Improve format string errors

 - Point at format string position inside the formatting string
 - Explain that argument names can't start with an underscore

1 files changed, 7 insertions, 0 deletions

diff --git a/src/libsyntax_pos/lib.rs b/src/libsyntax_pos/lib.rs
index 8b4a3ea26a1..e1693ff4db6 100644
--- a/src/libsyntax_pos/lib.rs
+++ b/src/libsyntax_pos/lib.rs
@@ -427,6 +427,13 @@ impl Span {
         )
     }
 
+    pub fn from_inner_byte_pos(self, start: usize, end: usize) -> Span {
+        let span = self.data();
+        Span::new(span.lo + BytePos::from_usize(start),
+                  span.lo + BytePos::from_usize(end),
+                  span.ctxt)
+    }
+
     #[inline]
     pub fn apply_mark(self, mark: Mark) -> Span {
         let span = self.data();