Rollup merge of #126353 - compiler-errors:move-match, r=lcnr

Move `MatchAgainstFreshVars` to old solver

Small change I noticed when trying to uplift the relations to the new trait solver.

2 files changed, 123 insertions, 2 deletions

diff --git a/compiler/rustc_trait_selection/src/traits/select/_match.rs b/compiler/rustc_trait_selection/src/traits/select/_match.rs
new file mode 100644
index 00000000000..50d8e96aaf9
--- /dev/null
+++ b/compiler/rustc_trait_selection/src/traits/select/_match.rs
@@ -0,0 +1,121 @@
+use rustc_infer::infer::relate::{
+    self, structurally_relate_tys, Relate, RelateResult, TypeRelation,
+};
+use rustc_middle::ty::error::{ExpectedFound, TypeError};
+use rustc_middle::ty::{self, InferConst, Ty, TyCtxt};
+use tracing::{debug, instrument};
+
+/// A type "A" *matches* "B" if the fresh types in B could be
+/// instantiated with values so as to make it equal to A. Matching is
+/// intended to be used only on freshened types, and it basically
+/// indicates if the non-freshened versions of A and B could have been
+/// unified.
+///
+/// It is only an approximation. If it yields false, unification would
+/// definitely fail, but a true result doesn't mean unification would
+/// succeed. This is because we don't track the "side-constraints" on
+/// type variables, nor do we track if the same freshened type appears
+/// more than once. To some extent these approximations could be
+/// fixed, given effort.
+///
+/// Like subtyping, matching is really a binary relation, so the only
+/// important thing about the result is Ok/Err. Also, matching never
+/// affects any type variables or unification state.
+pub struct MatchAgainstFreshVars<'tcx> {
+    tcx: TyCtxt<'tcx>,
+}
+
+impl<'tcx> MatchAgainstFreshVars<'tcx> {
+    pub fn new(tcx: TyCtxt<'tcx>) -> MatchAgainstFreshVars<'tcx> {
+        MatchAgainstFreshVars { tcx }
+    }
+}
+
+impl<'tcx> TypeRelation<TyCtxt<'tcx>> for MatchAgainstFreshVars<'tcx> {
+    fn tag(&self) -> &'static str {
+        "MatchAgainstFreshVars"
+    }
+
+    fn tcx(&self) -> TyCtxt<'tcx> {
+        self.tcx
+    }
+
+    fn relate_with_variance<T: Relate<TyCtxt<'tcx>>>(
+        &mut self,
+        _: ty::Variance,
+        _: ty::VarianceDiagInfo<TyCtxt<'tcx>>,
+        a: T,
+        b: T,
+    ) -> RelateResult<'tcx, T> {
+        self.relate(a, b)
+    }
+
+    #[instrument(skip(self), level = "debug")]
+    fn regions(
+        &mut self,
+        a: ty::Region<'tcx>,
+        _b: ty::Region<'tcx>,
+    ) -> RelateResult<'tcx, ty::Region<'tcx>> {
+        Ok(a)
+    }
+
+    #[instrument(skip(self), level = "debug")]
+    fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
+        if a == b {
+            return Ok(a);
+        }
+
+        match (a.kind(), b.kind()) {
+            (
+                _,
+                &ty::Infer(ty::FreshTy(_))
+                | &ty::Infer(ty::FreshIntTy(_))
+                | &ty::Infer(ty::FreshFloatTy(_)),
+            ) => Ok(a),
+
+            (&ty::Infer(_), _) | (_, &ty::Infer(_)) => {
+                Err(TypeError::Sorts(ExpectedFound::new(true, a, b)))
+            }
+
+            (&ty::Error(guar), _) | (_, &ty::Error(guar)) => Ok(Ty::new_error(self.tcx(), guar)),
+
+            _ => structurally_relate_tys(self, a, b),
+        }
+    }
+
+    fn consts(
+        &mut self,
+        a: ty::Const<'tcx>,
+        b: ty::Const<'tcx>,
+    ) -> RelateResult<'tcx, ty::Const<'tcx>> {
+        debug!("{}.consts({:?}, {:?})", self.tag(), a, b);
+        if a == b {
+            return Ok(a);
+        }
+
+        match (a.kind(), b.kind()) {
+            (_, ty::ConstKind::Infer(InferConst::Fresh(_))) => {
+                return Ok(a);
+            }
+
+            (ty::ConstKind::Infer(_), _) | (_, ty::ConstKind::Infer(_)) => {
+                return Err(TypeError::ConstMismatch(ExpectedFound::new(true, a, b)));
+            }
+
+            _ => {}
+        }
+
+        relate::structurally_relate_consts(self, a, b)
+    }
+
+    fn binders<T>(
+        &mut self,
+        a: ty::Binder<'tcx, T>,
+        b: ty::Binder<'tcx, T>,
+    ) -> RelateResult<'tcx, ty::Binder<'tcx, T>>
+    where
+        T: Relate<TyCtxt<'tcx>>,
+    {
+        Ok(a.rebind(self.relate(a.skip_binder(), b.skip_binder())?))
+    }
+}
diff --git a/compiler/rustc_trait_selection/src/traits/select/mod.rs b/compiler/rustc_trait_selection/src/traits/select/mod.rs
index 4487ba60faa..212ef2e4b2b 100644
--- a/compiler/rustc_trait_selection/src/traits/select/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/mod.rs
@@ -32,7 +32,6 @@ use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_errors::{Diag, EmissionGuarantee};
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
-use rustc_infer::infer::relate::MatchAgainstFreshVars;
 use rustc_infer::infer::relate::TypeRelation;
 use rustc_infer::infer::BoundRegionConversionTime;
 use rustc_infer::infer::BoundRegionConversionTime::HigherRankedType;
@@ -60,6 +59,7 @@ use std::ops::ControlFlow;
 pub use rustc_middle::traits::select::*;
 use rustc_middle::ty::print::with_no_trimmed_paths;
 
+mod _match;
 mod candidate_assembly;
 mod confirmation;
 
@@ -2719,7 +2719,7 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
         previous: ty::PolyTraitPredicate<'tcx>,
         current: ty::PolyTraitPredicate<'tcx>,
     ) -> bool {
-        let mut matcher = MatchAgainstFreshVars::new(self.tcx());
+        let mut matcher = _match::MatchAgainstFreshVars::new(self.tcx());
         matcher.relate(previous, current).is_ok()
     }