Resolve some minor parsing bugs

src/formatting/syntux/parser.rs

@@ -66,8 +66,11 @@ impl<'a> ParserBuilder<'a> {
         let parser = match Self::parser(sess.inner(), input) {
             Ok(p) => p,
             Err(db) => {
-                sess.emit_diagnostics(db);
-                return Err(ParserError::ParserCreationError);
+                if let Some(diagnostics) = db {
+                    sess.emit_diagnostics(diagnostics);
+                    return Err(ParserError::ParserCreationError);
+                }
+                return Err(ParserError::ParsePanicError);
             }
         };
 
@@ -77,14 +80,18 @@ impl<'a> ParserBuilder<'a> {
     fn parser(
         sess: &'a rustc_session::parse::ParseSess,
         input: Input,
-    ) -> Result<rustc_parse::parser::Parser<'a>, Vec<Diagnostic>> {
+    ) -> Result<rustc_parse::parser::Parser<'a>, Option<Vec<Diagnostic>>> {
         match input {
-            Input::File(ref file) => Ok(new_parser_from_file(sess, file, None)),
+            Input::File(ref file) => catch_unwind(AssertUnwindSafe(move || {
+                new_parser_from_file(sess, file, None)
+            }))
+            .map_err(|_| None),
             Input::Text(text) => rustc_parse::maybe_new_parser_from_source_str(
                 sess,
                 rustc_span::FileName::Custom("stdin".to_owned()),
                 text,
-            ),
+            )
+            .map_err(|db| Some(db)),
         }
     }
 }
@@ -121,7 +128,7 @@ impl<'a> Parser<'a> {
             match parser.parse_mod(&TokenKind::Eof, ast::Unsafe::No) {
                 Ok(result) => Some(result),
                 Err(mut e) => {
-                    e.cancel();
+                    sess.emit_or_cancel_diagnostic(&mut e);
                     if sess.can_reset_errors() {
                         sess.reset_errors();
                     }

src/formatting/syntux/session.rs

@@ -219,6 +219,17 @@ impl ParseSess {
         }
     }
 
+    pub(crate) fn emit_or_cancel_diagnostic(&self, diagnostic: &mut Diagnostic) {
+        self.parse_sess.span_diagnostic.emit_diagnostic(diagnostic);
+        // The Handler will check whether the diagnostic should be emitted
+        // based on the user's rustfmt configuration and the originating file
+        // that caused the parser error. If the Handler determined it should skip
+        // emission then we need to ensure the diagnostic is cancelled.
+        if !diagnostic.cancelled() {
+            diagnostic.cancel();
+        }
+    }
+
     pub(super) fn can_reset_errors(&self) -> bool {
         *self.can_reset_errors.borrow()
     }

src/result.rs

@@ -118,7 +118,7 @@ pub enum OperationError {
     #[error("invalid glob pattern found in ignore list: {0}")]
     InvalidGlobPattern(ignore::Error),
     /// Parse error occurred while parsing the input.
-    #[error("failed to parse {input:?}")]
+    #[error("failed to parse {input}")]
     ParseError { input: FileName, is_panic: bool },
     /// Io error.
     #[error("{0}")]

src/test/mod.rs

@@ -545,6 +545,22 @@ fn parser_errors_in_submods_are_surfaced() {
     }
 }
 
+#[test]
+fn parser_creation_errors_on_entry_new_parser_from_file_panic() {
+    // See also https://github.com/rust-lang/rustfmt/issues/4418
+    let filename = "tests/parser/issue_4418.rs";
+    let file = PathBuf::from(filename);
+    let (config, operation, _) = read_config(&file);
+    if let Err(OperationError::ParseError { input, is_panic }) =
+        format_file(&file, operation, config)
+    {
+        assert_eq!(input.as_path().unwrap(), file);
+        assert!(is_panic);
+    } else {
+        panic!("Expected ParseError operation error");
+    }
+}
+
 // For each file, run rustfmt and collect the output.
 // Returns the number of files checked and the number of failures.
 fn check_files(files: Vec<PathBuf>, opt_config: &Option<PathBuf>) -> (Vec<FormatReport>, u32, u32) {

tests/parser/issue_4418.rs

@@ -0,0 +1 @@
+}