improve deprecation error message

Author: giacomocavalieri

CHANGELOG.md

@@ -23,6 +23,32 @@
   update syntax with variants that have no labelled fields.
   ([Giacomo Cavalieri](https://github.com/giacomocavalieri))
 
+- The error message for invalid deprecated attributes with no deprecation
+  message has been improved. For example, the following code:
+
+  ```gleam
+  pub type HashAlgorithm {
+    @deprecated
+    Md5
+    Sha224
+    Sha512
+  }
+  ```
+
+  Will raise the following error:
+
+  ```txt
+  error: Syntax error
+    ┌─ /src/parse/error.gleam:3:3
+    │
+  3 │   @deprecated
+    │   ^^^^^^^^^^^ A deprecation attribute must have a string message.
+
+  See: https://tour.gleam.run/functions/deprecations/
+  ```
+
+  ([Giacomo Cavalieri](https://github.com/giacomocavalieri))
+
 - The compiler now raises a warning on the JavaScript target when defining an
   integer segment with a size higher than 52 bits. For example, this code:
 

compiler-core/src/parse.rs

@@ -4488,10 +4488,7 @@ functions are declared separately from types.";
                 self.parse_external_attribute(start, end, attributes)
             }
             "target" => self.parse_target_attribute(start, end, attributes),
-            "deprecated" => {
-                let _ = self.expect_one(&Token::LeftParen)?;
-                self.parse_deprecated_attribute(start, end, attributes)
-            }
+            "deprecated" => self.parse_deprecated_attribute(start, end, attributes),
             "internal" => self.parse_internal_attribute(start, end, attributes),
             _ => parse_error(ParseErrorType::UnknownAttribute, SrcSpan { start, end }),
         }?;
@@ -4553,6 +4550,10 @@ functions are declared separately from types.";
         end: u32,
         attributes: &mut Attributes,
     ) -> Result<u32, ParseError> {
+        let _ = self.expect_one(&Token::LeftParen).map_err(|_| ParseError {
+            error: ParseErrorType::ExpectedDeprecationMessage,
+            location: SrcSpan { start, end },
+        })?;
         if attributes.deprecated.is_deprecated() {
             return parse_error(ParseErrorType::DuplicateAttribute, SrcSpan::new(start, end));
         }

compiler-core/src/parse/error.rs

@@ -216,7 +216,7 @@ impl ParseErrorType {
             },
 
             ParseErrorType::ExpectedDeprecationMessage => ParseErrorDetails {
-                text: "".into(),
+                text: "See: https://tour.gleam.run/functions/deprecations/".into(),
                 hint: None,
                 label_text: "A deprecation attribute must have a string message.".into(),
                 extra_labels: vec![],

compiler-core/src/parse/snapshots/gleam_core__parse__tests__deprecation_with_no_message.snap

@@ -0,0 +1,20 @@
+---
+source: compiler-core/src/parse/tests.rs
+expression: "\n@deprecated\npub fn main() -> Nil {\n  Nil\n}\n"
+---
+----- SOURCE CODE
+
+@deprecated
+pub fn main() -> Nil {
+  Nil
+}
+
+
+----- ERROR
+error: Syntax error
+  ┌─ /src/parse/error.gleam:2:1
+  │
+2 │ @deprecated
+  │ ^^^^^^^^^^^ A deprecation attribute must have a string message.
+
+See: https://tour.gleam.run/functions/deprecations/

compiler-core/src/parse/snapshots/gleam_core__parse__tests__deprecation_with_no_message_on_constructor.snap

@@ -0,0 +1,22 @@
+---
+source: compiler-core/src/parse/tests.rs
+expression: "\npub type HashAlgorithm {\n  @deprecated\n  Md5\n  Sha224\n  Sha512\n}\n"
+---
+----- SOURCE CODE
+
+pub type HashAlgorithm {
+  @deprecated
+  Md5
+  Sha224
+  Sha512
+}
+
+
+----- ERROR
+error: Syntax error
+  ┌─ /src/parse/error.gleam:3:3
+  │
+3 │   @deprecated
+  │   ^^^^^^^^^^^ A deprecation attribute must have a string message.
+
+See: https://tour.gleam.run/functions/deprecations/

compiler-core/src/parse/snapshots/gleam_core__parse__tests__deprecation_without_message.snap

@@ -16,3 +16,5 @@ error: Syntax error
   │
 2 │ @deprecated()
   │ ^^^^^^^^^^^ A deprecation attribute must have a string message.
+
+See: https://tour.gleam.run/functions/deprecations/

compiler-core/src/parse/tests.rs

@@ -770,6 +770,32 @@ pub fn main() -> Nil {
     );
 }
 
+#[test]
+fn deprecation_with_no_message() {
+    assert_module_error!(
+        r#"
+@deprecated
+pub fn main() -> Nil {
+  Nil
+}
+"#
+    );
+}
+
+#[test]
+fn deprecation_with_no_message_on_constructor() {
+    assert_module_error!(
+        r#"
+pub type HashAlgorithm {
+  @deprecated
+  Md5
+  Sha224
+  Sha512
+}
+"#
+    );
+}
+
 #[test]
 fn deprecation_without_message() {
     assert_module_error!(