Make rustc implicitly use panic=abort for the panic_abort crate

[bjorn3]

The panic_abort crate must be compiled with panic=abort, but cargo doesn't allow setting the panic strategy for a single crate. Bootstrap handles this in its rustc wrapper, but for example the build systems of cg_clif and cg_gcc don't use a rustc wrapper, so they would either need to add one or be unable to build a sysroot suitable for both panic=abort and panic=unwind (as is currently the case).

There is a crate called panic_abort on crates.io that would also be affected by this change. This crate uses unstable features and has last been updated in 2019 however and seems to be meant to be used when you want panic=abort behavior anyway.

Required for rust-lang/rustc_codegen_cranelift#1567

[rustbot]

r? @petrochenkov

rustbot has assigned @petrochenkov.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[petrochenkov]

It's less of a practical concern, but shouldn't panic_unwind also implicitly use panic=unwind?

I guess the proper solution would be to move fn panic_strategy from sess to tcx so it can use tcx.is_panic_runtime(), and extend the #![panic_runtime] attribute to #![panic_runtime = "abort|unwind"] to avoid relying on the crate name.
What do you think?

[petrochenkov]

Ah, crate loader still searches for panic_(unwind,abort) by crate name, but we can at least sanity check that the crate found that way is indeed #![panic_runtime = "unwind" or "abort"] respectively.

[bjorn3]

It's less of a practical concern, but shouldn't panic_unwind also implicitly use panic=unwind?

If panic=abort was used while compiling the standard library, panic_unwind won't be used anyway as any panic=abort dependency will force panic=abort to be used and thus panic_unwind doesn't get linked in.

I guess the proper solution would be to move fn panic_strategy from sess to tcx so it can use tcx.is_panic_runtime(), and extend the #![panic_runtime] attribute to #![panic_runtime = "abort|unwind"] to avoid relying on the crate name.
What do you think?

The LLVM backend needs to know if the current crate uses panic=unwind or panic=abort before any source code gets parsed:

rust/compiler/rustc_codegen_llvm/src/llvm_util.rs

Line 114 in 862156d

&& sess.panic_strategy() == PanicStrategy::Unwind

Ah, crate loader still searches for panic_(unwind,abort) by crate name, but we can at least sanity check that the crate found that way is indeed #![panic_runtime = "unwind" or "abort"] respectively.

There already is a sanity check that the found crate is a panic runtime:

rust/compiler/rustc_metadata/src/creader.rs

Lines 999 to 1001 in 862156d

	if !data.is_panic_runtime() {
	self.dcx().emit_err(errors::CrateNotPanicRuntime { crate_name: name });
	}