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unwind: don't build dependency when building for Miri #100532

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Aug 17, 2022
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unwind: don't build dependency when building for Miri #100532

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8 changes: 7 additions & 1 deletion library/unwind/build.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2,8 +2,14 @@ use std::env;

fn main() {
println!("cargo:rerun-if-changed=build.rs");
let target = env::var("TARGET").expect("TARGET was not set");
println!("cargo:rerun-if-env-changed=CARGO_CFG_MIRI");

if env::var_os("CARGO_CFG_MIRI").is_some() {
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Should there be a rerun-if-changed for this, or does cargo rerun anyway when the cfg flags change?

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A dedicated rerun-if-change definitely won't hurt, I think.

That said, presumably you actually don't want to re-run the script (and rebuild the world) when switching between --cfg=miri and not, if you've previously had it build successfully? RUST_CHECK has similar behavior where once we've built LLVM, it never gets passed down (even in check builds).

I admit I've not followed the story around cfg(miri) enough to know where we're setting that or how, can you point me to that?

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@RalfJung RalfJung Aug 15, 2022 •
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A dedicated rerun-if-change definitely won't hurt, I think.

Done.

I admit I've not followed the story around cfg(miri) enough to know where we're setting that or how, can you point me to that?

Miri sets --cfg miri on all things it builds: here the flag is defined, here it is added.
Miri also uses a separate target dir, so for a given target dir, it should never be the case that it is sometimes built with --cfg miri and sometimes without.

RUST_CHECK has similar behavior where once we've built LLVM, it never gets passed down (even in check builds).

So that is some further extra magic and rustbuild implements?

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Ah, okay, if this never differs for a given target dir then it should be OK -- no need for special magic.

Yes, RUST_CHECK is only set by rustbuild (not Cargo or others); definition here.

// Miri doesn't need the linker flags or a libunwind build.
return;
}

let target = env::var("TARGET").expect("TARGET was not set");
if target.contains("android") {
let build = cc::Build::new();

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