Auto merge of #96195 - sunfishcode:sunfishcode/handle-or-error-type, r=joshtriplett

 Define a dedicated error type for `HandleOrNull` and `HandleOrInvalid`.

Define `NullHandleError` and `InvalidHandleError` types, that implement std::error::Error, and use them as the error types in `HandleOrNull` and `HandleOrInvalid`,

This addresses [this concern](https://github.com/rust-lang/rust/issues/87074#issuecomment-1080031167).

This is the same as #95387.

r? `@joshtriplett`

4 files changed, 100 insertions, 2 deletions

diff --git a/src/test/ui/coercion/coerce-issue-49593-box-never-windows.nofallback.stderr b/src/test/ui/coercion/coerce-issue-49593-box-never-windows.nofallback.stderr
new file mode 100644
index 00000000000..ab9cc707aa0
--- /dev/null
+++ b/src/test/ui/coercion/coerce-issue-49593-box-never-windows.nofallback.stderr
@@ -0,0 +1,39 @@
+error[E0277]: the trait bound `(): std::error::Error` is not satisfied
+  --> $DIR/coerce-issue-49593-box-never-windows.rs:18:53
+   |
+LL |     /* *mut $0 is coerced to Box<dyn Error> here */ Box::<_ /* ! */>::new(x)
+   |                                                     ^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::error::Error` is not implemented for `()`
+   |
+   = help: the following other types implement trait `std::error::Error`:
+             !
+             &'a T
+             AccessError
+             AddrParseError
+             Arc<T>
+             BorrowError
+             BorrowMutError
+             Box<T>
+           and 45 others
+   = note: required for the cast to the object type `dyn std::error::Error`
+
+error[E0277]: the trait bound `(): std::error::Error` is not satisfied
+  --> $DIR/coerce-issue-49593-box-never-windows.rs:23:49
+   |
+LL |     /* *mut $0 is coerced to *mut Error here */ raw_ptr_box::<_ /* ! */>(x)
+   |                                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::error::Error` is not implemented for `()`
+   |
+   = help: the following other types implement trait `std::error::Error`:
+             !
+             &'a T
+             AccessError
+             AddrParseError
+             Arc<T>
+             BorrowError
+             BorrowMutError
+             Box<T>
+           and 45 others
+   = note: required for the cast to the object type `(dyn std::error::Error + 'static)`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.
diff --git a/src/test/ui/coercion/coerce-issue-49593-box-never-windows.rs b/src/test/ui/coercion/coerce-issue-49593-box-never-windows.rs
new file mode 100644
index 00000000000..95d3935caa9
--- /dev/null
+++ b/src/test/ui/coercion/coerce-issue-49593-box-never-windows.rs
@@ -0,0 +1,58 @@
+// revisions: nofallback fallback
+// only-windows - the number of `Error` impls is platform-dependent
+//[fallback] check-pass
+//[nofallback] check-fail
+
+#![feature(never_type)]
+#![cfg_attr(fallback, feature(never_type_fallback))]
+#![allow(unreachable_code)]
+
+use std::error::Error;
+use std::mem;
+
+fn raw_ptr_box<T>(t: T) -> *mut T {
+    panic!()
+}
+
+fn foo(x: !) -> Box<dyn Error> {
+    /* *mut $0 is coerced to Box<dyn Error> here */ Box::<_ /* ! */>::new(x)
+    //[nofallback]~^ 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);
+}
+
+fn main() {
+}
diff --git a/src/test/ui/coercion/coerce-issue-49593-box-never.nofallback.stderr b/src/test/ui/coercion/coerce-issue-49593-box-never.nofallback.stderr
index 1b1ce67cb0c..1daa91f025a 100644
--- a/src/test/ui/coercion/coerce-issue-49593-box-never.nofallback.stderr
+++ b/src/test/ui/coercion/coerce-issue-49593-box-never.nofallback.stderr
@@ -1,5 +1,5 @@
 error[E0277]: the trait bound `(): std::error::Error` is not satisfied
-  --> $DIR/coerce-issue-49593-box-never.rs:17:53
+  --> $DIR/coerce-issue-49593-box-never.rs:18:53
    |
 LL |     /* *mut $0 is coerced to Box<dyn Error> here */ Box::<_ /* ! */>::new(x)
    |                                                     ^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::error::Error` is not implemented for `()`
@@ -17,7 +17,7 @@ LL |     /* *mut $0 is coerced to Box<dyn Error> here */ Box::<_ /* ! */>::new(x
    = note: required for the cast to the object type `dyn std::error::Error`
 
 error[E0277]: the trait bound `(): std::error::Error` is not satisfied
-  --> $DIR/coerce-issue-49593-box-never.rs:22:49
+  --> $DIR/coerce-issue-49593-box-never.rs:23:49
    |
 LL |     /* *mut $0 is coerced to *mut Error here */ raw_ptr_box::<_ /* ! */>(x)
    |                                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::error::Error` is not implemented for `()`
diff --git a/src/test/ui/coercion/coerce-issue-49593-box-never.rs b/src/test/ui/coercion/coerce-issue-49593-box-never.rs
index 7a4324bd5ad..16efb65acb2 100644
--- a/src/test/ui/coercion/coerce-issue-49593-box-never.rs
+++ b/src/test/ui/coercion/coerce-issue-49593-box-never.rs
@@ -1,4 +1,5 @@
 // revisions: nofallback fallback
+// ignore-windows - the number of `Error` impls is platform-dependent
 //[fallback] check-pass
 //[nofallback] check-fail