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

   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::{
   5     fn_has_unsatisfiable_preds, is_entrypoint_fn, is_from_proc_macro, meets_msrv, msrvs, trait_ref_of_method,
   6 };
   7 use rustc_hir as hir;
   8 use rustc_hir::def_id::CRATE_DEF_ID;
   9 use rustc_hir::intravisit::FnKind;
  10 use rustc_hir::{Body, Constness, FnDecl, GenericParamKind, HirId};
  11 use rustc_hir_analysis::hir_ty_to_ty;
  12 use rustc_lint::{LateContext, LateLintPass};
  13 use rustc_middle::lint::in_external_macro;
  14 use rustc_semver::RustcVersion;
  15 use rustc_session::{declare_tool_lint, impl_lint_pass};
  16 use rustc_span::Span;
  17
  18 declare_clippy_lint! {
  19     /// ### What it does
  20     /// Suggests the use of `const` in functions and methods where possible.
  21     ///
  22     /// ### Why is this bad?
  23     /// Not having the function const prevents callers of the function from being const as well.
  24     ///
  25     /// ### Known problems
  26     /// Const functions are currently still being worked on, with some features only being available
  27     /// on nightly. This lint does not consider all edge cases currently and the suggestions may be
  28     /// incorrect if you are using this lint on stable.
  29     ///
  30     /// Also, the lint only runs one pass over the code. Consider these two non-const functions:
  31     ///
  32     /// ```rust
  33     /// fn a() -> i32 {
  34     ///     0
  35     /// }
  36     /// fn b() -> i32 {
  37     ///     a()
  38     /// }
  39     /// ```
  40     ///
  41     /// When running Clippy, the lint will only suggest to make `a` const, because `b` at this time
  42     /// can't be const as it calls a non-const function. Making `a` const and running Clippy again,
  43     /// will suggest to make `b` const, too.
  44     ///
  45     /// ### Example
  46     /// ```rust
  47     /// # struct Foo {
  48     /// #     random_number: usize,
  49     /// # }
  50     /// # impl Foo {
  51     /// fn new() -> Self {
  52     ///     Self { random_number: 42 }
  53     /// }
  54     /// # }
  55     /// ```
  56     ///
  57     /// Could be a const fn:
  58     ///
  59     /// ```rust
  60     /// # struct Foo {
  61     /// #     random_number: usize,
  62     /// # }
  63     /// # impl Foo {
  64     /// const fn new() -> Self {
  65     ///     Self { random_number: 42 }
  66     /// }
  67     /// # }
  68     /// ```
  69     #[clippy::version = "1.34.0"]
  70     pub MISSING_CONST_FOR_FN,
  71     nursery,
  72     "Lint functions definitions that could be made `const fn`"
  73 }
  74
  75 impl_lint_pass!(MissingConstForFn => [MISSING_CONST_FOR_FN]);
  76
  77 pub struct MissingConstForFn {
  78     msrv: Option<RustcVersion>,
  79 }
  80
  81 impl MissingConstForFn {
  82     #[must_use]
  83     pub fn new(msrv: Option<RustcVersion>) -> Self {
  84         Self { msrv }
  85     }
  86 }
  87
  88 impl<'tcx> LateLintPass<'tcx> for MissingConstForFn {
  89     fn check_fn(
  90         &mut self,
  91         cx: &LateContext<'tcx>,
  92         kind: FnKind<'tcx>,
  93         _: &FnDecl<'_>,
  94         body: &Body<'tcx>,
  95         span: Span,
  96         hir_id: HirId,
  97     ) {
  98         if !meets_msrv(self.msrv, msrvs::CONST_IF_MATCH) {
  99             return;
 100         }
 101
 102         let def_id = cx.tcx.hir().local_def_id(hir_id);
 103
 104         if in_external_macro(cx.tcx.sess, span) || is_entrypoint_fn(cx, def_id.to_def_id()) {
 105             return;
 106         }
 107
 108         // Building MIR for `fn`s with unsatisfiable preds results in ICE.
 109         if fn_has_unsatisfiable_preds(cx, def_id.to_def_id()) {
 110             return;
 111         }
 112
 113         // Perform some preliminary checks that rule out constness on the Clippy side. This way we
 114         // can skip the actual const check and return early.
 115         match kind {
 116             FnKind::ItemFn(_, generics, header, ..) => {
 117                 let has_const_generic_params = generics
 118                     .params
 119                     .iter()
 120                     .any(|param| matches!(param.kind, GenericParamKind::Const { .. }));
 121
 122                 if already_const(header) || has_const_generic_params {
 123                     return;
 124                 }
 125             },
 126             FnKind::Method(_, sig, ..) => {
 127                 if trait_ref_of_method(cx, def_id).is_some()
 128                     || already_const(sig.header)
 129                     || method_accepts_droppable(cx, sig.decl.inputs)
 130                 {
 131                     return;
 132                 }
 133             },
 134             FnKind::Closure => return,
 135         }
 136
 137         // Const fns are not allowed as methods in a trait.
 138         {
 139             let parent = cx.tcx.hir().get_parent_item(hir_id).def_id;
 140             if parent != CRATE_DEF_ID {
 141                 if let hir::Node::Item(item) = cx.tcx.hir().get_by_def_id(parent) {
 142                     if let hir::ItemKind::Trait(..) = &item.kind {
 143                         return;
 144                     }
 145                 }
 146             }
 147         }
 148
 149         if is_from_proc_macro(cx, &(&kind, body, hir_id, span)) {
 150             return;
 151         }
 152
 153         let mir = cx.tcx.optimized_mir(def_id);
 154
 155         if let Err((span, err)) = is_min_const_fn(cx.tcx, mir, self.msrv) {
 156             if cx.tcx.is_const_fn_raw(def_id.to_def_id()) {
 157                 cx.tcx.sess.span_err(span, err.as_ref());
 158             }
 159         } else {
 160             span_lint(cx, MISSING_CONST_FOR_FN, span, "this could be a `const fn`");
 161         }
 162     }
 163     extract_msrv_attr!(LateContext);
 164 }
 165
 166 /// Returns true if any of the method parameters is a type that implements `Drop`. The method
 167 /// can't be made const then, because `drop` can't be const-evaluated.
 168 fn method_accepts_droppable(cx: &LateContext<'_>, param_tys: &[hir::Ty<'_>]) -> bool {
 169     // If any of the params are droppable, return true
 170     param_tys.iter().any(|hir_ty| {
 171         let ty_ty = hir_ty_to_ty(cx.tcx, hir_ty);
 172         has_drop(cx, ty_ty)
 173     })
 174 }
 175
 176 // We don't have to lint on something that's already `const`
 177 #[must_use]
 178 fn already_const(header: hir::FnHeader) -> bool {
 179     header.constness == Constness::Const
 180 }