RUSTFLAGS is easy to misspell as RUST_FLAGS

[huonw]

Problem

I can never remember if it's RUSTFLAGS or RUST_FLAGS, and writing the latter silently does nothing. This is made worse because essentially every other environment variable cargo understands or declares uses _s: https://doc.rust-lang.org/cargo/reference/environment-variables.html

(This also applies to RUSTDOCFLAGS vs. the incorrect RUSTDOC_FLAGS.)

Steps

1. Be forgetful and write RUST_FLAGS instead of RUSTFLAGS
2. Run RUST_FLAGS=some-arg cargo build
3. Don't notice (and/or be surprised) that some-arg didn't take effect (for that specific example, the build "should" fail but doesn't).

Possible Solution(s)

Cargo could check for a RUST_FLAGS environment variable and emit a warning (or note) that RUSTFLAGS was meant.

Notes

Output of cargo version: cargo 1.31.0 (339d9f9 2018-11-16)

[pmespresso]

Hey! What about just changing it to RUSTFLAGS so that it's consistent with the rest of the environment variables?

[Ygg01]

@yjkimjunior Wouldn't that break all existing implementations? Huonw's suggestion makes me think it will add an alternative spelling for RUST_FLAGS.
@huonw Should this be a specific solution or is a more generic solution be needed? I.e. print warning for all env. variables at some Levenshtein distance.