clippy_lints/src/block_in_if_condition.rs

   1 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
   2 use rustc::hir::*;
   3 use rustc::hir::intravisit::{walk_expr, NestedVisitorMap, Visitor};
   4 use utils::*;
   5
   6 /// **What it does:** Checks for `if` conditions that use blocks to contain an
   7 /// expression.
   8 ///
   9 /// **Why is this bad?** It isn't really Rust style, same as using parentheses
  10 /// to contain expressions.
  11 ///
  12 /// **Known problems:** None.
  13 ///
  14 /// **Example:**
  15 /// ```rust
  16 /// if { true } ..
  17 /// ```
  18 declare_lint! {
  19     pub BLOCK_IN_IF_CONDITION_EXPR,
  20     Warn,
  21     "braces that can be eliminated in conditions, e.g. `if { true } ...`"
  22 }
  23
  24 /// **What it does:** Checks for `if` conditions that use blocks containing
  25 /// statements, or conditions that use closures with blocks.
  26 ///
  27 /// **Why is this bad?** Using blocks in the condition makes it hard to read.
  28 ///
  29 /// **Known problems:** None.
  30 ///
  31 /// **Example:**
  32 /// ```rust
  33 /// if { let x = somefunc(); x } ..
  34 /// // or
  35 /// if somefunc(|x| { x == 47 }) ..
  36 /// ```
  37 declare_lint! {
  38     pub BLOCK_IN_IF_CONDITION_STMT,
  39     Warn,
  40     "complex blocks in conditions, e.g. `if { let x = true; x } ...`"
  41 }
  42
  43 #[derive(Copy, Clone)]
  44 pub struct BlockInIfCondition;
  45
  46 impl LintPass for BlockInIfCondition {
  47     fn get_lints(&self) -> LintArray {
  48         lint_array!(BLOCK_IN_IF_CONDITION_EXPR, BLOCK_IN_IF_CONDITION_STMT)
  49     }
  50 }
  51
  52 struct ExVisitor<'a, 'tcx: 'a> {
  53     found_block: Option<&'tcx Expr>,
  54     cx: &'a LateContext<'a, 'tcx>,
  55 }
  56
  57 impl<'a, 'tcx: 'a> Visitor<'tcx> for ExVisitor<'a, 'tcx> {
  58     fn visit_expr(&mut self, expr: &'tcx Expr) {
  59         if let ExprClosure(_, _, eid, _, _) = expr.node {
  60             let body = self.cx.tcx.hir.body(eid);
  61             let ex = &body.value;
  62             if matches!(ex.node, ExprBlock(_)) {
  63                 self.found_block = Some(ex);
  64                 return;
  65             }
  66         }
  67         walk_expr(self, expr);
  68     }
  69     fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
  70         NestedVisitorMap::None
  71     }
  72 }
  73
  74 const BRACED_EXPR_MESSAGE: &str = "omit braces around single expression condition";
  75 const COMPLEX_BLOCK_MESSAGE: &str = "in an 'if' condition, avoid complex blocks or closures with blocks; \
  76                                      instead, move the block or closure higher and bind it with a 'let'";
  77
  78 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for BlockInIfCondition {
  79     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
  80         if let ExprIf(ref check, ref then, _) = expr.node {
  81             if let ExprBlock(ref block) = check.node {
  82                 if block.rules == DefaultBlock {
  83                     if block.stmts.is_empty() {
  84                         if let Some(ref ex) = block.expr {
  85                             // don't dig into the expression here, just suggest that they remove
  86                             // the block
  87                             if in_macro(expr.span) || differing_macro_contexts(expr.span, ex.span) {
  88                                 return;
  89                             }
  90                             span_help_and_lint(
  91                                 cx,
  92                                 BLOCK_IN_IF_CONDITION_EXPR,
  93                                 check.span,
  94                                 BRACED_EXPR_MESSAGE,
  95                                 &format!(
  96                                     "try\nif {} {} ... ",
  97                                     snippet_block(cx, ex.span, ".."),
  98                                     snippet_block(cx, then.span, "..")
  99                                 ),
 100                             );
 101                         }
 102                     } else {
 103                         let span = block
 104                             .expr
 105                             .as_ref()
 106                             .map_or_else(|| block.stmts[0].span, |e| e.span);
 107                         if in_macro(span) || differing_macro_contexts(expr.span, span) {
 108                             return;
 109                         }
 110                         // move block higher
 111                         span_help_and_lint(
 112                             cx,
 113                             BLOCK_IN_IF_CONDITION_STMT,
 114                             check.span,
 115                             COMPLEX_BLOCK_MESSAGE,
 116                             &format!(
 117                                 "try\nlet res = {};\nif res {} ... ",
 118                                 snippet_block(cx, block.span, ".."),
 119                                 snippet_block(cx, then.span, "..")
 120                             ),
 121                         );
 122                     }
 123                 }
 124             } else {
 125                 let mut visitor = ExVisitor {
 126                     found_block: None,
 127                     cx: cx,
 128                 };
 129                 walk_expr(&mut visitor, check);
 130                 if let Some(block) = visitor.found_block {
 131                     span_lint(cx, BLOCK_IN_IF_CONDITION_STMT, block.span, COMPLEX_BLOCK_MESSAGE);
 132                 }
 133             }
 134         }
 135     }
 136 }