Check style of messages in rustc_macros

Author: JonathanBrouwer

compiler/rustc_macros/src/diagnostics/message.rs

@@ -23,9 +23,7 @@ impl Message {
                 quote! { crate::fluent_generated::#slug }
             }
             Message::Inline(message_span, message) => {
-                if let Some(variant) = variant {
-                    verify_fluent_message(*message_span, &message, variant);
-                }
+                verify_fluent_message(*message_span, &message, variant);
                 quote! { rustc_errors::DiagMessage::Inline(std::borrow::Cow::Borrowed(#message)) }
             }
         }
@@ -86,30 +84,37 @@ impl Message {
     }
 }
 
-fn verify_fluent_message(msg_span: Span, message: &str, variant: &VariantInfo<'_>) {
+fn verify_fluent_message(msg_span: Span, message_str: &str, variant: Option<&VariantInfo<'_>>) {
     // Parse the fluent message
     const GENERATED_MSG_ID: &str = "generated_msg";
-    let resource = FluentResource::try_new(format!("{GENERATED_MSG_ID} = {message}\n")).unwrap();
+    let resource =
+        FluentResource::try_new(format!("{GENERATED_MSG_ID} = {message_str}\n")).unwrap();
     assert_eq!(resource.entries().count(), 1);
     let Some(fluent_syntax::ast::Entry::Message(message)) = resource.get_entry(0) else {
         panic!("Did not parse into a message")
     };
 
     // Check if all variables are used
-    let fields: Vec<String> = variant
-        .bindings()
-        .iter()
-        .flat_map(|b| b.ast().ident.as_ref())
-        .map(|id| id.to_string())
-        .collect();
-    for variable in variable_references(&message) {
-        if !fields.iter().any(|f| f == variable) {
-            span_err(msg_span.unwrap(), format!("Variable `{variable}` not found in diagnostic "))
+    if let Some(variant) = variant {
+        let fields: Vec<String> = variant
+            .bindings()
+            .iter()
+            .flat_map(|b| b.ast().ident.as_ref())
+            .map(|id| id.to_string())
+            .collect();
+        for variable in variable_references(&message) {
+            if !fields.iter().any(|f| f == variable) {
+                span_err(
+                    msg_span.unwrap(),
+                    format!("Variable `{variable}` not found in diagnostic "),
+                )
                 .help(format!("Available fields: {:?}", fields.join(", ")))
                 .emit();
+            }
         }
-        // assert!(, );
     }
+
+    verify_message_style(msg_span, message_str);
 }
 
 fn variable_references<'a>(msg: &fluent_syntax::ast::Message<&'a str>) -> Vec<&'a str> {
@@ -136,3 +141,36 @@ fn variable_references<'a>(msg: &fluent_syntax::ast::Message<&'a str>) -> Vec<&'
     }
     refs
 }
+
+const ALLOWED_CAPITALIZED_WORDS: &[&str] = &[
+    // tidy-alphabetical-start
+    "ABI",
+    "C-variadic",
+    "CGU-reuse",
+    "Cargo",
+    "Ferris",
+    "MIR",
+    "OK",
+    "VS",
+    // tidy-alphabetical-end
+];
+
+/// See: https://rustc-dev-guide.rust-lang.org/diagnostics.html#diagnostic-output-style-guide
+fn verify_message_style(msg_span: Span, message: &str) {
+    // Verify that message starts with lowercase char
+    let Some(first_word) = message.split_whitespace().next() else {
+        span_err(msg_span.unwrap(), "message must not be empty").emit();
+        return;
+    };
+    let first_char = first_word.chars().next().expect("Word is not empty");
+    if first_char.is_uppercase() && !ALLOWED_CAPITALIZED_WORDS.contains(&first_word) {
+        span_err(msg_span.unwrap(), "message `{value}` starts with an uppercase letter. Fix it or add it to `ALLOWED_CAPITALIZED_WORDS`").emit();
+        return;
+    }
+
+    // Verify that message does not end in `.`
+    if message.ends_with(".") && !message.ends_with("...") {
+        span_err(msg_span.unwrap(), "message `{value}` ends with a period").emit();
+        return;
+    }
+}