[lang] Improve invalid selector error

crates/lang/ir/src/ir/attrs.rs

@@ -584,7 +584,8 @@ impl TryFrom<syn::NestedMeta> for AttributeArg {
                                     )
                                 })?;
                                 let str = lit_str.value();
-                                let cap = regex.captures(&str).unwrap();
+                                let cap = regex.captures(&str)
+                                    .expect("invalid selector - a selector must consist of four bytes in hex (e.g. `selector = \"0xCAFEBABE\"`)");
                                 let selector_bytes = [
                                     u8::from_str_radix(&cap[1], 16).expect(
                                         "encountered non-hex digit at position 0",

crates/lang/macro/tests/compile_tests.rs

@@ -44,6 +44,7 @@ fn compile_tests() {
     t.compile_fail("tests/ui/fail/M-02-message-missing-self-arg.rs");
     t.compile_fail("tests/ui/fail/M-03-message-returns-self.rs");
     t.compile_fail("tests/ui/fail/M-04-message-returns-non-codec.rs");
+    t.compile_fail("tests/ui/fail/M-05-message-invalid-selector.rs");
     t.compile_fail("tests/ui/fail/M-10-method-unknown-ink-marker.rs");
 
     t.compile_fail("tests/ui/fail/S-01-missing-storage-struct.rs");

crates/lang/macro/tests/ui/fail/M-05-message-invalid-selector.rs

@@ -0,0 +1,19 @@
+use ink_lang as ink;
+
+#[ink::contract]
+mod message_invalid_selector {
+    #[ink(storage)]
+    pub struct MessageInvalidSelector {}
+
+    impl MessageInvalidSelector {
+        #[ink(constructor)]
+        pub fn constructor() -> Self {
+            Self {}
+        }
+
+        #[ink(message, selector = "0x00")]
+        pub fn invalid_selector(&self) { }
+    }
+}
+
+fn main() {}

crates/lang/macro/tests/ui/fail/M-05-message-invalid-selector.stderr

@@ -0,0 +1,7 @@
+error: custom attribute panicked
+ --> $DIR/M-05-message-invalid-selector.rs:3:1
+  |
+3 | #[ink::contract]
+  | ^^^^^^^^^^^^^^^^
+  |
+  = help: message: invalid selector - a selector must consist of four bytes in hex (e.g. `selector = "0xCAFEBABE"`)