Add an env override to std_detect

[lu-zero]

Sometimes it can be useful to disable certain extensions even if the host cpu sports them.

• Refactor the cached check logic in src/detect/mod.rs
• Add a RUST_STD_DETECT_DISABLE env::var check
  • Takes a space separate list of strings
  • For each of them mapping to an arch Feature, turn it off.
• Document its existence

@gnzlbg Do you see any problem in this approach?

[gnzlbg]

Shall this env variable be read at run-time, or at build time ?

Either way, I think enabling this should be put behind a cargo feature that's disabled by default.

[bjorn3]

I think it should be during runtime, so you dont have to recompile the world.

[lu-zero]

Shall this env variable be read at run-time, or at build time ?

run-time

Either way, I think enabling this should be put behind a cargo feature that's disabled by default.

Not sure how to name the feature. (and how to advertise it)

[bjorn3]

Not sure how to name the feature. (and how to advertise it)

allow-runtime-detect-override?

[lu-zero]

I'm sold on it, now, which crate should expose it?

[lu-zero]

Also: should we make the cpu feature checks macro behave like log?

[gnzlbg]

I'm sold on it, now, which crate should expose it?

Maybe std_detect ?

Also: should we make the cpu feature checks macro behave like log?

There is an RFC that allows something like that: rust-lang/rfcs#2725 but I'm not sure if that's what you mean.

[lu-zero]

It is exactly what I meant.
Also this use-case might be mentioned here

I'll start sending the refactoring patches and the initial impl soon.

[lu-zero]

I started with #806

Where should I put the documentation about the env variable?

Can I use this to implement the mapping name -> feature mapping?