clippy_lints/src/needless_bool.rs

   1 //! Checks for needless boolean results of if-else expressions
   2 //!
   3 //! This lint is **warn** by default
   4
   5 use crate::utils::sugg::Sugg;
   6 use crate::utils::{in_macro, span_lint, span_lint_and_sugg};
   7 use rustc::hir::*;
   8 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
   9 use rustc::{declare_tool_lint, lint_array};
  10 use rustc_errors::Applicability;
  11 use syntax::ast::LitKind;
  12 use syntax::source_map::Spanned;
  13
  14 /// **What it does:** Checks for expressions of the form `if c { true } else {
  15 /// false }`
  16 /// (or vice versa) and suggest using the condition directly.
  17 ///
  18 /// **Why is this bad?** Redundant code.
  19 ///
  20 /// **Known problems:** Maybe false positives: Sometimes, the two branches are
  21 /// painstakingly documented (which we of course do not detect), so they *may*
  22 /// have some value. Even then, the documentation can be rewritten to match the
  23 /// shorter code.
  24 ///
  25 /// **Example:**
  26 /// ```rust
  27 /// if x {
  28 ///     false
  29 /// } else {
  30 ///     true
  31 /// }
  32 /// ```
  33 declare_clippy_lint! {
  34     pub NEEDLESS_BOOL,
  35     complexity,
  36     "if-statements with plain booleans in the then- and else-clause, e.g. `if p { true } else { false }`"
  37 }
  38
  39 /// **What it does:** Checks for expressions of the form `x == true`,
  40 /// `x != true` and order comparisons such as `x < true` (or vice versa) and
  41 /// suggest using the variable directly.
  42 ///
  43 /// **Why is this bad?** Unnecessary code.
  44 ///
  45 /// **Known problems:** None.
  46 ///
  47 /// **Example:**
  48 /// ```rust
  49 /// if x == true {} // could be `if x { }`
  50 /// ```
  51 declare_clippy_lint! {
  52     pub BOOL_COMPARISON,
  53     complexity,
  54     "comparing a variable to a boolean, e.g. `if x == true` or `if x != true`"
  55 }
  56
  57 #[derive(Copy, Clone)]
  58 pub struct NeedlessBool;
  59
  60 impl LintPass for NeedlessBool {
  61     fn get_lints(&self) -> LintArray {
  62         lint_array!(NEEDLESS_BOOL)
  63     }
  64 }
  65
  66 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NeedlessBool {
  67     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, e: &'tcx Expr) {
  68         use self::Expression::*;
  69         if let ExprKind::If(ref pred, ref then_block, Some(ref else_expr)) = e.node {
  70             let reduce = |ret, not| {
  71                 let mut applicability = Applicability::MachineApplicable;
  72                 let snip = Sugg::hir_with_applicability(cx, pred, "<predicate>", &mut applicability);
  73                 let snip = if not { !snip } else { snip };
  74
  75                 let hint = if ret {
  76                     format!("return {}", snip)
  77                 } else {
  78                     snip.to_string()
  79                 };
  80
  81                 span_lint_and_sugg(
  82                     cx,
  83                     NEEDLESS_BOOL,
  84                     e.span,
  85                     "this if-then-else expression returns a bool literal",
  86                     "you can reduce it to",
  87                     hint,
  88                     applicability,
  89                 );
  90             };
  91             if let ExprKind::Block(ref then_block, _) = then_block.node {
  92                 match (fetch_bool_block(then_block), fetch_bool_expr(else_expr)) {
  93                     (RetBool(true), RetBool(true)) | (Bool(true), Bool(true)) => {
  94                         span_lint(
  95                             cx,
  96                             NEEDLESS_BOOL,
  97                             e.span,
  98                             "this if-then-else expression will always return true",
  99                         );
 100                     },
 101                     (RetBool(false), RetBool(false)) | (Bool(false), Bool(false)) => {
 102                         span_lint(
 103                             cx,
 104                             NEEDLESS_BOOL,
 105                             e.span,
 106                             "this if-then-else expression will always return false",
 107                         );
 108                     },
 109                     (RetBool(true), RetBool(false)) => reduce(true, false),
 110                     (Bool(true), Bool(false)) => reduce(false, false),
 111                     (RetBool(false), RetBool(true)) => reduce(true, true),
 112                     (Bool(false), Bool(true)) => reduce(false, true),
 113                     _ => (),
 114                 }
 115             } else {
 116                 panic!("IfExpr 'then' node is not an ExprKind::Block");
 117             }
 118         }
 119     }
 120 }
 121
 122 #[derive(Copy, Clone)]
 123 pub struct BoolComparison;
 124
 125 impl LintPass for BoolComparison {
 126     fn get_lints(&self) -> LintArray {
 127         lint_array!(BOOL_COMPARISON)
 128     }
 129 }
 130
 131 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for BoolComparison {
 132     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, e: &'tcx Expr) {
 133         if in_macro(e.span) {
 134             return;
 135         }
 136
 137         if let ExprKind::Binary(Spanned { node, .. }, ..) = e.node {
 138             let ignore_case = None::<(fn(_) -> _, &str)>;
 139             let ignore_no_literal = None::<(fn(_, _) -> _, &str)>;
 140             match node {
 141                 BinOpKind::Eq => {
 142                     let true_case = Some((|h| h, "equality checks against true are unnecessary"));
 143                     let false_case = Some((
 144                         |h: Sugg<'_>| !h,
 145                         "equality checks against false can be replaced by a negation",
 146                     ));
 147                     check_comparison(cx, e, true_case, false_case, true_case, false_case, ignore_no_literal)
 148                 },
 149                 BinOpKind::Ne => {
 150                     let true_case = Some((
 151                         |h: Sugg<'_>| !h,
 152                         "inequality checks against true can be replaced by a negation",
 153                     ));
 154                     let false_case = Some((|h| h, "inequality checks against false are unnecessary"));
 155                     check_comparison(cx, e, true_case, false_case, true_case, false_case, ignore_no_literal)
 156                 },
 157                 BinOpKind::Lt => check_comparison(
 158                     cx,
 159                     e,
 160                     ignore_case,
 161                     Some((|h| h, "greater than checks against false are unnecessary")),
 162                     Some((
 163                         |h: Sugg<'_>| !h,
 164                         "less than comparison against true can be replaced by a negation",
 165                     )),
 166                     ignore_case,
 167                     Some((
 168                         |l: Sugg<'_>, r: Sugg<'_>| (!l).bit_and(&r),
 169                         "order comparisons between booleans can be simplified",
 170                     )),
 171                 ),
 172                 BinOpKind::Gt => check_comparison(
 173                     cx,
 174                     e,
 175                     Some((
 176                         |h: Sugg<'_>| !h,
 177                         "less than comparison against true can be replaced by a negation",
 178                     )),
 179                     ignore_case,
 180                     ignore_case,
 181                     Some((|h| h, "greater than checks against false are unnecessary")),
 182                     Some((
 183                         |l: Sugg<'_>, r: Sugg<'_>| l.bit_and(&(!r)),
 184                         "order comparisons between booleans can be simplified",
 185                     )),
 186                 ),
 187                 _ => (),
 188             }
 189         }
 190     }
 191 }
 192
 193 fn check_comparison<'a, 'tcx>(
 194     cx: &LateContext<'a, 'tcx>,
 195     e: &'tcx Expr,
 196     left_true: Option<(impl FnOnce(Sugg<'a>) -> Sugg<'a>, &str)>,
 197     left_false: Option<(impl FnOnce(Sugg<'a>) -> Sugg<'a>, &str)>,
 198     right_true: Option<(impl FnOnce(Sugg<'a>) -> Sugg<'a>, &str)>,
 199     right_false: Option<(impl FnOnce(Sugg<'a>) -> Sugg<'a>, &str)>,
 200     no_literal: Option<(impl FnOnce(Sugg<'a>, Sugg<'a>) -> Sugg<'a>, &str)>,
 201 ) {
 202     use self::Expression::*;
 203
 204     if let ExprKind::Binary(_, ref left_side, ref right_side) = e.node {
 205         let mut applicability = Applicability::MachineApplicable;
 206         match (fetch_bool_expr(left_side), fetch_bool_expr(right_side)) {
 207             (Bool(true), Other) => left_true.map_or((), |(h, m)| {
 208                 suggest_bool_comparison(cx, e, right_side, applicability, m, h)
 209             }),
 210             (Other, Bool(true)) => right_true.map_or((), |(h, m)| {
 211                 suggest_bool_comparison(cx, e, left_side, applicability, m, h)
 212             }),
 213             (Bool(false), Other) => left_false.map_or((), |(h, m)| {
 214                 suggest_bool_comparison(cx, e, right_side, applicability, m, h)
 215             }),
 216             (Other, Bool(false)) => right_false.map_or((), |(h, m)| {
 217                 suggest_bool_comparison(cx, e, left_side, applicability, m, h)
 218             }),
 219             (Other, Other) => no_literal.map_or((), |(h, m)| {
 220                 let (l_ty, r_ty) = (cx.tables.expr_ty(left_side), cx.tables.expr_ty(right_side));
 221                 if l_ty.is_bool() && r_ty.is_bool() {
 222                     let left_side = Sugg::hir_with_applicability(cx, left_side, "..", &mut applicability);
 223                     let right_side = Sugg::hir_with_applicability(cx, right_side, "..", &mut applicability);
 224                     span_lint_and_sugg(
 225                         cx,
 226                         BOOL_COMPARISON,
 227                         e.span,
 228                         m,
 229                         "try simplifying it as shown",
 230                         h(left_side, right_side).to_string(),
 231                         applicability,
 232                     )
 233                 }
 234             }),
 235             _ => (),
 236         }
 237     }
 238 }
 239
 240 fn suggest_bool_comparison<'a, 'tcx>(
 241     cx: &LateContext<'a, 'tcx>,
 242     e: &'tcx Expr,
 243     expr: &Expr,
 244     mut applicability: Applicability,
 245     message: &str,
 246     conv_hint: impl FnOnce(Sugg<'a>) -> Sugg<'a>,
 247 ) {
 248     let hint = Sugg::hir_with_applicability(cx, expr, "..", &mut applicability);
 249     span_lint_and_sugg(
 250         cx,
 251         BOOL_COMPARISON,
 252         e.span,
 253         message,
 254         "try simplifying it as shown",
 255         conv_hint(hint).to_string(),
 256         applicability,
 257     );
 258 }
 259
 260 enum Expression {
 261     Bool(bool),
 262     RetBool(bool),
 263     Other,
 264 }
 265
 266 fn fetch_bool_block(block: &Block) -> Expression {
 267     match (&*block.stmts, block.expr.as_ref()) {
 268         (&[], Some(e)) => fetch_bool_expr(&**e),
 269         (&[ref e], None) => {
 270             if let StmtKind::Semi(ref e) = e.node {
 271                 if let ExprKind::Ret(_) = e.node {
 272                     fetch_bool_expr(&**e)
 273                 } else {
 274                     Expression::Other
 275                 }
 276             } else {
 277                 Expression::Other
 278             }
 279         },
 280         _ => Expression::Other,
 281     }
 282 }
 283
 284 fn fetch_bool_expr(expr: &Expr) -> Expression {
 285     match expr.node {
 286         ExprKind::Block(ref block, _) => fetch_bool_block(block),
 287         ExprKind::Lit(ref lit_ptr) => {
 288             if let LitKind::Bool(value) = lit_ptr.node {
 289                 Expression::Bool(value)
 290             } else {
 291                 Expression::Other
 292             }
 293         },
 294         ExprKind::Ret(Some(ref expr)) => match fetch_bool_expr(expr) {
 295             Expression::Bool(value) => Expression::RetBool(value),
 296             _ => Expression::Other,
 297         },
 298         _ => Expression::Other,
 299     }
 300 }