Modify libsyntax/diagnostics to not be so persnickety. The scheme

doesn't work in a multi-crate context. We'll need to come up with
something better.

src/librustc_typeck/diagnostics.rs

+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![allow(non_snake_case)]
+
+register_diagnostic!(E0001, r##"
+    This error suggests that the expression arm corresponding to the noted pattern
+    will never be reached as for all possible values of the expression being matched,
+    one of the preceeding patterns will match.
+
+    This means that perhaps some of the preceeding patterns are too general, this
+    one is too specific or the ordering is incorrect.
+"##)
+
+register_diagnostics!(
+    E0002,
+    E0003,
+    E0004,
+    E0005,
+    E0006,
+    E0007,
+    E0008,
+    E0009,
+    E0010,
+    E0011,
+    E0012,
+    E0013,
+    E0014,
+    E0015,
+    E0016,
+    E0017,
+    E0018,
+    E0019,
+    E0020,
+    E0022,
+    E0023,
+    E0024,
+    E0025,
+    E0026,
+    E0027,
+    E0029,
+    E0030,
+    E0031,
+    E0033,
+    E0034,
+    E0035,
+    E0036,
+    E0038,
+    E0040,
+    E0044,
+    E0045,
+    E0046,
+    E0049,
+    E0050,
+    E0051,
+    E0052,
+    E0053,
+    E0054,
+    E0055,
+    E0056,
+    E0057,
+    E0059,
+    E0060,
+    E0061,
+    E0062,
+    E0063,
+    E0066,
+    E0067,
+    E0068,
+    E0069,
+    E0070,
+    E0071,
+    E0072,
+    E0073,
+    E0074,
+    E0075,
+    E0076,
+    E0077,
+    E0079,
+    E0080,
+    E0081,
+    E0082,
+    E0083,
+    E0084,
+    E0085,
+    E0086,
+    E0087,
+    E0088,
+    E0089,
+    E0090,
+    E0091,
+    E0092,
+    E0093,
+    E0094,
+    E0100,
+    E0101,
+    E0102,
+    E0103,
+    E0104,
+    E0106,
+    E0107,
+    E0108,
+    E0109,
+    E0110,
+    E0116,
+    E0117,
+    E0118,
+    E0119,
+    E0120,
+    E0121,
+    E0122,
+    E0124,
+    E0127,
+    E0128,
+    E0129,
+    E0130,
+    E0131,
+    E0132,
+    E0133,
+    E0134,
+    E0135,
+    E0136,
+    E0137,
+    E0138,
+    E0139,
+    E0140,
+    E0141,
+    E0152,
+    E0153,
+    E0157,
+    E0158,
+    E0159,
+    E0161,
+    E0162,
+    E0163,
+    E0164,
+    E0165,
+    E0166,
+    E0167,
+    E0168,
+    E0169
+)

src/libsyntax/diagnostics/plugin.rs

@@ -45,15 +45,6 @@ pub fn expand_diagnostic_used<'cx>(ecx: &'cx mut ExtCtxt,
         [ast::TtToken(_, token::Ident(code, _))] => code,
         _ => unreachable!()
     };
-    with_registered_diagnostics(|diagnostics| {
-        if !diagnostics.contains_key(&code.name) {
-            ecx.span_err(span, format!(
-                "unknown diagnostic code {}; add to librustc/diagnostics.rs",
-                token::get_ident(code).get()
-            ).as_slice());
-        }
-        ()
-    });
     with_used_diagnostics(|diagnostics| {
         match diagnostics.insert(code.name, span) {
             Some(previous_span) => {
@@ -106,25 +97,19 @@ pub fn expand_build_diagnostic_array<'cx>(ecx: &'cx mut ExtCtxt,
         _ => unreachable!()
     };
 
-    let (count, expr) = with_used_diagnostics(|diagnostics_in_use| {
+    let (count, expr) =
         with_registered_diagnostics(|diagnostics| {
-            let descriptions: Vec<P<ast::Expr>> = diagnostics
-                .iter().filter_map(|(code, description)| {
-                if !diagnostics_in_use.contains_key(code) {
-                    ecx.span_warn(span, format!(
-                        "diagnostic code {} never used", token::get_name(*code).get()
-                    ).as_slice());
-                }
-                description.map(|description| {
-                    ecx.expr_tuple(span, vec![
-                        ecx.expr_str(span, token::get_name(*code)),
-                        ecx.expr_str(span, token::get_name(description))
-                    ])
-                })
-            }).collect();
+            let descriptions: Vec<P<ast::Expr>> =
+                diagnostics.iter().filter_map(|(code, description)| {
+                    description.map(|description| {
+                        ecx.expr_tuple(span, vec![
+                            ecx.expr_str(span, token::get_name(*code)),
+                            ecx.expr_str(span, token::get_name(description))])
+                    })
+                }).collect();
             (descriptions.len(), ecx.expr_vec(span, descriptions))
-        })
-    });
+        });
+
     MacItems::new(vec![quote_item!(ecx,
         pub static $name: [(&'static str, &'static str), ..$count] = $expr;
     ).unwrap()].into_iter())