src/tools/clippy/clippy_lints/src/unit_types/let_unit_value.rs

   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;
  11 use rustc_middle::ty;
  12
  13 use super::LET_UNIT_VALUE;
  14
  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()
  20     {
  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)
  24         {
  25             if !matches!(local.pat.kind, PatKind::Wild | PatKind::Tuple([], None)) {
  26                 span_lint_and_then(
  27                     cx,
  28                     LET_UNIT_VALUE,
  29                     local.span,
  30                     "this let-binding has unit value",
  31                     |diag| {
  32                         diag.span_suggestion(
  33                             local.pat.span,
  34                             "use a wild (`_`) binding",
  35                             "_",
  36                             Applicability::MaybeIncorrect, // snippet
  37                         );
  38                     },
  39                 );
  40             }
  41         } else {
  42             span_lint_and_then(
  43                 cx,
  44                 LET_UNIT_VALUE,
  45                 local.span,
  46                 "this let-binding has unit value",
  47                 |diag| {
  48                     if let Some(expr) = &local.init {
  49                         let snip = snippet_with_macro_callsite(cx, expr.span, "()");
  50                         diag.span_suggestion(
  51                             local.span,
  52                             "omit the `let` binding",
  53                             format!("{snip};"),
  54                             Applicability::MachineApplicable, // snippet
  55                         );
  56                     }
  57                 },
  58             );
  59         }
  60     }
  61 }
  62
  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
  65 /// at this point.
  66 ///
  67 /// e.g.
  68 /// ```rust,ignore
  69 /// let bar = foo();
  70 /// let x: u32 = if true { baz() } else { bar };
  71 /// ```
  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) {
  81         return false;
  82     }
  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)) {
  86                 return false;
  87             }
  88             if let Some(e) = l.init {
  89                 if !each_value_source_needs_inference(cx, e, &mut locals_to_check, &mut seen_locals) {
  90                     return false;
  91                 }
  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(())
  95                 } else {
  96                     ControlFlow::Break(())
  97                 }
  98             })
  99             .is_break()
 100             {
 101                 return false;
 102             }
 103         }
 104     }
 105
 106     true
 107 }
 108
 109 fn each_value_source_needs_inference(
 110     cx: &LateContext<'_>,
 111     e: &Expr<'_>,
 112     locals_to_check: &mut Vec<HirId>,
 113     seen_locals: &mut HirIdSet,
 114 ) -> bool {
 115     for_each_value_source(e, &mut |e| {
 116         if needs_inferred_result_ty(cx, e, locals_to_check, seen_locals) {
 117             ControlFlow::Continue(())
 118         } else {
 119             ControlFlow::Break(())
 120         }
 121     })
 122     .is_continue()
 123 }
 124
 125 fn needs_inferred_result_ty(
 126     cx: &LateContext<'_>,
 127     e: &Expr<'_>,
 128     locals_to_check: &mut Vec<HirId>,
 129     seen_locals: &mut HirIdSet,
 130 ) -> bool {
 131     let (id, receiver, args) = match e.kind {
 132         ExprKind::Call(
 133             Expr {
 134                 kind: ExprKind::Path(ref path),
 135                 hir_id,
 136                 ..
 137             },
 138             args,
 139         ) => match cx.qpath_res(path, *hir_id) {
 140             Res::Def(DefKind::AssocFn | DefKind::Fn, id) => (id, None, args),
 141             _ => return false,
 142         },
 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,
 146         },
 147         ExprKind::Path(QPath::Resolved(None, path)) => {
 148             if let Res::Local(id) = path.res
 149                 && seen_locals.insert(id)
 150             {
 151                 locals_to_check.push(id);
 152             }
 153             return true;
 154         },
 155         _ => return false,
 156     };
 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()
 161         } else {
 162             args.iter().collect()
 163         };
 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)
 166         })
 167     } else {
 168         false
 169     }
 170 }