Use ty::Error for opaque types with errors in its bounds.

This reduces unhelpful diagnostics down the road.

compiler/rustc_trait_selection/src/opaque_types.rs

@@ -1022,7 +1022,7 @@ impl<'a, 'tcx> Instantiator<'a, 'tcx> {
             if let ty::PredicateKind::Projection(projection) = predicate.kind().skip_binder() {
                 if projection.ty.references_error() {
                     // No point on adding these obligations since there's a type error involved.
-                    return ty_var;
+                    return tcx.ty_error();
                 }
             }
             // Change the predicate to refer to the type variable,

src/test/ui/async-await/issues/issue-65159.rs

@@ -6,7 +6,6 @@ async fn copy() -> Result<()>
 //~^ ERROR this enum takes 2 generic arguments
 {
     Ok(())
-    //~^ ERROR type annotations needed
 }
 
 fn main() { }

src/test/ui/async-await/issues/issue-65159.stderr

@@ -16,13 +16,6 @@ help: add missing generic argument
 LL | async fn copy() -> Result<(), E>
    |                             +++
 
-error[E0282]: type annotations needed
-  --> $DIR/issue-65159.rs:8:5
-   |
-LL |     Ok(())
-   |     ^^ cannot infer type for type parameter `E` declared on the enum `Result`
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
-Some errors have detailed explanations: E0107, E0282.
-For more information about an error, try `rustc --explain E0107`.
+For more information about this error, try `rustc --explain E0107`.

src/test/ui/borrowck/issue-82126-mismatched-subst-and-hir.rs

@@ -17,7 +17,6 @@ async fn buy_lock(generator: &Mutex<MarketMultiplier>) -> LockedMarket<'_> {
     //~^ ERROR this struct takes 0 lifetime arguments but 1 lifetime argument was supplied
     //~^^ ERROR this struct takes 1 generic argument but 0 generic arguments were supplied
     LockedMarket(generator.lock().unwrap().buy())
-    //~^ ERROR cannot return value referencing temporary value
 }
 
 struct LockedMarket<T>(T);

src/test/ui/borrowck/issue-82126-mismatched-subst-and-hir.stderr

@@ -7,7 +7,7 @@ LL | async fn buy_lock(generator: &Mutex<MarketMultiplier>) -> LockedMarket<'_>
    |                                                           expected 0 lifetime arguments
    |
 note: struct defined here, with 0 lifetime parameters
-  --> $DIR/issue-82126-mismatched-subst-and-hir.rs:23:8
+  --> $DIR/issue-82126-mismatched-subst-and-hir.rs:22:8
    |
 LL | struct LockedMarket<T>(T);
    |        ^^^^^^^^^^^^
@@ -19,7 +19,7 @@ LL | async fn buy_lock(generator: &Mutex<MarketMultiplier>) -> LockedMarket<'_>
    |                                                           ^^^^^^^^^^^^ expected 1 generic argument
    |
 note: struct defined here, with 1 generic parameter: `T`
-  --> $DIR/issue-82126-mismatched-subst-and-hir.rs:23:8
+  --> $DIR/issue-82126-mismatched-subst-and-hir.rs:22:8
    |
 LL | struct LockedMarket<T>(T);
    |        ^^^^^^^^^^^^ -
@@ -28,16 +28,6 @@ help: add missing generic argument
 LL | async fn buy_lock(generator: &Mutex<MarketMultiplier>) -> LockedMarket<'_, T> {
    |                                                                          +++
 
-error[E0515]: cannot return value referencing temporary value
-  --> $DIR/issue-82126-mismatched-subst-and-hir.rs:19:5
-   |
-LL |     LockedMarket(generator.lock().unwrap().buy())
-   |     ^^^^^^^^^^^^^-------------------------^^^^^^^
-   |     |            |
-   |     |            temporary value created here
-   |     returns a value referencing data owned by the current function
-
-error: aborting due to 3 previous errors
+error: aborting due to 2 previous errors
 
-Some errors have detailed explanations: E0107, E0515.
-For more information about an error, try `rustc --explain E0107`.
+For more information about this error, try `rustc --explain E0107`.

src/test/ui/impl-trait/issue-72911.rs

@@ -16,7 +16,6 @@ fn gather_from_file(dir_entry: &foo::MissingItem) -> impl Iterator<Item = Lint>
 
 fn lint_files() -> impl Iterator<Item = foo::MissingItem> {
     //~^ ERROR: failed to resolve
-    //~| ERROR: `()` is not an iterator
     unimplemented!()
 }
 

src/test/ui/impl-trait/issue-72911.stderr

@@ -28,15 +28,7 @@ LL | fn gather_from_file(dir_entry: &foo::MissingItem) -> impl Iterator<Item = L
 LL | fn lint_files() -> impl Iterator<Item = foo::MissingItem> {
    |                    -------------------------------------- returning this opaque type `FlatMap<impl Iterator, [type error], [closure@$DIR/issue-72911.rs:9:27: 9:51]>`
 
-error[E0277]: `()` is not an iterator
-  --> $DIR/issue-72911.rs:17:20
-   |
-LL | fn lint_files() -> impl Iterator<Item = foo::MissingItem> {
-   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `()` is not an iterator
-   |
-   = help: the trait `Iterator` is not implemented for `()`
-
-error: aborting due to 4 previous errors
+error: aborting due to 3 previous errors
 
-Some errors have detailed explanations: E0277, E0433, E0720.
-For more information about an error, try `rustc --explain E0277`.
+Some errors have detailed explanations: E0433, E0720.
+For more information about an error, try `rustc --explain E0433`.