Always set the OUT_DIR environment variable

[JOOpdenhoevel]

Describe the problem you are trying to solve
I want to create a procedural attribute macro that analyzes a function signature and exports this information into a file. This file will be used later by another program, so it's definitely needed. However, the environment variable OUT_DIR isn't always set, which would tell me a good spot to write the file to. If the crate that uses the macro has a build script, OUT_DIR is set, but if it doesn't have a build script, OUT_DIR isn't set. Therefore, I would need to tell my users to always have a build script so that the macro works. This does not work well when there are many users of that macro.

Describe the solution you'd like
My requested solution is simple: Always set the OUT_DIR environment variable to a valid output directory, if the built crate has a build script or not.

Notes
One point against this is that OUT_DIR might collide with other uses of the variable, or that the name OUT_DIR isn't a good name for that in general. However, this would be a whole other discussion since it is already set for and used by build scripts.

[ehuss]

I think there may be a misunderstanding about how proc-macros work and how they are intended to work and how build scripts work. Generally, proc-macros should not persist any data to disk, and should be idempotent transformations on the input, without side-effects. OUT_DIR is specifically intended for the build script of the package being built, which for the case of a proc-macro is the macro itself, not the packages that are using the proc-macro.

It is not clear from the description why you are using proc-macros to interface with other tooling, but I recommend considering other approaches to what you are trying to accomplish. If you want help discussing different ideas, I would suggest one of the user forums.

[notdanilo]

IMO OUT_DIR should be available in every place possible. I am using proc-macros to generate language bindings and it would be helpful.

[Technohacker]

If this isn't to be changed, it'd be helpful to document this behaviour, that OUT_DIR (EDIT: Or maybe more than just OUT_DIR) is only set if there's a build script. We had an issue where we accidentally removed our build script (that generated Rust source files), but our include code panicked saying OUT_DIR was not set. It took me a while to realise this was the source of the issue

I could open a PR on the Cargo Book to make this change if that's alright

[weihanglo]

Thanks for your advice! It seems to be documented already.

cargo/src/doc/src/reference/environment-variables.md

Lines 230 to 232 in 8494149

	* `OUT_DIR` — If the package has a build script, this is set to the folder where the build
	script should place its output. See below for more information.
	(Only set during compilation.)