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