Remove by-mut-ref mode altogether #3513

nikomatsakis opened this Issue Sep 17, 2012 · 3 comments


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The handling of "by mutable reference" parameters is unsound because it doesn't affect variance. This is not news. One place we take advantage of this is vec::push(), which has the type

fn push<T>(&vec: ~[const T], +val: T) { ... }

This is unsound because we accept any sort of vector for the argument vec, but the type system only guarantees that we'll write back a const vector. I thought this was basically harmless, since vectors are uniques, but I forgot that it also implies that we will be covariant with respect to T. When combined with regions, this leads to bugs like #3501. However you can create problems without regions too. Any place where we have subtyping.

Anyway, we had always planned to remove by-mut-ref mode as part of the general "de-moding", but I think we should up the priority for this particular mode. It is not that widely used in any case, though vec::push() is certainly frequent. I am checking out how hard it will be to just purge it altogether.

graydon commented Sep 17, 2012

Yeah, this is the really major one.


I'm not sure if we decided this was for 0.4 or not, but I'll try to do it.

@catamorphism catamorphism was assigned Oct 2, 2012

I am working on this, but ran into a strange bug where apparently, the code after a nested pure fn gets treated as if it should be pure. I'll try to isolate it tomorrow.

@catamorphism catamorphism added a commit that closed this issue Oct 6, 2012
@catamorphism catamorphism Remove by-mutable-ref mode from the compiler
and test cases. Closes #3513
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