Auto merge of #52397 - estebank:println-comma, r=oli-obk

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

Fix #49370.

1 files changed, 25 insertions, 0 deletions

diff --git a/src/libsyntax/tokenstream.rs b/src/libsyntax/tokenstream.rs
index 1a4236b280b..f84b5307a11 100644
--- a/src/libsyntax/tokenstream.rs
+++ b/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 to 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,