Handle negative literals in cast overflow warning

compiler/rustc_lint/src/types.rs

@@ -217,7 +217,11 @@ fn report_bin_hex_error(
  cx.struct_span_lint(OVERFLOWING_LITERALS, expr.span, |lint| {
  let (t, actually) = match ty {
  attr::IntType::SignedInt(t) => {
- let actually = size.sign_extend(val) as i128;
+ let actually = if negative {
+ -(size.sign_extend(val) as i128)
+ } else {
+ size.sign_extend(val) as i128
+ };
  (t.name_str(), actually.to_string())
  }
  attr::IntType::UnsignedInt(t) => {
@@ -226,11 +230,22 @@ fn report_bin_hex_error(
  }
  };
  let mut err = lint.build(&format!("literal out of range for `{}`", t));
- err.note(&format!(
- "the literal `{}` (decimal `{}`) does not fit into \
- the type `{}` and will become `{}{}`",
- repr_str, val, t, actually, t
- ));
+ if negative {
+ // If the value is negative,
+ // emits a note about the value itself, apart from the literal.
+ err.note(&format!(
+ "the literal `{}` (decimal `{}`) does not fit into \
+ the type `{}`",
+ repr_str, val, t
+ ));
+ err.note(&format!("and the value `-{}` will become `{}{}`", repr_str, actually, t));
+ } else {
+ err.note(&format!(
+ "the literal `{}` (decimal `{}`) does not fit into \
+ the type `{}` and will become `{}{}`",
+ repr_str, val, t, actually, t
+ ));
+ }
  if let Some(sugg_ty) =
  get_type_suggestion(&cx.typeck_results().node_type(expr.hir_id), val, negative)
  {

src/test/ui/lint/type-overflow.stderr

@@ -60,7 +60,8 @@ warning: literal out of range for `i8`
 LL | let fail = -0b1111_1111i8;
  | ^^^^^^^^^^^^^ help: consider using the type `i16` instead: `0b1111_1111i16`
  |
- = note: the literal `0b1111_1111i8` (decimal `255`) does not fit into the type `i8` and will become `-1i8`
+ = note: the literal `0b1111_1111i8` (decimal `255`) does not fit into the type `i8`
+ = note: and the value `-0b1111_1111i8` will become `1i8`
 
 warning: 7 warnings emitted