rustdoc: improve refdef handling in the unresolved link lint

This commit takes advantage of a feature in pulldown-cmark that
makes the list of link definitions available to the consuming
application. It produces unresolved link warnings for refdefs
that aren't used, and can now produce exact spans for the dest
even when it has escapes.

compiler/rustc_resolve/src/rustdoc.rs

@@ -7,7 +7,7 @@ use pulldown_cmark::{
 use rustc_ast as ast;
 use rustc_ast::attr::AttributeExt;
 use rustc_ast::util::comments::beautify_doc_string;
-use rustc_data_structures::fx::FxIndexMap;
+use rustc_data_structures::fx::{FxHashSet, FxIndexMap};
 use rustc_middle::ty::TyCtxt;
 use rustc_span::def_id::DefId;
 use rustc_span::{DUMMY_SP, InnerSpan, Span, Symbol, kw, sym};
@@ -422,9 +422,11 @@ fn parse_links<'md>(doc: &'md str) -> Vec<Box<str>> {
     );
     let mut links = Vec::new();
 
+    let mut refids = FxHashSet::default();
+
     while let Some(event) = event_iter.next() {
         match event {
-            Event::Start(Tag::Link { link_type, dest_url, title: _, id: _ })
+            Event::Start(Tag::Link { link_type, dest_url, title: _, id })
                 if may_be_doc_link(link_type) =>
             {
                 if matches!(
@@ -439,6 +441,12 @@ fn parse_links<'md>(doc: &'md str) -> Vec<Box<str>> {
                         links.push(display_text);
                     }
                 }
+                if matches!(
+                    link_type,
+                    LinkType::Reference | LinkType::Shortcut | LinkType::Collapsed
+                ) {
+                    refids.insert(id);
+                }
 
                 links.push(preprocess_link(&dest_url));
             }
@@ -446,6 +454,12 @@ fn parse_links<'md>(doc: &'md str) -> Vec<Box<str>> {
         }
     }
 
+    for (label, refdef) in event_iter.reference_definitions().iter() {
+        if !refids.contains(label) {
+            links.push(preprocess_link(&refdef.dest));
+        }
+    }
+
     links
 }