Add trailing comma support

Author: dtolnay

src/algorithm.rs

@@ -16,6 +16,7 @@ pub enum Breaks {
 pub struct BreakToken {
     pub offset: isize,
     pub blank_space: usize,
+    pub trailing_comma: bool,
 }
 
 #[derive(Clone, Copy)]
@@ -255,6 +256,9 @@ impl Printer {
             self.pending_indentation += token.blank_space;
             self.space -= token.blank_space as isize;
         } else {
+            if token.trailing_comma {
+                self.out.push(',');
+            }
             self.out.push('\n');
             let indent = self.indent as isize + token.offset;
             self.pending_indentation = indent as usize;

src/convenience.rs

@@ -26,6 +26,7 @@ impl Printer {
         self.scan_break(BreakToken {
             offset: off,
             blank_space: n,
+            trailing_comma: false,
         });
     }
 
@@ -62,6 +63,15 @@ impl Printer {
         Token::Break(BreakToken {
             offset: off,
             blank_space: algorithm::SIZE_INFINITY as usize,
+            trailing_comma: false,
         })
     }
+
+    pub fn trailing_comma(&mut self) {
+        self.scan_break(BreakToken {
+            offset: 0,
+            blank_space: 0,
+            trailing_comma: true,
+        });
+    }
 }

src/data.rs

@@ -47,10 +47,14 @@ impl Printer {
         self.word("(");
         self.cbox(INDENT);
         self.zerobreak();
-        for field in &fields.unnamed {
+        for (i, field) in fields.unnamed.iter().enumerate() {
             self.field(field);
-            self.word(",");
-            self.space();
+            if i < fields.unnamed.len() - 1 {
+                self.word(",");
+                self.space();
+            } else {
+                self.trailing_comma();
+            }
         }
         self.offset(-INDENT);
         self.end();