Profile rustflags are not propagated to target dependencies

[stormshield-fabs]

Problem

When using the profile-rustflags Cargo feature with a --cfg flag, dependency resolution appears to be performed without the additional flag.

Steps

Minimal reproduction:

# Cargo.toml

cargo-features = ["profile-rustflags"]

[package]
name = "cargo-profile-rustflags-bug"
version = "0.1.0"
edition = "2021"

[lints.rust]
unexpected_cfgs = { level = "warn", check-cfg = ['cfg(unstable)'] }

[target.'cfg(unstable)'.dependencies]
thiserror = "1"

[profile.dev-unstable]
inherits = "dev"
rustflags = ["--cfg=unstable"]

// src/main.rs

#[cfg(unstable)]
use thiserror;

fn main() {
    println!("Hello, world!");
}

• cargo check and RUSTFLAGS="--cfg=unstable" both work fine
• cargo check --profile=dev-unstable fails (no external crate thiserror).

Possible Solution(s)

No response

Notes

No response

Version

cargo 1.86.0-nightly (cecde95c1 2025-01-24)
release: 1.86.0-nightly
commit-hash: cecde95c119a456c30e57d3e4b31fff5a7d83df4
commit-date: 2025-01-24
host: x86_64-unknown-linux-gnu
libgit2: 1.9.0 (sys:0.20.0 vendored)
libcurl: 8.9.0-DEV (sys:0.4.74+curl-8.9.0 vendored ssl:OpenSSL/1.1.1w)
ssl: OpenSSL 1.1.1w  11 Sep 2023
os: Ubuntu 22.4.0 (jammy) [64-bit]

[stormshield-fabs]

Possibly related to #12437

[epage]

If I'm reading the code correctly, resolving has no knowledge of the profile today

cargo/src/cargo/ops/resolve.rs

Lines 137 to 146 in 776129a

	pub fn resolve_ws_with_opts<'gctx>(
	ws: &Workspace<'gctx>,
	target_data: &mut RustcTargetData<'gctx>,
	requested_targets: &[CompileKind],
	cli_features: &CliFeatures,
	specs: &[PackageIdSpec],
	has_dev_units: HasDevUnits,
	force_all_targets: ForceAllTargets,
	dry_run: bool,
	) -> CargoResult<WorkspaceResolve<'gctx>> {

I'm not even sure if we can get that to work, at least if we were to also support this for profile overrides
https://doc.rust-lang.org/cargo/reference/profiles.html#overrides