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