Introduce substitute method on Binder

Author: KiChjang

chalk-ir/src/lib.rs

@@ -25,7 +25,7 @@ macro_rules! impl_debugs {
 use crate::cast::Cast;
 use crate::fold::shift::Shift;
 use crate::fold::{
-    DefaultInferenceFolder, DefaultPlaceholderFolder, DefaultTypeFolder, Fold, FreeVarFolder,
+    DefaultInferenceFolder, DefaultPlaceholderFolder, DefaultTypeFolder, Fold, FreeVarFolder, Subst,
 };
 use chalk_engine::fallible::*;
 use lalrpop_intern::InternedString;
@@ -823,6 +823,13 @@ impl<T> Binders<T> {
     }
 }
 
+impl<T> Binders<T> where T: Fold {
+    pub fn substitute(&self, parameters: &[Parameter]) -> T::Result {
+        assert_eq!(self.binders.len(), parameters.len());
+        Subst::apply(parameters, &self.value)
+    }
+}
+
 /// Allows iterating over a Binders<Vec<T>>, for instance.
 /// Each element will include the same set of parameter bounds.
 impl<V: IntoIterator> IntoIterator for Binders<V> {

chalk-solve/src/clauses.rs

@@ -3,7 +3,6 @@ use self::program_clauses::ToProgramClauses;
 use crate::RustIrDatabase;
 use chalk_ir::cast::{Cast, Caster};
 use chalk_ir::could_match::CouldMatch;
-use chalk_ir::fold::Subst;
 use chalk_ir::*;
 use rustc_hash::FxHashSet;
 use std::sync::Arc;
@@ -172,7 +171,7 @@ fn program_clauses_that_could_match(
             match trait_ref.self_type_parameter() {
                 Some(Ty::Opaque(qwc)) | Some(Ty::Dyn(qwc)) => {
                     let self_ty = trait_ref.self_type_parameter().unwrap(); // This cannot be None
-                    let wc = Subst::apply(&[self_ty.cast()], &qwc.value);
+                    let wc = qwc.substitute(&[self_ty.cast()]);
                     clauses.extend(wc.into_iter().casted());
                 }
                 _ => {}