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         if in_macro(e.span) {
 137             return;
 138         }
 139         use self::Expression::*;
 140         if let ExprKind::Binary(Spanned { node: BinOpKind::Eq, .. }, ref left_side, ref right_side) = e.node {
 141             match (fetch_bool_expr(left_side), fetch_bool_expr(right_side)) {
 142                 (Bool(true), Other) => {
 143                     let hint = snippet(cx, right_side.span, "..").into_owned();
 144                     span_lint_and_sugg(
 145                         cx,
 146                         BOOL_COMPARISON,
 147                         e.span,
 148                         "equality checks against true are unnecessary",
 149                         "try simplifying it as shown",
 150                         hint,
 151                     );
 152                 },
 153                 (Other, Bool(true)) => {
 154                     let hint = snippet(cx, left_side.span, "..").into_owned();
 155                     span_lint_and_sugg(
 156                         cx,
 157                         BOOL_COMPARISON,
 158                         e.span,
 159                         "equality checks against true are unnecessary",
 160                         "try simplifying it as shown",
 161                         hint,
 162                     );
 163                 },
 164                 (Bool(false), Other) => {
 165                     let hint = Sugg::hir(cx, right_side, "..");
 166                     span_lint_and_sugg(
 167                         cx,
 168                         BOOL_COMPARISON,
 169                         e.span,
 170                         "equality checks against false can be replaced by a negation",
 171                         "try simplifying it as shown",
 172                         (!hint).to_string(),
 173                     );
 174                 },
 175                 (Other, Bool(false)) => {
 176                     let hint = Sugg::hir(cx, left_side, "..");
 177                     span_lint_and_sugg(
 178                         cx,
 179                         BOOL_COMPARISON,
 180                         e.span,
 181                         "equality checks against false can be replaced by a negation",
 182                         "try simplifying it as shown",
 183                         (!hint).to_string(),
 184                     );
 185                 },
 186                 _ => (),
 187             }
 188         }
 189     }
 190 }
 191
 192 enum Expression {
 193     Bool(bool),
 194     RetBool(bool),
 195     Other,
 196 }
 197
 198 fn fetch_bool_block(block: &Block) -> Expression {
 199     match (&*block.stmts, block.expr.as_ref()) {
 200         (&[], Some(e)) => fetch_bool_expr(&**e),
 201         (&[ref e], None) => if let StmtKind::Semi(ref e, _) = e.node {
 202             if let ExprKind::Ret(_) = e.node {
 203                 fetch_bool_expr(&**e)
 204             } else {
 205                 Expression::Other
 206             }
 207         } else {
 208             Expression::Other
 209         },
 210         _ => Expression::Other,
 211     }
 212 }
 213
 214 fn fetch_bool_expr(expr: &Expr) -> Expression {
 215     match expr.node {
 216         ExprKind::Block(ref block, _) => fetch_bool_block(block),
 217         ExprKind::Lit(ref lit_ptr) => if let LitKind::Bool(value) = lit_ptr.node {
 218             Expression::Bool(value)
 219         } else {
 220             Expression::Other
 221         },
 222         ExprKind::Ret(Some(ref expr)) => match fetch_bool_expr(expr) {
 223             Expression::Bool(value) => Expression::RetBool(value),
 224             _ => Expression::Other,
 225         },
 226         _ => Expression::Other,
 227     }
 228 }