Turn the core's std off, which the std feature could never do

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This crate exists to be parking_lot's Mutex and RwLock without std. It has not been that in any build.

The wrapper depends on parking_lot_lite_hack_core without default-features = false, and that core carries default = ["std"]. So the core linked std unconditionally, and this crate's std feature — the whole point — could not turn it off.

before, --no-default-features   parking_lot_lite_hack_core [default,std]
after,  --no-default-features   parking_lot_lite_hack_core []
after,  --features std          parking_lot_lite_hack_core [std]

One line: default-features = false on that dependency.

Why nothing caught it

no-std.yml is not wrong, it is half right. The lines naming the core directly were testing the real thing:

cargo test  -p parking_lot_lite_hack_core --no-default-features --lib   <- real
cargo check -p parking_lot_lite_hack_core --no-default-features         <- real

Every line that goes through the wrapper was not:

cargo check --no-default-features                          <- std core
cargo check --no-default-features --target ...windows-gnu  <- std core
cargo test  --no-default-features --test no_std_backends   <- std core

tests/no_std_backends.rs, the file whose name is the claim, was exercising a std core and passing for the wrong reason. Those same commands are now checking what they say.

Who this was breaking

WorkTablesIndex is being ported to no_std and takes this crate for its Mutex and RwLock. With the leak, WorkTablesIndex --no-default-features still pulled a std core, so its own no_std claim would have been false in a real build no matter what it did.

Checks

cargo test                           36 + 1 + 1 + 4 passed, 0 failed
cargo check --no-default-features    clean
cargo check                          clean
cargo fmt --check                    clean

Version 0.12.6. Publishing here is manual (release-plz is workflow_dispatch in this fork), so this needs a deliberate publish before WorkTablesIndex can require it.