Rollup merge of rust-lang#120719 - compiler-errors:no-dyn-atb, r=lcnr

Remove support for `associated_type_bound` nested in `dyn` types

These necessarily desugar to `impl Trait`, which is inconsistent with the `associated_type_bound` feature after rust-lang#120584.

This PR keeps the `is_in_dyn_type` hack, which kind of makes me sad. Ideally, we'd be validating that no object types have associated type bounds somewhere else. Unfortunately, we can't do this later during astconv (i think?), nor can we do it earlier during ast validation (i think?) because of the feature gating of ATB being a *warning* rather than an *error*. Let me know if you have thoughts about this.

r? lcnr

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`.