Fix false positive in missing_const_for_fn

This fixes some additional false positives with functions or methods
that return types which implement drop. Since `drop` can't be
const-evaluated, these cases can't be made const.

Fixes rust-lang#4979

clippy_lints/src/missing_const_for_fn.rs

@@ -1,4 +1,4 @@
-use crate::utils::{has_drop, is_entrypoint_fn, span_lint, trait_ref_of_method};
+use crate::utils::{has_drop, is_entrypoint_fn, return_ty, span_lint, trait_ref_of_method};
 use rustc::declare_lint_pass;
 use rustc::hir;
 use rustc::hir::intravisit::FnKind;
@@ -91,14 +91,15 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for MissingConstForFn {
         // can skip the actual const check and return early.
         match kind {
             FnKind::ItemFn(_, _, header, ..) => {
-                if already_const(header) {
+                if already_const(header) || returns_dropable(cx, hir_id) {
                     return;
                 }
             },
             FnKind::Method(_, sig, ..) => {
                 if trait_ref_of_method(cx, hir_id).is_some()
                     || already_const(sig.header)
                     || method_accepts_dropable(cx, sig.decl.inputs)
+                    || returns_dropable(cx, hir_id)
                 {
                     return;
                 }
@@ -128,6 +129,11 @@ fn method_accepts_dropable(cx: &LateContext<'_, '_>, param_tys: &[hir::Ty<'_>])
     })
 }
 
+fn returns_dropable(cx: &LateContext<'_, '_>, ret_ty: hir::HirId) -> bool {
+    let return_ty = return_ty(cx, ret_ty);
+    has_drop(cx, return_ty)
+}
+
 // We don't have to lint on something that's already `const`
 #[must_use]
 fn already_const(header: hir::FnHeader) -> bool {

tests/ui/missing_const_for_fn/cant_be_const.rs

@@ -89,4 +89,29 @@ mod with_drop {
             B
         }
     }
+
+    struct CustomDrop {
+        field: i32,
+    }
+
+    impl Drop for CustomDrop {
+        fn drop(&mut self) {}
+    }
+
+    struct Bar {
+        field: CustomDrop,
+    }
+
+    impl Bar {
+        fn take(self) -> CustomDrop {
+            self.field
+        }
+    }
+
+    fn take() -> CustomDrop {
+        let x = Bar {
+            field: CustomDrop { field: 1 },
+        };
+        x.field
+    }
 }