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|
use clippy_utils::diagnostics::span_lint;
use clippy_utils::msrvs::{self, Msrv};
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, is_from_proc_macro, 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_hir_analysis::hir_ty_to_ty;
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::lint::in_external_macro;
use rustc_session::{declare_tool_lint, impl_lint_pass};
use rustc_span::Span;
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: Msrv,
}
impl MissingConstForFn {
#[must_use]
pub fn new(msrv: Msrv) -> Self {
Self { msrv }
}
}
impl<'tcx> LateLintPass<'tcx> for MissingConstForFn {
fn check_fn(
&mut self,
cx: &LateContext<'tcx>,
kind: FnKind<'tcx>,
_: &FnDecl<'_>,
body: &Body<'tcx>,
span: Span,
hir_id: HirId,
) {
if !self.msrv.meets(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_droppable(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).def_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;
}
}
}
}
if is_from_proc_macro(cx, &(&kind, body, hir_id, span)) {
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_droppable(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
}
|
