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use clippy_utils::diagnostics::span_lint_and_sugg;
use clippy_utils::is_ty_alias;
use hir::ExprKind;
use hir::def::Res;
use rustc_errors::Applicability;
use rustc_hir as hir;
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::ty;
use rustc_session::declare_lint_pass;
use rustc_span::sym;
declare_clippy_lint! {
/// ### What it does
/// Checks for construction on unit struct using `default`.
///
/// ### Why is this bad?
/// This adds code complexity and an unnecessary function call.
///
/// ### Example
/// ```no_run
/// # use std::marker::PhantomData;
/// #[derive(Default)]
/// struct S<T> {
/// _marker: PhantomData<T>
/// }
///
/// let _: S<i32> = S {
/// _marker: PhantomData::default()
/// };
/// ```
/// Use instead:
/// ```no_run
/// # use std::marker::PhantomData;
/// struct S<T> {
/// _marker: PhantomData<T>
/// }
///
/// let _: S<i32> = S {
/// _marker: PhantomData
/// };
/// ```
#[clippy::version = "1.71.0"]
pub DEFAULT_CONSTRUCTED_UNIT_STRUCTS,
complexity,
"unit structs can be constructed without calling `default`"
}
declare_lint_pass!(DefaultConstructedUnitStructs => [DEFAULT_CONSTRUCTED_UNIT_STRUCTS]);
fn is_alias(ty: hir::Ty<'_>) -> bool {
if let hir::TyKind::Path(ref qpath) = ty.kind {
is_ty_alias(qpath)
} else {
false
}
}
impl LateLintPass<'_> for DefaultConstructedUnitStructs {
fn check_expr<'tcx>(&mut self, cx: &LateContext<'tcx>, expr: &'tcx hir::Expr<'tcx>) {
if let ExprKind::Call(fn_expr, &[]) = expr.kind
// make sure we have a call to `Default::default`
&& let ExprKind::Path(ref qpath @ hir::QPath::TypeRelative(base, _)) = fn_expr.kind
// make sure this isn't a type alias:
// `<Foo as Bar>::Assoc` cannot be used as a constructor
&& !is_alias(*base)
&& let Res::Def(_, def_id) = cx.qpath_res(qpath, fn_expr.hir_id)
&& cx.tcx.is_diagnostic_item(sym::default_fn, def_id)
// make sure we have a struct with no fields (unit struct)
&& let ty::Adt(def, ..) = cx.typeck_results().expr_ty(expr).kind()
&& def.is_struct()
&& let var @ ty::VariantDef { ctor: Some((hir::def::CtorKind::Const, _)), .. } = def.non_enum_variant()
&& !var.is_field_list_non_exhaustive()
&& !expr.span.from_expansion() && !qpath.span().from_expansion()
{
span_lint_and_sugg(
cx,
DEFAULT_CONSTRUCTED_UNIT_STRUCTS,
expr.span.with_lo(qpath.qself_span().hi()),
"use of `default` to create a unit struct",
"remove this call to `default`",
String::new(),
Applicability::MachineApplicable,
);
}
}
}
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