move derefs_to_slice to methods/utils.rs

clippy_lints/src/methods/utils.rs

@@ -0,0 +1,46 @@
+use crate::utils::is_type_diagnostic_item;
+use rustc_hir as hir;
+use rustc_lint::LateContext;
+use rustc_middle::ty;
+use rustc_middle::ty::Ty;
+use rustc_span::symbol::sym;
+
+pub fn derefs_to_slice<'tcx>(
+    cx: &LateContext<'tcx>,
+    expr: &'tcx hir::Expr<'tcx>,
+    ty: Ty<'tcx>,
+) -> Option<&'tcx hir::Expr<'tcx>> {
+    fn may_slice<'a>(cx: &LateContext<'a>, ty: Ty<'a>) -> bool {
+        match ty.kind() {
+            ty::Slice(_) => true,
+            ty::Adt(def, _) if def.is_box() => may_slice(cx, ty.boxed_ty()),
+            ty::Adt(..) => is_type_diagnostic_item(cx, ty, sym::vec_type),
+            ty::Array(_, size) => size
+                .try_eval_usize(cx.tcx, cx.param_env)
+                .map_or(false, |size| size < 32),
+            ty::Ref(_, inner, _) => may_slice(cx, inner),
+            _ => false,
+        }
+    }
+
+    if let hir::ExprKind::MethodCall(ref path, _, ref args, _) = expr.kind {
+        if path.ident.name == sym::iter && may_slice(cx, cx.typeck_results().expr_ty(&args[0])) {
+            Some(&args[0])
+        } else {
+            None
+        }
+    } else {
+        match ty.kind() {
+            ty::Slice(_) => Some(expr),
+            ty::Adt(def, _) if def.is_box() && may_slice(cx, ty.boxed_ty()) => Some(expr),
+            ty::Ref(_, inner, _) => {
+                if may_slice(cx, inner) {
+                    Some(expr)
+                } else {
+                    None
+                }
+            },
+            _ => None,
+        }
+    }
+}