diff options
Diffstat (limited to 'src/tools/clippy/clippy_lints/src/missing_const_for_fn.rs')
-rw-r--r-- | src/tools/clippy/clippy_lints/src/missing_const_for_fn.rs | 174 |
1 files changed, 174 insertions, 0 deletions
diff --git a/src/tools/clippy/clippy_lints/src/missing_const_for_fn.rs b/src/tools/clippy/clippy_lints/src/missing_const_for_fn.rs new file mode 100644 index 000000000..16d65966c --- /dev/null +++ b/src/tools/clippy/clippy_lints/src/missing_const_for_fn.rs @@ -0,0 +1,174 @@ +use clippy_utils::diagnostics::span_lint; +use clippy_utils::qualify_min_const_fn::is_min_const_fn; +use clippy_utils::ty::has_drop; +use clippy_utils::{fn_has_unsatisfiable_preds, is_entrypoint_fn, meets_msrv, msrvs, trait_ref_of_method}; +use rustc_hir as hir; +use rustc_hir::def_id::CRATE_DEF_ID; +use rustc_hir::intravisit::FnKind; +use rustc_hir::{Body, Constness, FnDecl, GenericParamKind, HirId}; +use rustc_lint::{LateContext, LateLintPass}; +use rustc_middle::lint::in_external_macro; +use rustc_semver::RustcVersion; +use rustc_session::{declare_tool_lint, impl_lint_pass}; +use rustc_span::Span; +use rustc_typeck::hir_ty_to_ty; + +declare_clippy_lint! { + /// ### What it does + /// Suggests the use of `const` in functions and methods where possible. + /// + /// ### Why is this bad? + /// Not having the function const prevents callers of the function from being const as well. + /// + /// ### Known problems + /// Const functions are currently still being worked on, with some features only being available + /// on nightly. This lint does not consider all edge cases currently and the suggestions may be + /// incorrect if you are using this lint on stable. + /// + /// Also, the lint only runs one pass over the code. Consider these two non-const functions: + /// + /// ```rust + /// fn a() -> i32 { + /// 0 + /// } + /// fn b() -> i32 { + /// a() + /// } + /// ``` + /// + /// When running Clippy, the lint will only suggest to make `a` const, because `b` at this time + /// can't be const as it calls a non-const function. Making `a` const and running Clippy again, + /// will suggest to make `b` const, too. + /// + /// ### Example + /// ```rust + /// # struct Foo { + /// # random_number: usize, + /// # } + /// # impl Foo { + /// fn new() -> Self { + /// Self { random_number: 42 } + /// } + /// # } + /// ``` + /// + /// Could be a const fn: + /// + /// ```rust + /// # struct Foo { + /// # random_number: usize, + /// # } + /// # impl Foo { + /// const fn new() -> Self { + /// Self { random_number: 42 } + /// } + /// # } + /// ``` + #[clippy::version = "1.34.0"] + pub MISSING_CONST_FOR_FN, + nursery, + "Lint functions definitions that could be made `const fn`" +} + +impl_lint_pass!(MissingConstForFn => [MISSING_CONST_FOR_FN]); + +pub struct MissingConstForFn { + msrv: Option<RustcVersion>, +} + +impl MissingConstForFn { + #[must_use] + pub fn new(msrv: Option<RustcVersion>) -> Self { + Self { msrv } + } +} + +impl<'tcx> LateLintPass<'tcx> for MissingConstForFn { + fn check_fn( + &mut self, + cx: &LateContext<'_>, + kind: FnKind<'_>, + _: &FnDecl<'_>, + _: &Body<'_>, + span: Span, + hir_id: HirId, + ) { + if !meets_msrv(self.msrv, msrvs::CONST_IF_MATCH) { + return; + } + + let def_id = cx.tcx.hir().local_def_id(hir_id); + + if in_external_macro(cx.tcx.sess, span) || is_entrypoint_fn(cx, def_id.to_def_id()) { + return; + } + + // Building MIR for `fn`s with unsatisfiable preds results in ICE. + if fn_has_unsatisfiable_preds(cx, def_id.to_def_id()) { + return; + } + + // Perform some preliminary checks that rule out constness on the Clippy side. This way we + // can skip the actual const check and return early. + match kind { + FnKind::ItemFn(_, generics, header, ..) => { + let has_const_generic_params = generics + .params + .iter() + .any(|param| matches!(param.kind, GenericParamKind::Const { .. })); + + if already_const(header) || has_const_generic_params { + return; + } + }, + FnKind::Method(_, sig, ..) => { + if trait_ref_of_method(cx, def_id).is_some() + || already_const(sig.header) + || method_accepts_dropable(cx, sig.decl.inputs) + { + return; + } + }, + FnKind::Closure => return, + } + + // Const fns are not allowed as methods in a trait. + { + let parent = cx.tcx.hir().get_parent_item(hir_id); + if parent != CRATE_DEF_ID { + if let hir::Node::Item(item) = cx.tcx.hir().get_by_def_id(parent) { + if let hir::ItemKind::Trait(..) = &item.kind { + return; + } + } + } + } + + let mir = cx.tcx.optimized_mir(def_id); + + if let Err((span, err)) = is_min_const_fn(cx.tcx, mir, self.msrv) { + if cx.tcx.is_const_fn_raw(def_id.to_def_id()) { + cx.tcx.sess.span_err(span, err.as_ref()); + } + } else { + span_lint(cx, MISSING_CONST_FOR_FN, span, "this could be a `const fn`"); + } + } + extract_msrv_attr!(LateContext); +} + +/// Returns true if any of the method parameters is a type that implements `Drop`. The method +/// can't be made const then, because `drop` can't be const-evaluated. +fn method_accepts_dropable(cx: &LateContext<'_>, param_tys: &[hir::Ty<'_>]) -> bool { + // If any of the params are droppable, return true + param_tys.iter().any(|hir_ty| { + let ty_ty = hir_ty_to_ty(cx.tcx, hir_ty); + has_drop(cx, ty_ty) + }) +} + +// We don't have to lint on something that's already `const` +#[must_use] +fn already_const(header: hir::FnHeader) -> bool { + header.constness == Constness::Const +} |