update Display to carry index

Author: eopb

library/core/src/iter/traits/iterator.rs

@@ -1588,7 +1588,7 @@ pub trait Iterator {
     /// This will print:
     ///
     /// ```text
-    /// Parsing error: invalid digit found in string
+    /// Parsing error: invalid digit found at index 0
     /// Sum: 3
     /// ```
     #[inline]

library/core/src/num/error.rs

@@ -146,6 +146,11 @@ impl ParseIntError {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl fmt::Display for ParseIntError {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        self.__description().fmt(f)
+        match self.kind {
+            IntErrorKind::InvalidDigit(i) => {
+                write!(f, "invalid digit found at index {}", i)
+            }
+            _ => self.__description().fmt(f),
+        }
     }
 }

src/test/ui/feature-gates/unstable-attribute-allow-issue-0.stderr

@@ -12,7 +12,7 @@ error[E0545]: `issue` must be a non-zero numeric string or "none"
 LL | #[unstable(feature = "unstable_test_feature", issue = "something")]
    |                                               ^^^^^^^^-----------
    |                                                       |
-   |                                                       invalid digit found in string
+   |                                                       invalid digit found at index 0
 
 error: aborting due to 2 previous errors
 

src/test/ui/stability-attribute/stability-attribute-sanity-2.stderr

@@ -16,7 +16,7 @@ error[E0545]: `issue` must be a non-zero numeric string or "none"
 LL | #[unstable(feature = "a", issue = "no")]
    |                           ^^^^^^^^----
    |                                   |
-   |                                   invalid digit found in string
+   |                                   invalid digit found at index 0
 
 error: aborting due to 3 previous errors