Improve diagnostics

src/librustc_passes/ast_validation.rs

@@ -348,7 +348,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                                 let mut err = struct_span_err!(self.session, span, E0642,
                                     "patterns aren't allowed in trait methods");
                                 let suggestion = "give this argument a name or use an \
-                                                  underscore to ignore it, instead of a \
+                                                  underscore to ignore it instead of using a \
                                                   tuple pattern";
                                 err.span_suggestion(span, suggestion, "_".to_owned());
                                 err.emit();

src/librustc_passes/diagnostics.rs

@@ -269,7 +269,15 @@ Example of erroneous code:
 ```compile_fail,E0642
 trait Foo {
     fn foo((x, y): (i32, i32)); // error: patterns aren't allowed
-                                // in methods without bodies
+                                //        in trait methods
+}
+```
+
+You can instead use a single name for the argument:
+
+```
+trait Foo {
+    fn foo(x_and_y: (i32, i32)); // ok!
 }
 ```
 "##,

src/libsyntax/parse/parser.rs

@@ -1744,7 +1744,7 @@ impl<'a> Parser<'a> {
     fn parse_arg_general(&mut self, require_name: bool) -> PResult<'a, Arg> {
         maybe_whole!(self, NtArg, |x| x);
 
-        // If we see `ident :`, then we know that the argument is just of the
+        // If we see `ident :`, then we know that the argument is not just of the
         // form `type`, which means we won't need to recover from parsing a
         // pattern and so we don't need to store a parser snapshot.
         let parser_snapshot_before_pat = if

src/test/ui/E0642.stderr

@@ -3,7 +3,7 @@ error[E0642]: patterns aren't allowed in trait methods
    |
 LL |     fn foo((x, y): (i32, i32)); //~ ERROR patterns aren't allowed in trait methods
    |            ^^^^^^
-help: give this argument a name or use an underscore to ignore it, instead of a tuple pattern
+help: give this argument a name or use an underscore to ignore it instead of using a tuple pattern
    |
 LL |     fn foo(_: (i32, i32)); //~ ERROR patterns aren't allowed in trait methods
    |            ^
@@ -13,7 +13,7 @@ error[E0642]: patterns aren't allowed in trait methods
    |
 LL |     fn bar((x, y): (i32, i32)) {} //~ ERROR patterns aren't allowed in trait methods
    |            ^^^^^^
-help: give this argument a name or use an underscore to ignore it, instead of a tuple pattern
+help: give this argument a name or use an underscore to ignore it instead of using a tuple pattern
    |
 LL |     fn bar(_: (i32, i32)) {} //~ ERROR patterns aren't allowed in trait methods
    |            ^