GCC/Rust build, rustc: 'error[E0658]: `let...else` statements are unstable'

[tschwinge]

This already was mentioned a while ago in https://inbox.sourceware.org/87msrjghwy.fsf@euler.schwinge.ddns.net "Re: ☠ Buildbot (Sourceware): gccrust - failed compile (failure) (master)", but as of the 2024-08-01 push of GCC/Rust changes to GCC upstream (#2965 "[upstream] 2024-04-23"), I'm now no longer able to build GCC/Rust on one of my standard testing systems:

[...]
make[3]: Entering directory '[...]/build-gcc/libgrust/libformat_parser'
cargo \
  --config [...]/source-gcc/libgrust/libformat_parser/.cargo/config \
  build \
    --offline \
    --target-dir . \
    --manifest-path [...]/source-gcc/libgrust/libformat_parser/Cargo.toml \
    # FIXME: Not always '--release', right?
   Compiling generic_format_parser v0.1.0 ([...]/source-gcc/libgrust/libformat_parser/generic_format_parser)
error[E0658]: `let...else` statements are unstable
   --> generic_format_parser/src/lib.rs:994:5
    |
994 | /     let Some(unescaped) = unescape_string(snippet) else {
995 | |         return InputStringKind::NotALiteral;
996 | |     };
    | |______^
    |
    = note: see issue #87335 <https://github.com/rust-lang/rust/issues/87335> for more information

For more information about this error, try `rustc --explain E0658`.
error: could not compile `generic_format_parser` due to previous error
make[3]: *** [Makefile:431: debug/liblibformat_parser.a] Error 101
[...]

This is Debian GNU/Linux 12 (bookworm):

$  apt-cache madison cargo rustc
     cargo | 0.66.0+ds1-1 | http://deb.debian.org/debian bookworm/main ppc64el Packages
     cargo | 0.66.0+ds1-1 | http://deb.debian.org/debian bookworm/main Sources
     rustc | 1.63.0+dfsg1-2 | http://deb.debian.org/debian bookworm/main ppc64el Packages
     rustc | 1.63.0+dfsg1-2 | http://deb.debian.org/debian bookworm/main Sources

Is there (a) any (reasonable) way to keep the GCC/Rust build working with that version of rustc, and/or (b) what is the minimum versions of cargo and rustc that GCC/Rust requires? Let's please document this in gcc/doc/install.texi, as, for example, @rorth had also been requesting in https://inbox.sourceware.org/yddh6g0be4y.fsf@CeBiTec.Uni-Bielefeld.DE "Re: [PATCH] build: Check for cargo when building rust language".

Aha, and in that latter thread, in the next message by @CohenArthur: "We currently depend on Rust version 1.72", but "The goal is to reduce that Rust version further soon anyway - we are going to target Rust version 1.49" -- which would resolve my issue. What's the status of that?

---

I understand correctly this issue is different from the recent https://gcc-rust.zulipchat.com/#narrow/stream/266897-general/topic/rustc.20nightly.20needed.20by.20default "rustc nightly needed by default" discussion, #3101 "Remove dependency on nightly Rust", which resolved the requirement of an even-newer (nightly) version of rustc?

[tschwinge]

I have two small WIP patches to make this work; will share later. :smile:

[cooljeanius]

(b) what is the minimum versions of cargo and rustc that GCC/Rust requires? Let's please document this in gcc/doc/install.texi, as, for example, @rorth had also been requesting in inbox.sourceware.org/yddh6g0be4y.fsf@CeBiTec.Uni-Bielefeld.DE "Re: [PATCH] build: Check for cargo when building rust language".

Yeah, the cargo requirement in particular is a major problem, as a lot of the machines on GCC's compile farm are either missing it entirely, or have a version that doesn't recognize the --config flag...

[tschwinge]

I've finally submitted upstream: https://inbox.sourceware.org/87jzctww34.fsf@dem-tschwing-1.schwinge.ddns.net "Rust: Work around 'error[E0658]: let...else statements are unstable'" and https://inbox.sourceware.org/87h67xwvpz.fsf@dem-tschwing-1.schwinge.ddns.net "Rust: Work around 'error[E0599]: no method named leak found for struct std::string::String in the current scope'".