[wasm] Add the ability to provide a user-defined error;

cranelift/wasm/Cargo.toml

@@ -27,7 +27,7 @@ target-lexicon = "0.4.0"
 
 [features]
 default = ["std"]
-std = ["cranelift-codegen/std", "cranelift-frontend/std", "wasmparser/std"]
+std = ["cranelift-codegen/std", "cranelift-frontend/std", "wasmparser/std", "failure/std"]
 core = ["hashmap_core", "cranelift-codegen/core", "cranelift-frontend/core", "wasmparser/core"]
 
 [badges]

cranelift/wasm/src/environ/spec.rs

@@ -39,7 +39,7 @@ pub enum GlobalVariable {
 ///
 /// When a WebAssembly function can't be translated, one of these error codes will be returned
 /// to describe the failure.
-#[derive(Fail, Debug, PartialEq, Eq)]
+#[derive(Fail, Debug)]
 pub enum WasmError {
     /// The input WebAssembly code is invalid.
     ///
@@ -67,6 +67,18 @@ pub enum WasmError {
     /// [limits]: https://cranelift.readthedocs.io/en/latest/ir.html#implementation-limits
     #[fail(display = "Implementation limit exceeded")]
     ImplLimitExceeded,
+
+    /// Any user-defined error. Requires an std build, where failure::Error is defined.
+    #[cfg(feature = "std")]
+    #[fail(display = "User error: {}", _0)]
+    User(failure::Error),
+}
+
+#[cfg(feature = "std")]
+impl From<failure::Error> for WasmError {
+    fn from(err: failure::Error) -> Self {
+        WasmError::User(err)
+    }
 }
 
 impl From<BinaryReaderError> for WasmError {