support partial type hints #9508
Comments
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This seems useful and like it would not be too hard to implement -- we just have to be sure to disallow such hints in places where they are not legal. Would we permit you to omit the list of type parameters altogether? (i.e., |
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@nikomatsakis: I think it would probably make sense to start with the minimal feature (individual parameters) and enhance it if compelling use cases come up. |
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I have had some thoughts in the meantime as part of my work on #4846 and have a larger proposal to make that would encompass this issue. |
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See proposal for #5121 described here: http://smallcultfollowing.com/babysteps/blog/2013/10/29/intermingled-parameter-lists/ |
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+1. I recent ran into some code in Servo that would benefit from this. |
# Summary This patch introduces the `_` token into the type grammar, with the meaning "infer this type". With this change, the following two lines become equivalent: ``` let x = foo(); let x: _ = foo(); ``` But due to its composability, it enables partial type hints like this: ``` let x: Bar<_> = baz(); ``` Using it on the item level is explicitly forbidden, as the Rust language does not enable global type inference by design. This implements the feature requested in #9508. # Things requiring clarification - The change to enable it is very small, but I have only limited understanding of the related code, so the approach here might be wrong. - In particular, while this patch works, it does so in a way not originally intended according to the code comments. - This probably needs more tests, or rather feedback for which tests are still missing. - I'm unsure how this interacts with lifetime parameters, and whether it is correct in regard to them. - Partial type hints on the right side of `as` like `&foo as *_` work in both a normal function contexts and in constexprs like `static foo: *int = &'static 123 as *_`. The question is whether this should be allowed in general. # Todo for this PR - The manual and tutorial still needs updating. # Bugs I'm unsure how to fix - Requesting inference for the top level of the right hand side of a `as` fails to infer correctly, even if all possible hints are given: ``` .../type_hole_1.rs:35:18: 35:22 error: the type of this value must be known in this context .../type_hole_1.rs:35 let a: int = 1u32 as _; ^~~~ ```
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This is supported now. |
I want to be able to write this:
rather than:
I really don't like the idea of having special cases like
to_owned_vecinstd::iterto work around this issue:It doesn't fix the pain in the general case, and encourages using a specific container we special case rather than the right container for the use case. I think this problem will come up frequently with generic code, and the
_syntax would feel right at home with the usage elsewhere.The text was updated successfully, but these errors were encountered: