Add back the E0003 error code long explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes.rs

@@ -10,6 +10,7 @@
 register_diagnostics! {
 E0001: include_str!("./error_codes/E0001.md"),
 E0002: include_str!("./error_codes/E0002.md"),
+E0003: include_str!("./error_codes/E0003.md"),
 E0004: include_str!("./error_codes/E0004.md"),
 E0005: include_str!("./error_codes/E0005.md"),
 E0007: include_str!("./error_codes/E0007.md"),

src/librustc_error_codes/error_codes/E0003.md

@@ -0,0 +1,15 @@
+#### Note: this error code is no longer emitted by the compiler.
+
+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, like so:
+
+```
+let number = 0f64;
+
+match number {
+    // ...
+    x if x.is_nan() => { /* ... */ }
+    x => { /* ... */ }
+}
+```