clippy_lints/src/blocks_in_if_conditions.rs

   1 use clippy_utils::diagnostics::{span_lint, span_lint_and_sugg};
   2 use clippy_utils::source::snippet_block_with_applicability;
   3 use clippy_utils::ty::implements_trait;
   4 use clippy_utils::{differing_macro_contexts, get_parent_expr};
   5 use if_chain::if_chain;
   6 use rustc_errors::Applicability;
   7 use rustc_hir::intravisit::{walk_expr, NestedVisitorMap, Visitor};
   8 use rustc_hir::{BlockCheckMode, Expr, ExprKind};
   9 use rustc_lint::{LateContext, LateLintPass, LintContext};
  10 use rustc_middle::hir::map::Map;
  11 use rustc_middle::lint::in_external_macro;
  12 use rustc_session::{declare_lint_pass, declare_tool_lint};
  13 use rustc_span::sym;
  14
  15 declare_clippy_lint! {
  16     /// **What it does:** Checks for `if` conditions that use blocks containing an
  17     /// expression, statements or conditions that use closures with blocks.
  18     ///
  19     /// **Why is this bad?** Style, using blocks in the condition makes it hard to read.
  20     ///
  21     /// **Known problems:** None.
  22     ///
  23     /// **Examples:**
  24     /// ```rust
  25     /// // Bad
  26     /// if { true } { /* ... */ }
  27     ///
  28     /// // Good
  29     /// if true { /* ... */ }
  30     /// ```
  31     ///
  32     /// // or
  33     ///
  34     /// ```rust
  35     /// # fn somefunc() -> bool { true };
  36     /// // Bad
  37     /// if { let x = somefunc(); x } { /* ... */ }
  38     ///
  39     /// // Good
  40     /// let res = { let x = somefunc(); x };
  41     /// if res { /* ... */ }
  42     /// ```
  43     pub BLOCKS_IN_IF_CONDITIONS,
  44     style,
  45     "useless or complex blocks that can be eliminated in conditions"
  46 }
  47
  48 declare_lint_pass!(BlocksInIfConditions => [BLOCKS_IN_IF_CONDITIONS]);
  49
  50 struct ExVisitor<'a, 'tcx> {
  51     found_block: Option<&'tcx Expr<'tcx>>,
  52     cx: &'a LateContext<'tcx>,
  53 }
  54
  55 impl<'a, 'tcx> Visitor<'tcx> for ExVisitor<'a, 'tcx> {
  56     type Map = Map<'tcx>;
  57
  58     fn visit_expr(&mut self, expr: &'tcx Expr<'tcx>) {
  59         if let ExprKind::Closure(_, _, eid, _, _) = expr.kind {
  60             // do not lint if the closure is called using an iterator (see #1141)
  61             if_chain! {
  62                 if let Some(parent) = get_parent_expr(self.cx, expr);
  63                 if let ExprKind::MethodCall(_, _, args, _) = parent.kind;
  64                 let caller = self.cx.typeck_results().expr_ty(&args[0]);
  65                 if let Some(iter_id) = self.cx.tcx.get_diagnostic_item(sym::Iterator);
  66                 if implements_trait(self.cx, caller, iter_id, &[]);
  67                 then {
  68                     return;
  69                 }
  70             }
  71
  72             let body = self.cx.tcx.hir().body(eid);
  73             let ex = &body.value;
  74             if matches!(ex.kind, ExprKind::Block(_, _)) && !body.value.span.from_expansion() {
  75                 self.found_block = Some(ex);
  76                 return;
  77             }
  78         }
  79         walk_expr(self, expr);
  80     }
  81     fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
  82         NestedVisitorMap::None
  83     }
  84 }
  85
  86 const BRACED_EXPR_MESSAGE: &str = "omit braces around single expression condition";
  87 const COMPLEX_BLOCK_MESSAGE: &str = "in an `if` condition, avoid complex blocks or closures with blocks; \
  88                                     instead, move the block or closure higher and bind it with a `let`";
  89
  90 impl<'tcx> LateLintPass<'tcx> for BlocksInIfConditions {
  91     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
  92         if in_external_macro(cx.sess(), expr.span) {
  93             return;
  94         }
  95         if let ExprKind::If(cond, _, _) = &expr.kind {
  96             if let ExprKind::Block(block, _) = &cond.kind {
  97                 if block.rules == BlockCheckMode::DefaultBlock {
  98                     if block.stmts.is_empty() {
  99                         if let Some(ex) = &block.expr {
 100                             // don't dig into the expression here, just suggest that they remove
 101                             // the block
 102                             if expr.span.from_expansion() || differing_macro_contexts(expr.span, ex.span) {
 103                                 return;
 104                             }
 105                             let mut applicability = Applicability::MachineApplicable;
 106                             span_lint_and_sugg(
 107                                 cx,
 108                                 BLOCKS_IN_IF_CONDITIONS,
 109                                 cond.span,
 110                                 BRACED_EXPR_MESSAGE,
 111                                 "try",
 112                                 format!(
 113                                     "{}",
 114                                     snippet_block_with_applicability(
 115                                         cx,
 116                                         ex.span,
 117                                         "..",
 118                                         Some(expr.span),
 119                                         &mut applicability
 120                                     )
 121                                 ),
 122                                 applicability,
 123                             );
 124                         }
 125                     } else {
 126                         let span = block.expr.as_ref().map_or_else(|| block.stmts[0].span, |e| e.span);
 127                         if span.from_expansion() || differing_macro_contexts(expr.span, span) {
 128                             return;
 129                         }
 130                         // move block higher
 131                         let mut applicability = Applicability::MachineApplicable;
 132                         span_lint_and_sugg(
 133                             cx,
 134                             BLOCKS_IN_IF_CONDITIONS,
 135                             expr.span.with_hi(cond.span.hi()),
 136                             COMPLEX_BLOCK_MESSAGE,
 137                             "try",
 138                             format!(
 139                                 "let res = {}; if res",
 140                                 snippet_block_with_applicability(
 141                                     cx,
 142                                     block.span,
 143                                     "..",
 144                                     Some(expr.span),
 145                                     &mut applicability
 146                                 ),
 147                             ),
 148                             applicability,
 149                         );
 150                     }
 151                 }
 152             } else {
 153                 let mut visitor = ExVisitor { found_block: None, cx };
 154                 walk_expr(&mut visitor, cond);
 155                 if let Some(block) = visitor.found_block {
 156                     span_lint(cx, BLOCKS_IN_IF_CONDITIONS, block.span, COMPLEX_BLOCK_MESSAGE);
 157                 }
 158             }
 159         }
 160     }
 161 }