Rust 1.55 diagnostics can vary depending on whether rust-src is installed

[davidhewitt]

In PyO3/pyo3#1865 (comment) we ran into an issue where the diagnostic output changes depending on whether rust-src is available to the compiler.

Was quite a papercut until we worked it out - I'm not sure whether trybuild wants to be concerned about this, just bringing to your attention.

[dtolnay]

I had some crates in a similar situation and used a rust-toolchain.toml to make Rustup fill in the rust-src component when not already available. This way cargo test can always work regardless of the user's default toolchain or presence of rust-src.

[toolchain]
components = ["rust-src"]

https://github.com/dtolnay/anyhow/blob/3acbb335318b330a6c1d446b3a6d80e1e4e821b7/rust-toolchain.toml