1 use crate::utils::{fn_has_unsatisfiable_preds, has_drop, is_entrypoint_fn, span_lint, trait_ref_of_method};
2 use rustc::lint::in_external_macro;
4 use rustc_hir::intravisit::FnKind;
5 use rustc_hir::{Body, Constness, FnDecl, GenericParamKind, HirId};
6 use rustc_lint::{LateContext, LateLintPass};
7 use rustc_mir::transform::qualify_min_const_fn::is_min_const_fn;
8 use rustc_session::{declare_lint_pass, declare_tool_lint};
10 use rustc_typeck::hir_ty_to_ty;
13 declare_clippy_lint! {
16 /// Suggests the use of `const` in functions and methods where possible.
18 /// **Why is this bad?**
20 /// Not having the function const prevents callers of the function from being const as well.
22 /// **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.
47 /// # random_number: usize,
50 /// fn new() -> Self {
51 /// Self { random_number: 42 }
56 /// Could be a const fn:
60 /// # random_number: usize,
63 /// const fn new() -> Self {
64 /// Self { random_number: 42 }
68 pub MISSING_CONST_FOR_FN,
70 "Lint functions definitions that could be made `const fn`"
73 declare_lint_pass!(MissingConstForFn => [MISSING_CONST_FOR_FN]);
75 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for MissingConstForFn {
78 cx: &LateContext<'_, '_>,
85 let def_id = cx.tcx.hir().local_def_id(hir_id);
87 if in_external_macro(cx.tcx.sess, span) || is_entrypoint_fn(cx, def_id) {
91 // Building MIR for `fn`s with unsatisfiable preds results in ICE.
92 if fn_has_unsatisfiable_preds(cx, def_id) {
96 // Perform some preliminary checks that rule out constness on the Clippy side. This way we
97 // can skip the actual const check and return early.
99 FnKind::ItemFn(_, generics, header, ..) => {
100 let has_const_generic_params = generics
103 .any(|param| matches!(param.kind, GenericParamKind::Const{ .. }));
105 if already_const(header) || has_const_generic_params {
109 FnKind::Method(_, sig, ..) => {
110 if trait_ref_of_method(cx, hir_id).is_some()
111 || already_const(sig.header)
112 || method_accepts_dropable(cx, sig.decl.inputs)
120 let mir = cx.tcx.optimized_mir(def_id);
122 if let Err((span, err)) = is_min_const_fn(cx.tcx, def_id, &mir) {
123 if rustc_mir::const_eval::is_min_const_fn(cx.tcx, def_id) {
124 cx.tcx.sess.span_err(span, &err);
127 span_lint(cx, MISSING_CONST_FOR_FN, span, "this could be a `const fn`");
132 /// Returns true if any of the method parameters is a type that implements `Drop`. The method
133 /// can't be made const then, because `drop` can't be const-evaluated.
134 fn method_accepts_dropable(cx: &LateContext<'_, '_>, param_tys: &[hir::Ty<'_>]) -> bool {
135 // If any of the params are dropable, return true
136 param_tys.iter().any(|hir_ty| {
137 let ty_ty = hir_ty_to_ty(cx.tcx, hir_ty);
142 // We don't have to lint on something that's already `const`
144 fn already_const(header: hir::FnHeader) -> bool {
145 header.constness == Constness::Const