clippy_lints/src/needless_bool.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
  11 //! Checks for needless boolean results of if-else expressions
  12 //!
  13 //! This lint is **warn** by default
  14
  15 use crate::rustc::hir::*;
  16 use crate::rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
  17 use crate::rustc::{declare_tool_lint, lint_array};
  18 use crate::rustc_errors::Applicability;
  19 use crate::syntax::ast::LitKind;
  20 use crate::syntax::source_map::Spanned;
  21 use crate::utils::sugg::Sugg;
  22 use crate::utils::{in_macro, snippet_with_applicability, span_lint, span_lint_and_sugg};
  23
  24 /// **What it does:** Checks for expressions of the form `if c { true } else {
  25 /// false }`
  26 /// (or vice versa) and suggest using the condition directly.
  27 ///
  28 /// **Why is this bad?** Redundant code.
  29 ///
  30 /// **Known problems:** Maybe false positives: Sometimes, the two branches are
  31 /// painstakingly documented (which we of course do not detect), so they *may*
  32 /// have some value. Even then, the documentation can be rewritten to match the
  33 /// shorter code.
  34 ///
  35 /// **Example:**
  36 /// ```rust
  37 /// if x { false } else { true }
  38 /// ```
  39 declare_clippy_lint! {
  40     pub NEEDLESS_BOOL,
  41     complexity,
  42     "if-statements with plain booleans in the then- and else-clause, e.g. \
  43      `if p { true } else { false }`"
  44 }
  45
  46 /// **What it does:** Checks for expressions of the form `x == true` (or vice
  47 /// versa) and suggest using the variable directly.
  48 ///
  49 /// **Why is this bad?** Unnecessary code.
  50 ///
  51 /// **Known problems:** None.
  52 ///
  53 /// **Example:**
  54 /// ```rust
  55 /// if x == true { }  // could be `if x { }`
  56 /// ```
  57 declare_clippy_lint! {
  58     pub BOOL_COMPARISON,
  59     complexity,
  60     "comparing a variable to a boolean, e.g. `if x == true`"
  61 }
  62
  63 #[derive(Copy, Clone)]
  64 pub struct NeedlessBool;
  65
  66 impl LintPass for NeedlessBool {
  67     fn get_lints(&self) -> LintArray {
  68         lint_array!(NEEDLESS_BOOL)
  69     }
  70 }
  71
  72 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NeedlessBool {
  73     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, e: &'tcx Expr) {
  74         use self::Expression::*;
  75         if let ExprKind::If(ref pred, ref then_block, Some(ref else_expr)) = e.node {
  76             let reduce = |ret, not| {
  77                 let mut applicability = Applicability::MachineApplicable;
  78                 let snip = Sugg::hir_with_applicability(cx, pred, "<predicate>", &mut applicability);
  79                 let snip = if not { !snip } else { snip };
  80
  81                 let hint = if ret {
  82                     format!("return {}", snip)
  83                 } else {
  84                     snip.to_string()
  85                 };
  86
  87                 span_lint_and_sugg(
  88                     cx,
  89                     NEEDLESS_BOOL,
  90                     e.span,
  91                     "this if-then-else expression returns a bool literal",
  92                     "you can reduce it to",
  93                     hint,
  94                     applicability,
  95                 );
  96             };
  97             if let ExprKind::Block(ref then_block, _) = then_block.node {
  98                 match (fetch_bool_block(then_block), fetch_bool_expr(else_expr)) {
  99                     (RetBool(true), RetBool(true)) | (Bool(true), Bool(true)) => {
 100                         span_lint(
 101                             cx,
 102                             NEEDLESS_BOOL,
 103                             e.span,
 104                             "this if-then-else expression will always return true",
 105                         );
 106                     },
 107                     (RetBool(false), RetBool(false)) | (Bool(false), Bool(false)) => {
 108                         span_lint(
 109                             cx,
 110                             NEEDLESS_BOOL,
 111                             e.span,
 112                             "this if-then-else expression will always return false",
 113                         );
 114                     },
 115                     (RetBool(true), RetBool(false)) => reduce(true, false),
 116                     (Bool(true), Bool(false)) => reduce(false, false),
 117                     (RetBool(false), RetBool(true)) => reduce(true, true),
 118                     (Bool(false), Bool(true)) => reduce(false, true),
 119                     _ => (),
 120                 }
 121             } else {
 122                 panic!("IfExpr 'then' node is not an ExprKind::Block");
 123             }
 124         }
 125     }
 126 }
 127
 128 #[derive(Copy, Clone)]
 129 pub struct BoolComparison;
 130
 131 impl LintPass for BoolComparison {
 132     fn get_lints(&self) -> LintArray {
 133         lint_array!(BOOL_COMPARISON)
 134     }
 135 }
 136
 137 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for BoolComparison {
 138     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, e: &'tcx Expr) {
 139         use self::Expression::*;
 140
 141         if in_macro(e.span) {
 142             return;
 143         }
 144
 145         if let ExprKind::Binary(Spanned { node: BinOpKind::Eq, .. }, ref left_side, ref right_side) = e.node {
 146             let mut applicability = Applicability::MachineApplicable;
 147             match (fetch_bool_expr(left_side), fetch_bool_expr(right_side)) {
 148                 (Bool(true), Other) => {
 149                     let hint = snippet_with_applicability(cx, right_side.span, "..", &mut applicability);
 150                     span_lint_and_sugg(
 151                         cx,
 152                         BOOL_COMPARISON,
 153                         e.span,
 154                         "equality checks against true are unnecessary",
 155                         "try simplifying it as shown",
 156                         hint.to_string(),
 157                         applicability,
 158                     );
 159                 },
 160                 (Other, Bool(true)) => {
 161                     let hint = snippet_with_applicability(cx, left_side.span, "..", &mut applicability);
 162                     span_lint_and_sugg(
 163                         cx,
 164                         BOOL_COMPARISON,
 165                         e.span,
 166                         "equality checks against true are unnecessary",
 167                         "try simplifying it as shown",
 168                         hint.to_string(),
 169                         applicability,
 170                     );
 171                 },
 172                 (Bool(false), Other) => {
 173                     let hint = Sugg::hir_with_applicability(cx, right_side, "..", &mut applicability);
 174                     span_lint_and_sugg(
 175                         cx,
 176                         BOOL_COMPARISON,
 177                         e.span,
 178                         "equality checks against false can be replaced by a negation",
 179                         "try simplifying it as shown",
 180                         (!hint).to_string(),
 181                         applicability,
 182                     );
 183                 },
 184                 (Other, Bool(false)) => {
 185                     let hint = Sugg::hir_with_applicability(cx, left_side, "..", &mut applicability);
 186                     span_lint_and_sugg(
 187                         cx,
 188                         BOOL_COMPARISON,
 189                         e.span,
 190                         "equality checks against false can be replaced by a negation",
 191                         "try simplifying it as shown",
 192                         (!hint).to_string(),
 193                         applicability,
 194                     );
 195                 },
 196                 _ => (),
 197             }
 198         }
 199     }
 200 }
 201
 202 enum Expression {
 203     Bool(bool),
 204     RetBool(bool),
 205     Other,
 206 }
 207
 208 fn fetch_bool_block(block: &Block) -> Expression {
 209     match (&*block.stmts, block.expr.as_ref()) {
 210         (&[], Some(e)) => fetch_bool_expr(&**e),
 211         (&[ref e], None) => if let StmtKind::Semi(ref e, _) = e.node {
 212             if let ExprKind::Ret(_) = e.node {
 213                 fetch_bool_expr(&**e)
 214             } else {
 215                 Expression::Other
 216             }
 217         } else {
 218             Expression::Other
 219         },
 220         _ => Expression::Other,
 221     }
 222 }
 223
 224 fn fetch_bool_expr(expr: &Expr) -> Expression {
 225     match expr.node {
 226         ExprKind::Block(ref block, _) => fetch_bool_block(block),
 227         ExprKind::Lit(ref lit_ptr) => if let LitKind::Bool(value) = lit_ptr.node {
 228             Expression::Bool(value)
 229         } else {
 230             Expression::Other
 231         },
 232         ExprKind::Ret(Some(ref expr)) => match fetch_bool_expr(expr) {
 233             Expression::Bool(value) => Expression::RetBool(value),
 234             _ => Expression::Other,
 235         },
 236         _ => Expression::Other,
 237     }
 238 }