Make error_on_line_overflow false by default

And improve docs, the error message, etc.

I think false is the better default since such errors should only occur due to a bug in Rustfmt and therefore most users should not be notified of it happening (although to be clear, it might be a 'bug' which only occurs with pathological input and therefore we won't fix it). The error has proven to be confusing and annoying in the past.

Closes #1080

Configurations.md

@@ -579,9 +579,12 @@ Don't reformat anything
 
 ## `error_on_line_overflow`
 
-Error if unable to get all lines within `max_width`, except for comments and string literals.
+Error if Rustfmt is unable to get all lines within `max_width`, except for comments and string
+literals. If this happens, then it is a bug in Rustfmt. You might be able to work around the bug by
+refactoring your code to avoid long/complex expressions, usually by extracting a local variable or
+using a shorter name.
 
-- **Default value**: `true`
+- **Default value**: `false`
 - **Possible values**: `true`, `false`
 - **Stable**: No
 

src/config/mod.rs

@@ -134,7 +134,7 @@ create_config! {
     disable_all_formatting: bool, false, false, "Don't reformat anything";
     skip_children: bool, false, false, "Don't reformat out of line modules";
     hide_parse_errors: bool, false, false, "Hide errors from the parser";
-    error_on_line_overflow: bool, true, false, "Error if unable to get all lines within max_width";
+    error_on_line_overflow: bool, false, false, "Error if unable to get all lines within max_width";
     error_on_unformatted: bool, false, false,
         "Error if unable to get comments or string literals within max_width, \
          or they are left with trailing whitespaces";

src/lib.rs

@@ -101,7 +101,7 @@ pub enum ErrorKind {
     LineOverflow(usize, usize),
     // Line ends in whitespace
     TrailingWhitespace,
-    // TO-DO or FIX-ME item without an issue number
+    // TODO or FIXME item without an issue number
     BadIssue(Issue),
     // License check has failed
     LicenseCheck,
@@ -112,8 +112,8 @@ impl fmt::Display for ErrorKind {
         match *self {
             ErrorKind::LineOverflow(found, maximum) => write!(
                 fmt,
-                "line exceeded maximum width (maximum: {}, found: {})",
-                maximum, found
+                "line formatted, but exceeded maximum width (maximum: {} (see `max_width` option), found: {})",
+                maximum, found,
             ),
             ErrorKind::TrailingWhitespace => write!(fmt, "left behind trailing whitespace"),
             ErrorKind::BadIssue(issue) => write!(fmt, "found {}", issue),
@@ -134,10 +134,9 @@ pub struct FormattingError {
 impl FormattingError {
     fn msg_prefix(&self) -> &str {
         match self.kind {
-            ErrorKind::LineOverflow(..)
-            | ErrorKind::TrailingWhitespace
-            | ErrorKind::LicenseCheck => "error:",
-            ErrorKind::BadIssue(_) => "WARNING:",
+            ErrorKind::LineOverflow(..) | ErrorKind::TrailingWhitespace => "internal error:",
+            ErrorKind::LicenseCheck => "error:",
+            ErrorKind::BadIssue(_) => "warning:",
         }
     }
 

tests/lib.rs

@@ -283,7 +283,8 @@ fn stdin_formatting_smoke_test() {
 fn format_lines_errors_are_reported() {
     let long_identifier = String::from_utf8(vec![b'a'; 239]).unwrap();
     let input = Input::Text(format!("fn {}() {{}}", long_identifier));
-    let config = Config::default();
+    let mut config = Config::default();
+    config.set().error_on_line_overflow(true);
     let (error_summary, _file_map, _report) =
         format_input::<io::Stdout>(input, &config, None).unwrap();
     assert!(error_summary.has_formatting_errors());
@@ -293,7 +294,9 @@ fn format_lines_errors_are_reported() {
 fn format_lines_errors_are_reported_with_tabs() {
     let long_identifier = String::from_utf8(vec![b'a'; 97]).unwrap();
     let input = Input::Text(format!("fn a() {{\n\t{}\n}}", long_identifier));
-    let config = Config::from_toml("hard_tabs = true", Path::new("")).unwrap();
+    let mut config = Config::default();
+    config.set().error_on_line_overflow(true);
+    config.set().hard_tabs(true);
     let (error_summary, _file_map, _report) =
         format_input::<io::Stdout>(input, &config, None).unwrap();
     assert!(error_summary.has_formatting_errors());