src/tools/clippy/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 clippy_utils::diagnostics::{span_lint, span_lint_and_sugg};
   6 use clippy_utils::source::snippet_with_applicability;
   7 use clippy_utils::sugg::Sugg;
   8 use clippy_utils::{is_else_clause, is_expn_of};
   9 use rustc_ast::ast::LitKind;
  10 use rustc_errors::Applicability;
  11 use rustc_hir::{BinOpKind, Block, Expr, ExprKind, StmtKind, UnOp};
  12 use rustc_lint::{LateContext, LateLintPass};
  13 use rustc_session::{declare_lint_pass, declare_tool_lint};
  14 use rustc_span::source_map::Spanned;
  15 use rustc_span::Span;
  16
  17 declare_clippy_lint! {
  18     /// **What it does:** Checks for expressions of the form `if c { true } else {
  19     /// false }` (or vice versa) and suggests using the condition directly.
  20     ///
  21     /// **Why is this bad?** Redundant code.
  22     ///
  23     /// **Known problems:** Maybe false positives: Sometimes, the two branches are
  24     /// painstakingly documented (which we, of course, do not detect), so they *may*
  25     /// have some value. Even then, the documentation can be rewritten to match the
  26     /// shorter code.
  27     ///
  28     /// **Example:**
  29     /// ```rust,ignore
  30     /// if x {
  31     ///     false
  32     /// } else {
  33     ///     true
  34     /// }
  35     /// ```
  36     /// Could be written as
  37     /// ```rust,ignore
  38     /// !x
  39     /// ```
  40     pub NEEDLESS_BOOL,
  41     complexity,
  42     "if-statements with plain booleans in the then- and else-clause, e.g., `if p { true } else { false }`"
  43 }
  44
  45 declare_clippy_lint! {
  46     /// **What it does:** Checks for expressions of the form `x == true`,
  47     /// `x != true` and order comparisons such as `x < true` (or vice versa) and
  48     /// suggest using the variable directly.
  49     ///
  50     /// **Why is this bad?** Unnecessary code.
  51     ///
  52     /// **Known problems:** None.
  53     ///
  54     /// **Example:**
  55     /// ```rust,ignore
  56     /// if x == true {}
  57     /// if y == false {}
  58     /// ```
  59     /// use `x` directly:
  60     /// ```rust,ignore
  61     /// if x {}
  62     /// if !y {}
  63     /// ```
  64     pub BOOL_COMPARISON,
  65     complexity,
  66     "comparing a variable to a boolean, e.g., `if x == true` or `if x != true`"
  67 }
  68
  69 declare_lint_pass!(NeedlessBool => [NEEDLESS_BOOL]);
  70
  71 impl<'tcx> LateLintPass<'tcx> for NeedlessBool {
  72     fn check_expr(&mut self, cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) {
  73         use self::Expression::{Bool, RetBool};
  74         if let ExprKind::If(pred, then_block, Some(else_expr)) = e.kind {
  75             let reduce = |ret, not| {
  76                 let mut applicability = Applicability::MachineApplicable;
  77                 let snip = Sugg::hir_with_applicability(cx, pred, "<predicate>", &mut applicability);
  78                 let mut snip = if not { !snip } else { snip };
  79
  80                 if ret {
  81                     snip = snip.make_return();
  82                 }
  83
  84                 if is_else_clause(cx.tcx, e) {
  85                     snip = snip.blockify();
  86                 }
  87
  88                 span_lint_and_sugg(
  89                     cx,
  90                     NEEDLESS_BOOL,
  91                     e.span,
  92                     "this if-then-else expression returns a bool literal",
  93                     "you can reduce it to",
  94                     snip.to_string(),
  95                     applicability,
  96                 );
  97             };
  98             if let ExprKind::Block(then_block, _) = then_block.kind {
  99                 match (fetch_bool_block(then_block), fetch_bool_expr(else_expr)) {
 100                     (RetBool(true), RetBool(true)) | (Bool(true), Bool(true)) => {
 101                         span_lint(
 102                             cx,
 103                             NEEDLESS_BOOL,
 104                             e.span,
 105                             "this if-then-else expression will always return true",
 106                         );
 107                     },
 108                     (RetBool(false), RetBool(false)) | (Bool(false), Bool(false)) => {
 109                         span_lint(
 110                             cx,
 111                             NEEDLESS_BOOL,
 112                             e.span,
 113                             "this if-then-else expression will always return false",
 114                         );
 115                     },
 116                     (RetBool(true), RetBool(false)) => reduce(true, false),
 117                     (Bool(true), Bool(false)) => reduce(false, false),
 118                     (RetBool(false), RetBool(true)) => reduce(true, true),
 119                     (Bool(false), Bool(true)) => reduce(false, true),
 120                     _ => (),
 121                 }
 122             } else {
 123                 panic!("IfExpr `then` node is not an `ExprKind::Block`");
 124             }
 125         }
 126     }
 127 }
 128
 129 declare_lint_pass!(BoolComparison => [BOOL_COMPARISON]);
 130
 131 impl<'tcx> LateLintPass<'tcx> for BoolComparison {
 132     fn check_expr(&mut self, cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) {
 133         if e.span.from_expansion() {
 134             return;
 135         }
 136
 137         if let ExprKind::Binary(Spanned { node, .. }, ..) = e.kind {
 138             let ignore_case = None::<(fn(_) -> _, &str)>;
 139             let ignore_no_literal = None::<(fn(_, _) -> _, &str)>;
 140             match node {
 141                 BinOpKind::Eq => {
 142                     let true_case = Some((|h| h, "equality checks against true are unnecessary"));
 143                     let false_case = Some((
 144                         |h: Sugg<'_>| !h,
 145                         "equality checks against false can be replaced by a negation",
 146                     ));
 147                     check_comparison(cx, e, true_case, false_case, true_case, false_case, ignore_no_literal);
 148                 },
 149                 BinOpKind::Ne => {
 150                     let true_case = Some((
 151                         |h: Sugg<'_>| !h,
 152                         "inequality checks against true can be replaced by a negation",
 153                     ));
 154                     let false_case = Some((|h| h, "inequality checks against false are unnecessary"));
 155                     check_comparison(cx, e, true_case, false_case, true_case, false_case, ignore_no_literal);
 156                 },
 157                 BinOpKind::Lt => check_comparison(
 158                     cx,
 159                     e,
 160                     ignore_case,
 161                     Some((|h| h, "greater than checks against false are unnecessary")),
 162                     Some((
 163                         |h: Sugg<'_>| !h,
 164                         "less than comparison against true can be replaced by a negation",
 165                     )),
 166                     ignore_case,
 167                     Some((
 168                         |l: Sugg<'_>, r: Sugg<'_>| (!l).bit_and(&r),
 169                         "order comparisons between booleans can be simplified",
 170                     )),
 171                 ),
 172                 BinOpKind::Gt => check_comparison(
 173                     cx,
 174                     e,
 175                     Some((
 176                         |h: Sugg<'_>| !h,
 177                         "less than comparison against true can be replaced by a negation",
 178                     )),
 179                     ignore_case,
 180                     ignore_case,
 181                     Some((|h| h, "greater than checks against false are unnecessary")),
 182                     Some((
 183                         |l: Sugg<'_>, r: Sugg<'_>| l.bit_and(&(!r)),
 184                         "order comparisons between booleans can be simplified",
 185                     )),
 186                 ),
 187                 _ => (),
 188             }
 189         }
 190     }
 191 }
 192
 193 struct ExpressionInfoWithSpan {
 194     one_side_is_unary_not: bool,
 195     left_span: Span,
 196     right_span: Span,
 197 }
 198
 199 fn is_unary_not(e: &Expr<'_>) -> (bool, Span) {
 200     if let ExprKind::Unary(UnOp::Not, operand) = e.kind {
 201         return (true, operand.span);
 202     }
 203     (false, e.span)
 204 }
 205
 206 fn one_side_is_unary_not<'tcx>(left_side: &'tcx Expr<'_>, right_side: &'tcx Expr<'_>) -> ExpressionInfoWithSpan {
 207     let left = is_unary_not(left_side);
 208     let right = is_unary_not(right_side);
 209
 210     ExpressionInfoWithSpan {
 211         one_side_is_unary_not: left.0 != right.0,
 212         left_span: left.1,
 213         right_span: right.1,
 214     }
 215 }
 216
 217 fn check_comparison<'a, 'tcx>(
 218     cx: &LateContext<'tcx>,
 219     e: &'tcx Expr<'_>,
 220     left_true: Option<(impl FnOnce(Sugg<'a>) -> Sugg<'a>, &str)>,
 221     left_false: Option<(impl FnOnce(Sugg<'a>) -> Sugg<'a>, &str)>,
 222     right_true: Option<(impl FnOnce(Sugg<'a>) -> Sugg<'a>, &str)>,
 223     right_false: Option<(impl FnOnce(Sugg<'a>) -> Sugg<'a>, &str)>,
 224     no_literal: Option<(impl FnOnce(Sugg<'a>, Sugg<'a>) -> Sugg<'a>, &str)>,
 225 ) {
 226     use self::Expression::{Bool, Other};
 227
 228     if let ExprKind::Binary(op, left_side, right_side) = e.kind {
 229         let (l_ty, r_ty) = (
 230             cx.typeck_results().expr_ty(left_side),
 231             cx.typeck_results().expr_ty(right_side),
 232         );
 233         if is_expn_of(left_side.span, "cfg").is_some() || is_expn_of(right_side.span, "cfg").is_some() {
 234             return;
 235         }
 236         if l_ty.is_bool() && r_ty.is_bool() {
 237             let mut applicability = Applicability::MachineApplicable;
 238
 239             if let BinOpKind::Eq = op.node {
 240                 let expression_info = one_side_is_unary_not(left_side, right_side);
 241                 if expression_info.one_side_is_unary_not {
 242                     span_lint_and_sugg(
 243                         cx,
 244                         BOOL_COMPARISON,
 245                         e.span,
 246                         "this comparison might be written more concisely",
 247                         "try simplifying it as shown",
 248                         format!(
 249                             "{} != {}",
 250                             snippet_with_applicability(cx, expression_info.left_span, "..", &mut applicability),
 251                             snippet_with_applicability(cx, expression_info.right_span, "..", &mut applicability)
 252                         ),
 253                         applicability,
 254                     );
 255                 }
 256             }
 257
 258             match (fetch_bool_expr(left_side), fetch_bool_expr(right_side)) {
 259                 (Bool(true), Other) => left_true.map_or((), |(h, m)| {
 260                     suggest_bool_comparison(cx, e, right_side, applicability, m, h);
 261                 }),
 262                 (Other, Bool(true)) => right_true.map_or((), |(h, m)| {
 263                     suggest_bool_comparison(cx, e, left_side, applicability, m, h);
 264                 }),
 265                 (Bool(false), Other) => left_false.map_or((), |(h, m)| {
 266                     suggest_bool_comparison(cx, e, right_side, applicability, m, h);
 267                 }),
 268                 (Other, Bool(false)) => right_false.map_or((), |(h, m)| {
 269                     suggest_bool_comparison(cx, e, left_side, applicability, m, h);
 270                 }),
 271                 (Other, Other) => no_literal.map_or((), |(h, m)| {
 272                     let left_side = Sugg::hir_with_applicability(cx, left_side, "..", &mut applicability);
 273                     let right_side = Sugg::hir_with_applicability(cx, right_side, "..", &mut applicability);
 274                     span_lint_and_sugg(
 275                         cx,
 276                         BOOL_COMPARISON,
 277                         e.span,
 278                         m,
 279                         "try simplifying it as shown",
 280                         h(left_side, right_side).to_string(),
 281                         applicability,
 282                     );
 283                 }),
 284                 _ => (),
 285             }
 286         }
 287     }
 288 }
 289
 290 fn suggest_bool_comparison<'a, 'tcx>(
 291     cx: &LateContext<'tcx>,
 292     e: &'tcx Expr<'_>,
 293     expr: &Expr<'_>,
 294     mut applicability: Applicability,
 295     message: &str,
 296     conv_hint: impl FnOnce(Sugg<'a>) -> Sugg<'a>,
 297 ) {
 298     let hint = if expr.span.from_expansion() {
 299         if applicability != Applicability::Unspecified {
 300             applicability = Applicability::MaybeIncorrect;
 301         }
 302         Sugg::hir_with_macro_callsite(cx, expr, "..")
 303     } else {
 304         Sugg::hir_with_applicability(cx, expr, "..", &mut applicability)
 305     };
 306     span_lint_and_sugg(
 307         cx,
 308         BOOL_COMPARISON,
 309         e.span,
 310         message,
 311         "try simplifying it as shown",
 312         conv_hint(hint).to_string(),
 313         applicability,
 314     );
 315 }
 316
 317 enum Expression {
 318     Bool(bool),
 319     RetBool(bool),
 320     Other,
 321 }
 322
 323 fn fetch_bool_block(block: &Block<'_>) -> Expression {
 324     match (&*block.stmts, block.expr.as_ref()) {
 325         (&[], Some(e)) => fetch_bool_expr(&**e),
 326         (&[ref e], None) => {
 327             if let StmtKind::Semi(e) = e.kind {
 328                 if let ExprKind::Ret(_) = e.kind {
 329                     fetch_bool_expr(e)
 330                 } else {
 331                     Expression::Other
 332                 }
 333             } else {
 334                 Expression::Other
 335             }
 336         },
 337         _ => Expression::Other,
 338     }
 339 }
 340
 341 fn fetch_bool_expr(expr: &Expr<'_>) -> Expression {
 342     match expr.kind {
 343         ExprKind::Block(block, _) => fetch_bool_block(block),
 344         ExprKind::Lit(ref lit_ptr) => {
 345             if let LitKind::Bool(value) = lit_ptr.node {
 346                 Expression::Bool(value)
 347             } else {
 348                 Expression::Other
 349             }
 350         },
 351         ExprKind::Ret(Some(expr)) => match fetch_bool_expr(expr) {
 352             Expression::Bool(value) => Expression::RetBool(value),
 353             _ => Expression::Other,
 354         },
 355         _ => Expression::Other,
 356     }
 357 }