Two notes after a calming walk in the woods:

Tobias Geerinckx-Rice 写道：
> no package has a ‘gcc-toolchain’ input.

Not true: 2 do.  I meant that ‘the toolchain’ used to build 
everything…

> It is not used internally by Guix.

…is not the "gcc-toolchain" package as such.

> (package (inherit gcc->gcc-10 mesa) …)

Just turn it around:

  (define-public mesa/gcc-10
    (package
      (inherit mesa)
      (native-inputs
        `(("gcc" ,gcc-10)
          ,@(package-native-inputs mesa)))))

Kind regards,

T G-R