Add warn level lint redundant_explicit_links

- Currently it will panic due to the resolution's caching issue

src/doc/rustdoc/src/lints.md

@@ -412,3 +412,25 @@ help: if you meant to use a literal backtick, escape it
 
 warning: 1 warning emitted
 ```
+
+## `redundant_explicit_links`
+
+This lint is **warned by default**. It detects explicit links that are same
+as computed automatic links.
+This usually means the explicit links is removeable. For example:
+
+```rust
+#![warn(rustdoc::redundant_explicit_links)]
+
+pub fn dummy_target() {} // note: unnecessary - warns by default.
+
+/// [`dummy_target`](dummy_target)
+/// [dummy_target](dummy_target)
+pub fn c() {}
+```
+
+Which will give:
+
+```text
+
+```

src/librustdoc/lint.rs

@@ -185,6 +185,17 @@ macro_rules! declare_rustdoc_lint {
    "detects unescaped backticks in doc comments"
 }
 
+declare_rustdoc_lint! {
+    /// This lint is **warned by default**. It detects explicit links that are same
+    /// as computed automatic links. This usually means the explicit links is removeable.
+    /// This is a `rustdoc` only lint, see the documentation in the [rustdoc book].
+    ///
+    /// [rustdoc book]: ../../../rustdoc/lints.html#redundant_explicit_links
+    REDUNDANT_EXPLICIT_LINKS,
+    Warn,
+    "detects redundant explicit links in doc comments"
+}
+
 pub(crate) static RUSTDOC_LINTS: Lazy<Vec<&'static Lint>> = Lazy::new(|| {
     vec![
         BROKEN_INTRA_DOC_LINKS,
@@ -197,6 +208,7 @@ macro_rules! declare_rustdoc_lint {
         BARE_URLS,
         MISSING_CRATE_LEVEL_DOCS,
         UNESCAPED_BACKTICKS,
+        REDUNDANT_EXPLICIT_LINKS,
     ]
 });
 

src/librustdoc/passes/collect_intra_doc_links.rs

@@ -994,7 +994,15 @@ fn resolve_links(&mut self, item: &Item) {
                 _ => find_nearest_parent_module(self.cx.tcx, item_id).unwrap(),
             };
             for md_link in preprocessed_markdown_links(&doc) {
-                let link = self.resolve_link(item, item_id, module_id, &doc, &md_link);
+                let PreprocessedMarkdownLink(_pp_link, ori_link) = &md_link;
+                let diag_info = DiagnosticInfo {
+                    item,
+                    dox: &doc,
+                    ori_link: &ori_link.link,
+                    link_range: ori_link.range.clone(),
+                };
+
+                let link = self.resolve_link(item, item_id, module_id, &md_link, &diag_info);
                 if let Some(link) = link {
                     self.cx.cache.intra_doc_links.entry(item.item_id).or_default().insert(link);
                 }
@@ -1010,19 +1018,11 @@ fn resolve_link(
         item: &Item,
         item_id: DefId,
         module_id: DefId,
-        dox: &str,
-        link: &PreprocessedMarkdownLink,
+        PreprocessedMarkdownLink(pp_link, ori_link): &PreprocessedMarkdownLink,
+        diag_info: &DiagnosticInfo<'_>,
     ) -> Option<ItemLink> {
-        let PreprocessedMarkdownLink(pp_link, ori_link) = link;
         trace!("considering link '{}'", ori_link.link);
 
-        let diag_info = DiagnosticInfo {
-            item,
-            dox,
-            ori_link: &ori_link.link,
-            link_range: ori_link.range.clone(),
-        };
-
         let PreprocessingInfo { path_str, disambiguator, extra_fragment, link_text } =
             pp_link.as_ref().map_err(|err| err.report(self.cx, diag_info.clone())).ok()?;
         let disambiguator = *disambiguator;
@@ -1042,6 +1042,17 @@ fn resolve_link(
             matches!(ori_link.kind, LinkType::Reference | LinkType::Shortcut),
         )?;
 
+        self.check_redundant_explicit_link(
+            &res,
+            path_str,
+            item_id,
+            module_id,
+            disambiguator,
+            &ori_link,
+            extra_fragment,
+            &diag_info,
+        );
+
         // Check for a primitive which might conflict with a module
         // Report the ambiguity and require that the user specify which one they meant.
         // FIXME: could there ever be a primitive not in the type namespace?
@@ -1372,6 +1383,74 @@ fn resolve_with_disambiguator(
             }
         }
     }
+
+    fn check_redundant_explicit_link(
+        &mut self,
+        ex_res: &Res,
+        ex: &Box<str>,
+        item_id: DefId,
+        module_id: DefId,
+        dis: Option<Disambiguator>,
+        ori_link: &MarkdownLink,
+        extra_fragment: &Option<String>,
+        diag_info: &DiagnosticInfo<'_>,
+    ) {
+        // Check if explicit resolution's path is same as resolution of original link's display text path, e.g.
+        // [target](target)
+        // [`target`](target)
+        // [target](path::to::target)
+        // [`target`](path::to::target)
+        // [path::to::target](path::to::target)
+        // [`path::to::target`](path::to::target)
+        //
+        // To avoid disambiguator from panicking, we check if display text path is possible to be disambiguated
+        // into explicit path.
+        if ori_link.kind != LinkType::Inline {
+            return;
+        }
+
+        let di_text = &ori_link.display_text;
+        let di_len = di_text.len();
+        let ex_len = ex.len();
+
+        let intra_doc_links = std::mem::take(&mut self.cx.cache.intra_doc_links);
+
+        if ex_len >= di_len && &ex[(ex_len - di_len)..] == di_text {
+            let Some((di_res, _)) = self.resolve_with_disambiguator_cached(
+                ResolutionInfo {
+                    item_id,
+                    module_id,
+                    dis,
+                    path_str: di_text.clone().into_boxed_str(),
+                    extra_fragment: extra_fragment.clone(),
+                },
+                diag_info.clone(), // this struct should really be Copy, but Range is not :(
+                // For reference-style links we want to report only one error so unsuccessful
+                // resolutions are cached, for other links we want to report an error every
+                // time so they are not cached.
+                matches!(ori_link.kind, LinkType::Reference | LinkType::Shortcut),
+            ) else {
+                return;
+            };
+
+            if &di_res == ex_res {
+                use crate::lint::REDUNDANT_EXPLICIT_LINKS;
+
+                report_diagnostic(
+                    self.cx.tcx,
+                    REDUNDANT_EXPLICIT_LINKS,
+                    "redundant explicit rustdoc link",
+                    &diag_info,
+                    |diag, sp, _link_range| {
+                        if let Some(sp) = sp {
+                            diag.note("Explicit link does not affect the original link")
+                                .span_suggestion(sp, "Remove explicit link instead", format!("[{}]", ori_link.link), Applicability::MachineApplicable);
+                        }
+                    }
+                );
+            }
+        }
+    }
 }
 
 /// Get the section of a link between the backticks,

tests/rustdoc-ui/lints/redundant_explicit_links.rs

@@ -0,0 +1,6 @@
+#![deny(rustdoc::redundant_explicit_links)]
+
+pub fn dummy_target() {}
+
+/// [`Vec`](std::vec::Vec)
+pub fn c() {}