Fix #3373

Basically, we need to allow variables in the caller self type to unify with the
impl's declared self type. That requires some more contortions in the variable
handling. I'm looking forward to (hopefully) handling this in a cleaner way when
we switch to Chalk's types and unification code.

crates/ra_hir_ty/src/infer/unify.rs

@@ -142,12 +142,21 @@ impl<T> Canonicalized<T> {
 
 pub fn unify(ty1: &Canonical<Ty>, ty2: &Canonical<Ty>) -> Option<Substs> {
     let mut table = InferenceTable::new();
+    let num_vars = ty1.num_vars.max(ty2.num_vars);
     let vars =
-        Substs::builder(ty1.num_vars).fill(std::iter::repeat_with(|| table.new_type_var())).build();
-    let ty_with_vars = ty1.value.clone().subst_bound_vars(&vars);
-    if !table.unify(&ty_with_vars, &ty2.value) {
+        Substs::builder(num_vars).fill(std::iter::repeat_with(|| table.new_type_var())).build();
+    let ty1_with_vars = ty1.value.clone().subst_bound_vars(&vars);
+    let ty2_with_vars = ty2.value.clone().subst_bound_vars(&vars);
+    if !table.unify(&ty1_with_vars, &ty2_with_vars) {
         return None;
     }
+    // default any type vars that weren't unified back to their original bound vars
+    // (kind of hacky)
+    for (i, var) in vars.iter().enumerate() {
+        if &*table.resolve_ty_shallow(var) == var {
+            table.unify(var, &Ty::Bound(i as u32));
+        }
+    }
     Some(
         Substs::builder(ty1.num_vars)
             .fill(vars.iter().map(|v| table.resolve_ty_completely(v.clone())))

crates/ra_hir_ty/src/lib.rs

@@ -355,6 +355,10 @@ impl Substs {
         Substs(self.0[..std::cmp::min(self.0.len(), n)].into())
     }
 
+    pub fn suffix(&self, n: usize) -> Substs {
+        Substs(self.0[self.0.len() - std::cmp::min(self.0.len(), n)..].into())
+    }
+
     pub fn as_single(&self) -> &Ty {
         if self.0.len() != 1 {
             panic!("expected substs of len 1, got {:?}", self);

crates/ra_hir_ty/src/method_resolution.rs

@@ -508,10 +508,17 @@ pub(crate) fn inherent_impl_substs(
     impl_id: ImplId,
     self_ty: &Canonical<Ty>,
 ) -> Option<Substs> {
-    let vars = Substs::build_for_def(db, impl_id).fill_with_bound_vars(0).build();
+    // we create a var for each type parameter of the impl; we need to keep in
+    // mind here that `self_ty` might have vars of its own
+    let vars =
+        Substs::build_for_def(db, impl_id).fill_with_bound_vars(self_ty.num_vars as u32).build();
     let self_ty_with_vars = db.impl_self_ty(impl_id).subst(&vars);
-    let self_ty_with_vars = Canonical { num_vars: vars.len(), value: self_ty_with_vars };
-    super::infer::unify(&self_ty_with_vars, self_ty)
+    let self_ty_with_vars =
+        Canonical { num_vars: vars.len() + self_ty.num_vars, value: self_ty_with_vars };
+    let substs = super::infer::unify(&self_ty_with_vars, self_ty);
+    // we only want the substs for the vars we added, not the ones from self_ty
+    let result = substs.map(|s| s.suffix(vars.len()));
+    result
 }
 
 fn transform_receiver_ty(

crates/ra_hir_ty/src/tests/method_resolution.rs

@@ -1048,6 +1048,25 @@ where
     assert_eq!(t, "{unknown}");
 }
 
+#[test]
+fn method_resolution_3373() {
+    let t = type_at(
+        r#"
+//- /main.rs
+struct A<T>(T);
+
+impl A<i32> {
+    fn from(v: i32) -> A<i32> { A(v) }
+}
+
+fn main() {
+    A::from(3)<|>;
+}
+"#,
+    );
+    assert_eq!(t, "A<i32>");
+}
+
 #[test]
 fn method_resolution_slow() {
     // this can get quite slow if we set the solver size limit too high