manifest: Improve "wrong separator" error messages

Signed-off-by: Lann Martin <[email protected]>

Author: lann

crates/manifest/tests/ui/invalid/bad_component_id.err

@@ -2,4 +2,4 @@ TOML parse error at line 6, column 12
   |
 6 | [component.bad_id]
   |            ^^^^^^
-'-'-separated words may only contain alphanumeric ASCII; got '_'
+words must be separated with '-', not '_'

crates/serde/src/id.rs

@@ -33,6 +33,14 @@ impl<const DELIM: char> TryFrom<String> for Id<DELIM> {
         if id.is_empty() {
             return Err("empty".into());
         }
+        // Special-case common "wrong separator" errors
+        if let Some(wrong) = wrong_delim::<DELIM>() {
+            if id.contains(wrong) {
+                return Err(format!(
+                    "words must be separated with {DELIM:?}, not {wrong:?}"
+                ));
+            }
+        }
         for word in id.split(DELIM) {
             if word.is_empty() {
                 return Err(format!("{DELIM:?}-separated words must not be empty"));
@@ -59,3 +67,11 @@ impl<const DELIM: char> TryFrom<String> for Id<DELIM> {
         Ok(Self(id))
     }
 }
+
+const fn wrong_delim<const DELIM: char>() -> Option<char> {
+    match DELIM {
+        '_' => Some('-'),
+        '-' => Some('_'),
+        _ => None,
+    }
+}