1 use crate::utils::{is_entrypoint_fn, span_lint};
3 use rustc::hir::intravisit::FnKind;
4 use rustc::hir::{Body, Constness, FnDecl, HirId};
5 use rustc::lint::{in_external_macro, LateContext, LateLintPass, LintArray, LintPass};
6 use rustc::{declare_tool_lint, lint_array};
7 use rustc_mir::transform::qualify_min_const_fn::is_min_const_fn;
12 /// Suggests the use of `const` in functions and methods where possible.
14 /// **Why is this bad?**
16 /// Not having the function const prevents callers of the function from being const as well.
18 /// **Known problems:**
20 /// Const functions are currently still being worked on, with some features only being available
21 /// on nightly. This lint does not consider all edge cases currently and the suggestions may be
22 /// incorrect if you are using this lint on stable.
24 /// Also, the lint only runs one pass over the code. Consider these two non-const functions:
35 /// When running Clippy, the lint will only suggest to make `a` const, because `b` at this time
36 /// can't be const as it calls a non-const function. Making `a` const and running Clippy again,
37 /// will suggest to make `b` const, too.
42 /// fn new() -> Self {
43 /// Self { random_number: 42 }
47 /// Could be a const fn:
50 /// const fn new() -> Self {
51 /// Self { random_number: 42 }
54 declare_clippy_lint! {
55 pub MISSING_CONST_FOR_FN,
57 "Lint functions definitions that could be made `const fn`"
61 pub struct MissingConstForFn;
63 impl LintPass for MissingConstForFn {
64 fn get_lints(&self) -> LintArray {
65 lint_array!(MISSING_CONST_FOR_FN)
68 fn name(&self) -> &'static str {
73 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for MissingConstForFn {
76 cx: &LateContext<'_, '_>,
83 let def_id = cx.tcx.hir().local_def_id_from_hir_id(hir_id);
85 if in_external_macro(cx.tcx.sess, span) || is_entrypoint_fn(cx, def_id) {
89 // Perform some preliminary checks that rule out constness on the Clippy side. This way we
90 // can skip the actual const check and return early.
92 FnKind::ItemFn(_, _, header, ..) => {
93 if already_const(header) {
97 FnKind::Method(_, sig, ..) => {
98 if already_const(sig.header) {
105 let mir = cx.tcx.optimized_mir(def_id);
107 if let Err((span, err)) = is_min_const_fn(cx.tcx, def_id, &mir) {
108 if cx.tcx.is_min_const_fn(def_id) {
109 cx.tcx.sess.span_err(span, &err);
112 span_lint(cx, MISSING_CONST_FOR_FN, span, "this could be a const_fn");
117 // We don't have to lint on something that's already `const`
118 fn already_const(header: hir::FnHeader) -> bool {
119 header.constness == Constness::Const