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Fix two missing_const_for_fn false positives #3844

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Mar 5, 2019
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19 changes: 16 additions & 3 deletions clippy_lints/src/missing_const_for_fn.rs
Original file line number Diff line number Diff line change
@@ -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;
Expand Down Expand Up @@ -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) {
return;
}

Expand All @@ -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) {
return;
}
},
Expand All @@ -114,6 +115,18 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for MissingConstForFn {
}
}

fn is_trait_method(cx: &LateContext<'_, '_>, hir_id: HirId) -> bool {
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// 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; }
}
false
}

// We don't have to lint on something that's already `const`
fn already_const(header: hir::FnHeader) -> bool {
header.constness == Constness::Const
Expand Down
13 changes: 13 additions & 0 deletions tests/ui/missing_const_for_fn/cant_be_const.rs
Original file line number Diff line number Diff line change
Expand Up @@ -55,3 +55,16 @@ trait Foo {
33
}
}

// 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)
}
}