Unrolled build for #149541

Rollup merge of #149541 - WaffleLapkin:never-test, r=lcnr

Various never type test improvements

I want to make sure that the never type ui tests are actually sensible, and to do so I'm trying to clean them up. This mainly adds comments explaining test purposes and removes outdated stuff.

I imagine best reviewed commit-by-commit, I tried to write useful descriptions and group things into small commits.

cc `@lcnr` (I removed `fallback`/`nofallback` terminology in b5f82d4)

Author: rust-timer

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/coercion/coerce-to-bang.rs

@@ -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