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).

Author: Amanieu

src/allocator.rs

@@ -7,7 +7,7 @@ use rustc_span::symbol::sym;
 
 use crate::GccContext;
 
-pub(crate) unsafe fn codegen(tcx: TyCtxt<'_>, mods: &mut GccContext, _module_name: &str, kind: AllocatorKind, has_alloc_error_handler: bool) {
+pub(crate) unsafe fn codegen(tcx: TyCtxt<'_>, mods: &mut GccContext, _module_name: &str, kind: AllocatorKind, alloc_error_handler_kind: AllocatorKind) {
     let context = &mods.context;
     let usize =
         match tcx.sess.target.pointer_width {
@@ -90,14 +90,7 @@ pub(crate) unsafe fn codegen(tcx: TyCtxt<'_>, mods: &mut GccContext, _module_nam
         .collect();
     let func = context.new_function(None, FunctionType::Exported, void, &args, name, false);
 
-    let kind =
-        if has_alloc_error_handler {
-            AllocatorKind::Global
-        }
-        else {
-            AllocatorKind::Default
-        };
-    let callee = kind.fn_name(sym::oom);
+    let callee = alloc_error_handler_kind.fn_name(sym::oom);
     let args: Vec<_> = types.iter().enumerate()
         .map(|(index, typ)| context.new_parameter(None, *typ, &format!("param{}", index)))
         .collect();

src/lib.rs

@@ -153,11 +153,11 @@ impl CodegenBackend for GccCodegenBackend {
 }
 
 impl ExtraBackendMethods for GccCodegenBackend {
-    fn codegen_allocator<'tcx>(&self, tcx: TyCtxt<'tcx>, module_name: &str, kind: AllocatorKind, has_alloc_error_handler: bool) -> Self::Module {
+    fn codegen_allocator<'tcx>(&self, tcx: TyCtxt<'tcx>, module_name: &str, kind: AllocatorKind, alloc_error_handler_kind: AllocatorKind) -> Self::Module {
         let mut mods = GccContext {
             context: Context::default(),
         };
-        unsafe { allocator::codegen(tcx, &mut mods, module_name, kind, has_alloc_error_handler); }
+        unsafe { allocator::codegen(tcx, &mut mods, module_name, kind, alloc_error_handler_kind); }
         mods
     }