Improve error message for type mismatch in generator arguments

src/test/ui/generator/issue-88653.rs

@@ -0,0 +1,19 @@
+// Regression test for #88653, where a confusing warning about a
+// type mismatch in generator arguments was issued.
+
+#![feature(generators, generator_trait)]
+
+use std::ops::Generator;
+
+fn foo(bar: bool) -> impl Generator<(bool,)> {
+//~^ ERROR: type mismatch in generator arguments [E0631]
+//~| NOTE: expected signature of `fn((bool,)) -> _`
+    |bar| {
+    //~^ NOTE: found signature of `fn(bool) -> _`
+        if bar {
+            yield bar;
+        }
+    }
+}
+
+fn main() {}

src/test/ui/generator/issue-88653.stderr

@@ -0,0 +1,12 @@
+error[E0631]: type mismatch in generator arguments
+  --> $DIR/issue-88653.rs:8:22
+   |
+LL | fn foo(bar: bool) -> impl Generator<(bool,)> {
+   |                      ^^^^^^^^^^^^^^^^^^^^^^^ expected signature of `fn((bool,)) -> _`
+...
+LL |     |bar| {
+   |     ----- found signature of `fn(bool) -> _`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0631`.