keep indent state in fmt Buf

compiler/fmt/src/lib.rs

@@ -8,3 +8,53 @@ pub mod expr;
 pub mod module;
 pub mod pattern;
 pub mod spaces;
+
+use bumpalo::{collections::String, Bump};
+
+pub struct Buf<'a> {
+    text: String<'a>,
+    beginning_of_line: bool,
+}
+
+impl<'a> Buf<'a> {
+    pub fn new_in(arena: &'a Bump) -> Buf<'a> {
+        Buf {
+            text: String::new_in(arena),
+            beginning_of_line: true,
+        }
+    }
+
+    pub fn as_str(&'a self) -> &'a str {
+        self.text.as_str()
+    }
+
+    pub fn into_bump_str(self) -> &'a str {
+        self.text.into_bump_str()
+    }
+
+    pub fn indent(&mut self, indent: u16) {
+        if self.beginning_of_line {
+            for _ in 0..indent {
+                self.text.push(' ');
+            }
+        }
+        self.beginning_of_line = false;
+    }
+
+    pub fn push(&mut self, ch: char) {
+        debug_assert!(!self.beginning_of_line);
+        debug_assert!(ch != '\n');
+        self.text.push(ch);
+    }
+
+    pub fn push_str(&mut self, s: &str) {
+        debug_assert!(!self.beginning_of_line);
+        debug_assert!(!s.contains('\n'));
+        self.text.push_str(s);
+    }
+
+    pub fn newline(&mut self) {
+        self.text.push('\n');
+        self.beginning_of_line = true;
+    }
+}