Various never type test improvements

tests/ui/README.md

@@ -941,6 +941,8 @@ Contains a single test. It imports a massive amount of very similar types from a
 
 ## `tests/ui/never_type/`
 
+Tests relating to the never type. Most tests are specifically about the never type fallback behavior.
+
 See [Tracking issue for promoting `!` to a type (RFC 1216) #35121](https://github.com/rust-lang/rust/issues/35121).
 
 ## `tests/ui/new-range/`

tests/ui/coercion/coerce-issue-49593-box-never.e2021.stderr

@@ -0,0 +1,19 @@
+error[E0277]: the trait bound `(): std::error::Error` is not satisfied
+  --> $DIR/coerce-issue-49593-box-never.rs:28:5
+   |
+LL |     Box::new(x)
+   |     ^^^^^^^^^^^ the trait `std::error::Error` is not implemented for `()`
+   |
+   = note: required for the cast from `Box<()>` to `Box<(dyn std::error::Error + 'static)>`
+
+error[E0277]: the trait bound `(): std::error::Error` is not satisfied
+  --> $DIR/coerce-issue-49593-box-never.rs:33:5
+   |
+LL |     raw_ptr(x)
+   |     ^^^^^^^^^^ the trait `std::error::Error` is not implemented for `()`
+   |
+   = note: required for the cast from `*mut ()` to `*mut (dyn std::error::Error + 'static)`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/coercion/coerce-issue-49593-box-never.rs

@@ -1,57 +1,37 @@
-//@ revisions: nofallback fallback
-//@[fallback] edition: 2024
-//@[fallback] check-pass
+// Regression test for <https://github.com/rust-lang/rust/issues/49593>.
+//
+// This checks that we can construct `Box<dyn Error>` by calling `Box::new`
+// with a value of the never type. And similarly for raw pointers.
+//
+// This used to fail because we tried to coerce `! -> dyn Error`, which then
+// failed because we were trying to pass an unsized value by value, etc.
+//
+// On edition <= 2021 this currently fails because of never type fallback to
+// unit.
+//
+//@ revisions: e2021 e2024
+//@[e2021] edition: 2021
+//@[e2024] edition: 2024
+//
+//@[e2024] check-pass
 
 #![feature(never_type)]
 
 use std::error::Error;
 use std::mem;
 
-fn raw_ptr_box<T>(t: T) -> *mut T {
+fn raw_ptr<T>(t: T) -> *mut T {
     panic!()
 }
 
 fn foo(x: !) -> Box<dyn Error> {
-    // Method resolution will generate new inference vars and relate them.
-    // Thus fallback will not fall back to `!`, but `()` instead.
-    Box::<_ /* ! */>::new(x)
-    //[nofallback]~^ ERROR trait bound `(): std::error::Error` is not satisfied
+    Box::new(x)
+    //[e2021]~^ ERROR trait bound `(): std::error::Error` is not satisfied
 }
 
 fn foo_raw_ptr(x: !) -> *mut dyn Error {
-    /* *mut $0 is coerced to *mut Error here */
-    raw_ptr_box::<_ /* ! */>(x)
-    //[nofallback]~^ ERROR trait bound `(): std::error::Error` is not satisfied
-}
-
-fn no_coercion(d: *mut dyn Error) -> *mut dyn Error {
-    /* an unsize coercion won't compile here, and it is indeed not used
-    because there is nothing requiring the _ to be Sized */
-    d as *mut _
-}
-
-trait Xyz {}
-struct S;
-struct T;
-impl Xyz for S {}
-impl Xyz for T {}
-
-fn foo_no_never() {
-    let mut x /* : Option<S> */ = None;
-    let mut first_iter = false;
-    loop {
-        if !first_iter {
-            let y: Box<dyn Xyz>
-                = /* Box<$0> is coerced to Box<Xyz> here */ Box::new(x.unwrap());
-        }
-
-        x = Some(S);
-        first_iter = true;
-    }
-
-    let mut y: Option<S> = None;
-    // assert types are equal
-    mem::swap(&mut x, &mut y);
+    raw_ptr(x)
+    //[e2021]~^ ERROR trait bound `(): std::error::Error` is not satisfied
 }
 
 fn main() {}

tests/ui/editions/never-type-fallback-breaking.e2021.fixed

@@ -1,3 +1,7 @@
+// This is a test for various ways in which the change to the never type
+// fallback can break things and for the `dependency_on_unit_never_type_fallback`
+// lint.
+//
 //@ revisions: e2021 e2024
 //
 //@[e2021] edition: 2021