Add new unused_footnote_definition rustdoc lint

Author: GuillaumeGomez

src/librustdoc/lint.rs

@@ -211,6 +211,13 @@ declare_rustdoc_lint! {
     "footnote reference with no associated definition"
 }
 
+declare_rustdoc_lint! {
+    /// This lint checks if all footnote definitions are used.
+    UNUSED_FOOTNOTE_DEFINITION,
+    Warn,
+    "unused footnote definition"
+}
+
 pub(crate) static RUSTDOC_LINTS: Lazy<Vec<&'static Lint>> = Lazy::new(|| {
     vec![
         BROKEN_INTRA_DOC_LINKS,
@@ -226,6 +233,7 @@ pub(crate) static RUSTDOC_LINTS: Lazy<Vec<&'static Lint>> = Lazy::new(|| {
         REDUNDANT_EXPLICIT_LINKS,
         UNPORTABLE_MARKDOWN,
         BROKEN_FOOTNOTE,
+        UNUSED_FOOTNOTE_DEFINITION,
     ]
 });
 

src/librustdoc/passes/lint/unportable_markdown.rs

@@ -12,6 +12,7 @@
 
 use std::collections::{BTreeMap, BTreeSet};
 
+use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_hir::HirId;
 use rustc_lint_defs::Applicability;
 use rustc_resolve::rustdoc::source_span_for_markdown_range;
@@ -39,6 +40,8 @@ pub(crate) fn visit_item(cx: &DocContext<'_>, item: &Item, hir_id: HirId, dox: &
     // the new parser and old parser.
     let mut missing_footnote_references = BTreeMap::new();
     let mut found_footnote_references = BTreeSet::new();
+    let mut footnote_references = FxHashSet::default();
+    let mut footnote_definitions = FxHashMap::default();
 
     // populate problem cases from new parser
     {
@@ -51,13 +54,20 @@ pub(crate) fn visit_item(cx: &DocContext<'_>, item: &Item, hir_id: HirId, dox: &
         }
         let parser_new = cmarkn::Parser::new_ext(dox, main_body_opts_new()).into_offset_iter();
         for (event, span) in parser_new {
-            if let cmarkn::Event::Start(cmarkn::Tag::BlockQuote(_)) = event {
-                if !dox[span.clone()].starts_with("> ") {
-                    spaceless_block_quotes.insert(span.start);
+            match event {
+                cmarkn::Event::Start(cmarkn::Tag::BlockQuote(_)) => {
+                    if !dox[span.clone()].starts_with("> ") {
+                        spaceless_block_quotes.insert(span.start);
+                    }
                 }
-            }
-            if let cmarkn::Event::FootnoteReference(_) = event {
-                found_footnote_references.insert(span.start + 1);
+                cmarkn::Event::FootnoteReference(label) => {
+                    found_footnote_references.insert(span.start + 1);
+                    footnote_references.insert(label);
+                }
+                cmarkn::Event::Start(cmarkn::Tag::FootnoteDefinition(label)) => {
+                    footnote_definitions.insert(label, span.start + 1);
+                }
+                _ => {}
             }
         }
     }
@@ -86,6 +96,23 @@ pub(crate) fn visit_item(cx: &DocContext<'_>, item: &Item, hir_id: HirId, dox: &
         }
     }
 
+    #[allow(rustc::potential_query_instability)]
+    for (footnote, span) in footnote_definitions {
+        if !footnote_references.contains(&footnote) {
+            let span = source_span_for_markdown_range(
+                tcx,
+                dox,
+                &(span..span + 1),
+                &item.attrs.doc_strings,
+            )
+            .unwrap_or_else(|| item.attr_span(tcx));
+
+            tcx.node_span_lint(crate::lint::UNUSED_FOOTNOTE_DEFINITION, hir_id, span, |lint| {
+                lint.primary_message("unused footnote definition");
+            });
+        }
+    }
+
     for start in spaceless_block_quotes {
         let (span, precise) =
             source_span_for_markdown_range(tcx, dox, &(start..start + 1), &item.attrs.doc_strings)