gettext: enforce trimmed msgids

Now that we have trimmed our msgids, add an assertion to ensure that
they stay trimmed. Note that we don't check msgstrs. We could do so when
building the maps which get put into the executable, but there we also
include messages originating from fish scripts, and there we don't
enforce trimmed messages, so limiting the checks to only messages
originating from the Rust code there would not be trivial.

Closes #12405

crates/gettext-extraction/src/lib.rs

@@ -51,6 +51,14 @@ fn write_po_entry_to_file(message: &TokenStream, dir: &OsString) {
         !message_string.contains('\n'),
         "Gettext strings may not contain unescaped newlines. Unescaped newline found in '{message_string}'"
     );
+    let msgid_without_quotes = &message_string[1..(message_string.len() - 1)];
+    // We don't want leading or trailing whitespace in our messages.
+    let trimmed_msgid = msgid_without_quotes.trim();
+    assert_eq!(msgid_without_quotes, trimmed_msgid);
+    assert!(!trimmed_msgid.starts_with("\\n"));
+    assert!(!trimmed_msgid.ends_with("\\n"));
+    assert!(!trimmed_msgid.starts_with("\\t"));
+    assert!(!trimmed_msgid.ends_with("\\t"));
     // Crude check for format strings. This might result in false positives.
     let format_string_annotation = if message_string.contains('%') {
         "#, c-format\n"