136 lines
4.2 KiB
Rust
136 lines
4.2 KiB
Rust
use crate::utils::{has_drop, is_entrypoint_fn, span_lint, trait_ref_of_method};
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use rustc::declare_lint_pass;
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use rustc::hir;
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use rustc::hir::intravisit::FnKind;
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use rustc::hir::{Body, Constness, FnDecl, HirId, HirVec};
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use rustc::lint::{in_external_macro, LateContext, LateLintPass, LintArray, LintPass};
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use rustc_mir::transform::qualify_min_const_fn::is_min_const_fn;
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use rustc_session::declare_tool_lint;
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use rustc_typeck::hir_ty_to_ty;
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use syntax_pos::Span;
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declare_clippy_lint! {
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/// **What it does:**
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///
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/// Suggests the use of `const` in functions and methods where possible.
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///
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/// **Why is this bad?**
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///
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/// Not having the function const prevents callers of the function from being const as well.
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///
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/// **Known problems:**
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///
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/// Const functions are currently still being worked on, with some features only being available
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/// on nightly. This lint does not consider all edge cases currently and the suggestions may be
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/// incorrect if you are using this lint on stable.
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///
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/// Also, the lint only runs one pass over the code. Consider these two non-const functions:
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///
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/// ```rust
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/// fn a() -> i32 {
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/// 0
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/// }
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/// fn b() -> i32 {
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/// a()
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/// }
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/// ```
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///
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/// When running Clippy, the lint will only suggest to make `a` const, because `b` at this time
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/// can't be const as it calls a non-const function. Making `a` const and running Clippy again,
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/// will suggest to make `b` const, too.
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///
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/// **Example:**
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///
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/// ```rust
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/// # struct Foo {
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/// # random_number: usize,
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/// # }
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/// # impl Foo {
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/// fn new() -> Self {
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/// Self { random_number: 42 }
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/// }
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/// # }
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/// ```
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///
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/// Could be a const fn:
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///
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/// ```rust
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/// # struct Foo {
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/// # random_number: usize,
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/// # }
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/// # impl Foo {
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/// const fn new() -> Self {
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/// Self { random_number: 42 }
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/// }
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/// # }
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/// ```
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pub MISSING_CONST_FOR_FN,
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nursery,
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"Lint functions definitions that could be made `const fn`"
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}
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declare_lint_pass!(MissingConstForFn => [MISSING_CONST_FOR_FN]);
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impl<'a, 'tcx> LateLintPass<'a, 'tcx> for MissingConstForFn {
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fn check_fn(
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&mut self,
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cx: &LateContext<'_, '_>,
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kind: FnKind<'_>,
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_: &FnDecl,
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_: &Body<'_>,
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span: Span,
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hir_id: HirId,
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) {
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let def_id = cx.tcx.hir().local_def_id(hir_id);
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if in_external_macro(cx.tcx.sess, span) || is_entrypoint_fn(cx, def_id) {
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return;
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}
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// Perform some preliminary checks that rule out constness on the Clippy side. This way we
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// can skip the actual const check and return early.
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match kind {
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FnKind::ItemFn(_, _, header, ..) => {
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if already_const(header) {
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return;
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}
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},
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FnKind::Method(_, sig, ..) => {
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if trait_ref_of_method(cx, hir_id).is_some()
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|| already_const(sig.header)
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|| method_accepts_dropable(cx, &sig.decl.inputs)
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{
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return;
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}
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},
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_ => return,
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}
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let mir = cx.tcx.optimized_mir(def_id);
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if let Err((span, err)) = is_min_const_fn(cx.tcx, def_id, &mir) {
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if cx.tcx.is_min_const_fn(def_id) {
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cx.tcx.sess.span_err(span, &err);
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}
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} else {
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span_lint(cx, MISSING_CONST_FOR_FN, span, "this could be a const_fn");
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}
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}
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}
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/// Returns true if any of the method parameters is a type that implements `Drop`. The method
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/// can't be made const then, because `drop` can't be const-evaluated.
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fn method_accepts_dropable(cx: &LateContext<'_, '_>, param_tys: &HirVec<hir::Ty>) -> bool {
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// If any of the params are dropable, return true
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param_tys.iter().any(|hir_ty| {
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let ty_ty = hir_ty_to_ty(cx.tcx, hir_ty);
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has_drop(cx, ty_ty)
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})
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}
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// We don't have to lint on something that's already `const`
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#[must_use]
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fn already_const(header: hir::FnHeader) -> bool {
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header.constness == Constness::Const
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}
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