structural_match: non-structural-match ty closures

This commit adds a `Closure` variant to `NonStructuralMatchTy` in
`structural_match`, fixing an ICE which can occur when
`impl_trait_in_bindings` is used with constants.

Signed-off-by: David Wood <david@davidtw.co>

src/librustc_mir_build/hair/pattern/const_to_pat.rs

@@ -130,6 +130,9 @@ impl<'a, 'tcx> ConstToPat<'a, 'tcx> {
                     traits::NonStructuralMatchTy::Generator => {
                         "generators cannot be used in patterns".to_string()
                     }
+                    traits::NonStructuralMatchTy::Closure => {
+                        "closures cannot be used in patterns".to_string()
+                    }
                     traits::NonStructuralMatchTy::Param => {
                         bug!("use of a constant whose type is a parameter inside a pattern")
                     }

src/librustc_trait_selection/traits/structural_match.rs

@@ -18,6 +18,7 @@ pub enum NonStructuralMatchTy<'tcx> {
     Opaque,
     Generator,
     Projection,
+    Closure,
 }
 
 /// This method traverses the structure of `ty`, trying to find an
@@ -162,6 +163,10 @@ impl<'a, 'tcx> TypeVisitor<'tcx> for Search<'a, 'tcx> {
                 self.found = Some(NonStructuralMatchTy::Generator);
                 return true; // Stop visiting.
             }
+            ty::Closure(..) => {
+                self.found = Some(NonStructuralMatchTy::Closure);
+                return true; // Stop visiting.
+            }
             ty::RawPtr(..) => {
                 // structural-match ignores substructure of
                 // `*const _`/`*mut _`, so skip `super_visit_with`.
@@ -211,7 +216,7 @@ impl<'a, 'tcx> TypeVisitor<'tcx> for Search<'a, 'tcx> {
                 ty.super_visit_with(self);
                 return false;
             }
-            ty::Closure(..) | ty::Infer(_) | ty::Placeholder(_) | ty::Bound(..) => {
+            ty::Infer(_) | ty::Placeholder(_) | ty::Bound(..) => {
                 bug!("unexpected type during structural-match checking: {:?}", ty);
             }
             ty::Error => {

src/test/ui/impl-trait-in-bindings-issue-73003.rs

@@ -0,0 +1,8 @@
+// check-pass
+
+#![feature(impl_trait_in_bindings)]
+//~^ WARN the feature `impl_trait_in_bindings` is incomplete
+
+const _: impl Fn() = ||();
+
+fn main() {}

src/test/ui/impl-trait-in-bindings-issue-73003.stderr

@@ -0,0 +1,11 @@
+warning: the feature `impl_trait_in_bindings` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/impl-trait-in-bindings-issue-73003.rs:3:12
+   |
+LL | #![feature(impl_trait_in_bindings)]
+   |            ^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #63065 <https://github.com/rust-lang/rust/issues/63065> for more information
+
+warning: 1 warning emitted
+