pub async fn nightly_toolchain_with_rust_src(host: &Host) -> Result<String>Expand description
A nightly toolchain that can compile the standard library from source.
-Zbuild-std needs a nightly cargo and the rust-src component. The
Android preview needs both: the only shared libstd rustup ships is
4 KB-aligned, and a device with 16 KB pages refuses to map it, so the
preview runtime builds std itself under the page-size link flag.
The choice is deterministic — the plain nightly channel first, then the
newest dated nightly — because the support app and the preview module are
separate Cargo invocations that must land on the same libstd-<hash>.so.
§Errors
Returns an error when no nightly toolchain is installed, or when the
selected toolchain lacks rust-src — the error names the exact
rustup component add command rather than mutating the toolchain
silently.