// 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.