Rollup merge of #92487 - dtolnay:traitalias, r=matthewjasper

Fix unclosed boxes in pretty printing of TraitAlias

This was causing trait aliases to not even render at all in stringified / pretty printed output.

```rust
macro_rules! repro {
    ($item:item) => {
        stringify!($item)
    };
}

fn main() {
    println!("{:?}", repro!(pub trait Trait<T> = Sized where T: 'a;));
}
```

Before:&ensp;`""`
After:&ensp;`"pub trait Trait<T> = Sized where T: 'a;"`

The fix is copied from how `head`/`end` for `ItemKind::Use`, `ItemKind::ExternCrate`, and `ItemKind::Mod` are all done in the pretty printer:

https://github.com/rust-lang/rust/blob/dd3ac41495e85a9b7b5cb3186379d02ce17e51fe/compiler/rustc_ast_pretty/src/pprust/state.rs#L1178-L1184

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -1357,9 +1357,7 @@ impl<'a> State<'a> {
                 self.bclose(item.span, empty);
             }
             ast::ItemKind::TraitAlias(ref generics, ref bounds) => {
-                self.head("");
-                self.print_visibility(&item.vis);
-                self.word_nbsp("trait");
+                self.head(visibility_qualified(&item.vis, "trait"));
                 self.print_ident(item.ident);
                 self.print_generic_params(&generics.params);
                 let mut real_bounds = Vec::with_capacity(bounds.len());
@@ -1377,6 +1375,8 @@ impl<'a> State<'a> {
                 self.print_type_bounds("=", &real_bounds);
                 self.print_where_clause(&generics.where_clause);
                 self.word(";");
+                self.end(); // end inner head-block
+                self.end(); // end outer head-block
             }
             ast::ItemKind::MacCall(ref mac) => {
                 self.print_mac(mac);

compiler/rustc_hir_pretty/src/lib.rs

@@ -705,9 +705,7 @@ impl<'a> State<'a> {
                 self.bclose(item.span);
             }
             hir::ItemKind::TraitAlias(ref generics, ref bounds) => {
-                self.head("");
-                self.print_visibility(&item.vis);
-                self.word_nbsp("trait");
+                self.head(visibility_qualified(&item.vis, "trait"));
                 self.print_ident(item.ident);
                 self.print_generic_params(&generics.params);
                 let mut real_bounds = Vec::with_capacity(bounds.len());
@@ -725,6 +723,8 @@ impl<'a> State<'a> {
                 self.print_bounds("=", real_bounds);
                 self.print_where_clause(&generics.where_clause);
                 self.word(";");
+                self.end(); // end inner head-block
+                self.end(); // end outer head-block
             }
         }
         self.ann.post(self, AnnNode::Item(item))

src/test/ui/macros/stringify.rs

@@ -589,7 +589,7 @@ fn test_item() {
         stringify_item!(
             pub trait Trait<T> = Sized where T: 'a;
         ),
-        "", // FIXME
+        "pub trait Trait<T> = Sized where T: 'a;",
     );
 
     // ItemKind::Impl