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
```
|
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| .. | ||
| assembly | ||
| auxiliary | ||
| codegen | ||
| codegen-units | ||
| coverage | ||
| coverage-run-rustdoc | ||
| debuginfo | ||
| incremental | ||
| mir-opt | ||
| pretty | ||
| run-make | ||
| run-make-fulldeps | ||
| run-pass-valgrind | ||
| rustdoc | ||
| rustdoc-gui | ||
| rustdoc-js | ||
| rustdoc-js-std | ||
| rustdoc-json | ||
| rustdoc-ui | ||
| ui | ||
| ui-fulldeps | ||
| COMPILER_TESTS.md | ||