Improve E0001-E0003

diff --git a/src/librustc/diagnostics.rs b/src/librustc/diagnostics.rs
index 4e21efcf9eb6b12dbc9d825ea95e85d4242577ac..0c3c9b2ec8011faec42e2d38576e452da2166c65 100644 (file)
--- a/src/librustc/diagnostics.rs
+++ b/src/librustc/diagnostics.rs
@@ -22,6 +22,22 @@
 
 This means that perhaps some of the preceding patterns are too general, this one
 is too specific or the ordering is incorrect.
+
+For example, the following `match` block has too many arms:
+
+```
+match foo {
+    Some(bar) => {/* ... */}
+    None => {/* ... */}
+    _ => {/* ... */} // All possible cases have already been handled
+}
+```
+
+`match` blocks have their patterns matched in order, so, for example, putting
+a wildcard arm above a more specific arm will make the latter arm irrelevant.
+
+Ensure the ordering of the match arm is correct and remove any superfluous
+checks.
 "##,
 
 E0002: r##"
@@ -31,13 +47,40 @@
 expressions are almost never desired.  This error is typically fixed by adding
 one or more cases to the match expression.
 
-An example of an empty type is `enum Empty { }`.
+An example of an empty type is `enum Empty { }`. So, the following will work:
+
+```
+fn foo(x: Empty) {
+    match x {
+        // empty
+    }
+}
+
+```
+
+but this won't:
+
+```
+fn foo(x: Option<String>) {
+    match x {
+        // empty
+    }
+}
+```
 "##,
 
 E0003: r##"
 Not-a-Number (NaN) values cannot be compared for equality and hence can never
 match the input to a match expression. To match against NaN values, you should
-instead use the `is_nan` method in a guard, as in: `x if x.is_nan() => ...`
+instead use the `is_nan()` method in a guard, like so: 
+
+```
+match number {
+    // ...
+    x if x.is_nan() => { /* ... */ }
+    // ...
+}
+```
 "##,
 
 E0004: r##"