Auto merge of rust-lang#13382 - lowr:fix/reorder-dyn-bounds-on-render, r=lowr

fix: reorder dyn bounds on render

Fixes rust-lang#13368

rust-lang#13192 changed the order of dyn bounds, violating the [contract](https://github.com/rust-lang/rust-analyzer/blob/3a69435af7a1e6273744085cb251adb2b9c30a03/crates/hir-ty/src/display.rs#L896-L901) with `write_bounds_like_dyn_trait()` on render. The projection bounds are expected to come right after the trait bound they are accompanied with.

Although the reordering procedure can be made a bit more efficient, I opted for relying only on the [invariants](https://github.com/rust-lang/rust-analyzer/blob/3a69435af7a1e6273744085cb251adb2b9c30a03/crates/hir-ty/src/lower.rs#L995-L998) currently documented in `lower_dyn_trait()`. It's not the hottest path and dyn bounds tend to be short so I believe it shouldn't hurt performance noticeably.

Author: bors

crates/hir-ty/src/display.rs

@@ -751,9 +751,19 @@ impl HirDisplay for Ty {
             }
             TyKind::BoundVar(idx) => idx.hir_fmt(f)?,
             TyKind::Dyn(dyn_ty) => {
+                // Reorder bounds to satisfy `write_bounds_like_dyn_trait()`'s expectation.
+                // FIXME: `Iterator::partition_in_place()` or `Vec::drain_filter()` may make it
+                // more efficient when either of them hits stable.
+                let mut bounds: SmallVec<[_; 4]> =
+                    dyn_ty.bounds.skip_binders().iter(Interner).cloned().collect();
+                let (auto_traits, others): (SmallVec<[_; 4]>, _) =
+                    bounds.drain(1..).partition(|b| b.skip_binders().trait_id().is_some());
+                bounds.extend(others);
+                bounds.extend(auto_traits);
+
                 write_bounds_like_dyn_trait_with_prefix(
                     "dyn",
-                    dyn_ty.bounds.skip_binders().interned(),
+                    &bounds,
                     SizedByDefault::NotSized,
                     f,
                 )?;

crates/hir-ty/src/tests/display_source_code.rs

@@ -55,6 +55,28 @@ fn main() {
     );
 }
 
+#[test]
+fn render_dyn_ty_independent_of_order() {
+    check_types_source_code(
+        r#"
+auto trait Send {}
+trait A {
+    type Assoc;
+}
+trait B: A {}
+
+fn test(
+    _: &(dyn A<Assoc = ()> + Send),
+  //^ &(dyn A<Assoc = ()> + Send)
+    _: &(dyn Send + A<Assoc = ()>),
+  //^ &(dyn A<Assoc = ()> + Send)
+    _: &dyn B<Assoc = ()>,
+  //^ &(dyn B<Assoc = ()>)
+) {}
+        "#,
+    );
+}
+
 #[test]
 fn render_dyn_for_ty() {
     // FIXME