Add regression test for 134355

tests/ui/typeck/return-unsized-coercion.rs

@@ -0,0 +1,8 @@
+// Regression test for issue <https://github.com/rust-lang/rust/issues/134355>
+
+fn digit() -> str {
+    //~^ ERROR the size for values of type `str` cannot be known at compilation time
+    return { i32::MIN };
+}
+
+fn main() {}

tests/ui/typeck/return-unsized-coercion.stderr

@@ -0,0 +1,12 @@
+error[E0277]: the size for values of type `str` cannot be known at compilation time
+  --> $DIR/return-unsized-coercion.rs:3:15
+   |
+LL | fn digit() -> str {
+   |               ^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `str`
+   = note: the return type of a function must have a statically known size
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.