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([], None)))
23 && expr_needs_inferred_result(cx, init)
25 if !matches!(local.pat.kind, PatKind::Wild | PatKind::Tuple([], None)) {
30 "this let-binding has unit value",
34 "use a wild (`_`) binding",
36 Applicability::MaybeIncorrect, // snippet
46 "this let-binding has unit value",
48 if let Some(expr) = &local.init {
49 let snip = snippet_with_macro_callsite(cx, expr.span, "()");
52 "omit the `let` binding",
54 Applicability::MachineApplicable, // snippet
63 /// Checks sub-expressions which create the value returned by the given expression for whether
64 /// return value inference is needed. This checks through locals to see if they also need inference
70 /// let x: u32 = if true { baz() } else { bar };
72 /// Here the sources of the value assigned to `x` would be `baz()`, and `foo()` via the
73 /// initialization of `bar`. If both `foo` and `baz` have a return type which require type
74 /// inference then this function would return `true`.
75 fn expr_needs_inferred_result<'tcx>(cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) -> bool {
76 // The locals used for initialization which have yet to be checked.
77 let mut locals_to_check = Vec::new();
78 // All the locals which have been added to `locals_to_check`. Needed to prevent cycles.
79 let mut seen_locals = HirIdSet::default();
80 if !each_value_source_needs_inference(cx, e, &mut locals_to_check, &mut seen_locals) {
83 while let Some(id) = locals_to_check.pop() {
84 if let Some(Node::Local(l)) = get_parent_node(cx.tcx, id) {
85 if !l.ty.map_or(true, |ty| matches!(ty.kind, TyKind::Infer)) {
88 if let Some(e) = l.init {
89 if !each_value_source_needs_inference(cx, e, &mut locals_to_check, &mut seen_locals) {
92 } else if for_each_local_assignment(cx, id, |e| {
93 if each_value_source_needs_inference(cx, e, &mut locals_to_check, &mut seen_locals) {
94 ControlFlow::Continue(())
96 ControlFlow::Break(())
109 fn each_value_source_needs_inference(
110 cx: &LateContext<'_>,
112 locals_to_check: &mut Vec<HirId>,
113 seen_locals: &mut HirIdSet,
115 for_each_value_source(e, &mut |e| {
116 if needs_inferred_result_ty(cx, e, locals_to_check, seen_locals) {
117 ControlFlow::Continue(())
119 ControlFlow::Break(())
125 fn needs_inferred_result_ty(
126 cx: &LateContext<'_>,
128 locals_to_check: &mut Vec<HirId>,
129 seen_locals: &mut HirIdSet,
131 let (id, receiver, args) = match e.kind {
134 kind: ExprKind::Path(ref path),
139 ) => match cx.qpath_res(path, *hir_id) {
140 Res::Def(DefKind::AssocFn | DefKind::Fn, id) => (id, None, args),
143 ExprKind::MethodCall(_, receiver, args, _) => match cx.typeck_results().type_dependent_def_id(e.hir_id) {
144 Some(id) => (id, Some(receiver), args),
145 None => return false,
147 ExprKind::Path(QPath::Resolved(None, path)) => {
148 if let Res::Local(id) = path.res
149 && seen_locals.insert(id)
151 locals_to_check.push(id);
157 let sig = cx.tcx.fn_sig(id).skip_binder();
158 if let ty::Param(output_ty) = *sig.output().kind() {
159 let args: Vec<&Expr<'_>> = if let Some(receiver) = receiver {
160 std::iter::once(receiver).chain(args.iter()).collect()
162 args.iter().collect()
164 sig.inputs().iter().zip(args).all(|(&ty, arg)| {
165 !ty.is_param(output_ty.index) || each_value_source_needs_inference(cx, arg, locals_to_check, seen_locals)