copy` is not implemented for `String`
+LL |     type Out = Box<dyn Bar<Assoc: Copy>>;
+   |                            ^^^^^^^^^^^
+   |
+help: use `impl Trait` to introduce a type instead
+   |
+LL |     type Out = Box<dyn Bar<Assoc = impl Copy>>;
+   |                                  ~~~~~~
 
 error: aborting due to 1 previous error
 
-For more information about this error, try `rustc --explain E0277`.

tests/ui/associated-type-bounds/bad-universal-in-dyn-in-where-clause.rs

@@ -7,7 +7,7 @@ trait B {
 fn f()
 where
     dyn for<'j> B<AssocType: 'j>:,
-    //~^ ERROR associated type bounds are only allowed in where clauses and function signatures
+    //~^ ERROR associated type bounds are not allowed in `dyn` types
 {
 }
 

tests/ui/associated-type-bounds/bad-universal-in-dyn-in-where-clause.stderr

@@ -1,4 +1,4 @@
-error: associated type bounds are only allowed in where clauses and function signatures, not in bounds
+error: associated type bounds are not allowed in `dyn` types
   --> $DIR/bad-universal-in-dyn-in-where-clause.rs:9:19
    |
 LL |     dyn for<'j> B<AssocType: 'j>:,

tests/ui/associated-type-bounds/bad-universal-in-impl-sig.rs

@@ -8,6 +8,6 @@ trait Trait2 {}
 
 // It's not possible to insert a universal `impl Trait` here!
 impl dyn Trait<Item: Trait2> {}
-//~^ ERROR associated type bounds are only allowed in where clauses and function signatures
+//~^ ERROR associated type bounds are not allowed in `dyn` types
 
 fn main() {}

tests/ui/associated-type-bounds/bad-universal-in-impl-sig.stderr

@@ -1,4 +1,4 @@
-error: associated type bounds are only allowed in where clauses and function signatures, not in impl headers
+error: associated type bounds are not allowed in `dyn` types
   --> $DIR/bad-universal-in-impl-sig.rs:10:16
    |
 LL | impl dyn Trait<Item: Trait2> {}

tests/ui/associated-type-bounds/duplicate.rs

@@ -261,11 +261,4 @@ trait TRA3 {
     //~^ ERROR the value of the associated type `Item` in trait `Iterator` is already specified [E0719]
 }
 
-type TADyn1 = dyn Iterator<Item: Copy, Item: Send>;
-//~^ ERROR the value of the associated type `Item` in trait `Iterator` is already specified [E0719]
-type TADyn2 = Box<dyn Iterator<Item: Copy, Item: Copy>>;
-//~^ ERROR the value of the associated type `Item` in trait `Iterator` is already specified [E0719]
-type TADyn3 = dyn Iterator<Item: 'static, Item: 'static>;
-//~^ ERROR the value of the associated type `Item` in trait `Iterator` is already specified [E0719]
-
 fn main() {}

tests/ui/associated-type-bounds/duplicate.stderr

@@ -6,30 +6,6 @@ LL | struct SI1<T: Iterator<Item: Copy, Item: Send>> {
    |                        |
    |                        `Item` bound here first
 
-error[E0719]: the value of the associated type `Item` in trait `Iterator` is already specified
-  --> $DIR/duplicate.rs:264:40
-   |
-LL | type TADyn1 = dyn Iterator<Item: Copy, Item: Send>;
-   |                            ----------  ^^^^^^^^^^ re-bound here
-   |                            |
-   |                            `Item` bound here first
-
-error[E0719]: the value of the associated type `Item` in trait `Iterator` is already specified
-  --> $DIR/duplicate.rs:266:44
-   |
-LL | type TADyn2 = Box<dyn Iterator<Item: Copy, Item: Copy>>;
-   |                                ----------  ^^^^^^^^^^ re-bound here
-   |                                |
-   |                                `Item` bound here first
-
-error[E0719]: the value of the associated type `Item` in trait `Iterator` is already specified
-  --> $DIR/duplicate.rs:268:43
-   |
-LL | type TADyn3 = dyn Iterator<Item: 'static, Item: 'static>;
-   |                            -------------  ^^^^^^^^^^^^^ re-bound here
-   |                            |
-   |                            `Item` bound here first
-
 error[E0719]: the value of the associated type `Item` in trait `Iterator` is already specified
   --> $DIR/duplicate.rs:11:36
    |
@@ -631,7 +607,7 @@ LL |     Self: Iterator<Item: 'static, Item: 'static>,
    |
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
-error: aborting due to 75 previous errors
+error: aborting due to 72 previous errors
 
 Some errors have detailed explanations: E0282, E0719.
 For more information about an error, try `rustc --explain E0282`.