linker-features

The -Zlinker-features compiler flag allows enabling or disabling specific features used during linking, and is intended to be stabilized under the codegen options as -Clinker-features.

These feature flags are a flexible extension mechanism that is complementary to linker flavors, designed to avoid the combinatorial explosion of having to create a new set of flavors for each linker feature we'd want to use.

For example, this design allows:

• default feature sets for principal flavors, or for specific targets.
• flavor-specific features: for example, clang offers automatic cross-linking with --target, which gcc-style compilers don't support. The flavor is still a C/C++ compiler, and we don't want to multiply the number of flavors for this use-case. Instead, we can have a single +target feature.
• umbrella features: for example, if clang accumulates more features in the future than just the +target above. That could be modeled as +clang.
• niche features for resolving specific issues: for example, on Apple targets the linker flag implementing the as-needed native link modifier (#99424) is only possible on sufficiently recent linker versions.
• still allows for discovery and automation, for example via feature detection. This can be useful in exotic environments/build systems.

The flag accepts a comma-separated list of features, individually enabled (+features) or disabled (-features), though currently only one is exposed on the CLI:

• lld: to toggle using the lld linker, either the system-installed binary, or the self-contained rust-lld linker.

As described above, this list is intended to grow in the future.

One of the most common uses of this flag will be to toggle self-contained linking with rust-lld on and off: -Clinker-features=+lld -Clink-self-contained=+linker will use the toolchain's rust-lld as the linker. Inversely, -Clinker-features=-lld would opt out of that, if the current target had self-contained linking enabled by default.