clippy_lints/src/block_in_if_condition.rs

   1 use rustc::lint::{LateLintPass, LateContext, LintArray, LintPass};
   2 use rustc::hir::*;
   3 use rustc::hir::intravisit::{Visitor, walk_expr, NestedVisitorMap};
   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 expr = self.cx.tcx.map.body(eid).value;
  61             if matches!(expr.node, ExprBlock(_)) {
  62                 self.found_block = Some(&expr);
  63                 return;
  64             }
  65         }
  66         walk_expr(self, expr);
  67     }
  68     fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
  69         NestedVisitorMap::All(&self.cx.tcx.map)
  70     }
  71 }
  72
  73 const BRACED_EXPR_MESSAGE: &'static str = "omit braces around single expression condition";
  74 const COMPLEX_BLOCK_MESSAGE: &'static str = "in an 'if' condition, avoid complex blocks or closures with blocks; \
  75                                              instead, move the block or closure higher and bind it with a 'let'";
  76
  77 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for BlockInIfCondition {
  78     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
  79         if let ExprIf(ref check, ref then, _) = expr.node {
  80             if let ExprBlock(ref block) = check.node {
  81                 if block.rules == DefaultBlock {
  82                     if block.stmts.is_empty() {
  83                         if let Some(ref ex) = block.expr {
  84                             // don't dig into the expression here, just suggest that they remove
  85                             // the block
  86                             if in_macro(cx, expr.span) || differing_macro_contexts(expr.span, ex.span) {
  87                                 return;
  88                             }
  89                             span_help_and_lint(cx,
  90                                                BLOCK_IN_IF_CONDITION_EXPR,
  91                                                check.span,
  92                                                BRACED_EXPR_MESSAGE,
  93                                                &format!("try\nif {} {} ... ",
  94                                                         snippet_block(cx, ex.span, ".."),
  95                                                         snippet_block(cx, then.span, "..")));
  96                         }
  97                     } else {
  98                         let span = block.expr.as_ref().map_or_else(|| block.stmts[0].span, |e| e.span);
  99                         if in_macro(cx, span) || differing_macro_contexts(expr.span, span) {
 100                             return;
 101                         }
 102                         // move block higher
 103                         span_help_and_lint(cx,
 104                                            BLOCK_IN_IF_CONDITION_STMT,
 105                                            check.span,
 106                                            COMPLEX_BLOCK_MESSAGE,
 107                                            &format!("try\nlet res = {};\nif res {} ... ",
 108                                                     snippet_block(cx, block.span, ".."),
 109                                                     snippet_block(cx, then.span, "..")));
 110                     }
 111                 }
 112             } else {
 113                 let mut visitor = ExVisitor {
 114                     found_block: None,
 115                     cx: cx,
 116                 };
 117                 walk_expr(&mut visitor, check);
 118                 if let Some(block) = visitor.found_block {
 119                     span_lint(cx, BLOCK_IN_IF_CONDITION_STMT, block.span, COMPLEX_BLOCK_MESSAGE);
 120                 }
 121             }
 122         }
 123     }
 124 }