Don't build the rust-demangler binary for coverage tests

[Zalathar]

The coverage-run tests invoke llvm-cov, which requires us to specify a command-line demangler that it can use to demangle Rust symbol names.

Historically this used src/tools/rust-demangler, which means that we currently build two different command-line tools to help with the coverage tests (rust-demangler and coverage-dump).

However, it occurred to me that if we add a demangler mode to coverage-dump (which is only a handful of lines and no extra dependencies), then we only need to build one helper binary for the coverage tests, and there is no need for tests to build rust-demangler at all.

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Note that the rust-demangler binary is separate from the rustc-demangle crate (which both rust-demangler and coverage-dump use as a dependency to do the actual demangling).

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So the main benefits/motivations here are:

• Slightly faster builds after a fresh checkout or bootstrap bump.
• Making it clear that currently no tests actually need the rust-demangler binary, since the coverage tests can use their own tool instead.

[rustbot]

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[rustbot]

Some changes occurred in src/tools/compiletest

cc @jieyouxu

[jieyouxu]

This looks like mostly a T-bootstrap change
r? bootstrap

[Zalathar]

One of the reasons I want to do this is that the rust-demangler binary is pretty obscure, and I haven't seen anything outside of the coverage tests actually use it. There doesn't seem to be much documentation pointing to it, and I'm not sure how users would even get their hands on it.

We might be able to follow up by getting rid of that binary entirely, since the need for demangling seems to be well-served by rustfilt and by the rustc-demangle library.

[oli-obk]

Is the rust-demangler something we ship with releases or can we now rip it out?

[Zalathar]

Is the rust-demangler something we ship with releases or can we now rip it out?

I honestly don't know, and I was in the middle of trying to figure out who to even ask.

There does seem to be code in bootstrap for including it in source/binary tarballs, but I have no idea where it ends up, and so far I haven't seen any evidence of anyone actually using it.

[jieyouxu]

FWIW, the README says

rust-demangler is a Rust “extended tool”, used in Rust compiler tests, and optionally included in Rust distributions that enable coverage profiling. Symbol demangling is implemented using the rustc-demangle crate.

(Note, for Rust developers, the third-party tool rustfilt also supports llvm-cov symbol demangling. rustfilt is a more generalized tool that searches any body of text, using pattern matching, to find and demangle Rust symbols.)

But IDK how accurate that is now.

[oli-obk]

It's not shipped by default in nightly (or other releases). It may be in a component I don't have, but I can't think of which one should have it.

[oli-obk]

ok

@bors r+

let's MCP removing the tool separately just for visibility (in the hope if anyone still knows a need for it, they'll speak up)

[bors]

📌 Commit 54b6849 has been approved by oli-obk

It is now in the queue for this repository.