Support specifying #[cfg(panic = ...)]

[andylizi]

#[cfg(panic = "...")] was recently stablized in nightly. At the moment, rust-analyzer always uses panic = "unwind", making it difficult to write code for "abort", similar to #7225.

My first thought was to set [profile.dev] panic = "abort" in Cargo.toml, but that didn't work. As it turns out, RA gets the built-in cfgs from running cargo -Z unstable-options rustc --print cfg, which specifically ignores profile settings (rust-lang/cargo#9357):

• Profiles are ignored. This has historically caused confusion for people (for things like debug_assertions). However, all of cargo ignores profiles for the intended use case (--print=cfg). It's not clear what this should do.

But if we pass --print to rustc directly with cargo rustc --lib -- --print cfg, it will use the settings from profile.dev then. This kind of problem had never came up before, since panic is the first and only cfg that can be affected by cargo profiles (aside from debug_assertions which RA hardcodes anyway).

https://github.com/rust-analyzer/rust-analyzer/blob/b594f9c441cf12319d10c14ba6a511d5c9db1b87/crates/project_model/src/rustc_cfg.rs#L40-L44

[bjorn3]

cargo rustc --lib -- --print cfg will build all dependencies. It also doesn't work if your package doesn't have a lib target, but only a bin one.

[andylizi]

cargo rustc --lib -- --print cfg will build all dependencies

Ah I didn't notice that. Yeah that won't work then… 😂