Improve metadata code block parsing

Author: camsteffen

clippy_lints/src/utils/internal_lints/metadata_collector.rs

@@ -9,6 +9,7 @@
 //! a simple mistake)
 
 use if_chain::if_chain;
+use rustc_ast as ast;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_hir::{
     self as hir, def::DefKind, intravisit, intravisit::Visitor, ExprKind, Item, ItemKind, Mutability, QPath,
@@ -485,16 +486,44 @@ fn extract_attr_docs_or_lint(cx: &LateContext<'_>, item: &Item<'_>) -> Option<St
 ///
 /// Would result in `Hello world!\n=^.^=\n`
 fn extract_attr_docs(cx: &LateContext<'_>, item: &Item<'_>) -> Option<String> {
-    cx.tcx
+    let mut lines = cx
+        .tcx
         .hir()
         .attrs(item.hir_id())
         .iter()
-        .filter_map(|x| x.doc_str().map(|sym| sym.as_str().to_string()))
-        .reduce(|mut acc, sym| {
-            acc.push_str(&sym);
-            acc.push('\n');
-            acc
-        })
+        .filter_map(ast::Attribute::doc_str);
+    let mut docs = String::from(&*lines.next()?.as_str());
+    let mut in_code_block = false;
+    for line in lines {
+        docs.push('\n');
+        let line = line.as_str();
+        let line = &*line;
+        if let Some(info) = line.trim_start().strip_prefix("```") {
+            in_code_block = !in_code_block;
+            if in_code_block {
+                let mut parts = info
+                    .trim()
+                    .split(',')
+                    // remove rustdoc directives
+                    .filter(|&s| !matches!(s, "" | "ignore" | "no_run" | "should_panic"));
+                docs.push_str("```");
+                match parts.next() {
+                    // if no language is present, fill in "rust"
+                    None => docs.push_str("rust"),
+                    Some(first) => {
+                        docs.push_str(first);
+                        for part in parts {
+                            docs.push(',');
+                            docs.push_str(part);
+                        }
+                    },
+                }
+                continue;
+            }
+        }
+        docs.push_str(line);
+    }
+    Some(docs)
 }
 
 fn get_lint_group_and_level_or_lint(