Rollup merge of #42093 - llogiq:clippy-librustc_errors, r=arielb1

fix some clippy warnings in librustc_errors

Again, those are small changes to improve readability.

Author: Mark-Simulacrum

src/librustc_errors/diagnostic_builder.rs

@@ -192,8 +192,8 @@ impl<'a> Debug for DiagnosticBuilder<'a> {
     }
 }
 
-/// Destructor bomb - a DiagnosticBuilder must be either emitted or cancelled or
-/// we emit a bug.
+/// Destructor bomb - a `DiagnosticBuilder` must be either emitted or cancelled
+/// or we emit a bug.
 impl<'a> Drop for DiagnosticBuilder<'a> {
     fn drop(&mut self) {
         if !panicking() && !self.cancelled() {

src/librustc_errors/emitter.rs

@@ -1296,10 +1296,8 @@ impl Write for BufferedWriter {
     }
     fn flush(&mut self) -> io::Result<()> {
         let mut stderr = io::stderr();
-        let result = (|| {
-            stderr.write_all(&self.buffer)?;
-            stderr.flush()
-        })();
+        let result = stderr.write_all(&self.buffer)
+                           .and_then(|_| stderr.flush());
         self.buffer.clear();
         result
     }

src/librustc_errors/lib.rs

@@ -383,7 +383,7 @@ impl Handler {
     pub fn span_fatal<S: Into<MultiSpan>>(&self, sp: S, msg: &str) -> FatalError {
         self.emit(&sp.into(), msg, Fatal);
         self.panic_if_treat_err_as_bug();
-        return FatalError;
+        FatalError
     }
     pub fn span_fatal_with_code<S: Into<MultiSpan>>(&self,
                                                     sp: S,
@@ -392,7 +392,7 @@ impl Handler {
                                                     -> FatalError {
         self.emit_with_code(&sp.into(), msg, code, Fatal);
         self.panic_if_treat_err_as_bug();
-        return FatalError;
+        FatalError
     }
     pub fn span_err<S: Into<MultiSpan>>(&self, sp: S, msg: &str) {
         self.emit(&sp.into(), msg, Error);