Fix missing_const_for_fn false positive

We don't want to lint if the type of the method implements drop.

(constant functions cannot evaluate destructors)

clippy_lints/src/missing_const_for_fn.rs

@@ -1,10 +1,11 @@
-use crate::utils::{is_entrypoint_fn, span_lint, trait_ref_of_method};
+use crate::utils::{has_drop, is_entrypoint_fn, is_self_ty, span_lint, trait_ref_of_method};
 use rustc::hir;
 use rustc::hir::intravisit::FnKind;
 use rustc::hir::{Body, Constness, FnDecl, HirId};
 use rustc::lint::{in_external_macro, LateContext, LateLintPass, LintArray, LintPass};
 use rustc::{declare_lint_pass, declare_tool_lint};
 use rustc_mir::transform::qualify_min_const_fn::is_min_const_fn;
+use rustc_typeck::hir_ty_to_ty;
 use syntax_pos::Span;
 
 declare_clippy_lint! {
@@ -94,6 +95,15 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for MissingConstForFn {
                 }
             },
             FnKind::Method(_, sig, ..) => {
+                // Don't lint if `Self` implements drop
+                let input_tys = &sig.decl.inputs;
+                if input_tys.len() > 0 {
+                    let hir_ty = &input_tys[0];
+                    let ty_ty = hir_ty_to_ty(cx.tcx, hir_ty);
+                    if is_self_ty(hir_ty) && has_drop(cx, ty_ty) {
+                        return;
+                    }
+                }
                 if trait_ref_of_method(cx, hir_id).is_some() || already_const(sig.header) {
                     return;
                 }

tests/ui/missing_const_for_fn/cant_be_const.rs

@@ -68,3 +68,16 @@ impl std::ops::Add for Point {
         Point(self.0 + other.0, self.1 + other.1)
     }
 }
+
+pub struct A;
+impl A {
+    // This should not be const because the type implements `Drop`.
+    pub fn a(self) -> B {
+        B
+    }
+}
+impl Drop for A {
+    fn drop(&mut self) {}
+}
+
+pub struct B;