Auto merge of #119735 - lcnr:provisional-cache-readd, r=compiler-errors

next solver: provisional cache

this adds the cache removed in #115843. However, it should now correctly track whether a provisional result depends on an inductive or coinductive stack.

While working on this, I was using the following doc: https://hackmd.io/VsQPjW3wSTGUSlmgwrDKOA. I don't think it's too helpful to understanding this, but am somewhat hopeful that the inline comments are more useful.

There are quite a few future perf improvements here. Given that this is already very involved I don't believe it is worth it (for now). While working on this PR one of my few attempts to significantly improve perf ended up being unsound again because I was not careful enough ✨

r? `@compiler-errors`

Cargo.lock

@@ -4574,6 +4574,7 @@ checksum = "8ba09476327c4b70ccefb6180f046ef588c26a24cf5d269a9feba316eb4f029f"
 name = "rustc_trait_selection"
 version = "0.0.0"
 dependencies = [
+ "bitflags 2.4.1",
  "itertools",
  "rustc_ast",
  "rustc_attr",

compiler/rustc_middle/src/traits/solve/inspect.rs

@@ -73,6 +73,7 @@ pub struct CanonicalGoalEvaluation<'tcx> {
 pub enum CanonicalGoalEvaluationKind<'tcx> {
     Overflow,
     CycleInStack,
+    ProvisionalCacheHit,
     Evaluation { revisions: &'tcx [GoalEvaluationStep<'tcx>] },
 }
 impl Debug for GoalEvaluation<'_> {

compiler/rustc_middle/src/traits/solve/inspect/format.rs

@@ -77,6 +77,9 @@ impl<'a, 'b> ProofTreeFormatter<'a, 'b> {
             CanonicalGoalEvaluationKind::CycleInStack => {
                 writeln!(self.f, "CYCLE IN STACK: {:?}", eval.result)
             }
+            CanonicalGoalEvaluationKind::ProvisionalCacheHit => {
+                writeln!(self.f, "PROVISIONAL CACHE HIT: {:?}", eval.result)
+            }
             CanonicalGoalEvaluationKind::Evaluation { revisions } => {
                 for (n, step) in revisions.iter().enumerate() {
                     writeln!(self.f, "REVISION {n}")?;

compiler/rustc_trait_selection/Cargo.toml

@@ -5,6 +5,7 @@ edition = "2021"
 
 [dependencies]
 # tidy-alphabetical-start
+bitflags = "2.4.1"
 itertools = "0.11.0"
 rustc_ast = { path = "../rustc_ast" }
 rustc_attr = { path = "../rustc_attr" }

compiler/rustc_trait_selection/src/lib.rs

@@ -19,6 +19,7 @@
 #![feature(control_flow_enum)]
 #![feature(extract_if)]
 #![feature(let_chains)]
+#![feature(option_take_if)]
 #![feature(if_let_guard)]
 #![feature(never_type)]
 #![feature(type_alias_impl_trait)]

compiler/rustc_trait_selection/src/solve/inspect/analyse.rs

@@ -171,7 +171,8 @@ impl<'a, 'tcx> InspectGoal<'a, 'tcx> {
         let mut candidates = vec![];
         let last_eval_step = match self.evaluation.evaluation.kind {
             inspect::CanonicalGoalEvaluationKind::Overflow
-            | inspect::CanonicalGoalEvaluationKind::CycleInStack => {
+            | inspect::CanonicalGoalEvaluationKind::CycleInStack
+            | inspect::CanonicalGoalEvaluationKind::ProvisionalCacheHit => {
                 warn!("unexpected root evaluation: {:?}", self.evaluation);
                 return vec![];
             }

compiler/rustc_trait_selection/src/solve/inspect/build.rs

@@ -118,6 +118,7 @@ pub(in crate::solve) enum WipGoalEvaluationKind<'tcx> {
 pub(in crate::solve) enum WipCanonicalGoalEvaluationKind<'tcx> {
     Overflow,
     CycleInStack,
+    ProvisionalCacheHit,
     Interned { revisions: &'tcx [inspect::GoalEvaluationStep<'tcx>] },
 }
 
@@ -126,6 +127,7 @@ impl std::fmt::Debug for WipCanonicalGoalEvaluationKind<'_> {
         match self {
             Self::Overflow => write!(f, "Overflow"),
             Self::CycleInStack => write!(f, "CycleInStack"),
+            Self::ProvisionalCacheHit => write!(f, "ProvisionalCacheHit"),
             Self::Interned { revisions: _ } => f.debug_struct("Interned").finish_non_exhaustive(),
         }
     }
@@ -151,6 +153,9 @@ impl<'tcx> WipCanonicalGoalEvaluation<'tcx> {
             WipCanonicalGoalEvaluationKind::CycleInStack => {
                 inspect::CanonicalGoalEvaluationKind::CycleInStack
             }
+            WipCanonicalGoalEvaluationKind::ProvisionalCacheHit => {
+                inspect::CanonicalGoalEvaluationKind::ProvisionalCacheHit
+            }
             WipCanonicalGoalEvaluationKind::Interned { revisions } => {
                 inspect::CanonicalGoalEvaluationKind::Evaluation { revisions }
             }