[0.2] Add RUSTC_WRAPPER support to build script

[nathaniel-bennett]

(backport #3842)
(cherry picked from commit 8dba4a0)

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[tgross35]

This looks fine, but any reason it doesn't use the exact same pattern as #3842? if let rather than match

[nathaniel-bennett]

While backporting to 0.2, the Windows with rust 1.19 checks failed due to the filter() API not being stable yet for that version of rust. Using match rather than let... else with filter() fixed that.

[tgross35]

Makes sense, thanks for the clarification.

[nlflint]

This MR broke compilation of libc when using clippy-driver as my RUSTC_WRAPPER. I think the root cause is clippy-driver not this MR though. The clippy-driver doesn't "wrap" rustc properly, in that its output from --version isn't the same as rustc, so it can't be parsed.

For anyone else having this problem, I am currently able to workaround it by setting RUSTC_WORKSPACE_WRAPPER with my clippy-driver path, instead of RUSTC_WRAPPER. However, this brings the question to mind, by bypassing this, am I missing out on something?

[dtolnay]

Libc will need to use $RUSTC_WRAPPER $RUSTC --rustc --version when $RUSTC_WRAPPER $RUSTC --version produces output that begins with "clippy". I've need logic to handle the same thing in rustversion for a long time:
https://github.com/dtolnay/rustversion/blob/1.0.17/build/build.rs
https://github.com/dtolnay/rustversion/blob/1.0.17/build/rustc.rs

[tgross35]

Good point, it feels like we had this same discussion somewhere else very recently. @nathaniel-bennett would you be able to put up a fix?