Rollup merge of #99700 - est31:rustdoc_layout_heading, r=GuillaumeGomez

Add a clickable link to the layout section

The layout section (activated by `--show-type-layout`) is currently not linkable to (outside of chrome's link to text feature). This PR makes it linkable via `#layout`.

src/librustdoc/html/markdown.rs

@@ -1485,6 +1485,7 @@ fn init_id_map() -> FxHashMap<Cow<'static, str>, usize> {
     map.insert("synthetic-implementations-list".into(), 1);
     map.insert("blanket-implementations-list".into(), 1);
     map.insert("deref-methods".into(), 1);
+    map.insert("layout".into(), 1);
     map
 }
 

src/librustdoc/html/render/print_item.rs

@@ -1880,7 +1880,11 @@ fn document_type_layout(w: &mut Buffer, cx: &Context<'_>, ty_def_id: DefId) {
         return;
     }
 
-    writeln!(w, "<h2 class=\"small-section-header\">Layout</h2>");
+    writeln!(
+        w,
+        "<h2 id=\"layout\" class=\"small-section-header\"> \
+        Layout<a href=\"#layout\" class=\"anchor\"></a></h2>"
+    );
     writeln!(w, "<div class=\"docblock\">");
 
     let tcx = cx.tcx();

src/test/rustdoc/type-layout.rs

@@ -2,6 +2,7 @@
 
 // @has type_layout/struct.Foo.html 'Size: '
 // @has - ' bytes'
+// @has - '//*[@id="layout"]/a[@href="#layout"]' ''
 pub struct Foo {
     pub a: usize,
     b: Vec<String>,