std: Improve downstream codegen in Command::env by alexcrichton · Pull Request #64186 · rust-lang/rust (original) (raw)

This commit rejiggers the generics used in the implementation of
Command::env with the purpose of reducing the amount of codegen that
needs to happen in consumer crates, instead preferring to generate code
into libstd.

This was found when profiling the compile times of the cc crate where
the binary rlib produced had a lot of BTreeMap code compiled into it
but the crate doesn't actually use BTreeMap. It turns out that
Command::env is generic enough to codegen the entire implementation in
calling crates, but in this case there's no performance concern so it's
fine to compile the code into the standard library.

This change is done by removing the generic on the CommandEnv map
which is intended to handle case-insensitive variables on Windows.
Instead now a generic isn't used but rather a use statement defined
per-platform is used.

With this commit a debug build of Command::new("foo").env("a", "b")
drops from 21k lines of LLVM IR to 10k.