Add suggestion of similar macro names to macro undefined error message

src/libsyntax/ext/base.rs

@@ -24,6 +24,7 @@ use parse::token;
 use parse::token::{InternedString, intern, str_to_ident};
 use ptr::P;
 use util::small_vector::SmallVector;
+use util::lev_distance::lev_distance;
 use ext::mtwt;
 use fold::Folder;
 
@@ -776,6 +777,22 @@ impl<'a> ExtCtxt<'a> {
     pub fn name_of(&self, st: &str) -> ast::Name {
         token::intern(st)
     }
+
+    pub fn suggest_macro_name(&mut self, name: &str, span: Span) {
+        use std::cmp::max;
+
+        let mut min: Option<(Name, usize)> = None;
+        let max_dist = max(name.len() / 3, 1);
+        for macro_name in self.syntax_env.names.iter() {
+            let dist = lev_distance(name, &macro_name.as_str());
+            if dist <= max_dist && (min.is_none() || min.unwrap().1 > dist) {
+                min = Some((*macro_name, dist));
+            }
+        }
+        if let Some((suggestion, _)) = min {
+            self.span_help(span, &format!("did you mean `{}`?", suggestion));
+        }
+    }
 }
 
 /// Extract a string literal from the macro expanded version of `expr`,

src/libsyntax/ext/expand.rs

@@ -191,6 +191,7 @@ fn expand_mac_invoc<T, F, G>(mac: ast::Mac,
                 pth.span,
                 &format!("macro undefined: '{}!'",
                         &extname));
+            fld.cx.suggest_macro_name(&extname.as_str(), pth.span);
 
             // let compilation continue
             None