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