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