1 use crate::utils::{is_entrypoint_fn, span_lint};
2 use if_chain::if_chain;
4 use rustc::hir::intravisit::FnKind;
5 use rustc::hir::{Body, Constness, FnDecl, HirId};
6 use rustc::lint::{in_external_macro, LateContext, LateLintPass, LintArray, LintPass};
7 use rustc::{declare_tool_lint, lint_array};
8 use rustc_mir::transform::qualify_min_const_fn::is_min_const_fn;
13 /// Suggests the use of `const` in functions and methods where possible.
15 /// **Why is this bad?**
17 /// Not having the function const prevents callers of the function from being const as well.
19 /// **Known problems:**
21 /// Const functions are currently still being worked on, with some features only being available
22 /// on nightly. This lint does not consider all edge cases currently and the suggestions may be
23 /// incorrect if you are using this lint on stable.
25 /// Also, the lint only runs one pass over the code. Consider these two non-const functions:
36 /// When running Clippy, the lint will only suggest to make `a` const, because `b` at this time
37 /// can't be const as it calls a non-const function. Making `a` const and running Clippy again,
38 /// will suggest to make `b` const, too.
43 /// fn new() -> Self {
44 /// Self { random_number: 42 }
48 /// Could be a const fn:
51 /// const fn new() -> Self {
52 /// Self { random_number: 42 }
55 declare_clippy_lint! {
56 pub MISSING_CONST_FOR_FN,
58 "Lint functions definitions that could be made `const fn`"
62 pub struct MissingConstForFn;
64 impl LintPass for MissingConstForFn {
65 fn get_lints(&self) -> LintArray {
66 lint_array!(MISSING_CONST_FOR_FN)
69 fn name(&self) -> &'static str {
74 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for MissingConstForFn {
77 cx: &LateContext<'_, '_>,
84 let def_id = cx.tcx.hir().local_def_id_from_hir_id(hir_id);
86 if in_external_macro(cx.tcx.sess, span) || is_entrypoint_fn(cx, def_id) {
90 // Perform some preliminary checks that rule out constness on the Clippy side. This way we
91 // can skip the actual const check and return early.
93 FnKind::ItemFn(_, _, header, ..) => {
94 if already_const(header) {
98 FnKind::Method(_, sig, ..) => {
99 if is_trait_method(cx, hir_id) || already_const(sig.header) {
106 let mir = cx.tcx.optimized_mir(def_id);
108 if let Err((span, err)) = is_min_const_fn(cx.tcx, def_id, &mir) {
109 if cx.tcx.is_min_const_fn(def_id) {
110 cx.tcx.sess.span_err(span, &err);
113 span_lint(cx, MISSING_CONST_FOR_FN, span, "this could be a const_fn");
118 fn is_trait_method(cx: &LateContext<'_, '_>, hir_id: HirId) -> bool {
119 // Get the implemented trait for the current function
120 let parent_impl = cx.tcx.hir().get_parent_item(hir_id);
122 if parent_impl != hir::CRATE_HIR_ID;
123 if let hir::Node::Item(item) = cx.tcx.hir().get_by_hir_id(parent_impl);
124 if let hir::ItemKind::Impl(_, _, _, _, Some(_trait_ref), _, _) = &item.node;
125 then { return true; }
130 // We don't have to lint on something that's already `const`
131 fn already_const(header: hir::FnHeader) -> bool {
132 header.constness == Constness::Const