coerce_to_ref: no structural identity reliance

Author: BoxyUwU

compiler/rustc_hir_typeck/src/coercion.rs

@@ -251,8 +251,8 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
             ty::RawPtr(_, b_mutbl) => {
                 return self.coerce_to_raw_ptr(a, b, b_mutbl);
             }
-            ty::Ref(_, _, mutbl_b) => {
-                return self.coerce_to_ref(a, b, mutbl_b);
+            ty::Ref(r_b, _, mutbl_b) => {
+                return self.coerce_to_ref(a, b, mutbl_b, r_b);
             }
             ty::Adt(pin, _)
                 if self.tcx.features().pin_ergonomics()
@@ -343,15 +343,16 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
         a: Ty<'tcx>,
         b: Ty<'tcx>,
         mutbl_b: hir::Mutability,
+        r_b: ty::Region<'tcx>,
     ) -> CoerceResult<'tcx> {
         debug!("coerce_to_ref(a={:?}, b={:?})", a, b);
         debug_assert!(self.shallow_resolve(a) == a);
         debug_assert!(self.shallow_resolve(b) == b);
 
-        let mt_a = match *a.kind() {
-            ty::Ref(_, ty, mutbl) => {
+        let (r_a, mt_a) = match *a.kind() {
+            ty::Ref(r_a, ty, mutbl) => {
                 coerce_mutbls(mutbl, mutbl_b)?;
-                ty::TypeAndMut { ty, mutbl }
+                (r_a, ty::TypeAndMut { ty, mutbl })
             }
             _ => return self.unify(a, b),
         };
@@ -368,6 +369,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
         // 3. `[T]`, autoref'd ty: `&M [T]`
         //     - `&M [T]` does unify with `&M [T]`
         let mut first_error = None;
+        let mut r_borrow_var = None;
         let mut autoderef = self.autoderef(self.cause.span, a);
         let found = autoderef.by_ref().find_map(|(deref_ty, autoderefs)| {
             if autoderefs == 0 {
@@ -376,9 +378,29 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
                 return None;
             }
 
-            let coercion = RegionVariableOrigin::Coercion(self.cause.span);
-            let r_borrow = self.next_region_var(coercion);
-            let autorefd_deref_ty = Ty::new_ref(self.tcx, r_borrow, deref_ty, mutbl_b);
+            let r = if !self.use_lub {
+                r_b // [2] above
+            } else if autoderefs == 1 {
+                r_a // [3] above
+            } else {
+                if r_borrow_var.is_none() {
+                    // create var lazily, at most once
+                    let coercion = RegionVariableOrigin::Coercion(self.cause.span);
+                    let r = self.next_region_var(coercion);
+                    r_borrow_var = Some(r); // [4] above
+                }
+                r_borrow_var.unwrap()
+            };
+            let autorefd_deref_ty = Ty::new_ref(
+                self.tcx,
+                r,
+                deref_ty,
+                mutbl_b, // [1] above
+            );
+
+            // let coercion = RegionVariableOrigin::Coercion(self.cause.span);
+            // let r_borrow = self.next_region_var(coercion);
+            // let autorefd_deref_ty = Ty::new_ref(self.tcx, r_borrow, deref_ty, mutbl_b);
 
             // Note that we unify the autoref'd `Target` type with `b` rather than
             // the `Target` type with the pointee of `b`. This is necessary