src/tools/clippy/clippy_lints/src/unused_unit.rs

   1 use clippy_utils::diagnostics::span_lint_and_sugg;
   2 use clippy_utils::source::{position_before_rarrow, snippet_opt};
   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};
   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
  14     /// Checks for unit (`()`) expressions that can be removed.
  15     ///
  16     /// ### Why is this bad?
  17     /// Such expressions add no value, but can make the code
  18     /// less readable. Depending on formatting they can make a `break` or `return`
  19     /// statement look like a function call.
  20     ///
  21     /// ### Example
  22     /// ```rust
  23     /// fn return_unit() -> () {
  24     ///     ()
  25     /// }
  26     /// ```
  27     /// is equivalent to
  28     /// ```rust
  29     /// fn return_unit() {}
  30     /// ```
  31     pub UNUSED_UNIT,
  32     style,
  33     "needless unit expression"
  34 }
  35
  36 declare_lint_pass!(UnusedUnit => [UNUSED_UNIT]);
  37
  38 impl EarlyLintPass for UnusedUnit {
  39     fn check_fn(&mut self, cx: &EarlyContext<'_>, kind: FnKind<'_>, span: Span, _: ast::NodeId) {
  40         if_chain! {
  41             if let ast::FnRetTy::Ty(ref ty) = kind.decl().output;
  42             if let ast::TyKind::Tup(ref vals) = ty.kind;
  43             if vals.is_empty() && !ty.span.from_expansion() && get_def(span) == get_def(ty.span);
  44             then {
  45                 lint_unneeded_unit_return(cx, ty, span);
  46             }
  47         }
  48     }
  49
  50     fn check_block(&mut self, cx: &EarlyContext<'_>, block: &ast::Block) {
  51         if_chain! {
  52             if let Some(stmt) = block.stmts.last();
  53             if let ast::StmtKind::Expr(ref expr) = stmt.kind;
  54             if is_unit_expr(expr);
  55             let ctxt = block.span.ctxt();
  56             if stmt.span.ctxt() == ctxt && expr.span.ctxt() == ctxt;
  57             then {
  58                 let sp = expr.span;
  59                 span_lint_and_sugg(
  60                     cx,
  61                     UNUSED_UNIT,
  62                     sp,
  63                     "unneeded unit expression",
  64                     "remove the final `()`",
  65                     String::new(),
  66                     Applicability::MachineApplicable,
  67                 );
  68             }
  69         }
  70     }
  71
  72     fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
  73         match e.kind {
  74             ast::ExprKind::Ret(Some(ref expr)) | ast::ExprKind::Break(_, Some(ref expr)) => {
  75                 if is_unit_expr(expr) && !expr.span.from_expansion() {
  76                     span_lint_and_sugg(
  77                         cx,
  78                         UNUSED_UNIT,
  79                         expr.span,
  80                         "unneeded `()`",
  81                         "remove the `()`",
  82                         String::new(),
  83                         Applicability::MachineApplicable,
  84                     );
  85                 }
  86             },
  87             _ => (),
  88         }
  89     }
  90
  91     fn check_poly_trait_ref(&mut self, cx: &EarlyContext<'_>, poly: &ast::PolyTraitRef, _: &ast::TraitBoundModifier) {
  92         let segments = &poly.trait_ref.path.segments;
  93
  94         if_chain! {
  95             if segments.len() == 1;
  96             if ["Fn", "FnMut", "FnOnce"].contains(&&*segments[0].ident.name.as_str());
  97             if let Some(args) = &segments[0].args;
  98             if let ast::GenericArgs::Parenthesized(generic_args) = &**args;
  99             if let ast::FnRetTy::Ty(ty) = &generic_args.output;
 100             if ty.kind.is_unit();
 101             then {
 102                 lint_unneeded_unit_return(cx, ty, generic_args.span);
 103             }
 104         }
 105     }
 106 }
 107
 108 // get the def site
 109 #[must_use]
 110 fn get_def(span: Span) -> Option<Span> {
 111     if span.from_expansion() {
 112         Some(span.ctxt().outer_expn_data().def_site)
 113     } else {
 114         None
 115     }
 116 }
 117
 118 // is this expr a `()` unit?
 119 fn is_unit_expr(expr: &ast::Expr) -> bool {
 120     if let ast::ExprKind::Tup(ref vals) = expr.kind {
 121         vals.is_empty()
 122     } else {
 123         false
 124     }
 125 }
 126
 127 fn lint_unneeded_unit_return(cx: &EarlyContext<'_>, ty: &ast::Ty, span: Span) {
 128     let (ret_span, appl) =
 129         snippet_opt(cx, span.with_hi(ty.span.hi())).map_or((ty.span, Applicability::MaybeIncorrect), |fn_source| {
 130             position_before_rarrow(&fn_source).map_or((ty.span, Applicability::MaybeIncorrect), |rpos| {
 131                 (
 132                     #[allow(clippy::cast_possible_truncation)]
 133                     ty.span.with_lo(BytePos(span.lo().0 + rpos as u32)),
 134                     Applicability::MachineApplicable,
 135                 )
 136             })
 137         });
 138     span_lint_and_sugg(
 139         cx,
 140         UNUSED_UNIT,
 141         ret_span,
 142         "unneeded unit return type",
 143         "remove the `-> ()`",
 144         String::new(),
 145         appl,
 146     );
 147 }