Query about effect of `--target`

[phlummox]

The README says we always must pass --target:

rust-san/README.md

Line 59 in 6907e31

Be sure to **always** pass `--target x86_64-unknown-linux-gnu` to Cargo or else

Or else we may sanitize unwanted parts of our project (e.g. dynamic libs we depend on). And every other source I've seen agrees (e.g. the Rust Unstable Book).

And, indeed, I typically do run into build errors if I don't pass --target.

But is it possible to explain why? I thought --target was used to specify the target architecture and platform we're building for – there seems to be no obvious reason why that in itself should affect whether sanitizers instrument particular parts of a project and not others.