Don't resolve type var roots in point_at_expr_source_of_inferred_type

tests/ui/typeck/bad-type-in-vec-push.rs

@@ -0,0 +1,14 @@
+// The error message here still is pretty confusing.
+
+fn main() {
+    let mut result = vec![1];
+    // The type of `result` is constrained to be `Vec<{integer}>` here.
+    // But the logic we use to find what expression constrains a type
+    // is not sophisticated enough to know this.
+
+    let mut vector = Vec::new();
+    vector.sort();
+    result.push(vector);
+    //~^ ERROR mismatched types
+    // So it thinks that the type of `result` is constrained here.
+}

tests/ui/typeck/bad-type-in-vec-push.stderr

@@ -0,0 +1,20 @@
+error[E0308]: mismatched types
+  --> $DIR/bad-type-in-vec-push.rs:11:17
+   |
+LL |     vector.sort();
+   |     ------ here the type of `vector` is inferred to be `Vec<_>`
+LL |     result.push(vector);
+   |            ---- ^^^^^^
+   |            |    |
+   |            |    expected integer, found struct `Vec`
+   |            |    this is of type `Vec<_>`, which causes `result` to be inferred as `Vec<{integer}>`
+   |            arguments to this method are incorrect
+   |
+   = note: expected type `{integer}`
+            found struct `Vec<_>`
+note: associated function defined here
+  --> $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.