Enforce 80 char lines in extended errors.

src/librustc/diagnostics.rs

@@ -55,15 +55,17 @@ underscore `_` wildcard pattern can be added after all other patterns to match
 
 // FIXME: Remove duplication here?
 E0005: r##"
-Patterns used to bind names must be irrefutable, that is, they must guarantee that a
+Patterns used to bind names must be irrefutable, that is, they must guarantee
-name will be extracted in all cases. If you encounter this error you probably need
+that a name will be extracted in all cases. If you encounter this error you
-to use a `match` or `if let` to deal with the possibility of failure.
+probably need to use a `match` or `if let` to deal with the possibility of
+failure.
 "##,
 
 E0006: r##"
-Patterns used to bind names must be irrefutable, that is, they must guarantee that a
+Patterns used to bind names must be irrefutable, that is, they must guarantee
-name will be extracted in all cases. If you encounter this error you probably need
+that a name will be extracted in all cases. If you encounter this error you
-to use a `match` or `if let` to deal with the possibility of failure.
+probably need to use a `match` or `if let` to deal with the possibility of
+failure.
 "##,
 
 E0007: r##"

src/libsyntax/diagnostics/plugin.rs

@@ -20,6 +20,9 @@ use parse::token;
 use ptr::P;
 use util::small_vector::SmallVector;
 
+// Maximum width of any line in an extended error description (inclusive).
+const MAX_DESCRIPTION_WIDTH: usize = 80;
+
 thread_local! {
     static REGISTERED_DIAGNOSTICS: RefCell<BTreeMap<Name, Option<Name>>> = {
         RefCell::new(BTreeMap::new())
@@ -92,16 +95,22 @@ pub fn expand_register_diagnostic<'cx>(ecx: &'cx mut ExtCtxt,
         }
         _ => unreachable!()
     };
-    // Check that the description starts and ends with a newline.
+    // Check that the description starts and ends with a newline and doesn't
+    // overflow the maximum line width.
     description.map(|raw_msg| {
         let msg = raw_msg.as_str();
-        let last = msg.len() - 1;
+        if !msg.starts_with("\n") || !msg.ends_with("\n") {
-        if &msg[0..1] != "\n" || &msg[last..] != "\n" {
             ecx.span_err(span, &format!(
                 "description for error code {} doesn't start and end with a newline",
                 token::get_ident(*code)
             ));
         }
+        if msg.lines().any(|line| line.len() > MAX_DESCRIPTION_WIDTH) {
+            ecx.span_err(span, &format!(
+                "description for error code {} contains a line longer than {} characters",
+                token::get_ident(*code), MAX_DESCRIPTION_WIDTH
+            ));
+        }
         raw_msg
     });
     with_registered_diagnostics(|diagnostics| {