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Remove by-mut-ref mode altogether #3513

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nikomatsakis opened this Issue September 17, 2012 · 3 comments

3 participants

Niko Matsakis Tim Chevalier Graydon Hoare
Niko Matsakis
Collaborator

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 Hoare
Owner

Yeah, this is the really major one.

Tim Chevalier
Collaborator

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

Tim Chevalier
Collaborator

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.

Tim Chevalier catamorphism closed this in f96a2a2 October 05, 2012
Jay Anderson jayanderson referenced this issue from a commit November 10, 2013
Commit has since been removed from the repository and is no longer available.
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