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fix: don't suggest replacing env!("CARGO_BIN_NAME") with itself#152302

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ShE3py:cargo-envs
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fix: don't suggest replacing env!("CARGO_BIN_NAME") with itself#152302
ShE3py wants to merge 1 commit intorust-lang:mainfrom
ShE3py:cargo-envs

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@ShE3py ShE3py commented Feb 7, 2026 •
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Some environment variables (e.g. CARGO_BIN_NAME) are only available for some targets (Cargo Targets, § Environment variables Cargo sets for crates), and the current diagnostic when copy-pasting code from a binary to a library is kinda confusing:

const _: &str = env!("CARGO_BIN_NAME");

Before:

error: environment variable `CARGO_BIN_NAME` not defined at compile time
 --> lib.rs:1:17
  |
1 | const _: &str = env!("CARGO_BIN_NAME");
  |                 ^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: there is a similar Cargo environment variable: `CARGO_BIN_NAME`

After:

error: environment variable `CARGO_BIN_NAME` not defined at compile time
 --> lib.rs:1:17
  |
1 | const _: &str = env!("CARGO_BIN_NAME");
  |                 ^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: `CARGO_BIN_NAME` may not be available for the current Cargo target
  = help: Cargo sets build script variables at run time. Use `std::env::var("CARGO_BIN_NAME")` instead

@rustbot label +T-compiler +A-diagnostics +D-confusing

@rustbot rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Feb 7, 2026
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rustbot commented Feb 7, 2026

r? @davidtwco

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@rustbot rustbot added A-diagnostics Area: Messages for errors, warnings, and lints D-confusing Diagnostics: Confusing error or lint that should be reworked. labels Feb 7, 2026
davidtwco
davidtwco approved these changes Feb 16, 2026
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LGTM, one nit about the diagnostic wording but then we can merge

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compiler/rustc_builtin_macros/src/errors.rs
#[derive(Diagnostic)]
pub(crate) enum EnvNotDefined<'a> {
#[diag("environment variable `{$var}` not defined at compile time")]
#[help("`{$var}` may not be available for the current Cargo target")]
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"current Cargo target" reads strangely to me, I'm not entirely sure what I'd interpret this as referring to. Does this only come up with different crate types?

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@ShE3py ShE3py Feb 17, 2026

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It's what the Cargo Book call them, but they don't really map 1:1 to crate types (cf. TargetKind::rustc_crate_types).

E.g. CARGO_BIN_NAME is only available for binaries and example binaries, CARGO_MANIFEST_LINKS only for build scripts, and both are compiled with --crate-type=bin.

I'll try to improve the message with TargetKind::description, but not sure if it is exposed to Rustc.

I've also updated the PR description.

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A-diagnostics Area: Messages for errors, warnings, and lints D-confusing Diagnostics: Confusing error or lint that should be reworked. S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.

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