Path::file_stem mangles target names that contain a period

[saethlin]

$ cargo +nightly miri setup --target thumbv8m.base-none-eabi
Preparing a sysroot for Miri (target: thumbv8m.base-none-eabi)...
warning: Patch `rustc-std-workspace-alloc v1.99.0 (/home/ben/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/rustc-std-workspace-alloc)` was not used in the crate graph.
Patch `rustc-std-workspace-std v1.99.0 (/home/ben/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/rustc-std-workspace-std)` was not used in the crate graph.
Check that the patched package version and available features are compatible
with the dependency requirements. If the patch has a different version from
what is locked in the Cargo.lock file, run `cargo update` to use the new
version. This may also occur with an optional dependency that is not enabled.
    Checking core v0.0.0 (/home/ben/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core)
   Compiling compiler_builtins v0.1.103
    Checking rustc-std-workspace-core v1.99.0 (/home/ben/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/rustc-std-workspace-core)
    Checking alloc v0.0.0 (/home/ben/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc)
    Checking custom-local-sysroot v0.0.0 (/tmp/.tmpfH3QUJ)
    Finished release [optimized] target(s) in 7.85s
fatal error: failed to build sysroot: failed to read cargo out dir

Caused by:
    No such file or directory (os error 2)

The problem is that this comment is not entirely correct:

rustc-build-sysroot/src/lib.rs

Lines 171 to 173 in 18e2f2c

	// If this is a filename, the name is obtained by stripping directory and extension.
	// That will also work fine for built-in target names.
	path.file_stem()

because this does not work for all built-in target names.

[saethlin]

Ah, I think I've worked out how to fix this. It's a bit gnarly at the moment, I'll put up a PR in a bit when I'm happy with it.

[RalfJung]

Ouch, that's annoying :/ how does the compiler expect tools to tell apart built-in and custom targets?