Try filtering out non-const impls when we expect const impls #87375

fee1-dead · 2021-07-22T16:05:06Z

TL;DR: Associated types on const impls are now bounded; we now disallow calling a const function with bounds when the specified type param only has a non-const impl.

r? @oli-obk

rust-highfive · 2021-07-22T16:05:08Z

Some changes occurred in src/tools/clippy.

cc @rust-lang/clippy

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

compiler/rustc_trait_selection/src/traits/fulfill.rs

compiler/rustc_mir/src/borrow_check/type_check/mod.rs

oli-obk · 2021-07-22T16:46:06Z

compiler/rustc_trait_selection/src/traits/auto_trait.rs

@@ -285,6 +285,7 @@ impl AutoTraitFinder<'tcx> {
                def_id: trait_did,
                substs: infcx.tcx.mk_substs_trait(ty, &[]),
            },
+            constness: hir::Constness::NotConst,


similarly here, do we want Const if we are in a const context?

compiler/rustc_trait_selection/src/traits/auto_trait.rs

compiler/rustc_trait_selection/src/traits/select/mod.rs

oli-obk · 2021-07-22T16:53:02Z

compiler/rustc_traits/src/chalk/lowering.rs

@@ -702,7 +702,7 @@ impl<'tcx> LowerInto<'tcx, Option<chalk_solve::rust_ir::QuantifiedInlineBound<Ru
        let (predicate, binders, _named_regions) =
            collect_bound_vars(interner, interner.tcx, self.kind());
        match predicate {
-            ty::PredicateKind::Trait(predicate, _) => Some(chalk_ir::Binders::new(
+            ty::PredicateKind::Trait(predicate) => Some(chalk_ir::Binders::new(
                binders,
                chalk_solve::rust_ir::InlineBound::TraitBound(
                    predicate.trait_ref.lower_into(interner),


I think we need to start tracking this information in chalk, too

oli-obk · 2021-07-22T16:59:10Z

only got half way through, will look more tomorrow

compiler/rustc_typeck/src/check/compare_method.rs

fee1-dead · 2021-07-26T04:01:08Z

Aaaah why do we have these cursed tests of const + async

compiler/rustc_trait_selection/src/traits/query/evaluate_obligation.rs

oli-obk · 2021-07-26T13:56:36Z

@bors try @rust-timer queue

rust-timer · 2021-07-26T13:56:37Z

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

oli-obk · 2021-08-13T11:00:29Z

library/alloc/src/collections/btree/map.rs

-impl<K, V> BTreeMap<K, V> {
+// N.B. bound not on function to avoid unnecessary
+// const bound on `Ord`: see #87375
+impl<K: Ord, V> BTreeMap<K, V> {


this should not make a difference at all. All bounds on the impl must apply for functions defined inside, too.

if this truly fixes things, that is a bug. Instead we should use a ?const Ord bound

Hmmmm, are you saying bounds on impls should be treated as const bounds when the functions inside them are const? That would break a lot of existing code I think.

impl<T: Clone> MyType { fn a() { ... }; // T: Clone const fn b() { ... }; // T: const Clone? }

Only code using feature gates, stable code already doesn't allow this: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=428faddc9d2bce802ab7423fdfa7db33

Wow, I really didn't know this. But can we use ?const inside liballoc? That feature is marked as incomplete.

hmm... I guess we are just going to break ppl using btreemap in const contexts? Or try to remove the bound in a separate PR first?

From the crater run I think not many people are using these functions. I don't think we should remove the bound because it might require more work when re-adding the bound back. I'm fine with breaking BTree{Map,Set}::new for now.

oli-obk · 2021-08-14T09:49:28Z

@bors r+ rollup=never

bors · 2021-08-14T09:49:29Z

📌 Commit 74627c1 has been approved by oli-obk

ghost · 2021-08-14T10:10:22Z

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -293,6 +316,18 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
        self.infcx.tcx
    }

+    /// returns `true` if the predicate is considered `const` to


Suggested change

/// returns `true` if the predicate is considered `const` to

/// Returns `true` if the predicate is considered `const` to

I will change this after this lands or on this PR if CI fails

ghost · 2021-08-14T10:16:26Z

compiler/rustc_infer/src/traits/engine.rs

    fn select_where_possible(
        &mut self,
        infcx: &InferCtxt<'_, 'tcx>,
    ) -> Result<(), Vec<FulfillmentError<'tcx>>>;

+    // FIXME this should not provide a default body for chalk as chalk should be updated


Suggested change

// FIXME this should not provide a default body for chalk as chalk should be updated

// FIXME(fee1-dead): this should not provide a default body for chalk as chalk should be updated

ghost · 2021-08-14T10:16:48Z

compiler/rustc_traits/src/type_op.rs

+    fn prove_predicate(&mut self, mut predicate: Predicate<'tcx>) {
+        if let PredicateKind::Trait(mut tr) = predicate.kind().skip_binder() {
+            if let hir::Constness::Const = tr.constness {
+                // FIXME check if we actually want to prove const predicates inside AscribeUserType


Suggested change

// FIXME check if we actually want to prove const predicates inside AscribeUserType

// FIXME(fee1-dead): check if we actually want to prove const predicates inside AscribeUserType

ghost · 2021-08-14T10:17:03Z

library/alloc/tests/const_fns.rs

+#[allow(dead_code)]
+pub const MY_STRING: String = String::new();
+
+// FIXME remove this struct once we put `K: ?const Ord` on BTreeMap::new.


Suggested change

// FIXME remove this struct once we put `K: ?const Ord` on BTreeMap::new.

// FIXME(fee1-dead): remove this struct once we put `K: ?const Ord` on BTreeMap::new.

ghost · 2021-08-14T10:17:19Z

library/alloc/tests/lib.rs

@@ -23,6 +23,10 @@
 #![feature(slice_partition_dedup)]
 #![feature(vec_spare_capacity)]
 #![feature(string_remove_matches)]
+#![feature(const_btree_new)]
+#![feature(const_trait_impl)]
+// FIXME remove this when const_trait_impl is not incomplete anymore


Suggested change

// FIXME remove this when const_trait_impl is not incomplete anymore

// FIXME(fee1-dead): remove this when const_trait_impl is not incomplete anymore

bors · 2021-08-14T12:06:37Z

⌛ Testing commit 74627c1 with merge 136eaa1...

bors · 2021-08-14T14:52:21Z

☀️ Test successful - checks-actions
Approved by: oli-obk
Pushing 136eaa1 to master...

rust-highfive assigned oli-obk Jul 22, 2021

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fee1-dead force-pushed the move-constness-to-traitpred branch from 2ea3e1d to 663dfe6 Compare July 24, 2021 11:13

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fee1-dead force-pushed the move-constness-to-traitpred branch from 6e9dbcb to 52d8288 Compare July 26, 2021 03:16

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fee1-dead force-pushed the move-constness-to-traitpred branch from 9023ae5 to 5f19f12 Compare July 26, 2021 06:48

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fee1-dead marked this pull request as ready for review July 26, 2021 09:29

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fee1-dead force-pushed the move-constness-to-traitpred branch from fdc54d1 to 48a4868 Compare August 13, 2021 11:23

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Fix Cargo.lock and ui test

74627c1

fee1-dead force-pushed the move-constness-to-traitpred branch from 48a4868 to 74627c1 Compare August 13, 2021 11:31

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 Aug 14, 2021

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bors added the merged-by-bors This PR was explicitly merged by bors. label Aug 14, 2021

bors merged commit 136eaa1 into rust-lang:master Aug 14, 2021

rustbot added this to the 1.56.0 milestone Aug 14, 2021

Aaron1011 mentioned this pull request Aug 15, 2021

Copy not implemented for (u32, u32) error #88047

Closed

fee1-dead deleted the move-constness-to-traitpred branch August 16, 2021 11:56

steffahn mentioned this pull request Aug 27, 2021

False negative for "the trait bound K: Ord is not satisfied" #88244

Closed

JohnTitor added a commit to JohnTitor/rust-semverver that referenced this pull request Oct 13, 2021

Rustup to rust-lang/rust#87375

2d603a2

	/// returns `true` if the predicate is considered `const` to
	/// Returns `true` if the predicate is considered `const` to

	// FIXME this should not provide a default body for chalk as chalk should be updated
	// FIXME(fee1-dead): this should not provide a default body for chalk as chalk should be updated

	// FIXME check if we actually want to prove const predicates inside AscribeUserType
	// FIXME(fee1-dead): check if we actually want to prove const predicates inside AscribeUserType

	// FIXME remove this struct once we put `K: ?const Ord` on BTreeMap::new.
	// FIXME(fee1-dead): remove this struct once we put `K: ?const Ord` on BTreeMap::new.

	// FIXME remove this when const_trait_impl is not incomplete anymore
	// FIXME(fee1-dead): remove this when const_trait_impl is not incomplete anymore

Try filtering out non-const impls when we expect const impls #87375

Try filtering out non-const impls when we expect const impls #87375

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