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 e.span.from_expansion() {
  75             return;
  76         }
  77         if let ExprKind::If(pred, then_block, Some(else_expr)) = e.kind {
  78             let reduce = |ret, not| {
  79                 let mut applicability = Applicability::MachineApplicable;
  80                 let snip = Sugg::hir_with_applicability(cx, pred, "<predicate>", &mut applicability);
  81                 let mut snip = if not { !snip } else { snip };
  82
  83                 if ret {
  84                     snip = snip.make_return();
  85                 }
  86
  87                 if is_else_clause(cx.tcx, e) {
  88                     snip = snip.blockify();
  89                 }
  90
  91                 span_lint_and_sugg(
  92                     cx,
  93                     NEEDLESS_BOOL,
  94                     e.span,
  95                     "this if-then-else expression returns a bool literal",
  96                     "you can reduce it to",
  97                     snip.to_string(),
  98                     applicability,
  99                 );
 100             };
 101             if let ExprKind::Block(then_block, _) = then_block.kind {
 102                 match (fetch_bool_block(then_block), fetch_bool_expr(else_expr)) {
 103                     (RetBool(true), RetBool(true)) | (Bool(true), Bool(true)) => {
 104                         span_lint(
 105                             cx,
 106                             NEEDLESS_BOOL,
 107                             e.span,
 108                             "this if-then-else expression will always return true",
 109                         );
 110                     },
 111                     (RetBool(false), RetBool(false)) | (Bool(false), Bool(false)) => {
 112                         span_lint(
 113                             cx,
 114                             NEEDLESS_BOOL,
 115                             e.span,
 116                             "this if-then-else expression will always return false",
 117                         );
 118                     },
 119                     (RetBool(true), RetBool(false)) => reduce(true, false),
 120                     (Bool(true), Bool(false)) => reduce(false, false),
 121                     (RetBool(false), RetBool(true)) => reduce(true, true),
 122                     (Bool(false), Bool(true)) => reduce(false, true),
 123                     _ => (),
 124                 }
 125             } else {
 126                 panic!("IfExpr `then` node is not an `ExprKind::Block`");
 127             }
 128         }
 129     }
 130 }
 131
 132 declare_lint_pass!(BoolComparison => [BOOL_COMPARISON]);
 133
 134 impl<'tcx> LateLintPass<'tcx> for BoolComparison {
 135     fn check_expr(&mut self, cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) {
 136         if e.span.from_expansion() {
 137             return;
 138         }
 139
 140         if let ExprKind::Binary(Spanned { node, .. }, ..) = e.kind {
 141             let ignore_case = None::<(fn(_) -> _, &str)>;
 142             let ignore_no_literal = None::<(fn(_, _) -> _, &str)>;
 143             match node {
 144                 BinOpKind::Eq => {
 145                     let true_case = Some((|h| h, "equality checks against true are unnecessary"));
 146                     let false_case = Some((
 147                         |h: Sugg<'_>| !h,
 148                         "equality checks against false can be replaced by a negation",
 149                     ));
 150                     check_comparison(cx, e, true_case, false_case, true_case, false_case, ignore_no_literal);
 151                 },
 152                 BinOpKind::Ne => {
 153                     let true_case = Some((
 154                         |h: Sugg<'_>| !h,
 155                         "inequality checks against true can be replaced by a negation",
 156                     ));
 157                     let false_case = Some((|h| h, "inequality checks against false are unnecessary"));
 158                     check_comparison(cx, e, true_case, false_case, true_case, false_case, ignore_no_literal);
 159                 },
 160                 BinOpKind::Lt => check_comparison(
 161                     cx,
 162                     e,
 163                     ignore_case,
 164                     Some((|h| h, "greater than checks against false are unnecessary")),
 165                     Some((
 166                         |h: Sugg<'_>| !h,
 167                         "less than comparison against true can be replaced by a negation",
 168                     )),
 169                     ignore_case,
 170                     Some((
 171                         |l: Sugg<'_>, r: Sugg<'_>| (!l).bit_and(&r),
 172                         "order comparisons between booleans can be simplified",
 173                     )),
 174                 ),
 175                 BinOpKind::Gt => check_comparison(
 176                     cx,
 177                     e,
 178                     Some((
 179                         |h: Sugg<'_>| !h,
 180                         "less than comparison against true can be replaced by a negation",
 181                     )),
 182                     ignore_case,
 183                     ignore_case,
 184                     Some((|h| h, "greater than checks against false are unnecessary")),
 185                     Some((
 186                         |l: Sugg<'_>, r: Sugg<'_>| l.bit_and(&(!r)),
 187                         "order comparisons between booleans can be simplified",
 188                     )),
 189                 ),
 190                 _ => (),
 191             }
 192         }
 193     }
 194 }
 195
 196 struct ExpressionInfoWithSpan {
 197     one_side_is_unary_not: bool,
 198     left_span: Span,
 199     right_span: Span,
 200 }
 201
 202 fn is_unary_not(e: &Expr<'_>) -> (bool, Span) {
 203     if let ExprKind::Unary(UnOp::Not, operand) = e.kind {
 204         return (true, operand.span);
 205     }
 206     (false, e.span)
 207 }
 208
 209 fn one_side_is_unary_not<'tcx>(left_side: &'tcx Expr<'_>, right_side: &'tcx Expr<'_>) -> ExpressionInfoWithSpan {
 210     let left = is_unary_not(left_side);
 211     let right = is_unary_not(right_side);
 212
 213     ExpressionInfoWithSpan {
 214         one_side_is_unary_not: left.0 != right.0,
 215         left_span: left.1,
 216         right_span: right.1,
 217     }
 218 }
 219
 220 fn check_comparison<'a, 'tcx>(
 221     cx: &LateContext<'tcx>,
 222     e: &'tcx Expr<'_>,
 223     left_true: Option<(impl FnOnce(Sugg<'a>) -> Sugg<'a>, &str)>,
 224     left_false: Option<(impl FnOnce(Sugg<'a>) -> Sugg<'a>, &str)>,
 225     right_true: Option<(impl FnOnce(Sugg<'a>) -> Sugg<'a>, &str)>,
 226     right_false: Option<(impl FnOnce(Sugg<'a>) -> Sugg<'a>, &str)>,
 227     no_literal: Option<(impl FnOnce(Sugg<'a>, Sugg<'a>) -> Sugg<'a>, &str)>,
 228 ) {
 229     use self::Expression::{Bool, Other};
 230
 231     if let ExprKind::Binary(op, left_side, right_side) = e.kind {
 232         let (l_ty, r_ty) = (
 233             cx.typeck_results().expr_ty(left_side),
 234             cx.typeck_results().expr_ty(right_side),
 235         );
 236         if is_expn_of(left_side.span, "cfg").is_some() || is_expn_of(right_side.span, "cfg").is_some() {
 237             return;
 238         }
 239         if l_ty.is_bool() && r_ty.is_bool() {
 240             let mut applicability = Applicability::MachineApplicable;
 241
 242             if let BinOpKind::Eq = op.node {
 243                 let expression_info = one_side_is_unary_not(left_side, right_side);
 244                 if expression_info.one_side_is_unary_not {
 245                     span_lint_and_sugg(
 246                         cx,
 247                         BOOL_COMPARISON,
 248                         e.span,
 249                         "this comparison might be written more concisely",
 250                         "try simplifying it as shown",
 251                         format!(
 252                             "{} != {}",
 253                             snippet_with_applicability(cx, expression_info.left_span, "..", &mut applicability),
 254                             snippet_with_applicability(cx, expression_info.right_span, "..", &mut applicability)
 255                         ),
 256                         applicability,
 257                     );
 258                 }
 259             }
 260
 261             match (fetch_bool_expr(left_side), fetch_bool_expr(right_side)) {
 262                 (Bool(true), Other) => left_true.map_or((), |(h, m)| {
 263                     suggest_bool_comparison(cx, e, right_side, applicability, m, h);
 264                 }),
 265                 (Other, Bool(true)) => right_true.map_or((), |(h, m)| {
 266                     suggest_bool_comparison(cx, e, left_side, applicability, m, h);
 267                 }),
 268                 (Bool(false), Other) => left_false.map_or((), |(h, m)| {
 269                     suggest_bool_comparison(cx, e, right_side, applicability, m, h);
 270                 }),
 271                 (Other, Bool(false)) => right_false.map_or((), |(h, m)| {
 272                     suggest_bool_comparison(cx, e, left_side, applicability, m, h);
 273                 }),
 274                 (Other, Other) => no_literal.map_or((), |(h, m)| {
 275                     let left_side = Sugg::hir_with_applicability(cx, left_side, "..", &mut applicability);
 276                     let right_side = Sugg::hir_with_applicability(cx, right_side, "..", &mut applicability);
 277                     span_lint_and_sugg(
 278                         cx,
 279                         BOOL_COMPARISON,
 280                         e.span,
 281                         m,
 282                         "try simplifying it as shown",
 283                         h(left_side, right_side).to_string(),
 284                         applicability,
 285                     );
 286                 }),
 287                 _ => (),
 288             }
 289         }
 290     }
 291 }
 292
 293 fn suggest_bool_comparison<'a, 'tcx>(
 294     cx: &LateContext<'tcx>,
 295     e: &'tcx Expr<'_>,
 296     expr: &Expr<'_>,
 297     mut applicability: Applicability,
 298     message: &str,
 299     conv_hint: impl FnOnce(Sugg<'a>) -> Sugg<'a>,
 300 ) {
 301     let hint = if expr.span.from_expansion() {
 302         if applicability != Applicability::Unspecified {
 303             applicability = Applicability::MaybeIncorrect;
 304         }
 305         Sugg::hir_with_macro_callsite(cx, expr, "..")
 306     } else {
 307         Sugg::hir_with_applicability(cx, expr, "..", &mut applicability)
 308     };
 309     span_lint_and_sugg(
 310         cx,
 311         BOOL_COMPARISON,
 312         e.span,
 313         message,
 314         "try simplifying it as shown",
 315         conv_hint(hint).to_string(),
 316         applicability,
 317     );
 318 }
 319
 320 enum Expression {
 321     Bool(bool),
 322     RetBool(bool),
 323     Other,
 324 }
 325
 326 fn fetch_bool_block(block: &Block<'_>) -> Expression {
 327     match (&*block.stmts, block.expr.as_ref()) {
 328         (&[], Some(e)) => fetch_bool_expr(&**e),
 329         (&[ref e], None) => {
 330             if let StmtKind::Semi(e) = e.kind {
 331                 if let ExprKind::Ret(_) = e.kind {
 332                     fetch_bool_expr(e)
 333                 } else {
 334                     Expression::Other
 335                 }
 336             } else {
 337                 Expression::Other
 338             }
 339         },
 340         _ => Expression::Other,
 341     }
 342 }
 343
 344 fn fetch_bool_expr(expr: &Expr<'_>) -> Expression {
 345     match expr.kind {
 346         ExprKind::Block(block, _) => fetch_bool_block(block),
 347         ExprKind::Lit(ref lit_ptr) => {
 348             if let LitKind::Bool(value) = lit_ptr.node {
 349                 Expression::Bool(value)
 350             } else {
 351                 Expression::Other
 352             }
 353         },
 354         ExprKind::Ret(Some(expr)) => match fetch_bool_expr(expr) {
 355             Expression::Bool(value) => Expression::RetBool(value),
 356             _ => Expression::Other,
 357         },
 358         _ => Expression::Other,
 359     }
 360 }