Fix ambiguity issues in use of Borrow<T> #509
Merged
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There's a big problem with #506 which I missed during both authoring and testing. All of the following work fine:
However the following does not
It fails because
&usizeimplement bothBorrow<usize>andBorrow<&usize>. And so there are two solution togen_range's boundwhere B1: Borrow<T>, B2: Borrow<T>resulting in a disambiguation error. I.e. rustc doesn't know if we wantT=usizeorT=&usize.Even though
gen_rangealso sayswhere T: SampleUniform, which is only satisfied forT=usize, that apparently does not help.The problem could be solved by writing
gen_range::<usize, _, _>(&5, &10), but that's obviously not something we want to require.This PR instead implements a
SampleBorrowtrait. Which is essentially an exact copy ofBorrow, except that it's only implemented whereT=SampleUniform. So&usizeimplementsSampleBorrow<usize>, but notSampleBorrow<&usize>, resolving the ambiguity.This adds no complexity at callsites. Implementations of
SampleUniformwill have to writeSampleBorrowinstead ofBorrow, but otherwise will write exactly the same code.It's a bummer that we can't use the standard
Borrow, but I don't know of any way to solve that.And
SampleBorrowdoes give us more flexibility. For example we could enable passing&&5, or&Box::new(5)if we wanted (which can sometime occur with iterators). But that'd be a separate PR if so.