Don't leak inference variables in array unsizing

2 files changed, 29 insertions, 22 deletions

diff --git a/compiler/rustc_typeck/src/check/method/confirm.rs b/compiler/rustc_typeck/src/check/method/confirm.rs
index dabfe92190b..27c39934ba8 100644
--- a/compiler/rustc_typeck/src/check/method/confirm.rs
+++ b/compiler/rustc_typeck/src/check/method/confirm.rs
@@ -149,7 +149,7 @@ impl<'a, 'tcx> ConfirmContext<'a, 'tcx> {
         // time writing the results into the various typeck results.
         let mut autoderef =
             self.autoderef_overloaded_span(self.span, unadjusted_self_ty, self.call_expr.span);
-        let (_, n) = match autoderef.nth(pick.autoderefs) {
+        let (ty, n) = match autoderef.nth(pick.autoderefs) {
             Some(n) => n,
             None => {
                 return self.tcx.ty_error_with_message(
@@ -161,14 +161,15 @@ impl<'a, 'tcx> ConfirmContext<'a, 'tcx> {
         assert_eq!(n, pick.autoderefs);
 
         let mut adjustments = self.adjust_steps(&autoderef);
+        let mut target = self.structurally_resolved_type(autoderef.span(), ty);
 
-        let mut target =
-            self.structurally_resolved_type(autoderef.span(), autoderef.final_ty(false));
-
-        match &pick.autoref_or_ptr_adjustment {
+        match pick.autoref_or_ptr_adjustment {
             Some(probe::AutorefOrPtrAdjustment::Autoref { mutbl, unsize }) => {
                 let region = self.next_region_var(infer::Autoref(self.span));
-                target = self.tcx.mk_ref(region, ty::TypeAndMut { mutbl: *mutbl, ty: target });
+                // Type we're wrapping in a reference, used later for unsizing
+                let base_ty = target;
+
+                target = self.tcx.mk_ref(region, ty::TypeAndMut { mutbl, ty: target });
                 let mutbl = match mutbl {
                     hir::Mutability::Not => AutoBorrowMutability::Not,
                     hir::Mutability::Mut => AutoBorrowMutability::Mut {
@@ -182,10 +183,18 @@ impl<'a, 'tcx> ConfirmContext<'a, 'tcx> {
                     target,
                 });
 
-                if let Some(unsize_target) = unsize {
+                if unsize {
+                    let unsized_ty = if let ty::Array(elem_ty, _) = base_ty.kind() {
+                        self.tcx.mk_slice(elem_ty)
+                    } else {
+                        bug!(
+                            "AutorefOrPtrAdjustment's unsize flag should only be set for array ty, found {}",
+                            base_ty
+                        )
+                    };
                     target = self
                         .tcx
-                        .mk_ref(region, ty::TypeAndMut { mutbl: mutbl.into(), ty: unsize_target });
+                        .mk_ref(region, ty::TypeAndMut { mutbl: mutbl.into(), ty: unsized_ty });
                     adjustments
                         .push(Adjustment { kind: Adjust::Pointer(PointerCast::Unsize), target });
                 }
diff --git a/compiler/rustc_typeck/src/check/method/probe.rs b/compiler/rustc_typeck/src/check/method/probe.rs
index 5615a08369d..8ce1c27f81d 100644
--- a/compiler/rustc_typeck/src/check/method/probe.rs
+++ b/compiler/rustc_typeck/src/check/method/probe.rs
@@ -167,26 +167,26 @@ enum ProbeResult {
 /// T`, we could convert it to `*const T`, then autoref to `&*const T`. However, currently we do
 /// (at most) one of these. Either the receiver has type `T` and we convert it to `&T` (or with
 /// `mut`), or it has type `*mut T` and we convert it to `*const T`.
-#[derive(Debug, PartialEq, Clone)]
-pub enum AutorefOrPtrAdjustment<'tcx> {
+#[derive(Debug, PartialEq, Copy, Clone)]
+pub enum AutorefOrPtrAdjustment {
     /// Receiver has type `T`, add `&` or `&mut` (it `T` is `mut`), and maybe also "unsize" it.
     /// Unsizing is used to convert a `[T; N]` to `[T]`, which only makes sense when autorefing.
     Autoref {
         mutbl: hir::Mutability,
 
-        /// Indicates that the source expression should be "unsized" to a target type. This should
-        /// probably eventually go away in favor of just coercing method receivers.
-        unsize: Option<Ty<'tcx>>,
+        /// Indicates that the source expression should be "unsized" to a target type.
+        /// This is special-cased for just arrays unsizing to slices.
+        unsize: bool,
     },
     /// Receiver has type `*mut T`, convert to `*const T`
     ToConstPtr,
 }
 
-impl<'tcx> AutorefOrPtrAdjustment<'tcx> {
-    fn get_unsize(&self) -> Option<Ty<'tcx>> {
+impl AutorefOrPtrAdjustment {
+    fn get_unsize(&self) -> bool {
         match self {
             AutorefOrPtrAdjustment::Autoref { mutbl: _, unsize } => *unsize,
-            AutorefOrPtrAdjustment::ToConstPtr => None,
+            AutorefOrPtrAdjustment::ToConstPtr => false,
         }
     }
 }
@@ -204,7 +204,7 @@ pub struct Pick<'tcx> {
 
     /// Indicates that we want to add an autoref (and maybe also unsize it), or if the receiver is
     /// `*mut T`, convert it to `*const T`.
-    pub autoref_or_ptr_adjustment: Option<AutorefOrPtrAdjustment<'tcx>>,
+    pub autoref_or_ptr_adjustment: Option<AutorefOrPtrAdjustment>,
     pub self_ty: Ty<'tcx>,
 }
 
@@ -1202,7 +1202,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
                     pick.autoderefs += 1;
                     pick.autoref_or_ptr_adjustment = Some(AutorefOrPtrAdjustment::Autoref {
                         mutbl,
-                        unsize: pick.autoref_or_ptr_adjustment.and_then(|a| a.get_unsize()),
+                        unsize: pick.autoref_or_ptr_adjustment.map_or(false, |a| a.get_unsize()),
                     })
                 }
 
@@ -1227,10 +1227,8 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
         self.pick_method(autoref_ty, unstable_candidates).map(|r| {
             r.map(|mut pick| {
                 pick.autoderefs = step.autoderefs;
-                pick.autoref_or_ptr_adjustment = Some(AutorefOrPtrAdjustment::Autoref {
-                    mutbl,
-                    unsize: step.unsize.then_some(self_ty),
-                });
+                pick.autoref_or_ptr_adjustment =
+                    Some(AutorefOrPtrAdjustment::Autoref { mutbl, unsize: step.unsize });
                 pick
             })
         })