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

alloc/src/alloc.rs

@@ -402,19 +402,18 @@ pub use std::alloc::handle_alloc_error;
 #[allow(unused_attributes)]
 #[unstable(feature = "alloc_internals", issue = "none")]
 pub mod __alloc_error_handler {
-    use crate::alloc::Layout;
-
-    // called via generated `__rust_alloc_error_handler`
-
-    // if there is no `#[alloc_error_handler]`
+    // called via generated `__rust_alloc_error_handler` if there is no
+    // `#[alloc_error_handler]`.
     #[rustc_std_internal_symbol]
     pub unsafe fn __rdl_oom(size: usize, _align: usize) -> ! {
         panic!("memory allocation of {size} bytes failed")
     }
 
-    // if there is an `#[alloc_error_handler]`
+    #[cfg(bootstrap)]
     #[rustc_std_internal_symbol]
     pub unsafe fn __rg_oom(size: usize, align: usize) -> ! {
+        use crate::alloc::Layout;
+
         let layout = unsafe { Layout::from_size_align_unchecked(size, align) };
         extern "Rust" {
             #[lang = "oom"]

core/src/macros/mod.rs

@@ -1511,6 +1511,17 @@ pub(crate) mod builtin {
         /* compiler built-in */
     }
 
+    /// Attribute macro applied to a function to register it as a handler for allocation failure.
+    ///
+    /// See also [`std::alloc::handle_alloc_error`](../../../std/alloc/fn.handle_alloc_error.html).
+    #[cfg(not(bootstrap))]
+    #[unstable(feature = "alloc_error_handler", issue = "51540")]
+    #[allow_internal_unstable(rustc_attrs)]
+    #[rustc_builtin_macro]
+    pub macro alloc_error_handler($item:item) {
+        /* compiler built-in */
+    }
+
     /// Keeps the item it's applied to if the passed path is accessible, and removes it otherwise.
     #[unstable(
         feature = "cfg_accessible",

core/src/prelude/v1.rs

@@ -75,6 +75,9 @@ pub use crate::macros::builtin::{RustcDecodable, RustcEncodable};
 
 // Do not `doc(no_inline)` so that they become doc items on their own
 // (no public module for them to be re-exported from).
+#[cfg(not(bootstrap))]
+#[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
+pub use crate::macros::builtin::alloc_error_handler;
 #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
 pub use crate::macros::builtin::{bench, derive, global_allocator, test, test_case};
 

std/src/prelude/v1.rs

@@ -59,6 +59,9 @@ pub use core::prelude::v1::{RustcDecodable, RustcEncodable};
 
 // Do not `doc(no_inline)` so that they become doc items on their own
 // (no public module for them to be re-exported from).
+#[cfg(not(bootstrap))]
+#[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
+pub use core::prelude::v1::alloc_error_handler;
 #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
 pub use core::prelude::v1::{bench, derive, global_allocator, test, test_case};