Normalize all types when finishing inference

The new solver does not eagerly normalize, but things after inference expect types to be normalized. rustc does the same.

Also, I'm afraid other things in r-a don't expect results of the solver to be unnormalized. We'll need to handle that.

src/tools/rust-analyzer/crates/hir-ty/src/infer/unify.rs

@@ -621,6 +621,9 @@ impl<'a> InferenceTable<'a> {
     where
         T: HasInterner<Interner = Interner> + TypeFoldable<Interner>,
     {
+        let t = self.resolve_with_fallback(t, &|_, _, d, _| d);
+        let t = self.normalize_associated_types_in(t);
+        // Resolve again, because maybe normalization inserted infer vars.
         self.resolve_with_fallback(t, &|_, _, d, _| d)
     }
 

src/tools/rust-analyzer/crates/hir-ty/src/tests/regression/new_solver.rs

@@ -24,3 +24,29 @@ impl<'a> IntoIterator for &'a Grid {
         "#]],
     );
 }
+
+#[test]
+fn normalization() {
+    check_infer(
+        r#"
+//- minicore: iterator, iterators
+fn main() {
+    _ = [0i32].into_iter().filter_map(|_n| Some(1i32));
+}
+    "#,
+        expect![[r#"
+            10..69 '{     ...2)); }': ()
+            16..17 '_': FilterMap<IntoIter<i32, 1>, impl FnMut(i32) -> Option<i32>>
+            16..66 '_ = [0...1i32))': ()
+            20..26 '[0i32]': [i32; 1]
+            20..38 '[0i32]...iter()': IntoIter<i32, 1>
+            20..66 '[0i32]...1i32))': FilterMap<IntoIter<i32, 1>, impl FnMut(i32) -> Option<i32>>
+            21..25 '0i32': i32
+            50..65 '|_n| Some(1i32)': impl FnMut(i32) -> Option<i32>
+            51..53 '_n': i32
+            55..59 'Some': fn Some<i32>(i32) -> Option<i32>
+            55..65 'Some(1i32)': Option<i32>
+            60..64 '1i32': i32
+        "#]],
+    );
+}