Add back the E0024 error code long explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes.rs

@@ -28,6 +28,7 @@ E0019: include_str!("./error_codes/E0019.md"),
 E0020: include_str!("./error_codes/E0020.md"),
 E0022: include_str!("./error_codes/E0022.md"),
 E0023: include_str!("./error_codes/E0023.md"),
+E0024: include_str!("./error_codes/E0024.md"),
 E0025: include_str!("./error_codes/E0025.md"),
 E0026: include_str!("./error_codes/E0026.md"),
 E0027: include_str!("./error_codes/E0027.md"),

src/librustc_error_codes/error_codes/E0024.md

@@ -0,0 +1,24 @@
+#### Note: this error code is no longer emitted by the compiler.
+
+This error indicates that a pattern attempted to extract the fields of an enum
+variant with no fields. Here's a tiny example of this error:
+
+```compile_fail,E0532
+// This enum has two variants.
+enum Number {
+    // This variant has no fields.
+    Zero,
+    // This variant has one field.
+    One(u32)
+}
+
+// Assuming x is a Number we can pattern match on its contents.
+match Number::Zero {
+    Number::Zero(inside) => {}
+    Number::One(inside) => {}
+}
+```
+
+The pattern match `Zero(inside)` is incorrect because the `Zero` variant
+contains no fields, yet the `inside` name attempts to bind the first field of
+the enum.