use lint_unnecessary_cast for literals, suggest _ if not present

clippy_lints/src/casts/unnecessary_cast.rs

@@ -44,6 +44,15 @@ pub(super) fn check(
                 lint_unnecessary_cast(cx, expr, &literal_str, cast_from, cast_to);
             },
             LitKind::Int(_, LitIntType::Unsuffixed) | LitKind::Float(_, LitFloatType::Unsuffixed) => {},
+            LitKind::Int(_, LitIntType::Signed(_) | LitIntType::Unsigned(_))
+            | LitKind::Float(_, LitFloatType::Suffixed(_))
+                if cast_from.kind() == cast_to.kind() =>
+            {
+                if let Some(src) = snippet_opt(cx, lit.span) {
+                    let num_lit = NumericLiteral::from_lit_kind(&src, &lit.node).unwrap();
+                    lint_unnecessary_cast(cx, expr, num_lit.integer, cast_from, cast_to);
+                }
+            },
             _ => {
                 if cast_from.kind() == cast_to.kind() && !in_external_macro(cx.sess(), expr.span) {
                     span_lint_and_sugg(

tests/ui/unnecessary_cast.stderr

@@ -1,16 +1,16 @@
-error: casting to the same type is unnecessary (`i32` -> `i32`)
+error: casting integer literal to `i32` is unnecessary
   --> $DIR/unnecessary_cast.rs:6:5
    |
 LL |     1i32 as i32;
-   |     ^^^^^^^^^^^ help: try: `1i32`
+   |     ^^^^^^^^^^^ help: try: `1_i32`
    |
    = note: `-D clippy::unnecessary-cast` implied by `-D warnings`
 
-error: casting to the same type is unnecessary (`f32` -> `f32`)
+error: casting float literal to `f32` is unnecessary
   --> $DIR/unnecessary_cast.rs:7:5
    |
 LL |     1f32 as f32;
-   |     ^^^^^^^^^^^ help: try: `1f32`
+   |     ^^^^^^^^^^^ help: try: `1_f32`
 
 error: casting to the same type is unnecessary (`bool` -> `bool`)
   --> $DIR/unnecessary_cast.rs:8:5