RUSTC_OVERRIDE_VERSION_STRING

This feature is perma-unstable and has no tracking issue.

The RUSTC_OVERRIDE_VERSION_STRING environment variable overrides the version reported by rustc --version. For example:

$ rustc --version
rustc 1.87.0-nightly (43f0014ef 2025-03-25)
$ env RUSTC_OVERRIDE_VERSION_STRING=1.81.0-nightly rustc --version
rustc 1.81.0-nightly

Note that the version string is completely overwritten; i.e. rustc discards commit hash and commit date information unless it is explicitly included in the environment variable. The string only applies to the "release" part of the version; for example:

$ RUSTC_OVERRIDE_VERSION_STRING="1.81.0-nightly (aaaaaaaaa 2025-03-22)" rustc -vV
rustc 1.81.0-nightly (aaaaaaaaa 2025-03-22)
binary: rustc
commit-hash: 43f0014ef0f242418674f49052ed39b70f73bc1c
commit-date: 2025-03-25
host: x86_64-unknown-linux-gnu
release: 1.81.0-nightly (aaaaaaaaa 2025-03-22)
LLVM version: 20.1.1

Note here that commit-hash and commit-date do not match the values in the string, and release includes the fake hash and date.

This variable has no effect on whether or not unstable features are allowed to be used. It only affects the output of --version.

Why does this environment variable exist?

Various library crates have incomplete or incorrect feature detection. This environment variable allows bisecting crates that do incorrect detection with version_check::supports_feature.

This is not intended to be used for any other case (and, except for bisection, is not particularly useful).

See https://github.com/rust-lang/rust/pull/124339 for further discussion.