Add fail!() macro and test (#4699)

Co-authored-by: Tobie Morgan Hitchcock <tobie@surrealdb.com>

core/src/doc/compute.rs

@@ -88,7 +88,7 @@ impl Document {
 		// two separtate UNIQUE index definitions, and it
 		// wasn't possible to detect which record was the
 		// correct one to be updated
-		let _ = chn.send(Err(Error::Unreachable("Internal error"))).await;
+		let _ = chn.send(Err(fail!("Internal error"))).await;
 		// Break the loop
 		Ok(())
 	}

core/src/doc/process.rs

@@ -80,6 +80,6 @@ impl Document {
 		// two separtate UNIQUE index definitions, and it
 		// wasn't possible to detect which record was the
 		// correct one to be updated
-		Err(Error::Unreachable("Internal error"))
+		Err(fail!("Internal error"))
 	}
 }

core/src/mac/mod.rs

@@ -1,3 +1,10 @@
+/// Throws an unreachable error with location details
+macro_rules! fail {
+	($msg: literal) => {
+		$crate::err::Error::Unreachable(concat!(file!(), ":", line!(), ": ", $msg))
+	};
+}
+
 /// Converts some text into a new line byte string
 macro_rules! bytes {
 	($expression:expr) => {
@@ -203,6 +210,8 @@ macro_rules! async_defer{
 
 #[cfg(test)]
 mod test {
+	use crate::err::Error;
+
 	#[tokio::test]
 	async fn async_defer_basic() {
 		let mut counter = 0;
@@ -241,4 +250,12 @@ mod test {
 		})
 		.await;
 	}
+
+	#[test]
+	fn fail() {
+		let Error::Unreachable(msg) = fail!("Reached unreachable code") else {
+			panic!()
+		};
+		assert_eq!("core/src/mac/mod.rs:256: Reached unreachable code", msg);
+	}
 }