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, 25 insertions, 0 deletions

diff --git a/compiler/rustc_metadata/src/errors.rs b/compiler/rustc_metadata/src/errors.rs
index 7c387b9a9ec..289fa53aa5e 100644
--- a/compiler/rustc_metadata/src/errors.rs
+++ b/compiler/rustc_metadata/src/errors.rs
@@ -344,6 +344,16 @@ pub struct NoMultipleGlobalAlloc {
 }
 
 #[derive(Diagnostic)]
+#[diag(metadata_no_multiple_alloc_error_handler)]
+pub struct NoMultipleAllocErrorHandler {
+    #[primary_span]
+    #[label]
+    pub span2: Span,
+    #[label(metadata_prev_alloc_error_handler)]
+    pub span1: Span,
+}
+
+#[derive(Diagnostic)]
 #[diag(metadata_conflicting_global_alloc)]
 pub struct ConflictingGlobalAlloc {
     pub crate_name: Symbol,
@@ -351,10 +361,25 @@ pub struct ConflictingGlobalAlloc {
 }
 
 #[derive(Diagnostic)]
+#[diag(metadata_conflicting_alloc_error_handler)]
+pub struct ConflictingAllocErrorHandler {
+    pub crate_name: Symbol,
+    pub other_crate_name: Symbol,
+}
+
+#[derive(Diagnostic)]
 #[diag(metadata_global_alloc_required)]
 pub struct GlobalAllocRequired;
 
 #[derive(Diagnostic)]
+#[diag(metadata_alloc_func_required)]
+pub struct AllocFuncRequired;
+
+#[derive(Diagnostic)]
+#[diag(metadata_missing_alloc_error_handler)]
+pub struct MissingAllocErrorHandler;
+
+#[derive(Diagnostic)]
 #[diag(metadata_no_transitive_needs_dep)]
 pub struct NoTransitiveNeedsDep<'a> {
     pub crate_name: Symbol,