1 use clippy_utils::diagnostics::span_lint;
2 use clippy_utils::qualify_min_const_fn::is_min_const_fn;
3 use clippy_utils::ty::has_drop;
4 use clippy_utils::{fn_has_unsatisfiable_preds, is_entrypoint_fn, meets_msrv, msrvs, trait_ref_of_method};
6 use rustc_hir::def_id::CRATE_DEF_ID;
7 use rustc_hir::intravisit::FnKind;
8 use rustc_hir::{Body, Constness, FnDecl, GenericParamKind, HirId};
9 use rustc_lint::{LateContext, LateLintPass};
10 use rustc_middle::lint::in_external_macro;
11 use rustc_semver::RustcVersion;
12 use rustc_session::{declare_tool_lint, impl_lint_pass};
14 use rustc_typeck::hir_ty_to_ty;
16 declare_clippy_lint! {
18 /// Suggests the use of `const` in functions and methods where possible.
20 /// ### Why is this bad?
21 /// Not having the function const prevents callers of the function from being const as well.
23 /// ### Known problems
24 /// Const functions are currently still being worked on, with some features only being available
25 /// on nightly. This lint does not consider all edge cases currently and the suggestions may be
26 /// incorrect if you are using this lint on stable.
28 /// Also, the lint only runs one pass over the code. Consider these two non-const functions:
39 /// When running Clippy, the lint will only suggest to make `a` const, because `b` at this time
40 /// can't be const as it calls a non-const function. Making `a` const and running Clippy again,
41 /// will suggest to make `b` const, too.
46 /// # random_number: usize,
49 /// fn new() -> Self {
50 /// Self { random_number: 42 }
55 /// Could be a const fn:
59 /// # random_number: usize,
62 /// const fn new() -> Self {
63 /// Self { random_number: 42 }
67 #[clippy::version = "1.34.0"]
68 pub MISSING_CONST_FOR_FN,
70 "Lint functions definitions that could be made `const fn`"
73 impl_lint_pass!(MissingConstForFn => [MISSING_CONST_FOR_FN]);
75 pub struct MissingConstForFn {
76 msrv: Option<RustcVersion>,
79 impl MissingConstForFn {
81 pub fn new(msrv: Option<RustcVersion>) -> Self {
86 impl<'tcx> LateLintPass<'tcx> for MissingConstForFn {
96 if !meets_msrv(self.msrv, msrvs::CONST_IF_MATCH) {
100 let def_id = cx.tcx.hir().local_def_id(hir_id);
102 if in_external_macro(cx.tcx.sess, span) || is_entrypoint_fn(cx, def_id.to_def_id()) {
106 // Building MIR for `fn`s with unsatisfiable preds results in ICE.
107 if fn_has_unsatisfiable_preds(cx, def_id.to_def_id()) {
111 // Perform some preliminary checks that rule out constness on the Clippy side. This way we
112 // can skip the actual const check and return early.
114 FnKind::ItemFn(_, generics, header, ..) => {
115 let has_const_generic_params = generics
118 .any(|param| matches!(param.kind, GenericParamKind::Const { .. }));
120 if already_const(header) || has_const_generic_params {
124 FnKind::Method(_, sig, ..) => {
125 if trait_ref_of_method(cx, def_id).is_some()
126 || already_const(sig.header)
127 || method_accepts_dropable(cx, sig.decl.inputs)
132 FnKind::Closure => return,
135 // Const fns are not allowed as methods in a trait.
137 let parent = cx.tcx.hir().get_parent_item(hir_id);
138 if parent != CRATE_DEF_ID {
139 if let hir::Node::Item(item) = cx.tcx.hir().get_by_def_id(parent) {
140 if let hir::ItemKind::Trait(..) = &item.kind {
147 let mir = cx.tcx.optimized_mir(def_id);
149 if let Err((span, err)) = is_min_const_fn(cx.tcx, mir, self.msrv) {
150 if cx.tcx.is_const_fn_raw(def_id.to_def_id()) {
151 cx.tcx.sess.span_err(span, err.as_ref());
154 span_lint(cx, MISSING_CONST_FOR_FN, span, "this could be a `const fn`");
157 extract_msrv_attr!(LateContext);
160 /// Returns true if any of the method parameters is a type that implements `Drop`. The method
161 /// can't be made const then, because `drop` can't be const-evaluated.
162 fn method_accepts_dropable(cx: &LateContext<'_>, param_tys: &[hir::Ty<'_>]) -> bool {
163 // If any of the params are droppable, return true
164 param_tys.iter().any(|hir_ty| {
165 let ty_ty = hir_ty_to_ty(cx.tcx, hir_ty);
170 // We don't have to lint on something that's already `const`
172 fn already_const(header: hir::FnHeader) -> bool {
173 header.constness == Constness::Const