Don't unnecessarily clone some fields in Context

There was no need to clone `id_map` because it was reset before each
item was rendered. `deref_id_map` was not reset, but it was keyed by
`DefId` and thus was unlikely to have collisions (at least for now).

Now we just clone the fields that need to be cloned, and instead create
fresh versions of the others.

src/librustdoc/formats/renderer.rs

@@ -9,7 +9,7 @@ use crate::formats::cache::Cache;
 /// Allows for different backends to rustdoc to be used with the `run_format()` function. Each
 /// backend renderer has hooks for initialization, documenting an item, entering and exiting a
 /// module, and cleanup/finalizing output.
-crate trait FormatRenderer<'tcx>: Clone {
+crate trait FormatRenderer<'tcx>: Sized {
     /// Gives a description of the renderer. Used for performance profiling.
     fn descr() -> &'static str;
 
@@ -23,6 +23,9 @@ crate trait FormatRenderer<'tcx>: Clone {
         tcx: TyCtxt<'tcx>,
     ) -> Result<(Self, clean::Crate), Error>;
 
+    /// Make a new renderer to render a child of the item currently being rendered.
+    fn make_child_renderer(&self) -> Self;
+
     /// Renders a single non-module item. This means no recursive sub-item rendering is required.
     fn item(&mut self, item: clean::Item) -> Result<(), Error>;
 
@@ -67,7 +70,7 @@ crate fn run_format<'tcx, T: FormatRenderer<'tcx>>(
     item.name = Some(krate.name);
 
     // Render the crate documentation
-    let mut work = vec![(format_renderer.clone(), item)];
+    let mut work = vec![(format_renderer.make_child_renderer(), item)];
 
     let unknown = rustc_span::Symbol::intern("<unknown item>");
     while let Some((mut cx, item)) = work.pop() {
@@ -87,7 +90,7 @@ crate fn run_format<'tcx, T: FormatRenderer<'tcx>>(
             };
             for it in module.items {
                 debug!("Adding {:?} to worklist", it.name);
-                work.push((cx.clone(), it));
+                work.push((cx.make_child_renderer(), it));
             }
 
             cx.mod_item_out(&name)?;

src/librustdoc/html/markdown.rs

@@ -1373,10 +1373,6 @@ impl IdMap {
         }
     }
 
-    crate fn reset(&mut self) {
-        self.map = init_id_map();
-    }
-
     crate fn derive<S: AsRef<str> + ToString>(&mut self, candidate: S) -> String {
         let id = match self.map.get_mut(candidate.as_ref()) {
             None => candidate.to_string(),

src/librustdoc/html/markdown/tests.rs

@@ -38,15 +38,9 @@ fn test_unique_id() {
         "assoc_type.Item-1",
     ];
 
-    let map = RefCell::new(IdMap::new());
-    let test = || {
-        let mut map = map.borrow_mut();
-        let actual: Vec<String> = input.iter().map(|s| map.derive(s.to_string())).collect();
-        assert_eq!(&actual[..], expected);
-    };
-    test();
-    map.borrow_mut().reset();
-    test();
+    let mut map = IdMap::new();
+    let actual: Vec<String> = input.iter().map(|s| map.derive(s.to_string())).collect();
+    assert_eq!(&actual[..], expected);
 }
 
 #[test]

src/librustdoc/html/render/context.rs

@@ -40,7 +40,6 @@ use crate::html::{layout, sources};
 /// It is intended that this context is a lightweight object which can be fairly
 /// easily cloned because it is cloned per work-job (about once per item in the
 /// rustdoc tree).
-#[derive(Clone)]
 crate struct Context<'tcx> {
     /// Current hierarchy of components leading down to what's currently being
     /// rendered
@@ -157,11 +156,6 @@ impl<'tcx> Context<'tcx> {
             static_extra_scripts: &[],
         };
 
-        {
-            self.id_map.borrow_mut().reset();
-            self.id_map.borrow_mut().populate(&INITIAL_IDS);
-        }
-
         if !self.render_redirect_pages {
             layout::render(
                 &self.shared.layout,
@@ -436,6 +430,21 @@ impl<'tcx> FormatRenderer<'tcx> for Context<'tcx> {
         Ok((cx, krate))
     }
 
+    fn make_child_renderer(&self) -> Self {
+        let mut id_map = IdMap::new();
+        id_map.populate(&INITIAL_IDS);
+
+        Self {
+            current: self.current.clone(),
+            dst: self.dst.clone(),
+            render_redirect_pages: self.render_redirect_pages,
+            id_map: Box::new(RefCell::new(id_map)),
+            deref_id_map: Box::new(RefCell::new(FxHashMap::default())),
+            shared: Rc::clone(&self.shared),
+            cache: Rc::clone(&self.cache),
+        }
+    }
+
     fn after_krate(
         &mut self,
         krate: &clean::Crate,

src/librustdoc/json/mod.rs

@@ -149,6 +149,10 @@ impl<'tcx> FormatRenderer<'tcx> for JsonRenderer<'tcx> {
         ))
     }
 
+    fn make_child_renderer(&self) -> Self {
+        self.clone()
+    }
+
     /// Inserts an item into the index. This should be used rather than directly calling insert on
     /// the hashmap because certain items (traits and types) need to have their mappings for trait
     /// implementations filled out before they're inserted.