Don't cancel stashed TraitMissingMethod errors.

This gives one extra error message on two tests, but is necessary to fix
bigger problems caused by the cancellation of stashed errors.

(Note: why not just avoid stashing altogether? Because that resulted in
additional output changes.)

Author: nnethercote

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -879,17 +879,14 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     );
                 }
 
-                // emit or cancel the diagnostic for bare traits
+                // Emit the diagnostic for bare traits. (We used to cancel for slightly better
+                // error messages, but cancelling stashed diagnostics is no longer allowed because
+                // it causes problems when tracking whether errors have actually occurred.)
                 if span.edition().at_least_rust_2021()
                     && let Some(diag) =
                         self.dcx().steal_diagnostic(qself.span, StashKey::TraitMissingMethod)
                 {
-                    if trait_missing_method {
-                        // cancel the diag for bare traits when meeting `MyTrait::missing_method`
-                        diag.cancel();
-                    } else {
-                        diag.emit();
-                    }
+                    diag.emit();
                 }
 
                 if item_name.name != kw::Empty {

tests/ui/resolve/issue-111312.rs

@@ -7,5 +7,7 @@ trait Has {
 trait HasNot {}
 
 fn main() {
-    HasNot::has(); //~ ERROR
+    HasNot::has();
+    //~^ ERROR trait objects must include the `dyn` keyword
+    //~| ERROR no function or associated item named `has` found for trait `HasNot`
 }

tests/ui/resolve/issue-111312.stderr

@@ -1,3 +1,14 @@
+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
    |
@@ -10,6 +21,7 @@ note: `Has` defines an item `has`
 LL | trait Has {
    | ^^^^^^^^^
 
-error: aborting due to 1 previous error
+error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0599`.
+Some errors have detailed explanations: E0599, E0782.
+For more information about an error, try `rustc --explain E0599`.

tests/ui/resolve/issue-111727.rs

@@ -1,5 +1,7 @@
 //@ edition: 2021
 
 fn main() {
-    std::any::Any::create(); //~ ERROR
+    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

@@ -1,9 +1,21 @@
+error[E0782]: trait objects must include the `dyn` keyword
+  --> $DIR/issue-111727.rs:4:5
+   |
+LL |     std::any::Any::create();
+   |     ^^^^^^^^^^^^^
+   |
+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 1 previous error
+error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0599`.
+Some errors have detailed explanations: E0599, E0782.
+For more information about an error, try `rustc --explain E0599`.