Weird sentence in comments in compiler/rustc_trait_selection/src/traits/coherence.rs

[steffahn]

rust/compiler/rustc_trait_selection/src/traits/coherence.rs

Lines 364 to 383 in e9cdccc

	/// 3. For non-`#[fundamental]` traits, they guarantee that parent crates can
	/// add "non-blanket" impls without breaking negative reasoning in dependent
	/// crates. This is the "rebalancing coherence" (RFC 1023) restriction.
	///
	/// For that, we only a allow crate to perform negative reasoning on
	/// non-local-non-`#[fundamental]` only if there's a local key parameter as per (2).
	///
	/// Because we never perform negative reasoning generically (coherence does
	/// not involve type parameters), this can be interpreted as doing the full
	/// orphan check (using InCrate::Local mode), substituting non-local known
	/// types for all inference variables.
	///
	/// This allows for crates to future-compatibly add impls as long as they
	/// can't apply to types with a key parameter in a child crate - applying
	/// the rules, this basically means that every type parameter in the impl
	/// must appear behind a non-fundamental type (because this is not a
	/// type-system requirement, crate owners might also go for "semantic
	/// future-compatibility" involving things such as sealed traits, but
	/// the above requirement is sufficient, and is necessary in "open world"
	/// cases).

The sentence

For that, we only a allow crate to perform negative reasoning on non-local-non-#[fundamental] only if there's a local key parameter as per (2).

Has a weirdly placed “a” and twice “only”. I’m not immediately sure how to best fix it without changing the meaning to something that isn’t meant.

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Found while doing #88230

@rustbot label T-compiler

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I suppose it might be correct as

 ///    For that, we only allow a crate to perform negative reasoning on 
 ///    non-local-non-`#[fundamental]` if there's a local key parameter as per (2). 

but it would be nice to get confirmation from someone who knows what this comment is talking about.

[jieyouxu]

@rustbot claim