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Rollup merge of #107799 - lcnr:update-provisional-result, r=oli-obk
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correctly update goals in the cache

we may want to actually write the response for our goal into the provisional or global cache instead of simply using the result from the last iteration '^^

r? ```@rust-lang/initiative-trait-system-refactor```
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compiler-errors authored Feb 9, 2023
2 parents 04ebaba + 4c7c5e5 commit 46c7c91
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13 changes: 8 additions & 5 deletions compiler/rustc_trait_selection/src/solve/search_graph/mod.rs
Original file line number Diff line number Diff line change
Expand Up @@ -7,7 +7,7 @@ use cache::ProvisionalCache;
use overflow::OverflowData;
use rustc_index::vec::IndexVec;
use rustc_middle::ty::TyCtxt;
use std::collections::hash_map::Entry;
use std::{collections::hash_map::Entry, mem};

rustc_index::newtype_index! {
pub struct StackDepth {}
Expand Down Expand Up @@ -134,12 +134,15 @@ impl<'tcx> SearchGraph<'tcx> {
let provisional_entry_index = *cache.lookup_table.get(&goal).unwrap();
let provisional_entry = &mut cache.entries[provisional_entry_index];
let depth = provisional_entry.depth;
// We eagerly update the response in the cache here. If we have to reevaluate
// this goal we use the new response when hitting a cycle, and we definitely
// want to access the final response whenever we look at the cache.
let prev_response = mem::replace(&mut provisional_entry.response, response);

// Was the current goal the root of a cycle and was the provisional response
// different from the final one.
if has_been_used && provisional_entry.response != response {
// If so, update the provisional reponse for this goal...
provisional_entry.response = response;
// ...remove all entries whose result depends on this goal
if has_been_used && prev_response != response {
// If so, remove all entries whose result depends on this goal
// from the provisional cache...
//
// That's not completely correct, as a nested goal can also
Expand Down
37 changes: 37 additions & 0 deletions tests/ui/traits/new-solver/provisional-result-done.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,37 @@
// known-bug: unknown
// compile-flags: -Ztrait-solver=next
// failure-status: 101
// normalize-stderr-test "note: .*\n\n" -> ""
// normalize-stderr-test "thread 'rustc' panicked.*\n" -> ""
// rustc-env:RUST_BACKTRACE=0

// This tests checks that we update results in the provisional cache when
// we pop a goal from the stack.
#![feature(auto_traits)]
auto trait Coinductive {}
struct Foo<T>(T);
struct Bar<T>(T);

impl<T> Coinductive for Foo<T>
where
Bar<T>: Coinductive
{}

impl<T> Coinductive for Bar<T>
where
Foo<T>: Coinductive,
Bar<T>: ConstrainInfer,
{}

trait ConstrainInfer {}
impl ConstrainInfer for Bar<u8> {}
impl ConstrainInfer for Foo<u16> {}

fn impls<T: Coinductive>() -> T { todo!() }

fn constrain<T: ConstrainInfer>(_: T) {}

fn main() {
// This should constrain `_` to `u8`.
impls::<Foo<_>>();
}
6 changes: 6 additions & 0 deletions tests/ui/traits/new-solver/provisional-result-done.stderr
Original file line number Diff line number Diff line change
@@ -0,0 +1,6 @@
error: the compiler unexpectedly panicked. this is a bug.

query stack during panic:
#0 [check_well_formed] checking that `<impl at $DIR/provisional-result-done.rs:20:1: 20:31>` is well-formed
#1 [check_mod_type_wf] checking that types are well-formed in top-level module
end of query stack

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