Type checker doesn't consider trait duality when reasoning about type parameters for associated types

@Manishearth

See the testcase here: https://is.gd/WZh43I

I'd normally run this by a few people before filing an issue, but #rust is quiet on a Saturday night. ;-)

@nox

Trait resolution basically doesn't know how to figure out that two types are equal in the case of "dual traits".

This is one of the inevitable issues @nox is going to hit in his Servo script bindings refactor 😉

I do wish we had a more formalized notion of type families that could be used to better instruct the compiler in these cases. @nikomatsakis was talking about this in his blog, and I've personally found the need for a family-trait type situation more than once, both when modeling dualities and when modeling generic abstractions over sets of types like you're trying to do.

Bug 1325734 - Parameterize the style traversal on TElement instead of…

This appears to work today; E-needstest.

@bholley

I don't think it works necessarily -- if you remove the bound that @bholley highlighted, you still get an error.

@bholley

The example that @bholley expected to pass (and I think he is right that it should) is this one:

use std::borrow::Borrow;

trait TNode: Sized {
  type ConcreteElement: TElement<ConcreteNode = Self>;
}

trait TElement: Sized {
    type ConcreteNode: TNode<ConcreteElement = Self>;
}

trait DomTraversal<N: TNode> {
    type BorrowElement: Borrow<N::ConcreteElement>;
}

fn recalc_style_at<E, D>()
    where E: TElement,
          D: DomTraversal<E::ConcreteNode>,
{
}


fn main() {
}

Current output:

error[E0277]: the trait bound `<D as DomTraversal<<E as TElement>::ConcreteNode>>::BorrowElement: std::borrow::Borrow<E>` is not satisfied
  --> src/main.rs:15:1
   |
11 |   trait DomTraversal<N: TNode> {
   |   ---------------------------- required by `DomTraversal`
...
15 | / fn recalc_style_at<E, D>()
16 | |     where E: TElement,
17 | |           D: DomTraversal<E::ConcreteNode>,
   | |                                            - help: consider further restricting the associated type: `, <D as DomTraversal<<E as TElement>::ConcreteNode>>::BorrowElement: std::borrow::Borrow<E>`
18 | | {
19 | | }
   | |_^ the trait `std::borrow::Borrow<E>` is not implemented for `<D as DomTraversal<<E as TElement>::ConcreteNode>>::BorrowElement`

Rollup merge of rust-lang#81485 - jackh726:atb-issues, r=Mark-Simulacrum

Navigation Menu

Repository navigation

Type checker doesn't consider trait duality when reasoning about type parameters for associated types #38917

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Type checker doesn't consider trait duality when reasoning about type parameters for associated types #38917

Description

Activity

Manishearth commented on Jan 8, 2017

Mark-Simulacrum commented on May 19, 2017

nikomatsakis commented on May 25, 2017

nikomatsakis commented on May 25, 2017

estebank commented on Nov 5, 2019

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions