Allow `cfg(target="..")`

[newpavlov]

Looks like right now it's impossible to match on target triplet directly in cfg expressions. Example (compile to WASM). Is it intentional?

[Centril]

This issue description is rather vague; it would be good to elaborate on what exactly you want.

[newpavlov]

I want the following code to give a compilation error when compiled for wasm32-unknown-unknown target:

#[cfg(target = "wasm32-unknown-unknown")]
compile_error!("foo");

Meaning target = "<target_triplet>" should evaluate to true when code is compiled for the specified target. Right now IIUC it's simply ignored and always evaluated to false.

[newpavlov]

Am I correct that for implementing this feature I will need to add the following changes:

• Push target value to the return of default_configuration.
• Add target field to Target struct (we can't reuse llvm_target here, correct?) and modify specs accordingly.
• Add unstable feature to the declare_features!.
• Add tests.

Or have I missed something?