refactor(cheddar): Extract code block highlighting into function

Since I am going down the path of adding additional Markdown
extensions it makes sense to avoid letting `format_markdown` turn into
a giant beast of a function.

Therefore this commit extracts the logic for rendering code blocks via
syntect and changes the innards of `format_markdown` to instead
provide arbitrary AST value replacements.

tools/cheddar/src/main.rs

@@ -1,4 +1,4 @@
-use comrak::nodes::{AstNode, NodeValue, NodeHtmlBlock};
+use comrak::nodes::{Ast, AstNode, NodeValue, NodeCodeBlock, NodeHtmlBlock};
 use comrak::{Arena, parse_document, format_html, ComrakOptions};
 use lazy_static::lazy_static;
 use std::env;
@@ -80,27 +80,14 @@ fn iter_nodes<'a, F>(node: &'a AstNode<'a>, f: &F) where F : Fn(&'a AstNode<'a>)
 // Instead, try finding a syntax match by comparing case insensitively (for
 // ASCII characters, anyways).
 fn find_syntax_case_insensitive(info: &str) -> Option<&'static SyntaxReference> {
+    // TODO(tazjin): memoize this lookup
     SYNTAXES.syntaxes().iter().rev().find(|&s| info.eq_ignore_ascii_case(&s.name))
 }
 
-fn format_markdown() {
+// Replaces code-block inside of a Markdown AST with HTML blocks rendered by
-    let document = {
+// syntect. This enables static (i.e. no JavaScript) syntax highlighting, even
-        let mut buffer = String::new();
+// of complex languages.
-        let stdin = io::stdin();
+fn highlight_code_block(code_block: &NodeCodeBlock) -> NodeValue {
-        let mut stdin = stdin.lock();
-        stdin.read_to_string(&mut buffer).expect("failed to read stdin");
-        buffer
-    };
-
-    let arena = Arena::new();
-    let root = parse_document(&arena, &document, &MD_OPTS);
-
-    // Syntax highlighting is implemented by traversing the arena and
-    // replacing all code blocks with HTML blocks rendered by syntect.
-    iter_nodes(root, &|node| {
-        let mut ast = node.data.borrow_mut();
-        match &ast.value {
-            NodeValue::CodeBlock(code_block) => {
     let theme = &THEMES.themes["InspiredGitHub"];
     let info = String::from_utf8_lossy(&code_block.info);
 
@@ -132,10 +119,33 @@ fn format_markdown() {
         literal: rendered.into_bytes(),
     };
 
-                ast.value = NodeValue::HtmlBlock(block);
+    NodeValue::HtmlBlock(block)
-            },
+}
-            _ => (),
+
+fn format_markdown() {
+    let document = {
+        let mut buffer = String::new();
+        let stdin = io::stdin();
+        let mut stdin = stdin.lock();
+        stdin.read_to_string(&mut buffer).expect("failed to read stdin");
+        buffer
     };
+
+    let arena = Arena::new();
+    let root = parse_document(&arena, &document, &MD_OPTS);
+
+    // Syntax highlighting is implemented by traversing the arena and
+    // replacing all code blocks with HTML blocks rendered by syntect.
+    iter_nodes(root, &|node| {
+        let mut ast = node.data.borrow_mut();
+        let new = match &ast.value {
+            NodeValue::CodeBlock(code) => Some(highlight_code_block(code)),
+            _ => None,
+        };
+
+        if let Some(new_value) = new {
+            ast.value = new_value
+        }
     });
 
     format_html(root, &MD_OPTS, &mut io::stdout())