Don't bail out of trait selection when predicate references an error

Fixes #72590

With PR #70551, observing a `ty::Error` guarantees that compilation is
going to fail. Therefore, there are no soundness impliciations to
continuing on when we encounter a `ty::Error` - we can only affect
whether or not additional error messags are emitted.

By not bailing out, we avoid incorrectly determining that types are
`!Sized` when a type error is present, which allows us to avoid emitting
additional spurious error messages.

The original comment mentioned this code being shared by coherence -
howver, this change resulted in no diagnostic changes in any of the
existing tests.

src/librustc_trait_selection/traits/select.rs

@@ -1040,17 +1040,6 @@ fn candidate_from_obligation_no_cache<'o>(
         &mut self,
         stack: &TraitObligationStack<'o, 'tcx>,
     ) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
-        if stack.obligation.predicate.references_error() {
-            // If we encounter a `Error`, we generally prefer the
-            // most "optimistic" result in response -- that is, the
-            // one least likely to report downstream errors. But
-            // because this routine is shared by coherence and by
-            // trait selection, there isn't an obvious "right" choice
-            // here in that respect, so we opt to just return
-            // ambiguity and let the upstream clients sort it out.
-            return Ok(None);
-        }
-
         if let Some(conflict) = self.is_knowable(stack) {
             debug!("coherence stage: not knowable");
             if self.intercrate_ambiguity_causes.is_some() {

src/test/ui/async-await/issue-72590-type-error-sized.rs

@@ -0,0 +1,22 @@
+// Regression test for issue #72590
+// Tests that we don't emit a spurious "size cannot be statically determined" error
+// edition:2018
+
+struct Foo {
+    foo: Nonexistent, //~ ERROR cannot find
+    other: str
+}
+
+struct Bar {
+    test: Missing //~ ERROR cannot find
+}
+
+impl Foo {
+    async fn frob(self) {} //~ ERROR the size
+}
+
+impl Bar {
+    async fn myfn(self) {}
+}
+
+fn main() {}

src/test/ui/async-await/issue-72590-type-error-sized.stderr

@@ -0,0 +1,28 @@
+error[E0412]: cannot find type `Nonexistent` in this scope
+  --> $DIR/issue-72590-type-error-sized.rs:6:10
+   |
+LL |     foo: Nonexistent,
+   |          ^^^^^^^^^^^ not found in this scope
+
+error[E0412]: cannot find type `Missing` in this scope
+  --> $DIR/issue-72590-type-error-sized.rs:11:11
+   |
+LL |     test: Missing
+   |           ^^^^^^^ not found in this scope
+
+error[E0277]: the size for values of type `str` cannot be known at compilation time
+  --> $DIR/issue-72590-type-error-sized.rs:15:19
+   |
+LL |     async fn frob(self) {}
+   |                   ^^^^ doesn't have a size known at compile-time
+   |
+   = help: within `Foo`, the trait `std::marker::Sized` is not implemented for `str`
+   = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
+   = note: required because it appears within the type `Foo`
+   = note: all local variables must have a statically known size
+   = help: unsized locals are gated as an unstable feature
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0277, E0412.
+For more information about an error, try `rustc --explain E0277`.