Auto merge of #54599 - nikomatsakis:issue-54593-impl-Trait, r=eddyb

use closure def-id in returns, but base def-id in locals

The refactorings to  handle `let x: impl Trait`  wound up breaking `impl Trait` in closure return types. I think there are some deeper problems with the code in question, but this a least should make @eddyb's example work.

Fixes #54593

r? @eddyb

src/test/ui/impl-trait/closure-calling-parent-fn.rs

@@ -0,0 +1,14 @@
+#![feature(nll)]
+
+// Regression test for #54593: the MIR type checker was going wrong
+// when a closure returns the `impl Copy` from its parent fn. It was
+// (incorrectly) replacing the `impl Copy` in its return type with the
+// hidden type (`()`) but that type resulted from a recursive call to
+// `foo` and hence is treated opaquely within the closure body.  This
+// resulted in a failed subtype relationship.
+//
+// run-pass
+
+fn foo() -> impl Copy { || foo(); }
+fn bar() -> impl Copy { || bar(); }
+fn main() { }