Auto merge of #6369 - camsteffen:cast-cfg, r=Manishearth

Disable unnecessary_cast for cfg-dependant types

changelog: Disable unnecessary_cast for cfg-dependant types

Fix  #6331

clippy_lints/src/types.rs

@@ -8,6 +8,7 @@ use if_chain::if_chain;
 use rustc_ast::{FloatTy, IntTy, LitFloatType, LitIntType, LitKind, UintTy};
 use rustc_errors::{Applicability, DiagnosticBuilder};
 use rustc_hir as hir;
+use rustc_hir::def::Res;
 use rustc_hir::intravisit::{walk_body, walk_expr, walk_ty, FnKind, NestedVisitorMap, Visitor};
 use rustc_hir::{
     BinOpKind, Block, Body, Expr, ExprKind, FnDecl, FnRetTy, FnSig, GenericArg, GenericBounds, GenericParamKind, HirId,
@@ -1632,7 +1633,14 @@ impl<'tcx> LateLintPass<'tcx> for Casts {
         if expr.span.from_expansion() {
             return;
         }
-        if let ExprKind::Cast(ref ex, _) = expr.kind {
+        if let ExprKind::Cast(ref ex, cast_to) = expr.kind {
+            if let TyKind::Path(QPath::Resolved(_, path)) = cast_to.kind {
+                if let Res::Def(_, def_id) = path.res {
+                    if cx.tcx.has_attr(def_id, sym::cfg) || cx.tcx.has_attr(def_id, sym::cfg_attr) {
+                        return;
+                    }
+                }
+            }
             let (cast_from, cast_to) = (cx.typeck_results().expr_ty(ex), cx.typeck_results().expr_ty(expr));
             lint_fn_to_numeric_cast(cx, expr, ex, cast_from, cast_to);
             if let Some(lit) = get_numeric_literal(ex) {

tests/ui/unnecessary_cast.rs

@@ -20,4 +20,7 @@ fn main() {
     foo!(a, i32);
     foo!(b, f32);
     foo!(c, f64);
+
+    // do not lint cast to cfg-dependant type
+    1 as std::os::raw::c_char;
 }