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Self::EnumVariant not considered const fn #64247

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marmistrz opened this issue Sep 7, 2019 · 5 comments · Fixed by #65188
Closed

Self::EnumVariant not considered const fn #64247

marmistrz opened this issue Sep 7, 2019 · 5 comments · Fixed by #65188
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A-const-eval Area: Constant evaluation (MIR interpretation) A-const-fn Area: const fn foo(..) {..}. Pure functions which can be applied at compile time. A-diagnostics Area: Messages for errors, warnings, and lints C-feature-request Category: A feature request, i.e: not implemented / a PR. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-lang Relevant to the language team, which will review and decide on the PR/issue.

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@marmistrz
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marmistrz commented Sep 7, 2019 •
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When using Self to create an enum variant in Rust 1.37+, we get an error:

const VAL: Self = Self::X(42);

results in:

error: `Foo::X` is not yet stable as a const fn

but if the enum is constructed explicitly, the compilation succeeds:

const VAL: Self = Foo::X(42)

Looking at #53555, it looks like Self::EnumVariant should be considered a const fn with min_const_fn.

Playground links for both variants: error, no-error
@Centril
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Centril commented Sep 7, 2019

Duplicate of #61456 (will close in a bit).

@oli-obk It seems like folks open this issue because on stable, the error message does not point to the tracking issue, e.g. you don't see:

  = note: for more information, see https://github.com/rust-lang/rust/issues/61456

can we fix this?

@Centril Centril added A-const-eval Area: Constant evaluation (MIR interpretation) A-const-fn Area: const fn foo(..) {..}. Pure functions which can be applied at compile time. A-diagnostics Area: Messages for errors, warnings, and lints C-feature-request Category: A feature request, i.e: not implemented / a PR. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-lang Relevant to the language team, which will review and decide on the PR/issue. labels Sep 7, 2019
@oli-obk
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oli-obk commented Sep 7, 2019

I think we should just stabilize it

@Centril
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Centril commented Sep 7, 2019

@oli-obk Is this diagnostics problem specific to this feature gate? I meant a more general fix to const-fn related gates.

@Centril Centril closed this as completed Sep 7, 2019
@oli-obk
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oli-obk commented Sep 7, 2019

Not sure. I thought we're using the normal feature gate infra

@matthewjasper matthewjasper mentioned this issue Sep 15, 2019
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@Centril
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Centril commented Sep 15, 2019

Reopening this to make sure we add a test per #61456 (comment).

@Centril Centril reopened this Sep 15, 2019
@marmistrz marmistrz mentioned this issue Oct 3, 2019
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@matthewjasper matthewjasper mentioned this issue Oct 7, 2019
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bors added a commit that referenced this issue Oct 27, 2019
@bors
Auto merge of #65188 - matthewjasper:stabilize-const-constructor, r=C…
077619e 
…entril

Stabilize `const_constructor`

# Stabilization proposal

I propose that we stabilize `#![feature(const_constructor)]`.

Tracking issue: #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
@bors bors closed this as completed in 03a50ae Oct 28, 2019
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A-const-eval Area: Constant evaluation (MIR interpretation) A-const-fn Area: const fn foo(..) {..}. Pure functions which can be applied at compile time. A-diagnostics Area: Messages for errors, warnings, and lints C-feature-request Category: A feature request, i.e: not implemented / a PR. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-lang Relevant to the language team, which will review and decide on the PR/issue.
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Stabilize const_constructor matthewjasper/rust
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@oli-obk @Centril @marmistrz