[WIP] Add const qualifier in FnSig (for const fn pointers) #74553

filtsin · 2020-07-20T12:05:07Z

Add const qualifier in FnSig like unsafe or extern.

With this changes we can write something like this:

const fn foo() {}
const FOO: const fn() = foo;
const fn bar() { FOO() }
const fn baz(x: const fn()) { x() }
const fn bazz() { baz(FOO) }

or we can work around of const traits:

trait Bar { const F: const fn(Self) -> Self; }

const fn map_i32(x: i32) -> i32 { x * 2 }
impl Bar for i32 { const F: const fn(Self) -> Self = map_i32; } 
const fn map_u32(x: u32) -> u32 { x * 3 }
impl Bar for u32 { const F: const fn(Self) -> Self = map_u32; }

const fn foo(f: fn()) {
  f(); // <- is not allowed becase fn() is not const fn()
}
const fn foo(f: const fn()) {
  f(); // allow with this pr
}
fn foo(f: const fn()) {
  f(); // allow 
}

rust-highfive · 2020-07-20T12:05:11Z

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @matthewjasper (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

src/librustc_ast_passes/ast_validation.rs

matthewjasper · 2020-07-20T18:44:41Z

src/librustc_middle/ty/relate.rs

+        a: ast::Constness,
+        b: ast::Constness,
+    ) -> RelateResult<'tcx, ast::Constness> {
+        if a == b { Ok(a) } else { Ok(ast::Constness::NotConst) }


This should be if a == b { Ok(a) } else { Err(...) }, coercing from const fn() to fn() and from a const function item to a normal function pointer should be handled by adding a new variant to PointerCast in src/librustc_middle/ty/adjustment.rssrc/librustc_middle/ty/adjustment.rs and using it as appropriate in src/librustc_typeck/check/coercion.rs.

I added 2 types of pointer casting: NotConstFnPointer (const fn() => fn()), UnsafeNotConstFnPointer (const fn() => unsafe fn()) becase I can not find a better solution. But I think that it is not a good.

src/librustc_parse/parser/ty.rs

bors · 2020-07-21T07:14:30Z

☔ The latest upstream changes (presumably #69749) made this pull request unmergeable. Please resolve the merge conflicts.

filtsin · 2020-07-21T21:39:46Z

Can I ask? What's the best way for implementation of implicit casting?

matthewjasper

You should be able to chain an Unsafe and a NotConst coercion when you would use UnsafeNotConstFnPointer.

src/librustc_codegen_ssa/mir/rvalue.rs

src/librustc_mir/transform/check_consts/validation.rs

filtsin · 2020-07-28T18:57:44Z

I try to add implicit casting for drop const qualifier:

const fn foo() { }
fn bar() { }

fn main() {
 let x = if true { bar } else { foo };
}

What's the best way for implementing this feature? How can I do ReifyFnPointer to not const fn pointer in this situation?

filtsin · 2020-07-30T16:33:27Z

I added an example of how can I do implicit cast for drop const qualificator. Is it good?

Now I need edit check_match for this situation:

const fn foo() {}
const fn bar() {}
fn baz() {}

fn main() {
 let x = match 2 {
  2 => foo,
  3 => bar,
  _ => baz
 };
}

bors · 2020-08-08T04:08:00Z

☔ The latest upstream changes (presumably #74877) made this pull request unmergeable. Please resolve the merge conflicts.

bors · 2020-08-15T03:11:11Z

☔ The latest upstream changes (presumably #73851) made this pull request unmergeable. Please resolve the merge conflicts.

bors · 2020-08-22T02:47:26Z

☔ The latest upstream changes (presumably #75797) made this pull request unmergeable. Please resolve the merge conflicts.

bors · 2020-08-30T17:46:58Z

☔ The latest upstream changes (presumably #74862) made this pull request unmergeable. Please resolve the merge conflicts.

crlf0710 · 2020-09-18T11:19:40Z

@filtsin Ping from triage, any updates on this?

filtsin · 2020-09-18T11:59:57Z

I've no idea how can I resolve this question #74553 (comment)

matthewjasper

I've left some comments for how to get cases like that working correctly.

matthewjasper · 2020-10-04T20:50:34Z

src/librustc_typeck/check/coercion.rs

@@ -925,7 +984,18 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                            // We have a LUB of prev_ty and new_ty, just return it.


The patterns in the match on (&prev_ty.kind, &new_ty.kind) should use &ty::FnDef(..) | &ty::FnPtr(..) instead of &ty::FnDef(..)

matthewjasper · 2020-10-04T21:02:33Z

src/librustc_typeck/check/coercion.rs

+                    } else {
+                        Adjust::Pointer(PointerCast::ReifyFnPointer)
+                    }
+                }


This match and the next would be changed to return an Option and an arm for ty::FnPtr(..) should be added here. Some(Adjust::Pointer(PointerCast::NotConstFnPointer)) would be returned if prev_drop_const, otherwise None would be returned. The apply_adjustments calls would be skipped if None is returned from the match.

apply_adjustments would need to be changed to handle the case where a NotConstFnPointer adjustment is applied when there is already a ReifyFnPointer adjustment (by merging them to a NotConstFnPointer adjustment).

bors · 2020-10-14T02:50:44Z

☔ The latest upstream changes (presumably #77135) made this pull request unmergeable. Please resolve the merge conflicts.

Note that reviewers usually do not review pull requests until merge conflicts are resolved! Once you resolve the conflicts, you should change the labels applied by bors to indicate that your PR is ready for review. Post this as a comment to change the labels:

@rustbot modify labels: +S-waiting-on-review -S-waiting-on-author

bors · 2020-10-14T05:05:18Z

☔ The latest upstream changes (presumably #77926) made this pull request unmergeable. Please resolve the merge conflicts.

Note that reviewers usually do not review pull requests until merge conflicts are resolved! Once you resolve the conflicts, you should change the labels applied by bors to indicate that your PR is ready for review. Post this as a comment to change the labels:

@rustbot modify labels: +S-waiting-on-review -S-waiting-on-author

… Add pretty print

bors · 2020-10-17T12:39:18Z

☔ The latest upstream changes (presumably #77685) made this pull request unmergeable. Please resolve the merge conflicts.

Note that reviewers usually do not review pull requests until merge conflicts are resolved! Once you resolve the conflicts, you should change the labels applied by bors to indicate that your PR is ready for review. Post this as a comment to change the labels:

@rustbot modify labels: +S-waiting-on-review -S-waiting-on-author

matthewjasper · 2020-10-25T21:03:11Z

cc @rust-lang/wg-const-eval

RalfJung · 2020-10-26T07:51:51Z

compiler/rustc_mir/src/interpret/cast.rs

+                    ty::FnPtr(_) => {
+                        // No change to value
+                        self.write_immediate(*src, dest)?;
+                    }


Please merge these two new match arms into the existing one for UnsafeFnPointer... seems unnecessary to duplicate that code.

RalfJung · 2020-10-26T07:52:47Z

compiler/rustc_mir/src/transform/check_consts/validation.rs

+                    ty::FnPtr(fn_sig) => {
+                        // At this point, we are calling a function by raw pointer because
+                        // we know that it is const
+                        if self.ccx.tcx.features().const_fn_pointer


What do you mean with "by raw pointer"? A raw pointer would be *const T/*mut T, and I do not think there is such a pointer here.

RalfJung · 2020-10-26T07:54:00Z

src/test/ui/consts/const_fn_ptr_cast.rs

+const _: const unsafe fn() = unsafe_fn;
+//~^ ERROR mismatched types
+
+const _: unsafe fn() = const_unsafe_fn;


The file is called "const_fn_ptr_cast" but it only seems to check coercions, not casts (via as). Is that the same code in the compiler or should as-casts be tested explicitly as well?

crlf0710 · 2020-11-13T12:34:31Z

@filtsin Ping from triage, any updates on this?

Dylan-DPC-zz · 2020-11-23T02:01:43Z

@filtsin thanks for taking the time to contribute. I have to close this due to inactivity. If you wish and you have the time you can open a new PR with these changes and we'll take it from there. Thanks

rust-highfive assigned matthewjasper Jul 20, 2020

rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Jul 20, 2020

filtsin changed the title ~~[WIP] Add const qualifier to fn header (for const fn pointers)~~ [WIP] Add const qualifier in FnSig (for const fn pointers) Jul 20, 2020

filtsin marked this pull request as draft July 20, 2020 18:00

matthewjasper reviewed Jul 20, 2020

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filtsin force-pushed the master branch from af2b713 to 77d4b93 Compare July 21, 2020 15:21

filtsin requested a review from matthewjasper July 21, 2020 15:37

matthewjasper reviewed Jul 25, 2020

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filtsin requested a review from matthewjasper July 30, 2020 16:33

filtsin force-pushed the master branch from 062d7bc to 2632fc5 Compare August 9, 2020 11:16

filtsin force-pushed the master branch from 2632fc5 to ba41598 Compare August 21, 2020 09:11

filtsin marked this pull request as ready for review August 21, 2020 09:12

filtsin force-pushed the master branch from ba41598 to 3c643ca Compare August 26, 2020 16:14

crlf0710 added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Sep 18, 2020

matthewjasper reviewed Oct 4, 2020

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filtsin force-pushed the master branch 3 times, most recently from e90d74c to 0b0be68 Compare October 12, 2020 06:29

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Oct 14, 2020

filtsin force-pushed the master branch from b1d5c6f to f2183b4 Compare October 14, 2020 02:56

filtsin added 15 commits October 14, 2020 18:09

Add constness qualifier to fn pointer

008daf0

Add constness qualifier to function signature

855eca0

Allow call const function pointer in const methods

05f4924

Add feature for const pointers

97d388a

x.py fmt

7374ae7

Fix coerce-unify test

4d9b253

Add NotConstPointerCast + for unsafe. Add new tests for const fn ptr.…

80ef1af

… Add pretty print

Check const_fn_pointer feature in mir context

d72e63b

Add example of reifyfnpointer with const drop

270bf4c

Resolve conflict...

d982864

Match coercion

bb4ab24

Modified apply_adjustments, add unsafety coercion

55add43

Fix typo

ed42405

Add coercion unsafe

be46b44

Resolve conflict

c772728

filtsin force-pushed the master branch from f2183b4 to c772728 Compare October 14, 2020 15:19

RalfJung reviewed Oct 26, 2020

View reviewed changes

Dylan-DPC-zz closed this Nov 23, 2020

Dylan-DPC-zz added S-inactive Status: Inactive and waiting on the author. This is often applied to closed PRs. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Nov 23, 2020

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[WIP] Add const qualifier in FnSig (for const fn pointers) #74553

[WIP] Add const qualifier in FnSig (for const fn pointers) #74553

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Dylan-DPC-zz commented Nov 23, 2020

		@@ -925,7 +984,18 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
		// We have a LUB of prev_ty and new_ty, just return it.

[WIP] Add const qualifier in FnSig (for const fn pointers) #74553

[WIP] Add const qualifier in FnSig (for const fn pointers) #74553

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