Rewrite implementation of `#[alloc_error_handler]`

The new implementation doesn't use weak lang items and instead changes
`#[alloc_error_handler]` to an attribute macro just like
`#[global_allocator]`.

The attribute will generate the `__rg_oom` function which is called by
the compiler-generated `__rust_alloc_error_handler`. If no `__rg_oom`
function is defined in any crate then the compiler shim will call
`__rdl_oom` in the alloc crate which will simply panic.

This also fixes link errors with `-C link-dead-code` with
`default_alloc_error_handler`: `__rg_oom` was previously defined in the
alloc crate and would attempt to reference the `oom` lang item, even if
it didn't exist. This worked as long as `__rg_oom` was excluded from
linking since it was not called.

This is a prerequisite for the stabilization of
`default_alloc_error_handler` (#102318).

1 files changed, 2 insertions, 2 deletions

diff --git a/src/test/ui/alloc-error/alloc-error-handler-bad-signature-2.rs b/src/test/ui/alloc-error/alloc-error-handler-bad-signature-2.rs
index 49ea3105fbd..4f76257fc72 100644
--- a/src/test/ui/alloc-error/alloc-error-handler-bad-signature-2.rs
+++ b/src/test/ui/alloc-error/alloc-error-handler-bad-signature-2.rs
@@ -8,8 +8,8 @@ struct Layout;
 
 #[alloc_error_handler]
 fn oom(
-    info: Layout, //~ ERROR argument should be `Layout`
-) { //~ ERROR return type should be `!`
+    info: Layout, //~^ ERROR mismatched types
+) { //~^^ ERROR mismatched types
     loop {}
 }