src/tools/clippy/clippy_lints/src/returns.rs

   1 use clippy_utils::diagnostics::{span_lint_and_then, span_lint_hir_and_then};
   2 use clippy_utils::source::{snippet_opt, snippet_with_context};
   3 use clippy_utils::visitors::{for_each_expr, Descend};
   4 use clippy_utils::{fn_def_id, path_to_local_id};
   5 use core::ops::ControlFlow;
   6 use if_chain::if_chain;
   7 use rustc_errors::Applicability;
   8 use rustc_hir::intravisit::FnKind;
   9 use rustc_hir::{Block, Body, Expr, ExprKind, FnDecl, HirId, MatchSource, PatKind, StmtKind};
  10 use rustc_lint::{LateContext, LateLintPass, LintContext};
  11 use rustc_middle::lint::in_external_macro;
  12 use rustc_middle::ty::subst::GenericArgKind;
  13 use rustc_session::{declare_lint_pass, declare_tool_lint};
  14 use rustc_span::source_map::Span;
  15 use rustc_span::{BytePos, Pos};
  16
  17 declare_clippy_lint! {
  18     /// ### What it does
  19     /// Checks for `let`-bindings, which are subsequently
  20     /// returned.
  21     ///
  22     /// ### Why is this bad?
  23     /// It is just extraneous code. Remove it to make your code
  24     /// more rusty.
  25     ///
  26     /// ### Example
  27     /// ```rust
  28     /// fn foo() -> String {
  29     ///     let x = String::new();
  30     ///     x
  31     /// }
  32     /// ```
  33     /// instead, use
  34     /// ```
  35     /// fn foo() -> String {
  36     ///     String::new()
  37     /// }
  38     /// ```
  39     #[clippy::version = "pre 1.29.0"]
  40     pub LET_AND_RETURN,
  41     style,
  42     "creating a let-binding and then immediately returning it like `let x = expr; x` at the end of a block"
  43 }
  44
  45 declare_clippy_lint! {
  46     /// ### What it does
  47     /// Checks for return statements at the end of a block.
  48     ///
  49     /// ### Why is this bad?
  50     /// Removing the `return` and semicolon will make the code
  51     /// more rusty.
  52     ///
  53     /// ### Example
  54     /// ```rust
  55     /// fn foo(x: usize) -> usize {
  56     ///     return x;
  57     /// }
  58     /// ```
  59     /// simplify to
  60     /// ```rust
  61     /// fn foo(x: usize) -> usize {
  62     ///     x
  63     /// }
  64     /// ```
  65     #[clippy::version = "pre 1.29.0"]
  66     pub NEEDLESS_RETURN,
  67     style,
  68     "using a return statement like `return expr;` where an expression would suffice"
  69 }
  70
  71 #[derive(PartialEq, Eq, Copy, Clone)]
  72 enum RetReplacement {
  73     Empty,
  74     Block,
  75     Unit,
  76 }
  77
  78 impl RetReplacement {
  79     fn sugg_help(self) -> &'static str {
  80         match self {
  81             Self::Empty => "remove `return`",
  82             Self::Block => "replace `return` with an empty block",
  83             Self::Unit => "replace `return` with a unit value",
  84         }
  85     }
  86 }
  87
  88 impl ToString for RetReplacement {
  89     fn to_string(&self) -> String {
  90         match *self {
  91             Self::Empty => "",
  92             Self::Block => "{}",
  93             Self::Unit => "()",
  94         }
  95         .to_string()
  96     }
  97 }
  98
  99 declare_lint_pass!(Return => [LET_AND_RETURN, NEEDLESS_RETURN]);
 100
 101 impl<'tcx> LateLintPass<'tcx> for Return {
 102     fn check_block(&mut self, cx: &LateContext<'tcx>, block: &'tcx Block<'_>) {
 103         // we need both a let-binding stmt and an expr
 104         if_chain! {
 105             if let Some(retexpr) = block.expr;
 106             if let Some(stmt) = block.stmts.iter().last();
 107             if let StmtKind::Local(local) = &stmt.kind;
 108             if local.ty.is_none();
 109             if cx.tcx.hir().attrs(local.hir_id).is_empty();
 110             if let Some(initexpr) = &local.init;
 111             if let PatKind::Binding(_, local_id, _, _) = local.pat.kind;
 112             if path_to_local_id(retexpr, local_id);
 113             if !last_statement_borrows(cx, initexpr);
 114             if !in_external_macro(cx.sess(), initexpr.span);
 115             if !in_external_macro(cx.sess(), retexpr.span);
 116             if !local.span.from_expansion();
 117             then {
 118                 span_lint_hir_and_then(
 119                     cx,
 120                     LET_AND_RETURN,
 121                     retexpr.hir_id,
 122                     retexpr.span,
 123                     "returning the result of a `let` binding from a block",
 124                     |err| {
 125                         err.span_label(local.span, "unnecessary `let` binding");
 126
 127                         if let Some(mut snippet) = snippet_opt(cx, initexpr.span) {
 128                             if !cx.typeck_results().expr_adjustments(retexpr).is_empty() {
 129                                 snippet.push_str(" as _");
 130                             }
 131                             err.multipart_suggestion(
 132                                 "return the expression directly",
 133                                 vec![
 134                                     (local.span, String::new()),
 135                                     (retexpr.span, snippet),
 136                                 ],
 137                                 Applicability::MachineApplicable,
 138                             );
 139                         } else {
 140                             err.span_help(initexpr.span, "this expression can be directly returned");
 141                         }
 142                     },
 143                 );
 144             }
 145         }
 146     }
 147
 148     fn check_fn(
 149         &mut self,
 150         cx: &LateContext<'tcx>,
 151         kind: FnKind<'tcx>,
 152         _: &'tcx FnDecl<'tcx>,
 153         body: &'tcx Body<'tcx>,
 154         _: Span,
 155         _: HirId,
 156     ) {
 157         match kind {
 158             FnKind::Closure => {
 159                 // when returning without value in closure, replace this `return`
 160                 // with an empty block to prevent invalid suggestion (see #6501)
 161                 let replacement = if let ExprKind::Ret(None) = &body.value.kind {
 162                     RetReplacement::Block
 163                 } else {
 164                     RetReplacement::Empty
 165                 };
 166                 check_final_expr(cx, body.value, vec![], replacement);
 167             },
 168             FnKind::ItemFn(..) | FnKind::Method(..) => {
 169                 check_block_return(cx, &body.value.kind, vec![]);
 170             },
 171         }
 172     }
 173 }
 174
 175 // if `expr` is a block, check if there are needless returns in it
 176 fn check_block_return<'tcx>(cx: &LateContext<'tcx>, expr_kind: &ExprKind<'tcx>, semi_spans: Vec<Span>) {
 177     if let ExprKind::Block(block, _) = expr_kind {
 178         if let Some(block_expr) = block.expr {
 179             check_final_expr(cx, block_expr, semi_spans, RetReplacement::Empty);
 180         } else if let Some(stmt) = block.stmts.iter().last() {
 181             match stmt.kind {
 182                 StmtKind::Expr(expr) => {
 183                     check_final_expr(cx, expr, semi_spans, RetReplacement::Empty);
 184                 },
 185                 StmtKind::Semi(semi_expr) => {
 186                     let mut semi_spans_and_this_one = semi_spans;
 187                     // we only want the span containing the semicolon so we can remove it later. From `entry.rs:382`
 188                     if let Some(semicolon_span) = stmt.span.trim_start(semi_expr.span) {
 189                         semi_spans_and_this_one.push(semicolon_span);
 190                         check_final_expr(cx, semi_expr, semi_spans_and_this_one, RetReplacement::Empty);
 191                     }
 192                 },
 193                 _ => (),
 194             }
 195         }
 196     }
 197 }
 198
 199 fn check_final_expr<'tcx>(
 200     cx: &LateContext<'tcx>,
 201     expr: &'tcx Expr<'tcx>,
 202     semi_spans: Vec<Span>, /* containing all the places where we would need to remove semicolons if finding an
 203                             * needless return */
 204     replacement: RetReplacement,
 205 ) {
 206     let peeled_drop_expr = expr.peel_drop_temps();
 207     match &peeled_drop_expr.kind {
 208         // simple return is always "bad"
 209         ExprKind::Ret(ref inner) => {
 210             if cx.tcx.hir().attrs(expr.hir_id).is_empty() {
 211                 let borrows = inner.map_or(false, |inner| last_statement_borrows(cx, inner));
 212                 if !borrows {
 213                     // check if expr return nothing
 214                     let ret_span = if inner.is_none() && replacement == RetReplacement::Empty {
 215                         extend_span_to_previous_non_ws(cx, peeled_drop_expr.span)
 216                     } else {
 217                         peeled_drop_expr.span
 218                     };
 219
 220                     emit_return_lint(cx, ret_span, semi_spans, inner.as_ref().map(|i| i.span), replacement);
 221                 }
 222             }
 223         },
 224         ExprKind::If(_, then, else_clause_opt) => {
 225             check_block_return(cx, &then.kind, semi_spans.clone());
 226             if let Some(else_clause) = else_clause_opt {
 227                 check_block_return(cx, &else_clause.kind, semi_spans);
 228             }
 229         },
 230         // a match expr, check all arms
 231         // an if/if let expr, check both exprs
 232         // note, if without else is going to be a type checking error anyways
 233         // (except for unit type functions) so we don't match it
 234         ExprKind::Match(_, arms, MatchSource::Normal) => {
 235             for arm in arms.iter() {
 236                 check_final_expr(cx, arm.body, semi_spans.clone(), RetReplacement::Unit);
 237             }
 238         },
 239         // if it's a whole block, check it
 240         other_expr_kind => check_block_return(cx, other_expr_kind, semi_spans),
 241     }
 242 }
 243
 244 fn emit_return_lint(
 245     cx: &LateContext<'_>,
 246     ret_span: Span,
 247     semi_spans: Vec<Span>,
 248     inner_span: Option<Span>,
 249     replacement: RetReplacement,
 250 ) {
 251     if ret_span.from_expansion() {
 252         return;
 253     }
 254     let mut applicability = Applicability::MachineApplicable;
 255     let return_replacement = inner_span.map_or_else(
 256         || replacement.to_string(),
 257         |inner_span| {
 258             let (snippet, _) = snippet_with_context(cx, inner_span, ret_span.ctxt(), "..", &mut applicability);
 259             snippet.to_string()
 260         },
 261     );
 262     let sugg_help = if inner_span.is_some() {
 263         "remove `return`"
 264     } else {
 265         replacement.sugg_help()
 266     };
 267     span_lint_and_then(cx, NEEDLESS_RETURN, ret_span, "unneeded `return` statement", |diag| {
 268         diag.span_suggestion_hidden(ret_span, sugg_help, return_replacement, applicability);
 269         // for each parent statement, we need to remove the semicolon
 270         for semi_stmt_span in semi_spans {
 271             diag.tool_only_span_suggestion(semi_stmt_span, "remove this semicolon", "", applicability);
 272         }
 273     });
 274 }
 275
 276 fn last_statement_borrows<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) -> bool {
 277     for_each_expr(expr, |e| {
 278         if let Some(def_id) = fn_def_id(cx, e)
 279             && cx
 280                 .tcx
 281                 .fn_sig(def_id)
 282                 .skip_binder()
 283                 .output()
 284                 .walk()
 285                 .any(|arg| matches!(arg.unpack(), GenericArgKind::Lifetime(_)))
 286         {
 287             ControlFlow::Break(())
 288         } else {
 289             ControlFlow::Continue(Descend::from(!expr.span.from_expansion()))
 290         }
 291     })
 292     .is_some()
 293 }
 294
 295 // Go backwards while encountering whitespace and extend the given Span to that point.
 296 fn extend_span_to_previous_non_ws(cx: &LateContext<'_>, sp: Span) -> Span {
 297     if let Ok(prev_source) = cx.sess().source_map().span_to_prev_source(sp) {
 298         let ws = [' ', '\t', '\n'];
 299         if let Some(non_ws_pos) = prev_source.rfind(|c| !ws.contains(&c)) {
 300             let len = prev_source.len() - non_ws_pos - 1;
 301             return sp.with_lo(sp.lo() - BytePos::from_usize(len));
 302         }
 303     }
 304
 305     sp
 306 }