Re: [PATCH] kbuild, rust: use -fremap-path-prefix to make paths relative

[Miguel Ojeda]

On Mon, Feb 10, 2025 at 6:11 PM Thomas Weißschuh <linux@xxxxxxxxxxxxxx> wrote:
>
> Remap source path prefixes in all output, including compiler
> diagnostics, debug information, macro expansions, etc.

Hmm... We don't do all the cases in the C side -- the docs ask to use
`KCFLAGS` when one wants to remove them in the debug info:

    https://docs.kernel.org/kbuild/reproducible-builds.html#absolute-filenames

I am not sure if there is a reason not to cover all cases in C (Cc'ing Ben).

If there is a reason to not do it for the debug info by default (or if
we want to make it consistent with C even if there is no reason), then
I think we would need `-Zremap-path-scope=...` too, which is still
unstable, sadly (Cc'ing Urgau who implemented it for Rust 1.75.0):

    https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/remap-path-scope.html
    https://github.com/rust-lang/rust/issues/111540

In such case, we would also need probably to mention in the
`reproducible-builds` docs how to achieve the same as in C (e.g.
passing an extra `-Zremap-path-scope=debug`, since it aggregates with
the previous ones, according to compiler flag docs).

>  # change __FILE__ to the relative path to the source directory

Perhaps we could add a note to this comment with what we do in the Rust side.

> +KBUILD_RUSTFLAGS += $(call rustc-option,--remap-path-prefix=$(srcroot)/=)

I don't think we need `rustc-option`, since the flag is available
since a long time ago (Rust 1.26.0).

So we should be able to just do:

    KBUILD_RUSTFLAGS += --remap-path-prefix=$(srcroot)/=

Cheers,
Miguel