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use clippy_utils::diagnostics::span_lint;
use rustc_hir::{Expr, ExprKind};
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::ty;
use rustc_session::{declare_lint_pass, declare_tool_lint};
declare_clippy_lint! {
/// **What it does:** Checks for needlessly including a base struct on update
/// when all fields are changed anyway.
///
/// This lint is not applied to structs marked with
/// [non_exhaustive](https://doc.rust-lang.org/reference/attributes/type_system.html).
///
/// **Why is this bad?** This will cost resources (because the base has to be
/// somewhere), and make the code less readable.
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust
/// # struct Point {
/// # x: i32,
/// # y: i32,
/// # z: i32,
/// # }
/// # let zero_point = Point { x: 0, y: 0, z: 0 };
///
/// // Bad
/// Point {
/// x: 1,
/// y: 1,
/// z: 1,
/// ..zero_point
/// };
///
/// // Ok
/// Point {
/// x: 1,
/// y: 1,
/// ..zero_point
/// };
/// ```
pub NEEDLESS_UPDATE,
complexity,
"using `Foo { ..base }` when there are no missing fields"
}
declare_lint_pass!(NeedlessUpdate => [NEEDLESS_UPDATE]);
impl<'tcx> LateLintPass<'tcx> for NeedlessUpdate {
fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
if let ExprKind::Struct(_, fields, Some(base)) = expr.kind {
let ty = cx.typeck_results().expr_ty(expr);
if let ty::Adt(def, _) = ty.kind() {
if fields.len() == def.non_enum_variant().fields.len()
&& !def.variants[0_usize.into()].is_field_list_non_exhaustive()
{
span_lint(
cx,
NEEDLESS_UPDATE,
base.span,
"struct update has no effect, all the fields in the struct have already been specified",
);
}
}
}
}
}
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