Unknown keys in rust-toolchain.toml are silently ignored
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Since Jul 13, 2026.
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- Rust
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Description
I was adding a rust-toolchain.toml file to a project, and ended up in a very confusing state where it was clearly being picked up (e.g. rustup show had the overridden by .../rust-toolchain.toml text) but my selected channel wasn't being respected. I eventually realized that I had used channnel = "1.56.0" rather than channel = "1.56.0"!
It seems like rustup should complain more loudly about this kind of misconfiguration. If actually forbidding unknown fields isn't desirable, it'd be great to at least warn about unused keys found when parsing the configuration using serde_ignored.
This issue has been assigned to @ychampion via this comment.
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