Opt in for 32bit platforms (#44)

Cargo.toml

@@ -14,3 +14,9 @@ byteorder = "1.0"
 
 [dev-dependencies]
 wabt = "0.1.2"
+
+[features]
+# 32-bit platforms are not supported and not tested. Use this flag if you really want to use
+# wasmi on these platforms.
+# See https://github.com/pepyakin/wasmi/issues/43
+32bit_opt_in = []

src/lib.rs

@@ -101,6 +101,12 @@ extern crate wabt;
 extern crate parity_wasm;
 extern crate byteorder;
 
+#[cfg(all(not(feature = "32bit_opt_in"), target_pointer_width = "32"))]
+compile_error! {"32-bit targets are not supported at the moment.
+You can use '32bit_opt_in' feature.
+See https://github.com/pepyakin/wasmi/issues/43"
+}
+
 use std::fmt;
 use std::error;
 use std::collections::HashMap;