functions with keyword args, optional args, and/or variable-arity argument lists #17541

pnkfelix · 2014-09-25T22:16:19Z

A portion of the community (and of the core team) sees one or more of the following features as important for programmer ergonomics:

keyword-based parameters (as opposed to position-based parameters),
optional parameters (where one provides a default value for them, usually in the parameter list),
variable-arity functions (which can be seen as a generalization or variation on optional parameters, depending on how you look at it).

This issue is recording that we want to investigate designs for this, but not immediately. The main backwards compatibility concern is about premature commitment to library API's that would be simplified if one adds one or more of the above features. Nonetheless, We believe we produce a reasonable 1.0 version of Rust without support for this.

(This issue is also going to collect links to all of the useful RFC PR's and/or Rust issues that contain community discussion of these features.)

pnkfelix · 2014-09-25T22:17:35Z

crud I filed this in the wrong repo.

pnkfelix · 2014-09-25T22:19:52Z

(properly filed in rust-lang/rfcs#323 )

…ykril Disallow nested impl traits Fixes rust-lang#17498 The above issue is due to formatting self referencing, recursive bound like `Implemented(^0.0: TraitId(0)<[?0 := ^0.0]>)` on the codes like; ```rust trait Foo<T> {} trait Bar {} fn test(foo: impl Foo<impl Bar>) { ... } ``` When lowering predicate `impl Foo<impl Bar>` in `trait_environment_query`, the outer `impl Foo<...>` is treated as predicates, so the first `TypeRef` that passes the following code is `impl Bar`; https://github.com/rust-lang/rust-analyzer/blob/cae997e3380363a906588f14c7b4587f39cf09f5/crates/hir-ty/src/lower.rs#L376-L400 and thus the `idx` is `0` in the above context. But the following code sets `self_ty` as the `BoundVar` with `idx = 0` because the target param id for predicate `impl Foo<...>` is `0` since `impl Foo` is the first generic-like parameter of `fn test`; https://github.com/rust-lang/rust-analyzer/blob/cae997e3380363a906588f14c7b4587f39cf09f5/crates/hir-ty/src/lower.rs#L998-L1025 For the codes like; ```rust trait Foo { type Assoc; } trait Bar {} fn test(foo: impl Foo<Assoc = impl Bar>) { ... } ``` similar recursive bound doesn't happen because the following codes ***"count the number of `impl Trait` things that appear before the target of our `bound`."*** https://github.com/rust-lang/rust-analyzer/blob/cae997e3380363a906588f14c7b4587f39cf09f5/crates/hir-ty/src/lower.rs#L1168-L1199 Instead of doing similar thing like nested `impl Foo<impl Bar>` thing, this PR lowers such nested impl traits into error types in the similar manner as the rustc does in the following lines (and of course, allows lowering and inferencing nested impl traits for the cases that rustc permits); - https://github.com/rust-lang/rust/blob/e2cf31a6148725bde4ea48acf1e4fe72675257a2/compiler/rustc_ast_passes/src/ast_validation.rs#L802-L813 - https://github.com/rust-lang/rust/blob/7b21c18fe4de32a7d2faa468e6ef69abff013f85/compiler/rustc_ast_passes/src/ast_validation.rs#L1299-L1314 (Though rustc emits [E0666😈](https://doc.rust-lang.org/error_codes/E0666.html), I skipped diagnostics since gathering diagnostics in `hir-def` has no conventions so far 😅)

pnkfelix closed this as completed Sep 25, 2014

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functions with keyword args, optional args, and/or variable-arity argument lists #17541

functions with keyword args, optional args, and/or variable-arity argument lists #17541

pnkfelix commented Sep 25, 2014

pnkfelix commented Sep 25, 2014

pnkfelix commented Sep 25, 2014

functions with keyword args, optional args, and/or variable-arity argument lists #17541

functions with keyword args, optional args, and/or variable-arity argument lists #17541

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pnkfelix commented Sep 25, 2014

pnkfelix commented Sep 25, 2014

pnkfelix commented Sep 25, 2014