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
  71             let reduce = |ret, not| {
  72                 let mut applicability = Applicability::MachineApplicable;
  73                 let snip = Sugg::hir_with_applicability(cx, pred, "<predicate>", &mut applicability);
  74                 let snip = if not { !snip } else { snip };
  75
  76                 let mut hint = if ret {
  77                     format!("return {}", snip)
  78                 } else {
  79                     snip.to_string()
  80                 };
  81
  82                 if parent_node_is_if_expr(&e, &cx) {
  83                     hint = format!("{{ {} }}", hint);
  84                 }
  85
  86                 span_lint_and_sugg(
  87                     cx,
  88                     NEEDLESS_BOOL,
  89                     e.span,
  90                     "this if-then-else expression returns a bool literal",
  91                     "you can reduce it to",
  92                     hint,
  93                     applicability,
  94                 );
  95             };
  96             if let ExprKind::Block(ref then_block, _) = then_block.node {
  97                 match (fetch_bool_block(then_block), fetch_bool_expr(else_expr)) {
  98                     (RetBool(true), RetBool(true)) | (Bool(true), Bool(true)) => {
  99                         span_lint(
 100                             cx,
 101                             NEEDLESS_BOOL,
 102                             e.span,
 103                             "this if-then-else expression will always return true",
 104                         );
 105                     },
 106                     (RetBool(false), RetBool(false)) | (Bool(false), Bool(false)) => {
 107                         span_lint(
 108                             cx,
 109                             NEEDLESS_BOOL,
 110                             e.span,
 111                             "this if-then-else expression will always return false",
 112                         );
 113                     },
 114                     (RetBool(true), RetBool(false)) => reduce(true, false),
 115                     (Bool(true), Bool(false)) => reduce(false, false),
 116                     (RetBool(false), RetBool(true)) => reduce(true, true),
 117                     (Bool(false), Bool(true)) => reduce(false, true),
 118                     _ => (),
 119                 }
 120             } else {
 121                 panic!("IfExpr 'then' node is not an ExprKind::Block");
 122             }
 123         }
 124     }
 125 }
 126
 127 fn parent_node_is_if_expr<'a, 'b>(expr: &Expr, cx: &LateContext<'a, 'b>) -> bool {
 128     let parent_id = cx.tcx.hir().get_parent_node(expr.id);
 129     let parent_node = cx.tcx.hir().get(parent_id);
 130
 131     if let rustc::hir::Node::Expr(e) = parent_node {
 132         if let ExprKind::If(_,_,_) = e.node {
 133             return true;
 134         }
 135     }
 136
 137     false
 138 }
 139
 140 #[derive(Copy, Clone)]
 141 pub struct BoolComparison;
 142
 143 impl LintPass for BoolComparison {
 144     fn get_lints(&self) -> LintArray {
 145         lint_array!(BOOL_COMPARISON)
 146     }
 147 }
 148
 149 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for BoolComparison {
 150     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, e: &'tcx Expr) {
 151         if in_macro(e.span) {
 152             return;
 153         }
 154
 155         if let ExprKind::Binary(Spanned { node, .. }, ..) = e.node {
 156             let ignore_case = None::<(fn(_) -> _, &str)>;
 157             let ignore_no_literal = None::<(fn(_, _) -> _, &str)>;
 158             match node {
 159                 BinOpKind::Eq => {
 160                     let true_case = Some((|h| h, "equality checks against true are unnecessary"));
 161                     let false_case = Some((
 162                         |h: Sugg<'_>| !h,
 163                         "equality checks against false can be replaced by a negation",
 164                     ));
 165                     check_comparison(cx, e, true_case, false_case, true_case, false_case, ignore_no_literal)
 166                 },
 167                 BinOpKind::Ne => {
 168                     let true_case = Some((
 169                         |h: Sugg<'_>| !h,
 170                         "inequality checks against true can be replaced by a negation",
 171                     ));
 172                     let false_case = Some((|h| h, "inequality checks against false are unnecessary"));
 173                     check_comparison(cx, e, true_case, false_case, true_case, false_case, ignore_no_literal)
 174                 },
 175                 BinOpKind::Lt => check_comparison(
 176                     cx,
 177                     e,
 178                     ignore_case,
 179                     Some((|h| h, "greater than checks against false are unnecessary")),
 180                     Some((
 181                         |h: Sugg<'_>| !h,
 182                         "less than comparison against true can be replaced by a negation",
 183                     )),
 184                     ignore_case,
 185                     Some((
 186                         |l: Sugg<'_>, r: Sugg<'_>| (!l).bit_and(&r),
 187                         "order comparisons between booleans can be simplified",
 188                     )),
 189                 ),
 190                 BinOpKind::Gt => check_comparison(
 191                     cx,
 192                     e,
 193                     Some((
 194                         |h: Sugg<'_>| !h,
 195                         "less than comparison against true can be replaced by a negation",
 196                     )),
 197                     ignore_case,
 198                     ignore_case,
 199                     Some((|h| h, "greater than checks against false are unnecessary")),
 200                     Some((
 201                         |l: Sugg<'_>, r: Sugg<'_>| l.bit_and(&(!r)),
 202                         "order comparisons between booleans can be simplified",
 203                     )),
 204                 ),
 205                 _ => (),
 206             }
 207         }
 208     }
 209 }
 210
 211 fn check_comparison<'a, 'tcx>(
 212     cx: &LateContext<'a, 'tcx>,
 213     e: &'tcx Expr,
 214     left_true: Option<(impl FnOnce(Sugg<'a>) -> Sugg<'a>, &str)>,
 215     left_false: Option<(impl FnOnce(Sugg<'a>) -> Sugg<'a>, &str)>,
 216     right_true: Option<(impl FnOnce(Sugg<'a>) -> Sugg<'a>, &str)>,
 217     right_false: Option<(impl FnOnce(Sugg<'a>) -> Sugg<'a>, &str)>,
 218     no_literal: Option<(impl FnOnce(Sugg<'a>, Sugg<'a>) -> Sugg<'a>, &str)>,
 219 ) {
 220     use self::Expression::*;
 221
 222     if let ExprKind::Binary(_, ref left_side, ref right_side) = e.node {
 223         let mut applicability = Applicability::MachineApplicable;
 224         match (fetch_bool_expr(left_side), fetch_bool_expr(right_side)) {
 225             (Bool(true), Other) => left_true.map_or((), |(h, m)| {
 226                 suggest_bool_comparison(cx, e, right_side, applicability, m, h)
 227             }),
 228             (Other, Bool(true)) => right_true.map_or((), |(h, m)| {
 229                 suggest_bool_comparison(cx, e, left_side, applicability, m, h)
 230             }),
 231             (Bool(false), Other) => left_false.map_or((), |(h, m)| {
 232                 suggest_bool_comparison(cx, e, right_side, applicability, m, h)
 233             }),
 234             (Other, Bool(false)) => right_false.map_or((), |(h, m)| {
 235                 suggest_bool_comparison(cx, e, left_side, applicability, m, h)
 236             }),
 237             (Other, Other) => no_literal.map_or((), |(h, m)| {
 238                 let (l_ty, r_ty) = (cx.tables.expr_ty(left_side), cx.tables.expr_ty(right_side));
 239                 if l_ty.is_bool() && r_ty.is_bool() {
 240                     let left_side = Sugg::hir_with_applicability(cx, left_side, "..", &mut applicability);
 241                     let right_side = Sugg::hir_with_applicability(cx, right_side, "..", &mut applicability);
 242                     span_lint_and_sugg(
 243                         cx,
 244                         BOOL_COMPARISON,
 245                         e.span,
 246                         m,
 247                         "try simplifying it as shown",
 248                         h(left_side, right_side).to_string(),
 249                         applicability,
 250                     )
 251                 }
 252             }),
 253             _ => (),
 254         }
 255     }
 256 }
 257
 258 fn suggest_bool_comparison<'a, 'tcx>(
 259     cx: &LateContext<'a, 'tcx>,
 260     e: &'tcx Expr,
 261     expr: &Expr,
 262     mut applicability: Applicability,
 263     message: &str,
 264     conv_hint: impl FnOnce(Sugg<'a>) -> Sugg<'a>,
 265 ) {
 266     let hint = Sugg::hir_with_applicability(cx, expr, "..", &mut applicability);
 267     span_lint_and_sugg(
 268         cx,
 269         BOOL_COMPARISON,
 270         e.span,
 271         message,
 272         "try simplifying it as shown",
 273         conv_hint(hint).to_string(),
 274         applicability,
 275     );
 276 }
 277
 278 enum Expression {
 279     Bool(bool),
 280     RetBool(bool),
 281     Other,
 282 }
 283
 284 fn fetch_bool_block(block: &Block) -> Expression {
 285     match (&*block.stmts, block.expr.as_ref()) {
 286         (&[], Some(e)) => fetch_bool_expr(&**e),
 287         (&[ref e], None) => {
 288             if let StmtKind::Semi(ref e, _) = e.node {
 289                 if let ExprKind::Ret(_) = e.node {
 290                     fetch_bool_expr(&**e)
 291                 } else {
 292                     Expression::Other
 293                 }
 294             } else {
 295                 Expression::Other
 296             }
 297         },
 298         _ => Expression::Other,
 299     }
 300 }
 301
 302 fn fetch_bool_expr(expr: &Expr) -> Expression {
 303     match expr.node {
 304         ExprKind::Block(ref block, _) => fetch_bool_block(block),
 305         ExprKind::Lit(ref lit_ptr) => {
 306             if let LitKind::Bool(value) = lit_ptr.node {
 307                 Expression::Bool(value)
 308             } else {
 309                 Expression::Other
 310             }
 311         },
 312         ExprKind::Ret(Some(ref expr)) => match fetch_bool_expr(expr) {
 313             Expression::Bool(value) => Expression::RetBool(value),
 314             _ => Expression::Other,
 315         },
 316         _ => Expression::Other,
 317     }
 318 }