fixup! cast_possible_truncation: don't lint on valid constants

coauthor-by: [email protected]

Author: tesuji

clippy_lints/src/casts/cast_possible_truncation.rs

@@ -5,8 +5,9 @@ use clippy_utils::source::snippet;
 use clippy_utils::sugg::Sugg;
 use clippy_utils::ty::{get_discriminant_value, is_isize_or_usize};
 use rustc_errors::{Applicability, Diag, SuggestionStyle};
+use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
-use rustc_hir::{BinOpKind, Expr, ExprKind};
+use rustc_hir::{BinOpKind, Expr, ExprKind, QPath};
 use rustc_lint::LateContext;
 use rustc_middle::ty::{self, FloatTy, Ty};
 use rustc_span::symbol::sym;
@@ -80,7 +81,16 @@ fn apply_reductions(cx: &LateContext<'_>, nbits: u64, expr: &Expr<'_>, signed: b
                 nbits
             }
         },
-        ExprKind::Path(ref _qpath) => get_constant_bits(cx, expr).unwrap_or(nbits),
+        ExprKind::Path(QPath::Resolved(
+            None,
+            hir::Path {
+                res: Res::Def(DefKind::Const | DefKind::AssocConst, _id),
+                ..
+            },
+        )) => {
+            // `constant()` already checks if a const item is based on `cfg!`.
+            get_constant_bits(cx, expr).unwrap_or(nbits)
+        },
         // mem::size_of::<T>();
         ExprKind::Call(func, []) => {
             if let ExprKind::Path(ref func_qpath) = func.kind