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use clippy_utils::{diagnostics::span_lint_and_sugg, is_from_proc_macro, match_def_path, paths};
use hir::{def::Res, ExprKind};
use rustc_errors::Applicability;
use rustc_hir as hir;
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::ty;
use rustc_session::{declare_lint_pass, declare_tool_lint};
declare_clippy_lint! {
/// ### What it does
/// Check for construction on unit struct using `default`.
///
/// ### Why is this bad?
/// This adds code complexity and an unnecessary function call.
///
/// ### Example
/// ```rust
/// # use std::marker::PhantomData;
/// #[derive(Default)]
/// struct S<T> {
/// _marker: PhantomData<T>
/// }
///
/// let _: S<i32> = S {
/// _marker: PhantomData::default()
/// };
/// ```
/// Use instead:
/// ```rust
/// # 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 contructed without calling `default`"
}
declare_lint_pass!(DefaultConstructedUnitStructs => [DEFAULT_CONSTRUCTED_UNIT_STRUCTS]);
impl LateLintPass<'_> for DefaultConstructedUnitStructs {
fn check_expr<'tcx>(&mut self, cx: &LateContext<'tcx>, expr: &'tcx hir::Expr<'tcx>) {
if_chain!(
// make sure we have a call to `Default::default`
if let hir::ExprKind::Call(fn_expr, &[]) = expr.kind;
if let ExprKind::Path(ref qpath@ hir::QPath::TypeRelative(_,_)) = fn_expr.kind;
if let Res::Def(_, def_id) = cx.qpath_res(qpath, fn_expr.hir_id);
if match_def_path(cx, def_id, &paths::DEFAULT_TRAIT_METHOD);
// make sure we have a struct with no fields (unit struct)
if let ty::Adt(def, ..) = cx.typeck_results().expr_ty(expr).kind();
if def.is_struct();
if let var @ ty::VariantDef { ctor: Some((hir::def::CtorKind::Const, _)), .. } = def.non_enum_variant();
if !var.is_field_list_non_exhaustive() && !is_from_proc_macro(cx, expr);
then {
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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