1 files changed, 124 insertions, 0 deletions
diff --git a/compiler/rustc_middle/src/ty/_match.rs b/compiler/rustc_middle/src/ty/_match.rs
new file mode 100644
index 000000000..e6aab30a1
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/_match.rs
@@ -0,0 +1,124 @@
+use crate::ty::error::TypeError;
+use crate::ty::relate::{self, Relate, RelateResult, TypeRelation};
+use crate::ty::{self, InferConst, Ty, TyCtxt};
+
+/// A type "A" *matches* "B" if the fresh types in B could be
+/// substituted 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 Match<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    param_env: ty::ParamEnv<'tcx>,
+}
+
+impl<'tcx> Match<'tcx> {
+    pub fn new(tcx: TyCtxt<'tcx>, param_env: ty::ParamEnv<'tcx>) -> Match<'tcx> {
+        Match { tcx, param_env }
+    }
+}
+
+impl<'tcx> TypeRelation<'tcx> for Match<'tcx> {
+    fn tag(&self) -> &'static str {
+        "Match"
+    }
+    fn tcx(&self) -> TyCtxt<'tcx> {
+        self.tcx
+    }
+    fn param_env(&self) -> ty::ParamEnv<'tcx> {
+        self.param_env
+    }
+    fn a_is_expected(&self) -> bool {
+        true
+    } // irrelevant
+
+    fn relate_with_variance<T: Relate<'tcx>>(
+        &mut self,
+        _: ty::Variance,
+        _: ty::VarianceDiagInfo<'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(relate::expected_found(self, a, b)))
+            }
+
+            (&ty::Error(_), _) | (_, &ty::Error(_)) => Ok(self.tcx().ty_error()),
+
+            _ => relate::super_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(relate::expected_found(self, a, b)));
+            }
+
+            _ => {}
+        }
+
+        relate::super_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<'tcx>,
+    {
+        Ok(a.rebind(self.relate(a.skip_binder(), b.skip_binder())?))
+    }
+}