Add enum for find_width_map_from_snippet

This makes the relationship between the vec and the boolean clearer.

compiler/rustc_parse_format/src/lib.rs

@@ -57,6 +57,13 @@ impl InnerWidthMapping {
     }
 }
 
+/// Whether the input string is a literal. If yes, it contains the inner width mappings.
+#[derive(Clone, PartialEq, Eq)]
+enum InputStringKind {
+    NotALiteral,
+    Literal { width_mappings: Vec<InnerWidthMapping> },
+}
+
 /// The type of format string that we are parsing.
 #[derive(Copy, Clone, Debug, Eq, PartialEq)]
 pub enum ParseMode {
@@ -307,7 +314,11 @@ impl<'a> Parser<'a> {
         append_newline: bool,
         mode: ParseMode,
     ) -> Parser<'a> {
-        let (width_map, is_literal) = find_width_map_from_snippet(s, snippet, style);
+        let input_string_kind = find_width_map_from_snippet(s, snippet, style);
+        let (width_map, is_literal) = match input_string_kind {
+            InputStringKind::Literal { width_mappings } => (width_mappings, true),
+            InputStringKind::NotALiteral => (Vec::new(), false),
+        };
         Parser {
             mode,
             input: s,
@@ -848,14 +859,14 @@ fn find_width_map_from_snippet(
     input: &str,
     snippet: Option<string::String>,
     str_style: Option<usize>,
-) -> (Vec<InnerWidthMapping>, bool) {
+) -> InputStringKind {
     let snippet = match snippet {
         Some(ref s) if s.starts_with('"') || s.starts_with("r\"") || s.starts_with("r#") => s,
-        _ => return (vec![], false),
+        _ => return InputStringKind::NotALiteral,
     };
 
     if str_style.is_some() {
-        return (vec![], true);
+        return InputStringKind::Literal { width_mappings: Vec::new() };
     }
 
     // Strip quotes.
@@ -868,15 +879,15 @@ fn find_width_map_from_snippet(
     // Alternatively, we could just count the trailing newlines and only trim one from the input if they don't match up.
     let input_no_nl = input.trim_end_matches('\n');
     let Ok(unescaped) = unescape_string(snippet) else {
-        return (vec![], false);
+        return InputStringKind::NotALiteral;
     };
 
     let unescaped_no_nl = unescaped.trim_end_matches('\n');
 
     if unescaped_no_nl != input_no_nl {
         // The source string that we're pointing at isn't our input, so spans pointing at it will be incorrect.
         // This can for example happen with proc macros that respan generated literals.
-        return (vec![], false);
+        return InputStringKind::NotALiteral;
     }
 
     let mut s = snippet.char_indices();
@@ -958,7 +969,7 @@ fn find_width_map_from_snippet(
         }
     }
 
-    (width_mappings, true)
+    InputStringKind::Literal { width_mappings }
 }
 
 fn unescape_string(string: &str) -> Result<string::String, unescape::EscapeError> {