Fix backtraces with -C panic=abort on linux; emit unwind tables by default

[Enselic]

The linux backtrace unwinder relies on unwind tables to work properly. The default panic hook prints backtraces. Backtraces with -C panic=abort used to work in Rust 1.22 but broke in Rust 1.23, because in 1.23 we stopped emitting unwind tables with -C panic=abort (see #45031 and #81902 (comment)).

In 1.45 (see #69984) a workaround in the form of -C force-unwind-tables=yes was added.

More history

-C panic=abort was added in Rust 1.10 and the motivation was binary size and compile time. But given how confusing that behavior has turned out to be, it is better to make binary size optimzation opt-in with -C force-unwind-tables=no rather than default since the current default breaks backtraces.

Besides, if binary size is a primary concern, there are many other tricks that can be used that has a higher impact.

TODO

• check if all tests pass
• more tests?
• better commit message
• Update comment in must_emit_unwind_tables()
• Update comment in tests/run-make/panic-abort-eh_frame/rmake.rs
• sufficient to change this in compiler/rustc_target/src/spec/base/linux.rs ?
• figure out how to present this in release notes
• FCP probably

Closes

Closes #81902 which is regression-from-stable-to-stable
Closes #94815

[rustbot]

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[rustbot]

These commits modify compiler targets.
(See the Target Tier Policy.)

This PR modifies run-make tests.

cc @jieyouxu

[bjorn3]

Why did you remove the panic=unwind check? This will cause unwind tables to be disabled on some targets where they were previously enabled.

[Enselic]

Good catch, leftover from early prototyping and I didn't realize earlier it now is the wrong thing to do, will fix in the next iteration, thank you.

[hanna-kruppe]

Note that it’s currently pretty tricky to change this setting from whatever the default is on a given target, since there’s no Cargo profile setting for it (unlike the panic strategy), cc rust-lang/cargo#15333

This observation can probably be used as argument for or against this change.

[bjorn3]

If you don't want unwind tables to save space you probably want to strip it out after linking anyway. -Cpanic=abort still gives unwind tables for the standard library on targets supporting panic=unwind unless -Zbuild-std is used. Would be a good idea to document this stripping in the min-sized-rust guide. You have to strip .eh_frame, .eh_frame_hdr and .gcc_except_table. One complication however is that we do have to keep unwind tables for functions that call extern "C-unwind" functions for soundness though.

[hanna-kruppe]

If there's a way to drop all unwind tables that can safely be dropped but no more, then that would be a nice new feature (-Cstrip variant?) or addition to min-sized-rust. But if you're going all out on minimizing binary size, then you'll already want -Zbuild-std for a dozen other reasons anyway. I mostly brought it up from the POV of: if the default is changed, how easily can an ordinary user of stable Rust get back to the current behavior?