[lang] Improve invalid selector error (#561)

* [lang] Improve invalid selector message

* [lang] Improve span

* [lang] Return syn::Error instead of panicking

* Make clippy happy

* [lang] Reduce code dup with non_hex_err

* [lang] Rename fn

* [lang] Fix err functions

* [lang] Add comments

* [lang] Remove unnecessary lifetime

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

@@ -565,6 +565,19 @@ impl InkAttribute {
     }
 }
 
+/// Returns an error to notify about non-hex digits at a position.
+fn err_non_hex(meta: &syn::Meta, pos: usize) -> syn::Error {
+    format_err_spanned!(meta, "encountered non-hex digit at position {}", pos)
+}
+
+/// Returns an error to notify about an invalid ink! selector.
+fn invalid_selector_err_regex(meta: &syn::Meta) -> syn::Error {
+    format_err_spanned!(
+        meta,
+        "invalid selector - a selector must consist of four bytes in hex (e.g. `selector = \"0xCAFEBABE\"`)"
+    )
+}
+
 impl TryFrom<syn::NestedMeta> for AttributeArg {
     type Error = syn::Error;
 
@@ -577,27 +590,20 @@ impl TryFrom<syn::NestedMeta> for AttributeArg {
                             if let syn::Lit::Str(lit_str) = &name_value.lit {
                                 let regex = Regex::new(
                                     r"0x([\da-fA-F]{2})([\da-fA-F]{2})([\da-fA-F]{2})([\da-fA-F]{2})"
-                                ).map_err(|_| {
-                                    format_err_spanned!(
-                                        meta,
-                                        "invalid selector bytes"
-                                    )
-                                })?;
+                                ).map_err(|_| invalid_selector_err_regex(&meta))?;
                                 let str = lit_str.value();
-                                let cap = regex.captures(&str).unwrap();
+                                let cap = regex
+                                    .captures(&str)
+                                    .ok_or_else(|| invalid_selector_err_regex(&meta))?;
                                 let selector_bytes = [
-                                    u8::from_str_radix(&cap[1], 16).expect(
-                                        "encountered non-hex digit at position 0",
-                                    ),
-                                    u8::from_str_radix(&cap[2], 16).expect(
-                                        "encountered non-hex digit at position 1",
-                                    ),
-                                    u8::from_str_radix(&cap[3], 16).expect(
-                                        "encountered non-hex digit at position 2",
-                                    ),
-                                    u8::from_str_radix(&cap[4], 16).expect(
-                                        "encountered non-hex digit at position 3",
-                                    ),
+                                    u8::from_str_radix(&cap[1], 16)
+                                        .map_err(|_| err_non_hex(&meta, 0))?,
+                                    u8::from_str_radix(&cap[2], 16)
+                                        .map_err(|_| err_non_hex(&meta, 1))?,
+                                    u8::from_str_radix(&cap[3], 16)
+                                        .map_err(|_| err_non_hex(&meta, 2))?,
+                                    u8::from_str_radix(&cap[4], 16)
+                                        .map_err(|_| err_non_hex(&meta, 3))?,
                                 ];
                                 return Ok(AttributeArg {
                                     ast: meta,

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 +1,5 @@
+error: invalid selector - a selector must consist of four bytes in hex (e.g. `selector = "0xCAFEBABE"`)
+  --> $DIR/M-05-message-invalid-selector.rs:14:24
+   |
+14 |         #[ink(message, selector = "0x00")]
+   |                        ^^^^^^^^^^^^^^^^^