src/tools/clippy/clippy_lints/src/missing_const_for_fn.rs

   1 use clippy_utils::diagnostics::span_lint;
   2 use clippy_utils::msrvs::{self, Msrv};
   3 use clippy_utils::qualify_min_const_fn::is_min_const_fn;
   4 use clippy_utils::ty::has_drop;
   5 use clippy_utils::{fn_has_unsatisfiable_preds, is_entrypoint_fn, is_from_proc_macro, trait_ref_of_method};
   6 use rustc_hir as hir;
   7 use rustc_hir::def_id::CRATE_DEF_ID;
   8 use rustc_hir::intravisit::FnKind;
   9 use rustc_hir::{Body, Constness, FnDecl, GenericParamKind, HirId};
  10 use rustc_hir_analysis::hir_ty_to_ty;
  11 use rustc_lint::{LateContext, LateLintPass};
  12 use rustc_middle::lint::in_external_macro;
  13 use rustc_session::{declare_tool_lint, impl_lint_pass};
  14 use rustc_span::Span;
  15
  16 declare_clippy_lint! {
  17     /// ### What it does
  18     /// Suggests the use of `const` in functions and methods where possible.
  19     ///
  20     /// ### Why is this bad?
  21     /// Not having the function const prevents callers of the function from being const as well.
  22     ///
  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.
  27     ///
  28     /// Also, the lint only runs one pass over the code. Consider these two non-const functions:
  29     ///
  30     /// ```rust
  31     /// fn a() -> i32 {
  32     ///     0
  33     /// }
  34     /// fn b() -> i32 {
  35     ///     a()
  36     /// }
  37     /// ```
  38     ///
  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.
  42     ///
  43     /// ### Example
  44     /// ```rust
  45     /// # struct Foo {
  46     /// #     random_number: usize,
  47     /// # }
  48     /// # impl Foo {
  49     /// fn new() -> Self {
  50     ///     Self { random_number: 42 }
  51     /// }
  52     /// # }
  53     /// ```
  54     ///
  55     /// Could be a const fn:
  56     ///
  57     /// ```rust
  58     /// # struct Foo {
  59     /// #     random_number: usize,
  60     /// # }
  61     /// # impl Foo {
  62     /// const fn new() -> Self {
  63     ///     Self { random_number: 42 }
  64     /// }
  65     /// # }
  66     /// ```
  67     #[clippy::version = "1.34.0"]
  68     pub MISSING_CONST_FOR_FN,
  69     nursery,
  70     "Lint functions definitions that could be made `const fn`"
  71 }
  72
  73 impl_lint_pass!(MissingConstForFn => [MISSING_CONST_FOR_FN]);
  74
  75 pub struct MissingConstForFn {
  76     msrv: Msrv,
  77 }
  78
  79 impl MissingConstForFn {
  80     #[must_use]
  81     pub fn new(msrv: Msrv) -> Self {
  82         Self { msrv }
  83     }
  84 }
  85
  86 impl<'tcx> LateLintPass<'tcx> for MissingConstForFn {
  87     fn check_fn(
  88         &mut self,
  89         cx: &LateContext<'tcx>,
  90         kind: FnKind<'tcx>,
  91         _: &FnDecl<'_>,
  92         body: &Body<'tcx>,
  93         span: Span,
  94         hir_id: HirId,
  95     ) {
  96         if !self.msrv.meets(msrvs::CONST_IF_MATCH) {
  97             return;
  98         }
  99
 100         let def_id = cx.tcx.hir().local_def_id(hir_id);
 101
 102         if in_external_macro(cx.tcx.sess, span) || is_entrypoint_fn(cx, def_id.to_def_id()) {
 103             return;
 104         }
 105
 106         // Building MIR for `fn`s with unsatisfiable preds results in ICE.
 107         if fn_has_unsatisfiable_preds(cx, def_id.to_def_id()) {
 108             return;
 109         }
 110
 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.
 113         match kind {
 114             FnKind::ItemFn(_, generics, header, ..) => {
 115                 let has_const_generic_params = generics
 116                     .params
 117                     .iter()
 118                     .any(|param| matches!(param.kind, GenericParamKind::Const { .. }));
 119
 120                 if already_const(header) || has_const_generic_params {
 121                     return;
 122                 }
 123             },
 124             FnKind::Method(_, sig, ..) => {
 125                 if trait_ref_of_method(cx, def_id).is_some()
 126                     || already_const(sig.header)
 127                     || method_accepts_droppable(cx, sig.decl.inputs)
 128                 {
 129                     return;
 130                 }
 131             },
 132             FnKind::Closure => return,
 133         }
 134
 135         // Const fns are not allowed as methods in a trait.
 136         {
 137             let parent = cx.tcx.hir().get_parent_item(hir_id).def_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 {
 141                         return;
 142                     }
 143                 }
 144             }
 145         }
 146
 147         if is_from_proc_macro(cx, &(&kind, body, hir_id, span)) {
 148             return;
 149         }
 150
 151         let mir = cx.tcx.optimized_mir(def_id);
 152
 153         if let Err((span, err)) = is_min_const_fn(cx.tcx, mir, &self.msrv) {
 154             if cx.tcx.is_const_fn_raw(def_id.to_def_id()) {
 155                 cx.tcx.sess.span_err(span, err.as_ref());
 156             }
 157         } else {
 158             span_lint(cx, MISSING_CONST_FOR_FN, span, "this could be a `const fn`");
 159         }
 160     }
 161     extract_msrv_attr!(LateContext);
 162 }
 163
 164 /// Returns true if any of the method parameters is a type that implements `Drop`. The method
 165 /// can't be made const then, because `drop` can't be const-evaluated.
 166 fn method_accepts_droppable(cx: &LateContext<'_>, param_tys: &[hir::Ty<'_>]) -> bool {
 167     // If any of the params are droppable, return true
 168     param_tys.iter().any(|hir_ty| {
 169         let ty_ty = hir_ty_to_ty(cx.tcx, hir_ty);
 170         has_drop(cx, ty_ty)
 171     })
 172 }
 173
 174 // We don't have to lint on something that's already `const`
 175 #[must_use]
 176 fn already_const(header: hir::FnHeader) -> bool {
 177     header.constness == Constness::Const
 178 }