add force_instantiate_unchecked method

src/librustc/infer/mod.rs

@@ -1230,6 +1230,65 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
         self.inlined_shallow_resolve(typ)
     }
 
+    /// A hacky sort of method used by the NLL type-relating code:
+    ///
+    /// - `var` must be some unbound type variable.
+    /// - `value` must be a suitable type to use as its value.
+    ///
+    /// `var` will then be equated with `value`. Note that this
+    /// sidesteps a number of important checks, such as the "occurs
+    /// check" that prevents cyclic types, so it is important not to
+    /// use this method during regular type-check.
+    pub fn force_instantiate_unchecked(&self, var: Ty<'tcx>, value: Ty<'tcx>) {
+        match (&var.sty, &value.sty) {
+            (&ty::Infer(ty::TyVar(vid)), _) => {
+                let mut type_variables = self.type_variables.borrow_mut();
+
+                // In NLL, we don't have type inference variables
+                // floating around, so we can do this rather imprecise
+                // variant of the occurs-check.
+                assert!(!value.has_infer_types());
+
+                type_variables.instantiate(vid, value);
+            }
+
+            (&ty::Infer(ty::IntVar(vid)), &ty::Int(value)) => {
+                let mut int_unification_table = self.int_unification_table.borrow_mut();
+                int_unification_table
+                    .unify_var_value(vid, Some(ty::IntVarValue::IntType(value)))
+                    .unwrap_or_else(|_| {
+                        bug!("failed to unify int var `{:?}` with `{:?}`", vid, value);
+                    });
+            }
+
+            (&ty::Infer(ty::IntVar(vid)), &ty::Uint(value)) => {
+                let mut int_unification_table = self.int_unification_table.borrow_mut();
+                int_unification_table
+                    .unify_var_value(vid, Some(ty::IntVarValue::UintType(value)))
+                    .unwrap_or_else(|_| {
+                        bug!("failed to unify int var `{:?}` with `{:?}`", vid, value);
+                    });
+            }
+
+            (&ty::Infer(ty::FloatVar(vid)), &ty::Float(value)) => {
+                let mut float_unification_table = self.float_unification_table.borrow_mut();
+                float_unification_table
+                    .unify_var_value(vid, Some(ty::FloatVarValue(value)))
+                    .unwrap_or_else(|_| {
+                        bug!("failed to unify float var `{:?}` with `{:?}`", vid, value)
+                    });
+            }
+
+            _ => {
+                bug!(
+                    "force_instantiate_unchecked invoked with bad combination: var={:?} value={:?}",
+                    var,
+                    value,
+                );
+            }
+        }
+    }
+
     pub fn resolve_type_vars_if_possible<T>(&self, value: &T) -> T
     where
         T: TypeFoldable<'tcx>,