Always set result during finish() in debug builders

library/core/src/fmt/builders.rs

@@ -402,6 +402,7 @@ impl<'a, 'b: 'a> DebugTuple<'a, 'b> {
  }
 }
 
+/// A helper used to print list-like items with no special formatting.
 struct DebugInner<'a, 'b: 'a> {
  fmt: &'a mut fmt::Formatter<'b>,
  result: fmt::Result,
@@ -578,7 +579,8 @@ impl<'a, 'b: 'a> DebugSet<'a, 'b> {
  /// ```
  #[stable(feature = "debug_builders", since = "1.2.0")]
  pub fn finish(&mut self) -> fmt::Result {
- self.inner.result.and_then(|_| self.inner.fmt.write_str("}"))
+ self.inner.result = self.inner.result.and_then(|_| self.inner.fmt.write_str("}"));
+ self.inner.result
  }
 }
 
@@ -721,7 +723,8 @@ impl<'a, 'b: 'a> DebugList<'a, 'b> {
  /// ```
  #[stable(feature = "debug_builders", since = "1.2.0")]
  pub fn finish(&mut self) -> fmt::Result {
- self.inner.result.and_then(|_| self.inner.fmt.write_str("]"))
+ self.inner.result = self.inner.result.and_then(|_| self.inner.fmt.write_str("]"));
+ self.inner.result
  }
 }
 
@@ -1002,11 +1005,12 @@ impl<'a, 'b: 'a> DebugMap<'a, 'b> {
  /// ```
  #[stable(feature = "debug_builders", since = "1.2.0")]
  pub fn finish(&mut self) -> fmt::Result {
- self.result.and_then(|_| {
+ self.result = self.result.and_then(|_| {
  assert!(!self.has_key, "attempted to finish a map with a partial entry");
 
  self.fmt.write_str("}")
- })
+ });
+ self.result
  }
 
  fn is_pretty(&self) -> bool {