add lib.miri.rs file for miri-test-libstd

[RalfJung]

In rust-lang/rust#123317 we added lib.miri.rs files for core, alloc, and std to enable cargo miri test on those crates. Bootstrap has a comment that explains what this is about:

            // This hack helps bootstrap run standard library tests in Miri. The issue is as
            // follows: when running `cargo miri test` on libcore, cargo builds a local copy of core
            // and makes it a dependency of the integration test crate. This copy duplicates all the
            // lang items, so the build fails. (Regular testing avoids this because the sysroot is a
            // literal copy of what `cargo build` produces, but since Miri builds its own sysroot
            // this does not work for us.) So we need to make it so that the locally built libcore
            // contains all the items from `core`, but does not re-define them -- we want to replace
            // the entire crate but a re-export of the sysroot crate. We do this by swapping out the
            // source file: if `MIRI_REPLACE_LIBRS_IF_NOT_TEST` is set and we are building a
            // `lib.rs` file, and a `lib.miri.rs` file exists in the same folder, we build that
            // instead. But crucially we only do that for the library, not the test builds.

compiler_builtins has the same issue -- it defines global symbols via extern "C" fn, and then Miri complains that the same function is defined multiple times and calls to that function elsewhere are ambiguous.

This can be fixed by adding a lib.miri.rs here as well.

Fixes rust-lang/miri-test-libstd#59

[RalfJung]

What I am not sure about is whether this file will be picked up by cargo when creating the package for crates.io.

[Amanieu]

What I am not sure about is whether this file will be picked up by cargo when creating the package for crates.io.

Yes, it was picked up by Cargo.

[RalfJung]

Awesome, thanks for checking. :)