src/tools/clippy/clippy_lints/src/collapsible_if.rs

   1 //! Checks for if expressions that contain only an if expression.
   2 //!
   3 //! For example, the lint would catch:
   4 //!
   5 //! ```rust,ignore
   6 //! if x {
   7 //!     if y {
   8 //!         println!("Hello world");
   9 //!     }
  10 //! }
  11 //! ```
  12 //!
  13 //! This lint is **warn** by default
  14
  15 use rustc::lint::*;
  16 use syntax::ast;
  17
  18 use utils::{in_macro, snippet_block, span_lint_and_then, span_lint_and_sugg};
  19 use utils::sugg::Sugg;
  20
  21 /// **What it does:** Checks for nested `if` statements which can be collapsed
  22 /// by `&&`-combining their conditions and for `else { if ... }` expressions
  23 /// that
  24 /// can be collapsed to `else if ...`.
  25 ///
  26 /// **Why is this bad?** Each `if`-statement adds one level of nesting, which
  27 /// makes code look more complex than it really is.
  28 ///
  29 /// **Known problems:** None.
  30 ///
  31 /// **Example:**
  32 /// ```rust,ignore
  33 /// if x {
  34 ///     if y {
  35 ///         …
  36 ///     }
  37 /// }
  38 ///
  39 /// // or
  40 ///
  41 /// if x {
  42 ///     …
  43 /// } else {
  44 ///     if y {
  45 ///         …
  46 ///     }
  47 /// }
  48 /// ```
  49 ///
  50 /// Should be written:
  51 ///
  52 /// ```rust.ignore
  53 /// if x && y {
  54 ///     …
  55 /// }
  56 ///
  57 /// // or
  58 ///
  59 /// if x {
  60 ///     …
  61 /// } else if y {
  62 ///     …
  63 /// }
  64 /// ```
  65 declare_lint! {
  66     pub COLLAPSIBLE_IF,
  67     Warn,
  68     "`if`s that can be collapsed (e.g. `if x { if y { ... } }` and `else { if x { ... } }`)"
  69 }
  70
  71 #[derive(Copy, Clone)]
  72 pub struct CollapsibleIf;
  73
  74 impl LintPass for CollapsibleIf {
  75     fn get_lints(&self) -> LintArray {
  76         lint_array!(COLLAPSIBLE_IF)
  77     }
  78 }
  79
  80 impl EarlyLintPass for CollapsibleIf {
  81     fn check_expr(&mut self, cx: &EarlyContext, expr: &ast::Expr) {
  82         if !in_macro(expr.span) {
  83             check_if(cx, expr)
  84         }
  85     }
  86 }
  87
  88 fn check_if(cx: &EarlyContext, expr: &ast::Expr) {
  89     match expr.node {
  90         ast::ExprKind::If(ref check, ref then, ref else_) => {
  91             if let Some(ref else_) = *else_ {
  92                 check_collapsible_maybe_if_let(cx, else_);
  93             } else {
  94                 check_collapsible_no_if_let(cx, expr, check, then);
  95             }
  96         },
  97         ast::ExprKind::IfLet(_, _, _, Some(ref else_)) => {
  98             check_collapsible_maybe_if_let(cx, else_);
  99         },
 100         _ => (),
 101     }
 102 }
 103
 104 fn check_collapsible_maybe_if_let(cx: &EarlyContext, else_: &ast::Expr) {
 105     if_let_chain! {[
 106         let ast::ExprKind::Block(ref block) = else_.node,
 107         let Some(else_) = expr_block(block),
 108         !in_macro(else_.span),
 109     ], {
 110         match else_.node {
 111             ast::ExprKind::If(..) | ast::ExprKind::IfLet(..) => {
 112                 span_lint_and_sugg(cx,
 113                                    COLLAPSIBLE_IF,
 114                                    block.span,
 115                                    "this `else { if .. }` block can be collapsed",
 116                                    "try",
 117                                    snippet_block(cx, else_.span, "..").into_owned());
 118             }
 119             _ => (),
 120         }
 121     }}
 122 }
 123
 124 fn check_collapsible_no_if_let(cx: &EarlyContext, expr: &ast::Expr, check: &ast::Expr, then: &ast::Block) {
 125     if_let_chain! {[
 126         let Some(inner) = expr_block(then),
 127         let ast::ExprKind::If(ref check_inner, ref content, None) = inner.node,
 128     ], {
 129         if expr.span.ctxt() != inner.span.ctxt() {
 130             return;
 131         }
 132         span_lint_and_then(cx, COLLAPSIBLE_IF, expr.span, "this if statement can be collapsed", |db| {
 133             let lhs = Sugg::ast(cx, check, "..");
 134             let rhs = Sugg::ast(cx, check_inner, "..");
 135             db.span_suggestion(expr.span,
 136                                "try",
 137                                format!("if {} {}",
 138                                        lhs.and(rhs),
 139                                        snippet_block(cx, content.span, "..")));
 140         });
 141     }}
 142 }
 143
 144 /// If the block contains only one expression, return it.
 145 fn expr_block(block: &ast::Block) -> Option<&ast::Expr> {
 146     let mut it = block.stmts.iter();
 147
 148     if let (Some(stmt), None) = (it.next(), it.next()) {
 149         match stmt.node {
 150             ast::StmtKind::Expr(ref expr) |
 151             ast::StmtKind::Semi(ref expr) => Some(expr),
 152             _ => None,
 153         }
 154     } else {
 155         None
 156     }
 157 }