Problem

I have a crate with a .cargo/config.toml to enable tracing unstable features:

[build]
rustflags = ["--cfg", "tracing_unstable"]

However I use this crate as part of a workspace. So I also put the previous config.toml in my workspace .cargo/config.toml.

Now everytime I save, rust-analyzer build every crate if I did a cargo check in the crate directory.
I found that it's because the rustflags are not merged properly:

INFO cargo::core::compiler::fingerprint:     dirty: RustflagsChanged { old: ["--cfg", "tracing_unstable"], new: ["--cfg", "tracing_unstable", "--cfg", "tracing_unstable"] }

Steps

1. Create a workspace and put the following content in .cargo/config.toml:

[build]
rustflags = ["--cfg", "tracing_unstable"]

2. Create a crate foo and put the following content in foo/.cargo/config.toml:

[build]
rustflags = ["--cfg", "tracing_unstable"]

3. Now cargo check in the workspace and in the foo crate are rebuilding every crate instead of using previously built ones.

Possible Solution(s)

When appending rustflags, check for possible duplicates to avoid multiple same values counting as different ones.

Notes

No response

Version

cargo 1.78.0-nightly (ccc84ccec 2024-02-07)
release: 1.78.0-nightly
commit-hash: ccc84ccec4b7340eb916aefda1cb3e2fe17d8e7b
commit-date: 2024-02-07
host: x86_64-pc-windows-msvc
libgit2: 1.7.2 (sys:0.18.2 vendored)
libcurl: 8.6.0-DEV (sys:0.4.71+curl-8.6.0 vendored ssl:Schannel)
os: Windows 10.0.22621 (Windows 11 Professional) [64-bit]