Resolve grammar rules in link reference definitions

Currently, in our Markdown, we support `[text][RULE_NAME]` and
`[text][grammar-RULE_NAME]` for linking to grammar rules, but
we don't support this syntax within link reference definitions,
i.e., `[text]: grammar-RULE_NAME`, even though we do support
linking to (non-grammar) rule identifiers within link reference
definitions.  That's an inconsistency that continually surprises
us.  Let's fix that.

In this commit, we add `grammar_link_references`, which scans
link reference definitions for destinations that match a grammar
rule name -- either with a `grammar-` prefix or not.  When a match
is found, the destination is replaced with the resolved path and
anchor, just as `rule_link_references` does for rules.  Unrecognized
destinations pass through unchanged, falling through to `std_links`
for rustdoc resolution -- the same behavior as unresolved
`[text][NAME]` reference links.

We also update the dev-guide to document the new feature in both
`links.md` and `grammar.md`.

Author: traviscross

dev-guide/src/grammar.md

@@ -154,5 +154,19 @@ The [`mdbook-spec`] plugin automatically adds Markdown link definitions for all
 
 In some cases, there might be name collisions with the automatic linking of rule names. In that case, disambiguate with the `grammar-` prefix, such as `[Type][grammar-Type]`. The prefix can also be used when explicitness would aid clarity.
 
+Production names can also be used in link reference definitions to provide custom link text, both with and without the `grammar-` prefix.
+
+```markdown
+We accept any [type].
+
+[type]: grammar-Type
+```
+
+```markdown
+We accept any [type].
+
+[type]: Type
+```
+
 [`mdbook-spec`]: tooling/mdbook-spec.md
 [Notation]: https://doc.rust-lang.org/nightly/reference/notation.html

dev-guide/src/links.md

@@ -74,6 +74,10 @@ Link definitions are automatically generated for all grammar production names. S
 This attribute uses the [MetaWord] syntax.
 
 Explicit grammar links can have the `grammar-` prefix like [Type][grammar-Type].
+
+Grammar links can also appear in link reference definitions, e.g. [type].
+
+[type]: grammar-Type
 ```
 
 ## Outside book links

tools/mdbook-spec/src/grammar.rs

@@ -75,6 +75,47 @@ pub fn insert_grammar(grammar: &Grammar, chapter: &Chapter, diag: &mut Diagnosti
     content
 }
 
+/// Converts link reference definitions that point to a grammar rule
+/// to the correct link.
+///
+/// For example:
+///
+/// ```markdown
+/// We accept any [token].
+///
+/// [token]: grammar-Token
+/// ```
+///
+/// This will convert the `[token]` definition to point
+/// to the actual link.
+///
+/// This supports both a `grammar-` prefixed form (e.g.
+/// `grammar-Token`) and a bare rule name (e.g. `Token`).
+pub fn grammar_link_references(chapter: &Chapter, grammar: &Grammar) -> String {
+    let current_path = chapter.path.as_ref().unwrap().parent().unwrap();
+    let for_summary = is_summary(chapter);
+    crate::MD_LINK_REFERENCE_DEFINITION
+        .replace_all(&chapter.content, |caps: &Captures<'_>| {
+            let dest = &caps["dest"];
+            let name = dest.strip_prefix("grammar-").unwrap_or(dest);
+            if let Some(production) = grammar.productions.get(name) {
+                let label = &caps["label"];
+                let relative = pathdiff::diff_paths(&production.path, current_path).unwrap();
+                // Adjust paths for Windows.
+                let relative = relative.display().to_string().replace('\\', "/");
+                let id = render_markdown::markdown_id(name, for_summary);
+                if for_summary {
+                    format!("[{label}]: #{id}")
+                } else {
+                    format!("[{label}]: {relative}#{id}")
+                }
+            } else {
+                caps.get(0).unwrap().as_str().to_string()
+            }
+        })
+        .to_string()
+}
+
 /// Creates a map of production name -> relative link path.
 fn make_relative_link_map(grammar: &Grammar, chapter: &Chapter) -> HashMap<String, String> {
     let current_path = chapter.path.as_ref().unwrap().parent().unwrap();

tools/mdbook-spec/src/lib.rs

@@ -168,6 +168,7 @@ impl Preprocessor for Spec {
             }
             ch.content = admonitions::admonitions(&ch, &mut diag);
             ch.content = self.rule_link_references(&ch, &rules);
+            ch.content = grammar::grammar_link_references(&ch, &grammar);
             ch.content = self.auto_link_references(&ch, &rules);
             ch.content = self.render_rule_definitions(&ch.content, &tests, &git_ref);
             if ch.name == "Test summary" {