Re: [gentoo-dev] [RFC] Dealing with default upstream Rust flags (?)

[James Le Cuirot <chewi@gentoo.org>]

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On Sat, 2024-11-09 at 17:45 +0100, Michał Górny wrote:
> Hi,
> 
> Long story short, a lot of Rust upstreams are ricing in their
> Cargo.toml: enabling LTO, adjusting LTO jobs to exactly 1, and even
> weirder stuff.  With C, we generally have clear policies to override
> this and respect user flags.  With Rust, things aren't that clear.
> 
> Say, let's look at fractal:
> 
>   [profile.release]
>   debug = true
>   lto = "thin"
>   codegen-units = 1
> 
> Upstream is enabling LTO with 1 job (i.e. single threaded) to squeeze
> as much as they can from it.  Also, they are enabling debug symbols for
> some reason.  As a result, the build takes 45 minutes, if it doesn't OOM
> first.
> 
> How much of that stuff should we respect?  Or should we perhaps override
> it all?  Should we do some special magic via eclass, or perhaps expect
> users to set specific RUSTFLAGS for that?  Perhaps set default RUSTFLAGS
> in the profiles?

This stuff is unbelievably messy. Read the comments in cargo.eclass regarding
RUSTFLAGS. As things stand, user RUSTFLAGS are respected, as are some
project-specific flags but not all.

When I worked on this, I didn't look into these profile-related settings. Do
these actually have anything to do with RUSTFLAGS? I'm not sure. gyakovlev
experimented with a "gentoo" (as opposed to "release") profile a while back,
but that effort stalled.

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