Add SignextLowering pass to wasm-opt (#1280)

* Add `SignextLowering` pass to `wasm-opt`

* Remove check for Rust version

crates/build/src/lib.rs

@@ -308,16 +308,6 @@ fn exec_cargo_for_onchain_target(
             env.push(("RUSTC_BOOTSTRAP", Some("1".to_string())))
         }
 
-        // newer rustc versions might emit unsupported instructions
-        if rustc_version::version_meta()?.semver >= Version::new(1, 70, 0) {
-            eprintln!(
-                "{} {}",
-                "warning:".yellow().bold(),
-                "You are using a rustc version that might emit unsupported wasm instructions. Build using a version lower than 1.70.0 to make sure your contract will deploy."
-                    .bold()
-            );
-        }
-
         // merge target specific flags with the common flags (defined here)
         // We want to disable warnings here as they will be duplicates of the clippy pass.
         // However, if we want to do so with either `--cap-lints allow` or  `-A

crates/build/src/wasm_opt.rs

@@ -18,6 +18,7 @@ use anyhow::Result;
 use wasm_opt::{
     Feature,
     OptimizationOptions,
+    Pass,
 };
 
 use std::{
@@ -68,9 +69,14 @@ impl WasmOptHandler {
         );
 
         OptimizationOptions::from(self.optimization_level)
-            // Since rustc 1.70 `SignExt` can't be disabled anymore. Hence we have to allow it.
             .mvp_features_only()
+            // Since rustc 1.70 `SignExt` can't be disabled anymore. Hence we have to allow it,
+            // in order that the Wasm binary containing these instructions can be loaded.
             .enable_feature(Feature::SignExt)
+            // This pass will then remove any `signext` instructions in order that the resulting
+            // Wasm binary is compatible with older versions of `pallet-contracts` which do not
+            // support the `signext` instruction.
+            .add_pass(Pass::SignextLowering)
             // the memory in our module is imported, `wasm-opt` needs to be told that
             // the memory is initialized to zeroes, otherwise it won't run the
             // memory-packing pre-pass.