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Fix two missing_const_for_fn false positives #3844

merged 2 commits into from Mar 5, 2019
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Just for now

@@ -1,8 +1,9 @@
use crate::utils::{is_entrypoint_fn, span_lint};
use if_chain::if_chain;
use rustc::hir;
use rustc::hir::intravisit::FnKind;
use rustc::hir::{Body, Constness, FnDecl, HirId};
use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
use rustc::lint::{in_external_macro, LateContext, LateLintPass, LintArray, LintPass};
use rustc::{declare_tool_lint, lint_array};
use rustc_mir::transform::qualify_min_const_fn::is_min_const_fn;
use syntax_pos::Span;
@@ -82,7 +83,7 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for MissingConstForFn {
) {
let def_id = cx.tcx.hir().local_def_id_from_hir_id(hir_id);

if is_entrypoint_fn(cx, def_id) {
if in_external_macro(cx.tcx.sess, span) || is_entrypoint_fn(cx, def_id) {

@@ -95,7 +96,7 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for MissingConstForFn {
FnKind::Method(_, sig, ..) => {
if already_const(sig.header) {
if is_trait_method(cx, hir_id) || already_const(sig.header) {
@@ -114,6 +115,18 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for MissingConstForFn {

fn is_trait_method(cx: &LateContext<'_, '_>, hir_id: HirId) -> bool {
This conversation was marked as resolved by oli-obk

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oli-obk Mar 5, 2019


I think you may be able to use instead of this function. There's no docs on the function, so it might have panic cases, but as long as it's only used on methods, it should be fine

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phansch Mar 5, 2019

Author Collaborator

I tried that and it only reported the traits of the default impl methods, not of normal trait methods like in the example :/

// Get the implemented trait for the current function
let parent_impl = cx.tcx.hir().get_parent_item(hir_id);
if_chain! {
if parent_impl != hir::CRATE_HIR_ID;
if let hir::Node::Item(item) = cx.tcx.hir().get_by_hir_id(parent_impl);
if let hir::ItemKind::Impl(_, _, _, _, Some(_trait_ref), _, _) = &item.node;
then { return true; }

// We don't have to lint on something that's already `const`
fn already_const(header: hir::FnHeader) -> bool {
header.constness == Constness::Const
@@ -55,3 +55,16 @@ trait Foo {

// Don't lint in external macros (derive)
#[derive(PartialEq, Eq)]
struct Point(isize, isize);

impl std::ops::Add for Point {
type Output = Self;

// Don't lint in trait impls of derived methods
fn add(self, other: Self) -> Self {
Point(self.0 + other.0, self.1 + other.1)
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