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Rollup merge of #119115 - GuillaumeGomez:env-docs, r=Nilstrieb

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Update documentation for `--env` compilation flag

Part of https://github.com/rust-lang/rust/issues/80792.
As mentioned in https://github.com/rust-lang/rust/pull/118830.

It adds a mention to `tracked_env::var` and also clarifies what triggers a new compilation.

r? `@Nilstrieb`
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Guillaume Gomez 2023-12-20 16:29:54 +01:00 committed by GitHub
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21
src/doc/unstable-book/src/compiler-flags/env.md
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@ -5,7 +5,11 @@ The tracking issue for this feature is: [#118372](https://github.com/rust-lang/r
------------------------
This option flag allows to specify environment variables value at compile time to be
used by `env!` and `option_env!` macros.
used by `env!` and `option_env!` macros. It also impacts `tracked_env::var` function
from the `proc_macro` crate.
This information will be stored in the dep-info files. For more information about
dep-info files, take a look [here](https://doc.rust-lang.org/cargo/guide/build-cache.html#dep-info-files).
When retrieving an environment variable value, the one specified by `--env` will take
precedence. For example, if you want have `PATH=a` in your environment and pass:
@ -20,6 +24,21 @@ Then you will have:
assert_eq!(env!("PATH"), "env");
```
It will trigger a new compilation if any of the `--env` argument value is different.
So if you first passed:
```bash
--env A=B --env X=12
```
and then on next compilation:
```bash
--env A=B
```
`X` value is different (not set) so the code will be re-compiled.
Please note that on Windows, environment variables are case insensitive but case
preserving whereas `rustc`'s environment variables are case sensitive. For example,
having `Path` in your environment (case insensitive) is different than using