Some unstable features can't be used without CLI options

[Frago9876543210]

Problem
Since rust-lang/rust#83820 #[link_args] attribute was removed. According to specification of that PR new crates should use build scripts with code like that:

println!("cargo:rustc-link-arg-bins={}", arg);

However, the problem is that extra-link-arg is mentioned in the documentation of cargo unstable features, but can't used in cargo-features. So, you can only pass it as CLI option right now.

Steps (it is assumed that you are using x86_64-pc-windows-msvc toolchain)

1. git clone --branch=fix-link-args https://github.com/Frago9876543210/min-sized-rust-windows.git
2. cd min-sized-rust-windows
3. cargo build --release

Lot of warning: cargo:rustc-link-arg-bins requires -Zextra-link-arg flag as result.

The only way to bypass that problem is custom aliases or manually adding -Zextra-link-arg everywhere.

Possible Solution(s)
Make possible to fallback cargo-features to -Z <feature-name> instead of hardcoded values here?

cargo/src/cargo/core/features.rs

Lines 218 to 219 in e0f9643

	// Allow to specify whether binaries should be stripped.
	[unstable] strip: bool,

Notes
Output of cargo version:

cargo 1.53.0-nightly (65d57e6f3 2021-04-04)

Output of rustc -V --verbose:

rustc 1.53.0-nightly (b0c818c5e 2021-04-16)
binary: rustc
commit-hash: b0c818c5e0fa37251d9fda2f656bf1041a2e1e1d
commit-date: 2021-04-16
host: x86_64-pc-windows-msvc
release: 1.53.0-nightly
LLVM version: 12.0.0

[ehuss]

Hi! Using cargo-features still requires -Zunstable-options, so command-line flags will still be required. It might be possible to support cli flags in the config, but would that address your use case even if you still have to pass -Zunstable-options?

[PiDelport]

This can be configured in .cargo/config.toml:

[unstable]
extra-link-arg = true

See: #8127