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