Implement Make `handle_alloc_error` default to panic (for no_std + liballoc)

Related: https://github.com/rust-lang/rust/issues/66741

Guarded with `#![feature(default_alloc_error_handler)]` a default
`alloc_error_handler` is called, if a custom allocator is used and no
other custom `#[alloc_error_handler]` is defined.

The panic message does not contain the size anymore, because it would
pull in the fmt machinery, which would blow up the code size
significantly.

1 files changed, 4 insertions, 1 deletions

diff --git a/compiler/rustc_passes/src/weak_lang_items.rs b/compiler/rustc_passes/src/weak_lang_items.rs
index 6bc2110bfb3..effb25b0224 100644
--- a/compiler/rustc_passes/src/weak_lang_items.rs
+++ b/compiler/rustc_passes/src/weak_lang_items.rs
@@ -64,7 +64,10 @@ fn verify<'tcx>(tcx: TyCtxt<'tcx>, items: &lang_items::LanguageItems) {
             if item == LangItem::PanicImpl {
                 tcx.sess.err("`#[panic_handler]` function required, but not found");
             } else if item == LangItem::Oom {
-                tcx.sess.err("`#[alloc_error_handler]` function required, but not found");
+                if !tcx.features().default_alloc_error_handler {
+                    tcx.sess.err("`#[alloc_error_handler]` function required, but not found.");
+                    tcx.sess.note_without_error("Use `#![feature(default_alloc_error_handler)]` for a default error handler.");
+                }
             } else {
                 tcx.sess.err(&format!("language item required, but not found: `{}`", name));
             }