1 use clippy_utils::diagnostics::span_lint_and_then;
2 use clippy_utils::get_parent_node;
3 use clippy_utils::source::snippet_with_macro_callsite;
4 use clippy_utils::visitors::{for_each_local_assignment, for_each_value_source};
5 use core::ops::ControlFlow;
6 use rustc_errors::Applicability;
7 use rustc_hir::def::{DefKind, Res};
8 use rustc_hir::{Expr, ExprKind, HirId, HirIdSet, Local, Node, PatKind, QPath, TyKind};
9 use rustc_lint::{LateContext, LintContext};
10 use rustc_middle::lint::in_external_macro;
13 use super::LET_UNIT_VALUE;
15 pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, local: &'tcx Local<'_>) {
16 if let Some(init) = local.init
17 && !local.pat.span.from_expansion()
18 && !in_external_macro(cx.sess(), local.span)
19 && cx.typeck_results().pat_ty(local.pat).is_unit()
21 if (local.ty.map_or(false, |ty| !matches!(ty.kind, TyKind::Infer))
22 || matches!(local.pat.kind, PatKind::Tuple([], ddpos) if ddpos.as_opt_usize().is_none()))
23 && expr_needs_inferred_result(cx, init)
25 if !matches!(local.pat.kind, PatKind::Wild)
26 && !matches!(local.pat.kind, PatKind::Tuple([], ddpos) if ddpos.as_opt_usize().is_none())
32 "this let-binding has unit value",
36 "use a wild (`_`) binding",
38 Applicability::MaybeIncorrect, // snippet
48 "this let-binding has unit value",
50 if let Some(expr) = &local.init {
51 let snip = snippet_with_macro_callsite(cx, expr.span, "()");
54 "omit the `let` binding",
56 Applicability::MachineApplicable, // snippet
65 /// Checks sub-expressions which create the value returned by the given expression for whether
66 /// return value inference is needed. This checks through locals to see if they also need inference
72 /// let x: u32 = if true { baz() } else { bar };
74 /// Here the sources of the value assigned to `x` would be `baz()`, and `foo()` via the
75 /// initialization of `bar`. If both `foo` and `baz` have a return type which require type
76 /// inference then this function would return `true`.
77 fn expr_needs_inferred_result<'tcx>(cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) -> bool {
78 // The locals used for initialization which have yet to be checked.
79 let mut locals_to_check = Vec::new();
80 // All the locals which have been added to `locals_to_check`. Needed to prevent cycles.
81 let mut seen_locals = HirIdSet::default();
82 if !each_value_source_needs_inference(cx, e, &mut locals_to_check, &mut seen_locals) {
85 while let Some(id) = locals_to_check.pop() {
86 if let Some(Node::Local(l)) = get_parent_node(cx.tcx, id) {
87 if !l.ty.map_or(true, |ty| matches!(ty.kind, TyKind::Infer)) {
90 if let Some(e) = l.init {
91 if !each_value_source_needs_inference(cx, e, &mut locals_to_check, &mut seen_locals) {
94 } else if for_each_local_assignment(cx, id, |e| {
95 if each_value_source_needs_inference(cx, e, &mut locals_to_check, &mut seen_locals) {
96 ControlFlow::Continue(())
98 ControlFlow::Break(())
111 fn each_value_source_needs_inference(
112 cx: &LateContext<'_>,
114 locals_to_check: &mut Vec<HirId>,
115 seen_locals: &mut HirIdSet,
117 for_each_value_source(e, &mut |e| {
118 if needs_inferred_result_ty(cx, e, locals_to_check, seen_locals) {
119 ControlFlow::Continue(())
121 ControlFlow::Break(())
127 fn needs_inferred_result_ty(
128 cx: &LateContext<'_>,
130 locals_to_check: &mut Vec<HirId>,
131 seen_locals: &mut HirIdSet,
133 let (id, receiver, args) = match e.kind {
136 kind: ExprKind::Path(ref path),
141 ) => match cx.qpath_res(path, *hir_id) {
142 Res::Def(DefKind::AssocFn | DefKind::Fn, id) => (id, None, args),
145 ExprKind::MethodCall(_, receiver, args, _) => match cx.typeck_results().type_dependent_def_id(e.hir_id) {
146 Some(id) => (id, Some(receiver), args),
147 None => return false,
149 ExprKind::Path(QPath::Resolved(None, path)) => {
150 if let Res::Local(id) = path.res
151 && seen_locals.insert(id)
153 locals_to_check.push(id);
159 let sig = cx.tcx.fn_sig(id).skip_binder();
160 if let ty::Param(output_ty) = *sig.output().kind() {
161 let args: Vec<&Expr<'_>> = if let Some(receiver) = receiver {
162 std::iter::once(receiver).chain(args.iter()).collect()
164 args.iter().collect()
166 sig.inputs().iter().zip(args).all(|(&ty, arg)| {
167 !ty.is_param(output_ty.index) || each_value_source_needs_inference(cx, arg, locals_to_check, seen_locals)