clippy_lints/src/unused_unit.rs

   1 use clippy_utils::diagnostics::span_lint_and_sugg;
   2 use clippy_utils::source::position_before_rarrow;
   3 use if_chain::if_chain;
   4 use rustc_ast::ast;
   5 use rustc_ast::visit::FnKind;
   6 use rustc_errors::Applicability;
   7 use rustc_lint::{EarlyContext, EarlyLintPass, LintContext};
   8 use rustc_session::{declare_lint_pass, declare_tool_lint};
   9 use rustc_span::source_map::Span;
  10 use rustc_span::BytePos;
  11
  12 declare_clippy_lint! {
  13     /// **What it does:** Checks for unit (`()`) expressions that can be removed.
  14     ///
  15     /// **Why is this bad?** Such expressions add no value, but can make the code
  16     /// less readable. Depending on formatting they can make a `break` or `return`
  17     /// statement look like a function call.
  18     ///
  19     /// **Known problems:** None.
  20     ///
  21     /// **Example:**
  22     /// ```rust
  23     /// fn return_unit() -> () {
  24     ///     ()
  25     /// }
  26     /// ```
  27     pub UNUSED_UNIT,
  28     style,
  29     "needless unit expression"
  30 }
  31
  32 declare_lint_pass!(UnusedUnit => [UNUSED_UNIT]);
  33
  34 impl EarlyLintPass for UnusedUnit {
  35     fn check_fn(&mut self, cx: &EarlyContext<'_>, kind: FnKind<'_>, span: Span, _: ast::NodeId) {
  36         if_chain! {
  37             if let ast::FnRetTy::Ty(ref ty) = kind.decl().output;
  38             if let ast::TyKind::Tup(ref vals) = ty.kind;
  39             if vals.is_empty() && !ty.span.from_expansion() && get_def(span) == get_def(ty.span);
  40             then {
  41                 lint_unneeded_unit_return(cx, ty, span);
  42             }
  43         }
  44     }
  45
  46     fn check_block(&mut self, cx: &EarlyContext<'_>, block: &ast::Block) {
  47         if_chain! {
  48             if let Some(stmt) = block.stmts.last();
  49             if let ast::StmtKind::Expr(ref expr) = stmt.kind;
  50             if is_unit_expr(expr) && !stmt.span.from_expansion();
  51             then {
  52                 let sp = expr.span;
  53                 span_lint_and_sugg(
  54                     cx,
  55                     UNUSED_UNIT,
  56                     sp,
  57                     "unneeded unit expression",
  58                     "remove the final `()`",
  59                     String::new(),
  60                     Applicability::MachineApplicable,
  61                 );
  62             }
  63         }
  64     }
  65
  66     fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
  67         match e.kind {
  68             ast::ExprKind::Ret(Some(ref expr)) | ast::ExprKind::Break(_, Some(ref expr)) => {
  69                 if is_unit_expr(expr) && !expr.span.from_expansion() {
  70                     span_lint_and_sugg(
  71                         cx,
  72                         UNUSED_UNIT,
  73                         expr.span,
  74                         "unneeded `()`",
  75                         "remove the `()`",
  76                         String::new(),
  77                         Applicability::MachineApplicable,
  78                     );
  79                 }
  80             },
  81             _ => (),
  82         }
  83     }
  84
  85     fn check_poly_trait_ref(&mut self, cx: &EarlyContext<'_>, poly: &ast::PolyTraitRef, _: &ast::TraitBoundModifier) {
  86         let segments = &poly.trait_ref.path.segments;
  87
  88         if_chain! {
  89             if segments.len() == 1;
  90             if ["Fn", "FnMut", "FnOnce"].contains(&&*segments[0].ident.name.as_str());
  91             if let Some(args) = &segments[0].args;
  92             if let ast::GenericArgs::Parenthesized(generic_args) = &**args;
  93             if let ast::FnRetTy::Ty(ty) = &generic_args.output;
  94             if ty.kind.is_unit();
  95             then {
  96                 lint_unneeded_unit_return(cx, ty, generic_args.span);
  97             }
  98         }
  99     }
 100 }
 101
 102 // get the def site
 103 #[must_use]
 104 fn get_def(span: Span) -> Option<Span> {
 105     if span.from_expansion() {
 106         Some(span.ctxt().outer_expn_data().def_site)
 107     } else {
 108         None
 109     }
 110 }
 111
 112 // is this expr a `()` unit?
 113 fn is_unit_expr(expr: &ast::Expr) -> bool {
 114     if let ast::ExprKind::Tup(ref vals) = expr.kind {
 115         vals.is_empty()
 116     } else {
 117         false
 118     }
 119 }
 120
 121 fn lint_unneeded_unit_return(cx: &EarlyContext<'_>, ty: &ast::Ty, span: Span) {
 122     let (ret_span, appl) = if let Ok(fn_source) = cx.sess().source_map().span_to_snippet(span.with_hi(ty.span.hi())) {
 123         position_before_rarrow(&fn_source).map_or((ty.span, Applicability::MaybeIncorrect), |rpos| {
 124             (
 125                 #[allow(clippy::cast_possible_truncation)]
 126                 ty.span.with_lo(BytePos(span.lo().0 + rpos as u32)),
 127                 Applicability::MachineApplicable,
 128             )
 129         })
 130     } else {
 131         (ty.span, Applicability::MaybeIncorrect)
 132     };
 133     span_lint_and_sugg(
 134         cx,
 135         UNUSED_UNIT,
 136         ret_span,
 137         "unneeded unit return type",
 138         "remove the `-> ()`",
 139         String::new(),
 140         appl,
 141     );
 142 }