Rollup merge of rust-lang#73098 - jyn514:rustdoc-is-fake, r=GuillaumeGomez

Add Item::is_fake for rustdoc

I wasn't aware items _could_ be fake, so I think having a function
mentioning it could be helpful. Also, I'd need to make this change for
cross-crate intra-doc links anyway, so I figured it's better to make the
refactor separate.

Author: Dylan-DPC

src/librustdoc/clean/types.rs

@@ -85,9 +85,7 @@ pub struct Item {
 
 impl fmt::Debug for Item {
     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
-        let fake = MAX_DEF_ID.with(|m| {
-            m.borrow().get(&self.def_id.krate).map(|id| self.def_id >= *id).unwrap_or(false)
-        });
+        let fake = self.is_fake();
         let def_id: &dyn fmt::Debug = if fake { &"**FAKE**" } else { &self.def_id };
 
         fmt.debug_struct("Item")
@@ -238,6 +236,13 @@ impl Item {
             _ => false,
         }
     }
+
+    /// See comments on next_def_id
+    pub fn is_fake(&self) -> bool {
+        MAX_DEF_ID.with(|m| {
+            m.borrow().get(&self.def_id.krate).map(|id| self.def_id >= *id).unwrap_or(false)
+        })
+    }
 }
 
 #[derive(Clone, Debug)]