clippy_lints/src/implicit_return.rs

   1 // Copyright 2014-2018 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution.
   3 //
   4 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   5 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   6 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   7 // option. This file may not be copied, modified, or distributed
   8 // except according to those terms.
   9
  10 use crate::rustc::hir::{intravisit::FnKind, Body, ExprKind, FnDecl};
  11 use crate::rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
  12 use crate::rustc::{declare_tool_lint, lint_array};
  13 use crate::rustc_errors::Applicability;
  14 use crate::syntax::{ast::NodeId, source_map::Span};
  15 use crate::utils::{snippet_opt, span_lint_and_then};
  16
  17 /// **What it does:** Checks for missing return statements at the end of a block.
  18 ///
  19 /// **Why is this bad?** Actually omitting the return keyword is idiomatic Rust code. Programmers
  20 /// coming from other languages might prefer the expressiveness of `return`. It's possible to miss
  21 /// the last returning statement because the only difference is a missing `;`. Especially in bigger
  22 /// code with multiple return paths having a `return` keyword makes it easier to find the
  23 /// corresponding statements.
  24 ///
  25 /// **Known problems:** None.
  26 ///
  27 /// **Example:**
  28 /// ```rust
  29 /// fn foo(x: usize) {
  30 ///     x
  31 /// }
  32 /// ```
  33 /// add return
  34 /// ```rust
  35 /// fn foo(x: usize) {
  36 ///     return x;
  37 /// }
  38 /// ```
  39 declare_clippy_lint! {
  40     pub IMPLICIT_RETURN,
  41     restriction,
  42     "use a return statement like `return expr` instead of an expression"
  43 }
  44
  45 pub struct Pass;
  46
  47 impl Pass {
  48     fn expr_match(cx: &LateContext<'_, '_>, expr: &rustc::hir::Expr) {
  49         match &expr.node {
  50             // loops could be using `break` instead of `return`
  51             ExprKind::Block(block, ..) | ExprKind::Loop(block, ..) => {
  52                 if let Some(expr) = &block.expr {
  53                     Self::expr_match(cx, expr);
  54                 }
  55                 // only needed in the case of `break` with `;` at the end
  56                 else if let Some(stmt) = block.stmts.last() {
  57                     if let rustc::hir::StmtKind::Semi(expr, ..) = &stmt.node {
  58                         Self::expr_match(cx, expr);
  59                     }
  60                 }
  61             },
  62             // use `return` instead of `break`
  63             ExprKind::Break(.., break_expr) => {
  64                 if let Some(break_expr) = break_expr {
  65                     span_lint_and_then(cx, IMPLICIT_RETURN, expr.span, "missing return statement", |db| {
  66                         if let Some(snippet) = snippet_opt(cx, break_expr.span) {
  67                             db.span_suggestion_with_applicability(
  68                                 expr.span,
  69                                 "change `break` to `return` as shown",
  70                                 format!("return {}", snippet),
  71                                 Applicability::MachineApplicable,
  72                             );
  73                         }
  74                     });
  75                 }
  76             },
  77             ExprKind::If(.., if_expr, else_expr) => {
  78                 Self::expr_match(cx, if_expr);
  79
  80                 if let Some(else_expr) = else_expr {
  81                     Self::expr_match(cx, else_expr);
  82                 }
  83             },
  84             ExprKind::Match(_, arms, ..) => {
  85                 for arm in arms {
  86                     Self::expr_match(cx, &arm.body);
  87                 }
  88             },
  89             // skip if it already has a return statement
  90             ExprKind::Ret(..) => (),
  91             // everything else is missing `return`
  92             _ => span_lint_and_then(cx, IMPLICIT_RETURN, expr.span, "missing return statement", |db| {
  93                 if let Some(snippet) = snippet_opt(cx, expr.span) {
  94                     db.span_suggestion_with_applicability(
  95                         expr.span,
  96                         "add `return` as shown",
  97                         format!("return {}", snippet),
  98                         Applicability::MachineApplicable,
  99                     );
 100                 }
 101             }),
 102         }
 103     }
 104 }
 105
 106 impl LintPass for Pass {
 107     fn get_lints(&self) -> LintArray {
 108         lint_array!(IMPLICIT_RETURN)
 109     }
 110 }
 111
 112 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Pass {
 113     fn check_fn(
 114         &mut self,
 115         cx: &LateContext<'a, 'tcx>,
 116         _: FnKind<'tcx>,
 117         _: &'tcx FnDecl,
 118         body: &'tcx Body,
 119         _: Span,
 120         _: NodeId,
 121     ) {
 122         let def_id = cx.tcx.hir.body_owner_def_id(body.id());
 123         let mir = cx.tcx.optimized_mir(def_id);
 124
 125         // checking return type through MIR, HIR is not able to determine inferred closure return types
 126         if !mir.return_ty().is_unit() {
 127             Self::expr_match(cx, &body.value);
 128         }
 129     }
 130 }