Silence errors in experssions caused by bare traits in paths

Address rust-lang#51077 in editions>=2021 by treating `<Trait>` as a `<{type error}>` instead of `<dyn Trait>`, silencing subsequent errors. We new emit a single error ("missing `dyn`").

```
error[E0782]: trait objects must include the `dyn` keyword
 --> f800.rs:2:15
  |
2 | let x: u32 = <Default>::default();
  |               ^^^^^^^
  |
help: add `dyn` keyword before this trait
  |
2 | let x: u32 = <dyn Default>::default();
  |               +++
```

instead of 6

```
error[E0782]: trait objects must include the `dyn` keyword
 --> f800.rs:2:15
  |
2 | let x: u32 = <Default>::default();
  |               ^^^^^^^
  |
help: add `dyn` keyword before this trait
  |
2 | let x: u32 = <dyn Default>::default();
  |               +++

error[E0038]: the trait `Default` cannot be made into an object
 --> f800.rs:2:15
  |
2 | let x: u32 = <Default>::default();
  |               ^^^^^^^ `Default` cannot be made into an object
  |
  = note: the trait cannot be made into an object because it requires `Self: Sized`
  = note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>

error[E0277]: the size for values of type `dyn Default` cannot be known at compilation time
   --> f800.rs:2:15
    |
2   | let x: u32 = <Default>::default();
    |               ^^^^^^^ doesn't have a size known at compile-time
    |
    = help: the trait `Sized` is not implemented for `dyn Default`
note: required by a bound in `default`
   --> /home/gh-estebank/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/default.rs:104:20
    |
104 | pub trait Default: Sized {
    |                    ^^^^^ required by this bound in `Default::default`
...
136 |     fn default() -> Self;
    |        ------- required by a bound in this associated function

error[E0308]: mismatched types
 --> f800.rs:2:14
  |
2 | let x: u32 = <Default>::default();
  |        ---   ^^^^^^^^^^^^^^^^^^^^ expected `u32`, found `dyn Default`
  |        |
  |        expected due to this
  |
  = note:      expected type `u32`
          found trait object `dyn Default`
  = help: `u32` implements `Default` so you could change the expected type to `Box<dyn Default>`

error[E0038]: the trait `Default` cannot be made into an object
 --> f800.rs:2:14
  |
2 | let x: u32 = <Default>::default();
  |              ^^^^^^^^^^^^^^^^^^^^ `Default` cannot be made into an object
  |
  = note: the trait cannot be made into an object because it requires `Self: Sized`
  = note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>

error[E0277]: the size for values of type `dyn Default` cannot be known at compilation time
 --> f800.rs:2:14
  |
2 | let x: u32 = <Default>::default();
  |              ^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
  |
  = help: the trait `Sized` is not implemented for `dyn Default`
  = note: the return type of a function must have a statically known size
```

Author: estebank

compiler/rustc_hir_analysis/src/hir_ty_lowering/lint.rs

@@ -1,5 +1,5 @@
 use rustc_ast::TraitObjectSyntax;
-use rustc_errors::{codes::*, Diag, EmissionGuarantee, StashKey};
+use rustc_errors::{codes::*, Diag, EmissionGuarantee, ErrorGuaranteed, StashKey};
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
 use rustc_lint_defs::{builtin::BARE_TRAIT_OBJECTS, Applicability};
@@ -17,13 +17,13 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         &self,
         self_ty: &hir::Ty<'_>,
         in_path: bool,
-    ) {
+    ) -> Option<ErrorGuaranteed> {
         let tcx = self.tcx();
 
         let hir::TyKind::TraitObject([poly_trait_ref, ..], _, TraitObjectSyntax::None) =
             self_ty.kind
         else {
-            return;
+            return None;
         };
 
         let needs_bracket = in_path
@@ -70,7 +70,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             // Check if the impl trait that we are considering is an impl of a local trait.
             self.maybe_suggest_blanket_trait_impl(self_ty, &mut diag);
             self.maybe_suggest_assoc_ty_bound(self_ty, &mut diag);
-            diag.stash(self_ty.span, StashKey::TraitMissingMethod);
+            diag.stash(self_ty.span, StashKey::TraitMissingMethod)
         } else {
             tcx.node_span_lint(BARE_TRAIT_OBJECTS, self_ty.hir_id, self_ty.span, |lint| {
                 lint.primary_message("trait objects without an explicit `dyn` are deprecated");
@@ -83,6 +83,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 }
                 self.maybe_suggest_blanket_trait_impl(self_ty, lint);
             });
+            None
         }
     }
 

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

@@ -2080,7 +2080,18 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 )
             }
             hir::TyKind::TraitObject(bounds, lifetime, repr) => {
-                self.prohibit_or_lint_bare_trait_object_ty(hir_ty, in_path);
+                if let Some(guard) = self.prohibit_or_lint_bare_trait_object_ty(hir_ty, in_path) {
+                    if let hir::Node::Item(_) | hir::Node::TraitItem(_) | hir::Node::ImplItem(_) =
+                        tcx.parent_hir_node(hir_ty.hir_id)
+                    {
+                        // `tests/ui/suggestions/issue-116434-2021.rs` and others would otherwise
+                        // fail with no error beign emitted. We restrict the silencing only to
+                        // expressions, as those *will* emit the error *and* also produce further
+                        // knock-down errors.
+                    } else {
+                        return Ty::new_error(tcx, guard);
+                    }
+                }
 
                 let repr = match repr {
                     TraitObjectSyntax::Dyn | TraitObjectSyntax::None => ty::Dyn,

tests/ui/associated-type-bounds/suggest-assoc-ty-bound-on-eq-bound.rs

@@ -5,15 +5,11 @@ fn f(_: impl Trait<T = Copy>) {}
 //~^ ERROR trait objects must include the `dyn` keyword
 //~| HELP add `dyn` keyword before this trait
 //~| HELP you might have meant to write a bound here
-//~| ERROR the trait `Copy` cannot be made into an object
 
 fn g(_: impl Trait<T = std::fmt::Debug + Eq>) {}
 //~^ ERROR trait objects must include the `dyn` keyword
 //~| HELP add `dyn` keyword before this trait
 //~| HELP you might have meant to write a bound here
-//~| ERROR only auto traits can be used as additional traits in a trait object
-//~| HELP consider creating a new trait
-//~| ERROR the trait `Eq` cannot be made into an object
 
 fn h(_: impl Trait<T<> = 'static + for<'a> Fn(&'a ())>) {}
 //~^ ERROR trait objects must include the `dyn` keyword

tests/ui/associated-type-bounds/suggest-assoc-ty-bound-on-eq-bound.stderr

@@ -1,34 +1,3 @@
-error[E0038]: the trait `Copy` cannot be made into an object
-  --> $DIR/suggest-assoc-ty-bound-on-eq-bound.rs:4:20
-   |
-LL | fn f(_: impl Trait<T = Copy>) {}
-   |                    ^^^^^^^^ `Copy` cannot be made into an object
-   |
-   = note: the trait cannot be made into an object because it requires `Self: Sized`
-   = note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
-
-error[E0225]: only auto traits can be used as additional traits in a trait object
-  --> $DIR/suggest-assoc-ty-bound-on-eq-bound.rs:10:42
-   |
-LL | fn g(_: impl Trait<T = std::fmt::Debug + Eq>) {}
-   |                        ---------------   ^^ additional non-auto trait
-   |                        |
-   |                        first non-auto trait
-   |
-   = help: consider creating a new trait with all of these as supertraits and using that trait here instead: `trait NewTrait: Debug + Eq {}`
-   = note: auto-traits like `Send` and `Sync` are traits that have special properties; for more information on them, visit <https://doc.rust-lang.org/reference/special-types-and-traits.html#auto-traits>
-
-error[E0038]: the trait `Eq` cannot be made into an object
-  --> $DIR/suggest-assoc-ty-bound-on-eq-bound.rs:10:24
-   |
-LL | fn g(_: impl Trait<T = std::fmt::Debug + Eq>) {}
-   |                        ^^^^^^^^^^^^^^^^^^^^ `Eq` cannot be made into an object
-   |
-note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
-  --> $SRC_DIR/core/src/cmp.rs:LL:COL
-   |
-   = note: the trait cannot be made into an object because it uses `Self` as a type parameter
-
 error[E0782]: trait objects must include the `dyn` keyword
   --> $DIR/suggest-assoc-ty-bound-on-eq-bound.rs:4:24
    |
@@ -45,7 +14,7 @@ LL | fn f(_: impl Trait<T: Copy>) {}
    |                     ~
 
 error[E0782]: trait objects must include the `dyn` keyword
-  --> $DIR/suggest-assoc-ty-bound-on-eq-bound.rs:10:24
+  --> $DIR/suggest-assoc-ty-bound-on-eq-bound.rs:9:24
    |
 LL | fn g(_: impl Trait<T = std::fmt::Debug + Eq>) {}
    |                        ^^^^^^^^^^^^^^^^^^^^
@@ -60,7 +29,7 @@ LL | fn g(_: impl Trait<T: std::fmt::Debug + Eq>) {}
    |                     ~
 
 error[E0782]: trait objects must include the `dyn` keyword
-  --> $DIR/suggest-assoc-ty-bound-on-eq-bound.rs:18:26
+  --> $DIR/suggest-assoc-ty-bound-on-eq-bound.rs:14:26
    |
 LL | fn h(_: impl Trait<T<> = 'static + for<'a> Fn(&'a ())>) {}
    |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -75,7 +44,7 @@ LL | fn h(_: impl Trait<T<>: 'static + for<'a> Fn(&'a ())>) {}
    |                       ~
 
 error[E0782]: trait objects must include the `dyn` keyword
-  --> $DIR/suggest-assoc-ty-bound-on-eq-bound.rs:24:26
+  --> $DIR/suggest-assoc-ty-bound-on-eq-bound.rs:20:26
    |
 LL | type Obj = dyn Trait<T = Clone>;
    |                          ^^^^^
@@ -85,7 +54,6 @@ help: add `dyn` keyword before this trait
 LL | type Obj = dyn Trait<T = dyn Clone>;
    |                          +++
 
-error: aborting due to 7 previous errors
+error: aborting due to 4 previous errors
 
-Some errors have detailed explanations: E0038, E0225, E0782.
-For more information about an error, try `rustc --explain E0038`.
+For more information about this error, try `rustc --explain E0782`.

tests/ui/const-generics/generic_const_exprs/expected-type-of-closure-body-to-be-a-closure-or-coroutine-ice-113776.rs

@@ -14,10 +14,8 @@ fn f<T>(
          1
      }],
 ) -> impl Iterator<Item = SubAssign> {
-//~^ ERROR the type parameter `Rhs` must be explicitly specified
-//~| ERROR `()` is not an iterator
+//~^ ERROR `()` is not an iterator
 //~| ERROR trait objects must include the `dyn` keyword
-//~| ERROR the type parameter `Rhs` must be explicitly specified [E0393]
 }
 
 pub fn main() {}

tests/ui/const-generics/generic_const_exprs/expected-type-of-closure-body-to-be-a-closure-or-coroutine-ice-113776.stderr

@@ -16,29 +16,6 @@ help: you might be missing a type parameter
 LL | fn f<T, F>(
    |       +++
 
-error[E0393]: the type parameter `Rhs` must be explicitly specified
-  --> $DIR/expected-type-of-closure-body-to-be-a-closure-or-coroutine-ice-113776.rs:16:27
-   |
-LL | ) -> impl Iterator<Item = SubAssign> {
-   |                           ^^^^^^^^^ help: set the type parameter to the desired type: `SubAssign<Rhs>`
-  --> $SRC_DIR/core/src/ops/arith.rs:LL:COL
-   |
-   = note: type parameter `Rhs` must be specified for this
-   |
-   = note: because of the default `Self` reference, type parameters must be specified on object types
-
-error[E0393]: the type parameter `Rhs` must be explicitly specified
-  --> $DIR/expected-type-of-closure-body-to-be-a-closure-or-coroutine-ice-113776.rs:16:27
-   |
-LL | ) -> impl Iterator<Item = SubAssign> {
-   |                           ^^^^^^^^^ help: set the type parameter to the desired type: `SubAssign<Rhs>`
-  --> $SRC_DIR/core/src/ops/arith.rs:LL:COL
-   |
-   = note: type parameter `Rhs` must be specified for this
-   |
-   = note: because of the default `Self` reference, type parameters must be specified on object types
-   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
-
 error[E0277]: `()` is not an iterator
   --> $DIR/expected-type-of-closure-body-to-be-a-closure-or-coroutine-ice-113776.rs:16:6
    |
@@ -62,7 +39,7 @@ help: you might have meant to write a bound here
 LL | ) -> impl Iterator<Item: SubAssign> {
    |                        ~
 
-error: aborting due to 5 previous errors
+error: aborting due to 3 previous errors
 
-Some errors have detailed explanations: E0277, E0393, E0412, E0782.
+Some errors have detailed explanations: E0277, E0412, E0782.
 For more information about an error, try `rustc --explain E0277`.

tests/ui/dyn-keyword/trait-missing-dyn-in-qualified-path.edition2018.stderr

@@ -0,0 +1,67 @@
+warning: trait objects without an explicit `dyn` are deprecated
+  --> $DIR/trait-missing-dyn-in-qualified-path.rs:5:19
+   |
+LL |     let x: u32 = <Default>::default();
+   |                   ^^^^^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2018) but is a hard error in Rust 2021!
+   = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/warnings-promoted-to-error.html>
+   = note: `#[warn(bare_trait_objects)]` on by default
+help: if this is an object-safe trait, use `dyn`
+   |
+LL |     let x: u32 = <dyn Default>::default();
+   |                   +++
+
+error[E0038]: the trait `Default` cannot be made into an object
+  --> $DIR/trait-missing-dyn-in-qualified-path.rs:5:19
+   |
+LL |     let x: u32 = <Default>::default();
+   |                   ^^^^^^^ `Default` cannot be made into an object
+   |
+   = note: the trait cannot be made into an object because it requires `Self: Sized`
+   = note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
+
+error[E0277]: the size for values of type `dyn Default` cannot be known at compilation time
+  --> $DIR/trait-missing-dyn-in-qualified-path.rs:5:19
+   |
+LL |     let x: u32 = <Default>::default();
+   |                   ^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `dyn Default`
+note: required by a bound in `default`
+  --> $SRC_DIR/core/src/default.rs:LL:COL
+
+error[E0308]: mismatched types
+  --> $DIR/trait-missing-dyn-in-qualified-path.rs:5:18
+   |
+LL |     let x: u32 = <Default>::default();
+   |            ---   ^^^^^^^^^^^^^^^^^^^^ expected `u32`, found `dyn Default`
+   |            |
+   |            expected due to this
+   |
+   = note:      expected type `u32`
+           found trait object `dyn Default`
+   = help: `u32` implements `Default` so you could change the expected type to `Box<dyn Default>`
+
+error[E0038]: the trait `Default` cannot be made into an object
+  --> $DIR/trait-missing-dyn-in-qualified-path.rs:5:18
+   |
+LL |     let x: u32 = <Default>::default();
+   |                  ^^^^^^^^^^^^^^^^^^^^ `Default` cannot be made into an object
+   |
+   = note: the trait cannot be made into an object because it requires `Self: Sized`
+   = note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
+
+error[E0277]: the size for values of type `dyn Default` cannot be known at compilation time
+  --> $DIR/trait-missing-dyn-in-qualified-path.rs:5:18
+   |
+LL |     let x: u32 = <Default>::default();
+   |                  ^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `dyn Default`
+   = note: the return type of a function must have a statically known size
+
+error: aborting due to 5 previous errors; 1 warning emitted
+
+Some errors have detailed explanations: E0038, E0277, E0308.
+For more information about an error, try `rustc --explain E0038`.

tests/ui/dyn-keyword/trait-missing-dyn-in-qualified-path.edition2021.stderr

@@ -0,0 +1,14 @@
+error[E0782]: trait objects must include the `dyn` keyword
+  --> $DIR/trait-missing-dyn-in-qualified-path.rs:5:19
+   |
+LL |     let x: u32 = <Default>::default();
+   |                   ^^^^^^^
+   |
+help: add `dyn` keyword before this trait
+   |
+LL |     let x: u32 = <dyn Default>::default();
+   |                   +++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0782`.

tests/ui/dyn-keyword/trait-missing-dyn-in-qualified-path.rs

@@ -0,0 +1,14 @@
+//@revisions: edition2018 edition2021
+//@[edition2018] edition:2018
+//@[edition2021] edition:2021
+fn main() {
+    let x: u32 = <Default>::default();
+    //[edition2021]~^ ERROR trait objects must include the `dyn` keyword
+    //[edition2018]~^^ WARNING trait objects without an explicit `dyn` are deprecated
+    //[edition2018]~| WARNING this is accepted in the current edition (Rust 2018) but is a hard error in Rust 2021!
+    //[edition2018]~| ERROR trait `Default` cannot be made into an object
+    //[edition2018]~| ERROR trait `Default` cannot be made into an object
+    //[edition2018]~| ERROR the size for values of type `dyn Default` cannot be known at compilation time
+    //[edition2018]~| ERROR the size for values of type `dyn Default` cannot be known at compilation time
+    //[edition2018]~| ERROR mismatched types
+}

tests/ui/impl-trait/generic-with-implicit-hrtb-without-dyn.edition2021.stderr

@@ -1,9 +1,3 @@
-error[E0277]: the trait bound `(): AsRef<(dyn for<'a> Fn(&'a ()) + 'static)>` is not satisfied
-  --> $DIR/generic-with-implicit-hrtb-without-dyn.rs:6:13
-   |
-LL | fn ice() -> impl AsRef<Fn(&())> {
-   |             ^^^^^^^^^^^^^^^^^^^ the trait `AsRef<(dyn for<'a> Fn(&'a ()) + 'static)>` is not implemented for `()`
-
 error[E0782]: trait objects must include the `dyn` keyword
   --> $DIR/generic-with-implicit-hrtb-without-dyn.rs:6:24
    |
@@ -15,7 +9,6 @@ help: add `dyn` keyword before this trait
 LL | fn ice() -> impl AsRef<dyn Fn(&())> {
    |                        +++
 
-error: aborting due to 2 previous errors
+error: aborting due to 1 previous error
 
-Some errors have detailed explanations: E0277, E0782.
-For more information about an error, try `rustc --explain E0277`.
+For more information about this error, try `rustc --explain E0782`.

tests/ui/impl-trait/generic-with-implicit-hrtb-without-dyn.rs

@@ -6,7 +6,6 @@
 fn ice() -> impl AsRef<Fn(&())> {
     //[edition2015]~^ ERROR: the trait bound `(): AsRef<(dyn for<'a> Fn(&'a ()) + 'static)>` is not satisfied [E0277]
     //[edition2021]~^^ ERROR: trait objects must include the `dyn` keyword [E0782]
-    //[edition2021]~| ERROR: the trait bound `(): AsRef<(dyn for<'a> Fn(&'a ()) + 'static)>` is not satisfied [E0277]
     todo!()
 }
 

tests/ui/resolve/issue-111312.rs

@@ -7,7 +7,6 @@ trait Has {
 trait HasNot {}
 
 fn main() {
-    HasNot::has();
-    //~^ ERROR trait objects must include the `dyn` keyword
-    //~| ERROR no function or associated item named `has` found for trait `HasNot`
+    <dyn HasNot>::has();
+    //~^ ERROR no function or associated item named `has` found for trait `HasNot`
 }

tests/ui/resolve/issue-111312.stderr

@@ -1,27 +1,15 @@
-error[E0782]: trait objects must include the `dyn` keyword
-  --> $DIR/issue-111312.rs:10:5
-   |
-LL |     HasNot::has();
-   |     ^^^^^^
-   |
-help: add `dyn` keyword before this trait
-   |
-LL |     <dyn HasNot>::has();
-   |     ++++       +
-
 error[E0599]: no function or associated item named `has` found for trait `HasNot`
-  --> $DIR/issue-111312.rs:10:13
+  --> $DIR/issue-111312.rs:10:19
    |
-LL |     HasNot::has();
-   |             ^^^ function or associated item not found in `HasNot`
+LL |     <dyn HasNot>::has();
+   |                   ^^^ function or associated item not found in `HasNot`
    |
 note: `Has` defines an item `has`
   --> $DIR/issue-111312.rs:3:1
    |
 LL | trait Has {
    | ^^^^^^^^^
 
-error: aborting due to 2 previous errors
+error: aborting due to 1 previous error
 
-Some errors have detailed explanations: E0599, E0782.
-For more information about an error, try `rustc --explain E0599`.
+For more information about this error, try `rustc --explain E0599`.

tests/ui/resolve/issue-111727.rs

@@ -3,5 +3,4 @@
 fn main() {
     std::any::Any::create();
     //~^ ERROR trait objects must include the `dyn` keyword
-    //~| ERROR no function or associated item named `create` found for trait `Any`
 }

tests/ui/resolve/issue-111727.stderr

@@ -9,13 +9,6 @@ help: add `dyn` keyword before this trait
 LL |     <dyn std::any::Any>::create();
    |     ++++              +
 
-error[E0599]: no function or associated item named `create` found for trait `Any`
-  --> $DIR/issue-111727.rs:4:20
-   |
-LL |     std::any::Any::create();
-   |                    ^^^^^^ function or associated item not found in `Any`
-
-error: aborting due to 2 previous errors
+error: aborting due to 1 previous error
 
-Some errors have detailed explanations: E0599, E0782.
-For more information about an error, try `rustc --explain E0599`.
+For more information about this error, try `rustc --explain E0782`.