Fix unresolved inference variable ICE.

This commit moves well-formedness check for the
`UserTypeAnnotation::Ty(..)` case from always running to only when the
code is reachable. This solves the ICE that resulted from
`src/test/ui/issue-54943-1.rs` (a minimal repro of `dropck-eyepatch`
run-pass tests that failed).

The main well-formedness check that was intended to be run despite
unreachable code still is, that being the
`UserTypeAnnotation::TypeOf(..)` case. Before this PR, the other case
wasn't being checked at all.

It is possible to fix this ICE while still always checking
well-formedness for the `UserTypeAnnotation::Ty(..)` case but that
solution will ICE in unreachable code for that case, the diff for
that change [can be found here](0).

[0]: https://gist.github.com/davidtwco/f9751ffd9c0508f7251c0f17adc3af53

4 files changed, 35 insertions, 2 deletions

diff --git a/src/test/incremental/hashes/let_expressions.rs b/src/test/incremental/hashes/let_expressions.rs
index 3aa66527ca7..b6050f059c2 100644
--- a/src/test/incremental/hashes/let_expressions.rs
+++ b/src/test/incremental/hashes/let_expressions.rs
@@ -70,7 +70,7 @@ pub fn change_mutability_of_reference_type() {
 
 #[cfg(not(cfail1))]
 #[rustc_clean(cfg="cfail2",
-    except="HirBody,TypeckTables,MirValidated")]
+    except="HirBody,TypeckTables,MirValidated,MirOptimized")]
 #[rustc_clean(cfg="cfail3")]
 pub fn change_mutability_of_reference_type() {
     let _x: &mut u64;
diff --git a/src/test/ui/issue-54943-1.rs b/src/test/ui/issue-54943-1.rs
new file mode 100644
index 00000000000..7750e340361
--- /dev/null
+++ b/src/test/ui/issue-54943-1.rs
@@ -0,0 +1,15 @@
+#![feature(nll)]
+
+// This test is a minimal version of an ICE in the dropck-eyepatch tests
+// found in the fix for #54943.
+
+// compile-pass
+
+fn foo<T>(_t: T) {
+}
+
+fn main() {
+    struct A<'a, B: 'a>(&'a B);
+    let (a1, a2): (String, A<_>) = (String::from("auto"), A(&"this"));
+    foo((a1, a2));
+}
diff --git a/src/test/ui/issue-54943-2.rs b/src/test/ui/issue-54943-2.rs
new file mode 100644
index 00000000000..f829c38c35d
--- /dev/null
+++ b/src/test/ui/issue-54943-2.rs
@@ -0,0 +1,18 @@
+#![feature(nll)]
+
+// This test is a minimal version of an ICE in the dropck-eyepatch tests
+// found in the fix for #54943. In particular, this test is in unreachable
+// code as the initial fix for this ICE only worked if the code was reachable.
+
+// compile-pass
+
+fn foo<T>(_t: T) {
+}
+
+fn main() {
+    return;
+
+    struct A<'a, B: 'a>(&'a B);
+    let (a1, a2): (String, A<_>) = (String::from("auto"), A(&"this"));
+    foo((a1, a2));
+}
diff --git a/src/test/ui/regions/regions-free-region-ordering-caller1.nll.stderr b/src/test/ui/regions/regions-free-region-ordering-caller1.nll.stderr
index abda7ec5e07..539343a6829 100644
--- a/src/test/ui/regions/regions-free-region-ordering-caller1.nll.stderr
+++ b/src/test/ui/regions/regions-free-region-ordering-caller1.nll.stderr
@@ -21,7 +21,7 @@ LL | fn call1<'a>(x: &'a usize) {
 LL |     let z: &'a & usize = &(&y);
    |            -----------    ^^^^ borrowed value does not live long enough
    |            |
-   |            assignment requires that `y` is borrowed for `'a`
+   |            type annotation requires that `y` is borrowed for `'a`
 ...
 LL | }
    | - `y` dropped here while still borrowed