Auto merge of #122663 - beetrees:non-unicode-env-error, r=TaKO8Ki

Fix error message for `env!` when env var is not valid Unicode

Currently (without this PR) the `env!` macro emits an ```environment variable `name` not defined at compile time``` error when the environment variable is defined, but not a valid Unicode string. This PR introduces a separate more accurate error message, and a test to verify this behaviour.

For reference, before this PR, the new test would have outputted:
```
error: environment variable `NON_UNICODE_VAR` not defined at compile time
 --> non_unicode_env.rs:2:13
  |
2 |     let _ = env!("NON_UNICODE_VAR");
  |             ^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: use `std::env::var("NON_UNICODE_VAR")` to read the variable at run time
  = note: this error originates in the macro `env` (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to 1 previous error
```

whereas with this PR, the test ouputs:
```
error: environment variable `NON_UNICODE_VAR` is not a valid Unicode string
 --> non_unicode_env.rs:2:13
  |
2 |     let _ = env!("NON_UNICODE_VAR");
  |             ^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: this error originates in the macro `env` (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to 1 previous error
```

1 files changed, 2 insertions, 1 deletions

diff --git a/library/core/src/macros/mod.rs b/library/core/src/macros/mod.rs
index 574a357b44a..6da05a1ca86 100644
--- a/library/core/src/macros/mod.rs
+++ b/library/core/src/macros/mod.rs
@@ -1053,7 +1053,8 @@ pub(crate) mod builtin {
     ///
     /// If the environment variable is not defined, then a compilation error
     /// will be emitted. To not emit a compile error, use the [`option_env!`]
-    /// macro instead.
+    /// macro instead. A compilation error will also be emitted if the
+    /// environment variable is not a vaild Unicode string.
     ///
     /// # Examples
     ///