Suggest comma when writing println!("{}" a);

src/libsyntax/ext/tt/macro_rules.rs

@@ -174,7 +174,32 @@ fn generic_extension<'cx>(cx: &'cx mut ExtCtxt,
     }
 
     let best_fail_msg = parse_failure_msg(best_fail_tok.expect("ran no matchers"));
-    cx.span_err(best_fail_spot.substitute_dummy(sp), &best_fail_msg);
+    let mut err = cx.struct_span_err(best_fail_spot.substitute_dummy(sp), &best_fail_msg);
+
+    // Check whether there's a missing comma in this macro call, like `println!("{}" a);`
+    if let Some((arg, comma_span)) = arg.add_comma() {
+        for lhs in lhses { // try each arm's matchers
+            let lhs_tt = match *lhs {
+                quoted::TokenTree::Delimited(_, ref delim) => &delim.tts[..],
+                _ => cx.span_bug(sp, "malformed macro lhs")
+            };
+            match TokenTree::parse(cx, lhs_tt, arg.clone()) {
+                Success(_) => {
+                    if comma_span == DUMMY_SP {
+                        err.note("you might be missing a comma");
+                    } else {
+                        err.span_suggestion_short(
+                            comma_span,
+                            "missing comma here",
+                            ",".to_string(),
+                        );
+                    }
+                }
+                _ => {}
+            }
+        }
+    }
+    err.emit();
     cx.trace_macros_diag();
     DummyResult::any(sp)
 }

src/libsyntax/tokenstream.rs

@@ -182,6 +182,31 @@ pub struct TokenStream {
     kind: TokenStreamKind,
 }
 
+impl TokenStream {
+    /// Given a `TokenStream` with a `Stream` of only two arguments, return a new `TokenStream`
+    /// separating the two arguments with a comma for diagnostic suggestions.
+    pub(crate) fn add_comma(&self) -> Option<(TokenStream, Span)> {
+        // Used ot suggest if a user writes `println!("{}" a);`
+        if let TokenStreamKind::Stream(ref slice) = self.kind {
+            if slice.len() == 2 {
+                let comma_span = match slice[0] {
+                    TokenStream { kind: TokenStreamKind::Tree(TokenTree::Token(sp, _)) } |
+                    TokenStream { kind: TokenStreamKind::Tree(TokenTree::Delimited(sp, _)) } => {
+                        sp.shrink_to_hi()
+                    }
+                    _ => DUMMY_SP,
+                };
+                let comma = TokenStream {
+                    kind: TokenStreamKind::Tree(TokenTree::Token(comma_span, token::Comma)),
+                };
+                let slice = RcSlice::new(vec![slice[0].clone(), comma, slice[1].clone()]);
+                return Some((TokenStream { kind: TokenStreamKind::Stream(slice) }, comma_span));
+            }
+        }
+        None
+    }
+}
+
 #[derive(Clone, Debug)]
 enum TokenStreamKind {
     Empty,

src/test/ui/macros/missing-comma.rs

@@ -0,0 +1,14 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+fn main() {
+    println!("{}" a);
+    //~^ ERROR no rules expected the token `a`
+}

src/test/ui/macros/missing-comma.stderr

@@ -0,0 +1,10 @@
+error: no rules expected the token `a`
+  --> $DIR/missing-comma.rs:12:19
+   |
+LL |     println!("{}" a);
+   |                  -^
+   |                  |
+   |                  help: missing comma here
+
+error: aborting due to previous error
+