Where is the official documentation on for<'a>

Google returns Higher-Rank Trait Bounds - The Rustonomicon , which is a bit sparse.

Is there an 'official + comprehensive' documentation on for<'a> ? There seems to be tidbits here & there, but I can't find a comprehensive + complete explaination.

There is also the language reference and the original RFC.


One somewhat important aspect seems to be documented in none of these resources listed in this thread so far: a bound for<'a> (…bound using 'a'…) only quantifies over those 'a for which all the types / traits are valid for the lifetime 'a. To give some example:

A bound for<'a> Fn(&'a T) (or equivalently written “Fn(&T)”) only quantifies over such 'a that &'a T is valid, i.e. only those 'a such that T: 'a is fulfilled. E.g. for<'a> Fn(&'a &'b ()) only applies to those 'a such that 'b: 'a. You can observe this e.g. in something like

for<'a, 'b> Fn(&'a &'b (), &'b T) -> &'a T

which is a bound that |_, x| x can implement because of the 'b: 'a restriction which allows coercing &'b T into &'a T:

fn foo<T, F>(f: F)
    F: for<'a, 'b> Fn(&'a &'b (), &'b T) -> &'a T

fn bar<T>() {
    foo::<T, _>(|_, x| x);

This behavior, together with the fact that rustc itself doesn’t quite fully reason about these implied bounds properly is also a source of a few soundness issues in the language currently (e.g. #25860, #84591). So don’t feel too bad if you find these rules tricky or perhaps confusing, because rustc is actually feeling the same.

This knowledge is also important when designing unsafe APIs that rely on HRTBs for soundness. E.g. the issue Kimundi/owning-ref-rs#71 only exists because the type parameters T or U can limit the set of lifetimes that a HRTB FnOnce(&T) -> &U ranges over.


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