Shrink parser positions from usize to u32.

The number of source code bytes can't exceed a `u32`'s range, so a token
position also can't. This reduces the size of `Parser` and
`LazyAttrTokenStreamImpl` by eight bytes each.

compiler/rustc_expand/src/mbe/diagnostics.rs

@@ -120,21 +120,21 @@ struct CollectTrackerAndEmitter<'a, 'cx, 'matcher> {
 
 struct BestFailure {
     token: Token,
-    position_in_tokenstream: usize,
+    position_in_tokenstream: u32,
     msg: &'static str,
     remaining_matcher: MatcherLoc,
 }
 
 impl BestFailure {
-    fn is_better_position(&self, position: usize) -> bool {
+    fn is_better_position(&self, position: u32) -> bool {
         position > self.position_in_tokenstream
     }
 }
 
 impl<'a, 'cx, 'matcher> Tracker<'matcher> for CollectTrackerAndEmitter<'a, 'cx, 'matcher> {
-    type Failure = (Token, usize, &'static str);
+    type Failure = (Token, u32, &'static str);
 
-    fn build_failure(tok: Token, position: usize, msg: &'static str) -> Self::Failure {
+    fn build_failure(tok: Token, position: u32, msg: &'static str) -> Self::Failure {
         (tok, position, msg)
     }
 
@@ -211,9 +211,9 @@ impl<'matcher> FailureForwarder<'matcher> {
 }
 
 impl<'matcher> Tracker<'matcher> for FailureForwarder<'matcher> {
-    type Failure = (Token, usize, &'static str);
+    type Failure = (Token, u32, &'static str);
 
-    fn build_failure(tok: Token, position: usize, msg: &'static str) -> Self::Failure {
+    fn build_failure(tok: Token, position: u32, msg: &'static str) -> Self::Failure {
         (tok, position, msg)
     }
 

compiler/rustc_expand/src/mbe/macro_parser.rs

@@ -452,7 +452,7 @@ impl TtParser {
         &mut self,
         matcher: &'matcher [MatcherLoc],
         token: &Token,
-        approx_position: usize,
+        approx_position: u32,
         track: &mut T,
     ) -> Option<NamedParseResult<T::Failure>> {
         // Matcher positions that would be valid if the macro invocation was over now. Only

compiler/rustc_expand/src/mbe/macro_rules.rs

@@ -153,7 +153,7 @@ pub(super) trait Tracker<'matcher> {
     /// Arm failed to match. If the token is `token::Eof`, it indicates an unexpected
     /// end of macro invocation. Otherwise, it indicates that no rules expected the given token.
     /// The usize is the approximate position of the token in the input token stream.
-    fn build_failure(tok: Token, position: usize, msg: &'static str) -> Self::Failure;
+    fn build_failure(tok: Token, position: u32, msg: &'static str) -> Self::Failure;
 
     /// This is called before trying to match next MatcherLoc on the current token.
     fn before_match_loc(&mut self, _parser: &TtParser, _matcher: &'matcher MatcherLoc) {}
@@ -182,7 +182,7 @@ pub(super) struct NoopTracker;
 impl<'matcher> Tracker<'matcher> for NoopTracker {
     type Failure = ();
 
-    fn build_failure(_tok: Token, _position: usize, _msg: &'static str) -> Self::Failure {}
+    fn build_failure(_tok: Token, _position: u32, _msg: &'static str) -> Self::Failure {}
 
     fn description() -> &'static str {
         "none"