Rollup merge of #107344 - compiler-errors:new-solver-tweaks, r=lcnr

Minor tweaks in the new solver

1. `InferCtxt::probe` is not needed in `compute_subtype_goal` and `compute_well_formed_goal`.
2. Add a handful of comments.
3. Add a micro-optimization in `consider_assumption` where we check the def-ids of the assumption and goal match before instantiating any binders.

r? ``@lcnr``

compiler/rustc_trait_selection/src/solve/assembly.rs

@@ -1,6 +1,8 @@
 //! Code shared by trait and projection goals for candidate assembly.
 
 use super::infcx_ext::InferCtxtExt;
+#[cfg(doc)]
+use super::trait_goals::structural_traits::*;
 use super::{CanonicalResponse, Certainty, EvalCtxt, Goal, QueryResult};
 use rustc_hir::def_id::DefId;
 use rustc_infer::traits::query::NoSolution;
@@ -98,52 +100,75 @@ pub(super) trait GoalKind<'tcx>: TypeFoldable<'tcx> + Copy + Eq {
         assumption: ty::Predicate<'tcx>,
     ) -> QueryResult<'tcx>;
 
+    // A type implements an `auto trait` if its components do as well. These components
+    // are given by built-in rules from [`instantiate_constituent_tys_for_auto_trait`].
     fn consider_auto_trait_candidate(
         ecx: &mut EvalCtxt<'_, 'tcx>,
         goal: Goal<'tcx, Self>,
     ) -> QueryResult<'tcx>;
 
+    // A trait alias holds if the RHS traits and `where` clauses hold.
     fn consider_trait_alias_candidate(
         ecx: &mut EvalCtxt<'_, 'tcx>,
         goal: Goal<'tcx, Self>,
     ) -> QueryResult<'tcx>;
 
+    // A type is `Copy` or `Clone` if its components are `Sized`. These components
+    // are given by built-in rules from [`instantiate_constituent_tys_for_sized_trait`].
     fn consider_builtin_sized_candidate(
         ecx: &mut EvalCtxt<'_, 'tcx>,
         goal: Goal<'tcx, Self>,
     ) -> QueryResult<'tcx>;
 
+    // A type is `Copy` or `Clone` if its components are `Copy` or `Clone`. These
+    // components are given by built-in rules from [`instantiate_constituent_tys_for_copy_clone_trait`].
     fn consider_builtin_copy_clone_candidate(
         ecx: &mut EvalCtxt<'_, 'tcx>,
         goal: Goal<'tcx, Self>,
     ) -> QueryResult<'tcx>;
 
+    // A type is `PointerSized` if we can compute its layout, and that layout
+    // matches the layout of `usize`.
     fn consider_builtin_pointer_sized_candidate(
         ecx: &mut EvalCtxt<'_, 'tcx>,
         goal: Goal<'tcx, Self>,
     ) -> QueryResult<'tcx>;
 
+    // A callable type (a closure, fn def, or fn ptr) is known to implement the `Fn<A>`
+    // family of traits where `A` is given by the signature of the type.
     fn consider_builtin_fn_trait_candidates(
         ecx: &mut EvalCtxt<'_, 'tcx>,
         goal: Goal<'tcx, Self>,
         kind: ty::ClosureKind,
     ) -> QueryResult<'tcx>;
 
+    // `Tuple` is implemented if the `Self` type is a tuple.
     fn consider_builtin_tuple_candidate(
         ecx: &mut EvalCtxt<'_, 'tcx>,
         goal: Goal<'tcx, Self>,
     ) -> QueryResult<'tcx>;
 
+    // `Pointee` is always implemented.
+    //
+    // See the projection implementation for the `Metadata` types for all of
+    // the built-in types. For structs, the metadata type is given by the struct
+    // tail.
     fn consider_builtin_pointee_candidate(
         ecx: &mut EvalCtxt<'_, 'tcx>,
         goal: Goal<'tcx, Self>,
     ) -> QueryResult<'tcx>;
 
+    // A generator (that comes from an `async` desugaring) is known to implement
+    // `Future<Output = O>`, where `O` is given by the generator's return type
+    // that was computed during type-checking.
     fn consider_builtin_future_candidate(
         ecx: &mut EvalCtxt<'_, 'tcx>,
         goal: Goal<'tcx, Self>,
     ) -> QueryResult<'tcx>;
 
+    // A generator (that doesn't come from an `async` desugaring) is known to
+    // implement `Generator<R, Yield = Y, Return = O>`, given the resume, yield,
+    // and return types of the generator computed during type-checking.
     fn consider_builtin_generator_candidate(
         ecx: &mut EvalCtxt<'_, 'tcx>,
         goal: Goal<'tcx, Self>,

compiler/rustc_trait_selection/src/solve/mod.rs

@@ -335,7 +335,6 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
             // That won't actually reflect in the query response, so it seems moot.
             self.make_canonical_response(Certainty::AMBIGUOUS)
         } else {
-            self.infcx.probe(|_| {
             let InferOk { value: (), obligations } = self
                 .infcx
                 .at(&ObligationCause::dummy(), goal.param_env)
@@ -343,7 +342,6 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
             self.evaluate_all_and_make_canonical_response(
                 obligations.into_iter().map(|pred| pred.into()).collect(),
             )
-            })
         }
     }
 
@@ -376,7 +374,6 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
         &mut self,
         goal: Goal<'tcx, ty::GenericArg<'tcx>>,
     ) -> QueryResult<'tcx> {
-        self.infcx.probe(|_| {
         match crate::traits::wf::unnormalized_obligations(
             self.infcx,
             goal.param_env,
@@ -387,11 +384,12 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
             ),
             None => self.make_canonical_response(Certainty::AMBIGUOUS),
         }
-        })
     }
 }
 
 impl<'tcx> EvalCtxt<'_, 'tcx> {
+    // Recursively evaluates a list of goals to completion, returning the certainty
+    // of all of the goals.
     fn evaluate_all(
         &mut self,
         mut goals: Vec<Goal<'tcx, ty::Predicate<'tcx>>>,
@@ -428,6 +426,10 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
         })
     }
 
+    // Recursively evaluates a list of goals to completion, making a query response.
+    //
+    // This is just a convenient way of calling [`EvalCtxt::evaluate_all`],
+    // then [`EvalCtxt::make_canonical_response`].
     fn evaluate_all_and_make_canonical_response(
         &mut self,
         goals: Vec<Goal<'tcx, ty::Predicate<'tcx>>>,

compiler/rustc_trait_selection/src/solve/project_goals.rs

@@ -296,7 +296,9 @@ impl<'tcx> assembly::GoalKind<'tcx> for ProjectionPredicate<'tcx> {
         goal: Goal<'tcx, Self>,
         assumption: ty::Predicate<'tcx>,
     ) -> QueryResult<'tcx> {
-        if let Some(poly_projection_pred) = assumption.to_opt_poly_projection_pred() {
+        if let Some(poly_projection_pred) = assumption.to_opt_poly_projection_pred()
+            && poly_projection_pred.projection_def_id() == goal.predicate.def_id()
+        {
             ecx.infcx.probe(|_| {
                 let assumption_projection_pred =
                     ecx.infcx.instantiate_bound_vars_with_infer(poly_projection_pred);

compiler/rustc_trait_selection/src/solve/trait_goals.rs

@@ -65,7 +65,9 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
         goal: Goal<'tcx, Self>,
         assumption: ty::Predicate<'tcx>,
     ) -> QueryResult<'tcx> {
-        if let Some(poly_trait_pred) = assumption.to_opt_poly_trait_pred() {
+        if let Some(poly_trait_pred) = assumption.to_opt_poly_trait_pred()
+            && poly_trait_pred.def_id() == goal.predicate.def_id()
+        {
             // FIXME: Constness and polarity
             ecx.infcx.probe(|_| {
                 let assumption_trait_pred =