Project-wide `profile.debug` not taking preference over global setting?

[d-e-s-o]

Problem

To improve build performance, I am using

[profile.dev]
debug = "line-tables-only"

in the global ~/.cargo/config.toml configuration (see for example https://nnethercote.github.io/perf-book/build-configuration.html#disable-debug-info-generation).

I expect that a project-wide setting should supercede this setting, but that does not appear to be the case.

cargo new foobar
cd foobar
cat <<EOF >> Cargo.toml
[profile.dev]
debug = "full"
EOF

# With global configuration INACTIVE:
cargo build
llvm-dwarfdump target/debug/foobar | wc -l
> 1562

# With global configuration ACTIVE:
cargo clean # seems to be necessary because global profile setting change does not cause a rebuild?!
cargo build
llvm-dwarfdump /tmp/rust-targets/debug/foobar | wc -l
> 124

If the project-wide setting were to take preference, the result should be unchanged. If I change the global setting to none I see a line count of 3.

Steps

See above.

Possible Solution(s)

I believe the project profile settings should take preference over global configuration.

Notes

No response

Version

rustc --version --verbose
rustc 1.90.0-nightly (1159e78c4 2025-09-14) (gentoo)
binary: rustc
commit-hash: 1159e78c4747b02ef996e55082b704c09b970588
commit-date: 2025-09-14
host: x86_64-unknown-linux-gnu
release: 1.90.0-nightly
LLVM version: 20.1.8

cargo --version
cargo 1.90.0-nightly (840b83a10 2025-07-30) (gentoo)

[hanna-kruppe]

The current behavior is documented https://doc.rust-lang.org/cargo/reference/profiles.html#profiles

Additionally, profiles can be overridden from a config definition. Specifying a profile in a config file or environment variable will override the settings from Cargo.toml.

[d-e-s-o]

Thanks for the pointer. Okay, that describes the status quo, but do we really want to live with this behavior?

Consider, if I use CARGO_PROFILE_DEV_DEBUG=full cargo build then everything works as expected again:

llvm-dwarfdump /tmp/rust-targets/debug/foobar | wc -l
1562

Sorry, but this is horrible behavior. I can certainly see why it's implemented that way, but that doesn't make it any better.

[epage]

For framing to understand this, .cargo/config.toml is a transient, environmental config. This type of config can be set by having a file in CARGO_HOME, in a parent directory, in an env variable, or on the command-line. In contrast to this, Cargo.toml is static project config. One off overrides of Cargo.toml shouldn't be done by editing the file. We need some mechanism to do that overriding and Cargo's configuration fits that exactly.

Changing this would also be a breaking change. We have Editions to allow some subset of breaking changes to be opted into. Configuration and the command-line operate outside of the Cargo.toml and so don't have Editions apply.

Depending on your use case that this is causing problems, an isolated mode similar to what is discussed in #7621 might help.