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);
  51             let ctxt = block.span.ctxt();
  52             if stmt.span.ctxt() == ctxt && expr.span.ctxt() == ctxt;
  53             then {
  54                 let sp = expr.span;
  55                 span_lint_and_sugg(
  56                     cx,
  57                     UNUSED_UNIT,
  58                     sp,
  59                     "unneeded unit expression",
  60                     "remove the final `()`",
  61                     String::new(),
  62                     Applicability::MachineApplicable,
  63                 );
  64             }
  65         }
  66     }
  67
  68     fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
  69         match e.kind {
  70             ast::ExprKind::Ret(Some(ref expr)) | ast::ExprKind::Break(_, Some(ref expr)) => {
  71                 if is_unit_expr(expr) && !expr.span.from_expansion() {
  72                     span_lint_and_sugg(
  73                         cx,
  74                         UNUSED_UNIT,
  75                         expr.span,
  76                         "unneeded `()`",
  77                         "remove the `()`",
  78                         String::new(),
  79                         Applicability::MachineApplicable,
  80                     );
  81                 }
  82             },
  83             _ => (),
  84         }
  85     }
  86
  87     fn check_poly_trait_ref(&mut self, cx: &EarlyContext<'_>, poly: &ast::PolyTraitRef, _: &ast::TraitBoundModifier) {
  88         let segments = &poly.trait_ref.path.segments;
  89
  90         if_chain! {
  91             if segments.len() == 1;
  92             if ["Fn", "FnMut", "FnOnce"].contains(&&*segments[0].ident.name.as_str());
  93             if let Some(args) = &segments[0].args;
  94             if let ast::GenericArgs::Parenthesized(generic_args) = &**args;
  95             if let ast::FnRetTy::Ty(ty) = &generic_args.output;
  96             if ty.kind.is_unit();
  97             then {
  98                 lint_unneeded_unit_return(cx, ty, generic_args.span);
  99             }
 100         }
 101     }
 102 }
 103
 104 // get the def site
 105 #[must_use]
 106 fn get_def(span: Span) -> Option<Span> {
 107     if span.from_expansion() {
 108         Some(span.ctxt().outer_expn_data().def_site)
 109     } else {
 110         None
 111     }
 112 }
 113
 114 // is this expr a `()` unit?
 115 fn is_unit_expr(expr: &ast::Expr) -> bool {
 116     if let ast::ExprKind::Tup(ref vals) = expr.kind {
 117         vals.is_empty()
 118     } else {
 119         false
 120     }
 121 }
 122
 123 fn lint_unneeded_unit_return(cx: &EarlyContext<'_>, ty: &ast::Ty, span: Span) {
 124     let (ret_span, appl) = if let Ok(fn_source) = cx.sess().source_map().span_to_snippet(span.with_hi(ty.span.hi())) {
 125         position_before_rarrow(&fn_source).map_or((ty.span, Applicability::MaybeIncorrect), |rpos| {
 126             (
 127                 #[allow(clippy::cast_possible_truncation)]
 128                 ty.span.with_lo(BytePos(span.lo().0 + rpos as u32)),
 129                 Applicability::MachineApplicable,
 130             )
 131         })
 132     } else {
 133         (ty.span, Applicability::MaybeIncorrect)
 134     };
 135     span_lint_and_sugg(
 136         cx,
 137         UNUSED_UNIT,
 138         ret_span,
 139         "unneeded unit return type",
 140         "remove the `-> ()`",
 141         String::new(),
 142         appl,
 143     );
 144 }