Support codegen-backend and rustflags in profiles in config file

[weihanglo]

What does this PR try to resolve?

Add two -Z flags

• profile-rustflags
• codegen-backend

to support configuring them in .cargo/config.toml without touching Cargo.toml.

How should we test and review this PR?

A test is added for -Zprofile-rustflags. There is no test around codegen-backend. I followed that.

Additional information

Was trying to write an instruction for this fix but accidentally finished it…

Fixes #11552
Fixes #9926

[rustbot]

r? @epage

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[ehuss]

Thanks! Can you also update unstable.md? I would probably just mention in the Profile rustflags option section that if you are just setting the profile in a Cargo config that you can use -Z profile-rustflags or use the [unstable] table in the config itself. (And same in the codegen section.)

[ehuss]

Thanks!

@bors r+

[bors]

📌 Commit 5c87110 has been approved by ehuss

It is now in the queue for this repository.

[bors]

⌛ Testing commit 5c87110 with merge 48e0c53...

[bors]

☀️ Test successful - checks-actions
Approved by: ehuss
Pushing 48e0c53 to master...

[pickfire]

How does one use this? I added as per the instruction but it says failed to find the directory.

[unstable]
codegen-backend = true

[profile.dev]
codegen-backend = "cranelift"

Error

> cargo c
   Compiling proc-macro2 v1.0.55
   Compiling quote v1.0.26
   Compiling unicode-ident v1.0.8
   Compiling autocfg v1.1.0
   Compiling cfg-if v1.0.0
   Compiling log v0.4.17
error: failed to find a `codegen-backends` folder in the sysroot candidates:
       * /home/ivan/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu
       * /home/ivan/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu

Maybe I need to add it from rustup?

[weihanglo]

Hi, @pickfire. This unstable feature just passes the value to rustc -Zcodegen-backend flag, so I believe the value must be a path to your backend.

cargo/src/cargo/core/compiler/mod.rs

Line 1058 in 7bf43f0

cmd.arg("-Z").arg(&format!("codegen-backend={}", backend));

There is a pull request rust-lang/rust#81746 making clif a rustupcomponent though.