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+ # ` env `
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+ The tracking issue for this feature is: [ #118372 ] ( https://github.com/rust-lang/rust/issues/118372 ) .
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+
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+ ------------------------
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+ This option flag allows to specify environment variables value at compile time to be
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+ used by ` env! ` and ` option_env! ` macros.
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+ When retrieving an environment variable value, the one specified by ` --env ` will take
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+ precedence. For example, if you want have ` PATH=a ` in your environment and pass:
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+
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+ ``` bash
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+ rustc --env PATH=env
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+ ```
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+ Then you will have:
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+
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+ ``` rust,no_run
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+ assert_eq!(env!("PATH"), "env");
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+ ```
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+ Please note that on Windows, environment variables are case insensitive but case
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+ preserving whereas ` rustc ` 's environment variables are case sensitive. For example,
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+ having ` Path ` in your environment (case insensitive) is different than using
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+ ` rustc --env Path=... ` (case sensitive).
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