Add support for revert reason strings in assert statement

Closes #288

compiler/src/yul/mappers/functions.rs

@@ -188,10 +188,15 @@ fn emit(context: &Context, stmt: &Node<fe::FuncStmt>) -> yul::Statement {
 }
 
 fn assert(context: &Context, stmt: &Node<fe::FuncStmt>) -> yul::Statement {
-    if let fe::FuncStmt::Assert { test, msg: _ } = &stmt.kind {
+    if let fe::FuncStmt::Assert { test, msg } = &stmt.kind {
         let test = expressions::expr(context, test);
-
-        return statement! { if (iszero([test])) { (revert(0, 0)) } };
+        return match msg {
+            Some(val) => {
+                let msg = expressions::expr(context, val);
+                statement! { if (iszero([test])) { (revert_with_reason_string([msg])) } }
+            }
+            None => statement! { if (iszero([test])) { (revert(0, 0)) } },
+        };
     }
 
     unreachable!()

compiler/tests/features.rs

@@ -75,17 +75,41 @@ fn test_assert() {
 
         let exit1 = harness.capture_call(&mut executor, "bar", &[uint_token(4)]);
 
-        assert!(matches!(
-            exit1,
-            evm::Capture::Exit((evm::ExitReason::Revert(_), _))
-        ));
+        match exit1 {
+            evm::Capture::Exit((evm::ExitReason::Revert(_), output)) => assert_eq!(output.len(), 0),
+            _ => panic!("Did not revert correctly"),
+        }
 
         let exit2 = harness.capture_call(&mut executor, "bar", &[uint_token(42)]);
 
         assert!(matches!(
             exit2,
             evm::Capture::Exit((evm::ExitReason::Succeed(_), _))
-        ))
+        ));
+
+        let exit3 =
+            harness.capture_call(&mut executor, "revert_with_static_string", &[uint_token(4)]);
+
+        match exit3 {
+            evm::Capture::Exit((evm::ExitReason::Revert(_), output)) => {
+                assert_eq!(output, encode_error_reason("Must be greater than five"))
+            }
+            _ => panic!("Did not revert correctly"),
+        }
+
+        let reason = "A very looooooooooooooong reason that consumes multiple words";
+        let exit4 = harness.capture_call(
+            &mut executor,
+            "revert_with",
+            &[uint_token(4), string_token(&reason)],
+        );
+
+        match exit4 {
+            evm::Capture::Exit((evm::ExitReason::Revert(_), output)) => {
+                assert_eq!(output, encode_error_reason(reason))
+            }
+            _ => panic!("Did not revert correctly"),
+        }
     })
 }
 

compiler/tests/fixtures/features/assert.fe

@@ -1,3 +1,9 @@
 contract Foo:
     pub def bar(baz: u256):
-        assert baz > 5
+        assert baz > 5
+
+    pub def revert_with_static_string(baz: u256):
+        assert baz > 5, "Must be greater than five"
+
+    pub def revert_with(baz: u256, reason: string1000):
+        assert baz > 5, reason

newsfragments/288.feature.md

@@ -0,0 +1,17 @@
+Support for revert messages in assert statements
+
+E.g
+
+```
+assert a == b, "my revert statement"
+```
+
+The provided string is abi-encoded as if it were a call
+to a function `Error(string)`. For example, the revert string `"Not enough Ether provided."` returns the following hexadecimal as error return data:
+
+```
+0x08c379a0                                                         // Function selector for Error(string)
+0x0000000000000000000000000000000000000000000000000000000000000020 // Data offset
+0x000000000000000000000000000000000000000000000000000000000000001a // String length
+0x4e6f7420656e6f7567682045746865722070726f76696465642e000000000000 // String data
+```