build scripts: DEBUG environment variable is always "true" or "false"

[jrose-signal]

Problem

The docs for build scripts say that OPT_LEVEL and DEBUG are set to the "values of the corresponding variables for the profile currently being built". However, this is only true for OPT_LEVEL; DEBUG is always "true" or "false":

cargo/src/cargo/core/compiler/custom_build.rs

Lines 365 to 366 in 40086e8

	.env("DEBUG", debug.to_string())
	.env("OPT_LEVEL", &unit.profile.opt_level)

This prevents tools like cc-rs from making more fine-grained decisions about what debug info to include, which I implemented in rust-lang/cc-rs#1624 before realizing Cargo didn't actually support it.

Steps

No response

Possible Solution(s)

I would love to just change this, but of course there may be build scripts depending on the existing behavior. :-( If it is not to be changed, the docs should be adjusted to match the actual behavior.

Notes

The docs probably were correct way back in Rust/Cargo's history, when this was just a boolean option.

Version

(discovered in Cargo 1.89, but verified with code investigation in main)

[ehuss]

Correct. For backwards compatibility reasons, the environment variable can only be true or false. false corresponds to when the debug profile setting is 0, false, or "none". I thought the documentation was updated, but apparently it was missed.

#5891 is tracking whether or not more should be exposed.