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Stabilize `const_constructor` # Stabilization proposal I propose that we stabilize `#![feature(const_constructor)]`. Tracking issue: https://github.com/rust-lang/rust/issues/61456 Version target: 1.40 (2019-11-05 => beta, 2019-12-19 => stable). ## What is stabilized ### User guide Tuple struct and tuple variant constructors are now considered to be constant functions. As such a call expression where the callee has a tuple struct or variant constructor "function item" type can be called: ```rust const fn make_options() { // These already work because they are special cased: Some(0); (Option::Some)(1); // These also work now: let f = Option::Some; f(2); {Option::Some}(3); <Option<_>>::Some(5); } ``` ### Motivation Consistency with other `const fn`. Consistency between syntactic path forms. This should also ensure that constructors implement `const Fn` traits and can be coerced to `const fn` function pointers, if they are introduced. ## Tests * [ui/consts/const_constructor/const-construct-call.rs](https://github.com/rust-lang/rust/blob/0d75ab2293a106eb674ac01860910cfc1580837e/src/test/ui/consts/const_constructor/const-construct-call.rs) - Tests various syntactic forms, use in both `const fn` and `const` items, and constructors in both the current and extern crates. * [ui/consts/const_constructor/const_constructor_qpath.rs](https://github.com/rust-lang/rust/blob/1850dfcdabf8258a1f023f26c2c59e96b869dd95/src/test/ui/consts/const_constructor/const_constructor_qpath.rs) - Tests that type qualified paths to enum variants are also considered to be `const fn`.(#64247) r? @oli-obk Closes #61456 Closes #64247
The syntax crate contains those things concerned purely with syntax
– that is, the AST ("abstract syntax tree"), parser, pretty-printer,
lexer, macro expander, and utilities for traversing ASTs.
For more information about how these things work in rustc, see the rustc guide: