rustdoc: Preserve rendering of macro_rules matchers when possible

src/librustdoc/clean/utils.rs

@@ -16,6 +16,8 @@ use rustc_hir::def_id::{DefId, LOCAL_CRATE};
 use rustc_middle::mir::interpret::ConstValue;
 use rustc_middle::ty::subst::{GenericArgKind, SubstsRef};
 use rustc_middle::ty::{self, DefIdTree, TyCtxt};
+use rustc_session::parse::ParseSess;
+use rustc_span::source_map::FilePathMapping;
 use rustc_span::symbol::{kw, sym, Symbol};
 use std::fmt::Write as _;
 use std::mem;
@@ -484,21 +486,68 @@ crate const DOC_RUST_LANG_ORG_CHANNEL: &str = env!("DOC_RUST_LANG_ORG_CHANNEL");
 /// Render a sequence of macro arms in a format suitable for displaying to the user
 /// as part of an item declaration.
 pub(super) fn render_macro_arms<'a>(
+    cx: &DocContext<'_>,
     matchers: impl Iterator<Item = &'a TokenTree>,
     arm_delim: &str,
 ) -> String {
     let mut out = String::new();
     for matcher in matchers {
-        writeln!(out, "    {} => {{ ... }}{}", render_macro_matcher(matcher), arm_delim).unwrap();
+        writeln!(out, "    {} => {{ ... }}{}", render_macro_matcher(cx, matcher), arm_delim)
+            .unwrap();
     }
     out
 }
 
 /// Render a macro matcher in a format suitable for displaying to the user
 /// as part of an item declaration.
-pub(super) fn render_macro_matcher(matcher: &TokenTree) -> String {
+pub(super) fn render_macro_matcher(cx: &DocContext<'_>, matcher: &TokenTree) -> String {
+    if let Some(snippet) = snippet_equal_to_token(cx, matcher) {
+        snippet
+    } else {
         rustc_ast_pretty::pprust::tt_to_string(matcher)
     }
+}
+
+/// Find the source snippet for this token's Span, reparse it, and return the
+/// snippet if the reparsed TokenTree matches the argument TokenTree.
+fn snippet_equal_to_token(cx: &DocContext<'_>, matcher: &TokenTree) -> Option<String> {
+    // Find what rustc thinks is the source snippet.
+    // This may not actually be anything meaningful if this matcher was itself
+    // generated by a macro.
+    let source_map = cx.sess().source_map();
+    let span = matcher.span();
+    let snippet = source_map.span_to_snippet(span).ok()?;
+
+    // Create a Parser.
+    let sess = ParseSess::new(FilePathMapping::empty());
+    let file_name = source_map.span_to_filename(span);
+    let mut parser =
+        match rustc_parse::maybe_new_parser_from_source_str(&sess, file_name, snippet.clone()) {
+            Ok(parser) => parser,
+            Err(diagnostics) => {
+                for mut diagnostic in diagnostics {
+                    diagnostic.cancel();
+                }
+                return None;
+            }
+        };
+
+    // Reparse a single token tree.
+    let mut reparsed_trees = match parser.parse_all_token_trees() {
+        Ok(reparsed_trees) => reparsed_trees,
+        Err(mut diagnostic) => {
+            diagnostic.cancel();
+            return None;
+        }
+    };
+    if reparsed_trees.len() != 1 {
+        return None;
+    }
+    let reparsed_tree = reparsed_trees.pop().unwrap();
+
+    // Compare against the original tree.
+    if reparsed_tree.eq_unspanned(matcher) { Some(snippet) } else { None }
+}
 
 pub(super) fn display_macro_source(
     cx: &mut DocContext<'_>,
@@ -512,21 +561,21 @@ pub(super) fn display_macro_source(
     let matchers = tts.chunks(4).map(|arm| &arm[0]);
 
     if def.macro_rules {
-        format!("macro_rules! {} {{\n{}}}", name, render_macro_arms(matchers, ";"))
+        format!("macro_rules! {} {{\n{}}}", name, render_macro_arms(cx, matchers, ";"))
     } else {
         if matchers.len() <= 1 {
             format!(
                 "{}macro {}{} {{\n    ...\n}}",
                 vis.to_src_with_space(cx.tcx, def_id),
                 name,
-                matchers.map(render_macro_matcher).collect::<String>(),
+                matchers.map(|matcher| render_macro_matcher(cx, matcher)).collect::<String>(),
             )
         } else {
             format!(
                 "{}macro {} {{\n{}}}",
                 vis.to_src_with_space(cx.tcx, def_id),
                 name,
-                render_macro_arms(matchers, ","),
+                render_macro_arms(cx, matchers, ","),
             )
         }
     }