Allow using generic trait methods in `const fn` by jonas-schievink · Pull Request #79287 · rust-lang/rust

jonas-schievink · 2020-11-22T03:25:26Z

Next step for #67792, this now also allows code like the following:

structS;implconstPartialEqforS{fneq(&self, _:&S) -> bool{true}}constfnequals_self<T:PartialEq>(t:&T) -> bool{*t == *t
}pubconstEQ:bool = equals_self(&S);

This works by threading const-ness of trait predicates through trait selection, in particular through ParamCandidate, and exposing it in the resulting ImplSource.

Since this change makes two bounds T: Trait and T: ?const Trait that only differ in their const-ness be treated like different bounds, candidate winnowing has been changed to drop the ?const candidate in favor of the const candidate, to avoid ambiguities when both a const and a non-const bound is present.

rust-highfive · 2020-11-22T03:25:30Z

r? @oli-obk

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oli-obk · 2020-11-22T13:17:30Z

This is absolutely great! A few points for clarity:

Please add tests for mixes of ?const Trait bounds and Trait bounds, where there would be ambiguities for figuring out which bound should be called.
Please add a test for calling a generic method with bounds from withing a generic function with bounds. Ideally different tests for both with ?const and without on the method's bounds
- or will this just hit the same issue as src/test/ui/rfc-2632-const-trait-impl/call-generic-method-nonconst.rs?
Track the problem about the actual usage within Tracking issue for closed RFC 2632, impl const Trait for Ty and [const] (conditionally const) syntax (const_trait_impl) #67792

jonas-schievink · 2020-11-22T14:52:05Z

I've added more tests, but I'm not sure what you mean by this:

Track the problem about the actual usage within #67792

oli-obk · 2020-11-22T15:22:57Z

I added some text about the issue of using types that do not impl const Trait for const fn parameters that have Trait bounds and thus expect const Trait impls.

@bors r+

bors · 2020-11-22T15:22:58Z

📌 Commit cb40684 has been approved by oli-obk

RalfJung · 2020-11-22T15:38:13Z

Do we have tests to ensure that this only works when a feature flag is set?

oli-obk · 2020-11-22T15:54:26Z

We have https://github.com/rust-lang/rust/blob/6af388b25050bca26710be7e4030e17bf6d8d2f7/src/test/ui/rfc-2632-const-trait-impl/feature-gate.rs, but that only checks for impls, not actually using them.

jonas-schievink · 2020-11-22T16:05:36Z

There are no const impls in libcore/std, so using one currently also requires defining one. I'll add a note to the tracking issue.

…=oli-obk Allow using generic trait methods in `const fn` Next step for rust-lang#67792, this now also allows code like the following: ```rust struct S; impl const PartialEq for S { fn eq(&self, _: &S) -> bool { true } } const fn equals_self<T: PartialEq>(t: &T) -> bool { *t == *t } pub const EQ: bool = equals_self(&S); ``` This works by threading const-ness of trait predicates through trait selection, in particular through `ParamCandidate`, and exposing it in the resulting `ImplSource`. Since this change makes two bounds `T: Trait` and `T: ?const Trait` that only differ in their const-ness be treated like different bounds, candidate winnowing has been changed to drop the `?const` candidate in favor of the const candidate, to avoid ambiguities when both a const and a non-const bound is present.

…as-schievink Rollup of 10 pull requests Successful merges: - rust-lang#76829 (stabilize const_int_pow) - rust-lang#79080 (MIR visitor: Don't treat debuginfo field access as a use of the struct) - rust-lang#79236 (const_generics: assert resolve hack causes an error) - rust-lang#79287 (Allow using generic trait methods in `const fn`) - rust-lang#79324 (Use Option::and_then instead of open-coding it) - rust-lang#79325 (Reduce boilerplate with the `?` operator) - rust-lang#79330 (Fix typo in comment) - rust-lang#79333 (doc typo) - rust-lang#79337 (Use Option::map instead of open coding it) - rust-lang#79343 (Add my (`@flip1995)` work mail to the mailmap) Failed merges: r? `@ghost` `@rustbot` modify labels: rollup

leonardo-m · 2020-11-24T11:56:24Z

I think if you write the keywords in the wrong order "const impl PartialEq for S {" the compiler could give a bit better error message.

leonardo-m · 2020-11-24T13:04:47Z

See also: #79378

…trochenkov Recover on `const impl<> X for Y` `@leonardo-m` mentioned that `const impl Foo for Bar` could be recovered from in rust-lang#79287. I'm not sure about the error strings as they are, I think it should probably be something like the error that `expected_one_of_not_found` makes + the suggestion to flip the keywords, but I'm not sure how exactly to do that. Also, I decided not to try to handle `const unsafe impl` or `unsafe const impl` cause I figured that `unsafe impl const` would be pretty rare anyway (if it's even valid?), and it wouldn't be worth making the code more messy.

usbalbin · 2020-12-26T22:23:50Z

Is something like this supposed to work with this PR? Or is that something which might get implemented in a later stage? :)

impl<T,U>constInto<U>forTwhereU:From<T>,{fninto(self) -> U{U::from(self)}}

I currently get this error

error[E0015]: calls in constant functions are limited to constant functions, tuple structs and tuple variants

oli-obk · 2020-12-26T23:38:56Z

This is unimplemented as of now, but being looked into since around a week

usbalbin · 2020-12-26T23:46:11Z

Oh, sorry. Thanks :)

jonas-schievink added 3 commits November 22, 2020 02:13

Thread Constness through selection
ee6f42b

winnow: drop non-const cand. in favor of const
71d350e

const fn: allow use of trait impls from bounds
e69fcea

rust-highfive assigned oli-obk Nov 22, 2020

rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Nov 22, 2020

jyn514 added A-const-fn A-trait-system Area: Trait system T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Nov 22, 2020

oli-obk reviewed Nov 22, 2020
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Comment threadsrc/test/ui/rfc-2632-const-trait-impl/call-generic-method-nonconst.rs

oli-obk mentioned this pull request Nov 22, 2020
Calling methods on generic parameters of const fns rust-lang/rfcs#2632
Closed

Add some more tests
cb40684

oli-obk mentioned this pull request Nov 22, 2020
Tracking issue for closed RFC 2632, impl const Trait for Ty and [const] (conditionally const) syntax (const_trait_impl) #67792
Closed
13 tasks

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Nov 22, 2020

m-ou-se mentioned this pull request Nov 22, 2020
Rollup of 7 pull requests #79317
Closed

jonas-schievink mentioned this pull request Nov 23, 2020
Rollup of 10 pull requests #79345
Merged

bors merged commit c7a6720 into rust-lang:masterNov 23, 2020

rustbot added this to the 1.50.0 milestone Nov 23, 2020

jonas-schievink deleted the const-trait-impl branch November 23, 2020 23:09

Pratyush mentioned this pull request Nov 30, 2020
Upgrade field_new macro to take in signed integers arkworks-rs/algebra#96
Merged
6 tasks

coolreader18 mentioned this pull request Dec 3, 2020
Recover on const impl<> X for Y#79668
Merged

RalfJung added the A-const-eval Area: Constant evaluation, covers all const contexts (static, const fn, ...) label Dec 1, 2024

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