Avoid ICEs when recovering malformed function parameters

compiler/rustc_parse/src/parser/diagnostics.rs

@@ -2374,15 +2374,28 @@ impl<'a> Parser<'a> {
         let pat = self.parse_pat_no_top_alt(Some(Expected::ArgumentName), None)?;
         self.expect(exp!(Colon))?;
         let ty = self.parse_ty()?;
-        self.dcx().emit_err(PatternMethodParamWithoutBody {
-            span: pat.span,
-            target: match context {
-                FnContext::Trait => "methods without bodies",
-                FnContext::FunctionPtrType => "function pointer types",
-                FnContext::Free => unreachable!("This method is not called in free functions, as patterns are always allowed there"),
-                FnContext::Impl => unreachable!("This method is not called in impls, as patterns are always allowed there"),
-            },
-        });
+        match context {
+            FnContext::Trait | FnContext::FunctionPtrType => {
+                self.dcx().emit_err(PatternMethodParamWithoutBody {
+                    span: pat.span,
+                    target: if context == FnContext::Trait {
+                        "methods without bodies"
+                    } else {
+                        "function pointer types"
+                    },
+                });
+            }
+            FnContext::Free | FnContext::Impl => {
+                self.dcx().span_delayed_bug(
+                    pat.span,
+                    if context == FnContext::Free {
+                        "This method is not called in free functions, as patterns are always allowed there"
+                    } else {
+                        "This method is not called in impls, as patterns are always allowed there"
+                    },
+                );
+            }
+        }
 
         // Pretend the pattern is `_`, to avoid duplicate errors from AST validation.
         let pat = Box::new(Pat { kind: PatKind::Wild, span: pat.span, id: ast::DUMMY_NODE_ID });

tests/ui/parser/recover-invalid-fn-trait-bound-pattern.rs

@@ -0,0 +1,5 @@
+// Regression test for https://github.com/rust-lang/rust/issues/160337.
+
+struct Baz where U : fn(() : bool)
+//~^ ERROR expected identifier, found keyword `fn`
+//~| ERROR expected `{` after struct name, found `<eof>`

tests/ui/parser/recover-invalid-fn-trait-bound-pattern.stderr

@@ -0,0 +1,20 @@
+error: expected identifier, found keyword `fn`
+  --> $DIR/recover-invalid-fn-trait-bound-pattern.rs:3:22
+   |
+LL | struct Baz where U : fn(() : bool)
+   |                      ^^
+   |
+help: use `Fn` to refer to the trait (notice the capitalization)
+   |
+LL - struct Baz where U : fn(() : bool)
+LL + struct Baz where U : Fn(() : bool)
+   |
+
+error: expected `{` after struct name, found `<eof>`
+  --> $DIR/recover-invalid-fn-trait-bound-pattern.rs:3:34
+   |
+LL | struct Baz where U : fn(() : bool)
+   |                                  ^ expected `{` after struct name
+
+error: aborting due to 2 previous errors
+

tests/ui/parser/recover-invalid-function-parameter-pattern.rs

@@ -0,0 +1,6 @@
+// Regression test for https://github.com/rust-lang/rust/issues/160337.
+
+fn main(... : ...)
+//~^ ERROR unexpected `...`
+//~| ERROR unexpected `...`
+//~| ERROR expected one of `->`, `where`, or `{`, found `<eof>`

tests/ui/parser/recover-invalid-function-parameter-pattern.stderr

@@ -0,0 +1,28 @@
+error: unexpected `...`
+  --> $DIR/recover-invalid-function-parameter-pattern.rs:3:9
+   |
+LL | fn main(... : ...)
+   |         ^^^ not a valid pattern
+   |
+help: for a rest pattern, use `..` instead of `...`
+   |
+LL - fn main(... : ...)
+LL + fn main(.. : ...)
+   |
+
+error: unexpected `...`
+  --> $DIR/recover-invalid-function-parameter-pattern.rs:3:15
+   |
+LL | fn main(... : ...)
+   |               ^^^
+   |
+   = note: only `extern "C"` and `extern "C-unwind"` functions may have a C variable argument list
+
+error: expected one of `->`, `where`, or `{`, found `<eof>`
+  --> $DIR/recover-invalid-function-parameter-pattern.rs:3:18
+   |
+LL | fn main(... : ...)
+   |                  ^ expected one of `->`, `where`, or `{`
+
+error: aborting due to 3 previous errors
+