Sandbox::https://github.com/rust-lang/rust/pull/159016

bjorn3 · 2026-07-09T13:41:18Z

While we can't yet use EIIs for things like the global allocator due to missing platform support, we can safely use it for #[panic_handler] as that doesn't require any object file format features. Just matching symbol names.

This gets rid of one of the two remaining weak lang items.

TODO

Support EII without defaults in cg_clif and cg_gcc. (Rename symbols rather than add symbol aliases for EIIs when possible #159843)
#[no_mangle] is not denied on EII definitions #159015 to avoid accepting some invalid code.
Possibly improve the error for applying #[target_feature] on the panic handler.

bjorn3 · 2026-07-09T13:41:29Z

cc @jdonszelmann

bjorn3 · 2026-07-24T10:10:02Z


+/// Attribute macro applied to a function to register it as a panic handler.
+///
+/// See also [`core::panic::PanicInfo`](crate::panic::PanicInfo).


There are probably better pages to link to.

View changes since the review

The symbol name of the EII impl will now get changed to match the EII decl whenever there is no default impl. This already works across all backends and targets unlike the weak symbol aliases that we currently use. When there is a default impl, we will still use weak symbol aliases.

The only way it was possible to skip needing to declare #[panic_handler] is using #![no_core], which is unstable anyway. Always requiring \#[panic_handler] makes things more consistent and avoids having to special case several MIR optimizations for the absence of #[panic_handler].

rust-log-analyzer · 2026-11-12T12:38:39Z

The job test-pr-check-2 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

  --> library/core/src/panicking.rs:17:1
   |
17 | #[eii(panic_handler)]
   | ^^^^^^^^^^^^^^^^^^^^^
18 | fn panic_impl(pi: &core::panic::PanicInfo<'_>) -> !;
   |    ---------- `#[eii]` is used on this item, which is part of another item's local scope

error: free function without a body
  --> library/core/src/panicking.rs:18:1
   |
18 | fn panic_impl(pi: &core::panic::PanicInfo<'_>) -> !;
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^-
   |                                                    |
   |                                                    help: provide a definition for the function: `{ <body> }`

error: aborting due to 3 previous errors

For more information about this error, try `rustc --explain E0658`.
Couldn't compile the test.

mejrs mentioned this pull request Jul 9, 2026

Tracking issue for documenting individual built-in inert attributes in the standard library using #[doc(attribute = "…")] #157604

Open

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