Add diagnostic width span when '0$' is used as width.

compiler/rustc_parse_format/src/lib.rs

@@ -572,9 +572,10 @@ fn format(&mut self) -> FormatSpec<'a> {
             // '0' flag and then an ill-formatted format string with just a '$'
             // and no count, but this is better if we instead interpret this as
             // no '0' flag and '0$' as the width instead.
-            if self.consume('$') {
+            if let Some(end) = self.consume_pos('$') {
                 spec.width = CountIsParam(0);
                 havewidth = true;
+                spec.width_span = Some(self.to_span_index(end - 1).to(self.to_span_index(end + 1)));
             } else {
                 spec.flags |= 1 << (FlagSignAwareZeroPad as u32);
             }

compiler/rustc_parse_format/src/tests.rs

@@ -178,6 +178,23 @@ fn format_counts() {
             },
         })],
     );
+    same(
+        "{1:0$.10x}",
+        &[NextArgument(Argument {
+            position: ArgumentIs(1),
+            format: FormatSpec {
+                fill: None,
+                align: AlignUnknown,
+                flags: 0,
+                precision: CountIs(10),
+                width: CountIsParam(0),
+                precision_span: None,
+                width_span: Some(InnerSpan::new(4, 6)),
+                ty: "x",
+                ty_span: None,
+            },
+        })],
+    );
     same(
         "{:.*x}",
         &[NextArgument(Argument {