Introduce Trap struct.

src/host.rs

@@ -1,6 +1,6 @@
 use std::any::TypeId;
 use value::{RuntimeValue, TryInto};
-use Error;
+use {Error, Trap};
 
 /// Safe wrapper for list of arguments.
 #[derive(Debug)]
@@ -188,7 +188,7 @@ impl Externals for NopExternals {
 		_index: usize,
 		_args: RuntimeArgs,
 	) -> Result<Option<RuntimeValue>, Error> {
-		Err(Error::Trap("invoke index on no-op externals".into()))
+		Err(Error::Trap(Trap::Unreachable))
 	}
 }
 

src/lib.rs

@@ -106,6 +106,11 @@ use std::fmt;
 use std::error;
 use std::collections::HashMap;
 
+#[derive(Debug)]
+pub enum Trap {
+	Unreachable,
+}
+
 /// Internal interpreter error.
 #[derive(Debug)]
 pub enum Error {
@@ -127,7 +132,7 @@ pub enum Error {
 	/// Value-level error.
 	Value(String),
 	/// Trap.
-	Trap(String),
+	Trap(Trap),
 	/// Custom embedder error.
 	Host(Box<host::HostError>),
 }
@@ -143,7 +148,7 @@ impl Into<String> for Error {
 			Error::Global(s) => s,
 			Error::Stack(s) => s,
 			Error::Value(s) => s,
-			Error::Trap(s) => format!("trap: {}", s),
+			Error::Trap(s) => format!("trap: {:?}", s),
 			Error::Host(e) => format!("user: {}", e),
 		}
 	}
@@ -160,7 +165,7 @@ impl fmt::Display for Error {
 			Error::Global(ref s) => write!(f, "Global: {}", s),
 			Error::Stack(ref s) => write!(f, "Stack: {}", s),
 			Error::Value(ref s) => write!(f, "Value: {}", s),
-			Error::Trap(ref s) => write!(f, "Trap: {}", s),
+			Error::Trap(ref s) => write!(f, "Trap: {:?}", s),
 			Error::Host(ref e) => write!(f, "User: {}", e),
 		}
 	}
@@ -179,7 +184,7 @@ impl error::Error for Error {
 			Error::Global(ref s) => s,
 			Error::Stack(ref s) => s,
 			Error::Value(ref s) => s,
-			Error::Trap(ref s) => s,
+			Error::Trap(_) => "Trap",
 			Error::Host(_) => "Host error",
 		}
 	}

src/runner.rs

@@ -5,7 +5,7 @@ use std::fmt::{self, Display};
 use std::iter::repeat;
 use std::collections::{HashMap, VecDeque};
 use parity_wasm::elements::{Opcode, BlockType, Local};
-use {Error, Signature};
+use {Error, Trap, Signature};
 use module::ModuleRef;
 use func::{FuncRef, FuncInstance, FuncInstanceInternal};
 use value::{
@@ -351,7 +351,7 @@ impl<'a, E: Externals> Interpreter<'a, E> {
 	}
 
 	fn run_unreachable(&mut self, _context: &mut FunctionContext) -> Result<InstructionOutcome, Error> {
-		Err(Error::Trap("programmatic".into()))
+		Err(Error::Trap(Trap::Unreachable))
 	}
 
 	fn run_nop(&mut self, _context: &mut FunctionContext) -> Result<InstructionOutcome, Error> {