Add trailing comma support

compiler/rustc_ast_pretty/src/pp.rs

@@ -148,7 +148,7 @@ pub enum Breaks {
     Inconsistent,
 }
 
-#[derive(Clone, Copy)]
+#[derive(Clone, Copy, PartialEq)]
 enum IndentStyle {
     /// Vertically aligned under whatever column this block begins at.
     ///
@@ -164,19 +164,20 @@ enum IndentStyle {
     Block { offset: isize },
 }
 
-#[derive(Clone, Copy)]
+#[derive(Clone, Copy, Default, PartialEq)]
 pub struct BreakToken {
     offset: isize,
     blank_space: isize,
+    pre_break: Option<char>,
 }
 
-#[derive(Clone, Copy)]
+#[derive(Clone, Copy, PartialEq)]
 pub struct BeginToken {
     indent: IndentStyle,
     breaks: Breaks,
 }
 
-#[derive(Clone)]
+#[derive(Clone, PartialEq)]
 pub enum Token {
     // In practice a string token contains either a `&'static str` or a
     // `String`. `Cow` is overkill for this because we never modify the data,
@@ -415,6 +416,9 @@ impl Printer {
             self.pending_indentation += token.blank_space;
             self.space -= token.blank_space;
         } else {
+            if let Some(pre_break) = token.pre_break {
+                self.out.push(pre_break);
+            }
             self.out.push('\n');
             let indent = self.indent as isize + token.offset;
             self.pending_indentation = indent;

compiler/rustc_ast_pretty/src/pp/convenience.rs

@@ -25,7 +25,11 @@ impl Printer {
     }
 
     pub fn break_offset(&mut self, n: usize, off: isize) {
-        self.scan_break(BreakToken { offset: off, blank_space: n as isize })
+        self.scan_break(BreakToken {
+            offset: off,
+            blank_space: n as isize,
+            ..BreakToken::default()
+        });
     }
 
     pub fn end(&mut self) {
@@ -66,12 +70,24 @@ impl Printer {
     }
 
     pub fn hardbreak_tok_offset(off: isize) -> Token {
-        Token::Break(BreakToken { offset: off, blank_space: SIZE_INFINITY })
+        Token::Break(BreakToken {
+            offset: off,
+            blank_space: SIZE_INFINITY,
+            ..BreakToken::default()
+        })
+    }
+
+    pub fn trailing_comma(&mut self) {
+        self.scan_break(BreakToken {
+            blank_space: 1,
+            pre_break: Some(','),
+            ..BreakToken::default()
+        });
     }
 }
 
 impl Token {
     pub fn is_hardbreak_tok(&self) -> bool {
-        matches!(self, Token::Break(BreakToken { offset: 0, blank_space: SIZE_INFINITY }))
+        *self == Printer::hardbreak_tok_offset(0)
     }
 }