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::higher;
   7 use clippy_utils::source::snippet_with_applicability;
   8 use clippy_utils::sugg::Sugg;
   9 use clippy_utils::{get_parent_node, is_else_clause, is_expn_of, peel_blocks, peel_blocks_with_stmt};
  10 use rustc_ast::ast::LitKind;
  11 use rustc_errors::Applicability;
  12 use rustc_hir::{BinOpKind, Block, Expr, ExprKind, HirId, Node, UnOp};
  13 use rustc_lint::{LateContext, LateLintPass};
  14 use rustc_session::{declare_lint_pass, declare_tool_lint};
  15 use rustc_span::source_map::Spanned;
  16 use rustc_span::Span;
  17
  18 declare_clippy_lint! {
  19     /// ### What it does
  20     /// Checks for expressions of the form `if c { true } else {
  21     /// false }` (or vice versa) and suggests using the condition directly.
  22     ///
  23     /// ### Why is this bad?
  24     /// Redundant code.
  25     ///
  26     /// ### Known problems
  27     /// Maybe false positives: Sometimes, the two branches are
  28     /// painstakingly documented (which we, of course, do not detect), so they *may*
  29     /// have some value. Even then, the documentation can be rewritten to match the
  30     /// shorter code.
  31     ///
  32     /// ### Example
  33     /// ```rust
  34     /// # let x = true;
  35     /// if x {
  36     ///     false
  37     /// } else {
  38     ///     true
  39     /// }
  40     /// # ;
  41     /// ```
  42     ///
  43     /// Use instead:
  44     /// ```rust
  45     /// # let x = true;
  46     /// !x
  47     /// # ;
  48     /// ```
  49     #[clippy::version = "pre 1.29.0"]
  50     pub NEEDLESS_BOOL,
  51     complexity,
  52     "if-statements with plain booleans in the then- and else-clause, e.g., `if p { true } else { false }`"
  53 }
  54
  55 declare_clippy_lint! {
  56     /// ### What it does
  57     /// Checks for expressions of the form `x == true`,
  58     /// `x != true` and order comparisons such as `x < true` (or vice versa) and
  59     /// suggest using the variable directly.
  60     ///
  61     /// ### Why is this bad?
  62     /// Unnecessary code.
  63     ///
  64     /// ### Example
  65     /// ```rust,ignore
  66     /// if x == true {}
  67     /// if y == false {}
  68     /// ```
  69     /// use `x` directly:
  70     /// ```rust,ignore
  71     /// if x {}
  72     /// if !y {}
  73     /// ```
  74     #[clippy::version = "pre 1.29.0"]
  75     pub BOOL_COMPARISON,
  76     complexity,
  77     "comparing a variable to a boolean, e.g., `if x == true` or `if x != true`"
  78 }
  79
  80 declare_lint_pass!(NeedlessBool => [NEEDLESS_BOOL]);
  81
  82 fn condition_needs_parentheses(e: &Expr<'_>) -> bool {
  83     let mut inner = e;
  84     while let ExprKind::Binary(_, i, _)
  85     | ExprKind::Call(i, _)
  86     | ExprKind::Cast(i, _)
  87     | ExprKind::Type(i, _)
  88     | ExprKind::Index(i, _) = inner.kind
  89     {
  90         if matches!(
  91             i.kind,
  92             ExprKind::Block(..)
  93                 | ExprKind::ConstBlock(..)
  94                 | ExprKind::If(..)
  95                 | ExprKind::Loop(..)
  96                 | ExprKind::Match(..)
  97         ) {
  98             return true;
  99         }
 100         inner = i;
 101     }
 102     false
 103 }
 104
 105 fn is_parent_stmt(cx: &LateContext<'_>, id: HirId) -> bool {
 106     matches!(
 107         get_parent_node(cx.tcx, id),
 108         Some(Node::Stmt(..) | Node::Block(Block { stmts: &[], .. }))
 109     )
 110 }
 111
 112 impl<'tcx> LateLintPass<'tcx> for NeedlessBool {
 113     fn check_expr(&mut self, cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) {
 114         use self::Expression::{Bool, RetBool};
 115         if e.span.from_expansion() {
 116             return;
 117         }
 118         if let Some(higher::If {
 119             cond,
 120             then,
 121             r#else: Some(r#else),
 122         }) = higher::If::hir(e)
 123         {
 124             let reduce = |ret, not| {
 125                 let mut applicability = Applicability::MachineApplicable;
 126                 let snip = Sugg::hir_with_applicability(cx, cond, "<predicate>", &mut applicability);
 127                 let mut snip = if not { !snip } else { snip };
 128
 129                 if ret {
 130                     snip = snip.make_return();
 131                 }
 132
 133                 if is_else_clause(cx.tcx, e) {
 134                     snip = snip.blockify();
 135                 }
 136
 137                 if condition_needs_parentheses(cond) && is_parent_stmt(cx, e.hir_id) {
 138                     snip = snip.maybe_par();
 139                 }
 140
 141                 span_lint_and_sugg(
 142                     cx,
 143                     NEEDLESS_BOOL,
 144                     e.span,
 145                     "this if-then-else expression returns a bool literal",
 146                     "you can reduce it to",
 147                     snip.to_string(),
 148                     applicability,
 149                 );
 150             };
 151             if let Some((a, b)) = fetch_bool_block(then).and_then(|a| Some((a, fetch_bool_block(r#else)?))) {
 152                 match (a, b) {
 153                     (RetBool(true), RetBool(true)) | (Bool(true), Bool(true)) => {
 154                         span_lint(
 155                             cx,
 156                             NEEDLESS_BOOL,
 157                             e.span,
 158                             "this if-then-else expression will always return true",
 159                         );
 160                     },
 161                     (RetBool(false), RetBool(false)) | (Bool(false), Bool(false)) => {
 162                         span_lint(
 163                             cx,
 164                             NEEDLESS_BOOL,
 165                             e.span,
 166                             "this if-then-else expression will always return false",
 167                         );
 168                     },
 169                     (RetBool(true), RetBool(false)) => reduce(true, false),
 170                     (Bool(true), Bool(false)) => reduce(false, false),
 171                     (RetBool(false), RetBool(true)) => reduce(true, true),
 172                     (Bool(false), Bool(true)) => reduce(false, true),
 173                     _ => (),
 174                 }
 175             }
 176         }
 177     }
 178 }
 179
 180 declare_lint_pass!(BoolComparison => [BOOL_COMPARISON]);
 181
 182 impl<'tcx> LateLintPass<'tcx> for BoolComparison {
 183     fn check_expr(&mut self, cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) {
 184         if e.span.from_expansion() {
 185             return;
 186         }
 187
 188         if let ExprKind::Binary(Spanned { node, .. }, ..) = e.kind {
 189             let ignore_case = None::<(fn(_) -> _, &str)>;
 190             let ignore_no_literal = None::<(fn(_, _) -> _, &str)>;
 191             match node {
 192                 BinOpKind::Eq => {
 193                     let true_case = Some((|h| h, "equality checks against true are unnecessary"));
 194                     let false_case = Some((
 195                         |h: Sugg<'tcx>| !h,
 196                         "equality checks against false can be replaced by a negation",
 197                     ));
 198                     check_comparison(cx, e, true_case, false_case, true_case, false_case, ignore_no_literal);
 199                 },
 200                 BinOpKind::Ne => {
 201                     let true_case = Some((
 202                         |h: Sugg<'tcx>| !h,
 203                         "inequality checks against true can be replaced by a negation",
 204                     ));
 205                     let false_case = Some((|h| h, "inequality checks against false are unnecessary"));
 206                     check_comparison(cx, e, true_case, false_case, true_case, false_case, ignore_no_literal);
 207                 },
 208                 BinOpKind::Lt => check_comparison(
 209                     cx,
 210                     e,
 211                     ignore_case,
 212                     Some((|h| h, "greater than checks against false are unnecessary")),
 213                     Some((
 214                         |h: Sugg<'tcx>| !h,
 215                         "less than comparison against true can be replaced by a negation",
 216                     )),
 217                     ignore_case,
 218                     Some((
 219                         |l: Sugg<'tcx>, r: Sugg<'tcx>| (!l).bit_and(&r),
 220                         "order comparisons between booleans can be simplified",
 221                     )),
 222                 ),
 223                 BinOpKind::Gt => check_comparison(
 224                     cx,
 225                     e,
 226                     Some((
 227                         |h: Sugg<'tcx>| !h,
 228                         "less than comparison against true can be replaced by a negation",
 229                     )),
 230                     ignore_case,
 231                     ignore_case,
 232                     Some((|h| h, "greater than checks against false are unnecessary")),
 233                     Some((
 234                         |l: Sugg<'tcx>, r: Sugg<'tcx>| l.bit_and(&(!r)),
 235                         "order comparisons between booleans can be simplified",
 236                     )),
 237                 ),
 238                 _ => (),
 239             }
 240         }
 241     }
 242 }
 243
 244 struct ExpressionInfoWithSpan {
 245     one_side_is_unary_not: bool,
 246     left_span: Span,
 247     right_span: Span,
 248 }
 249
 250 fn is_unary_not(e: &Expr<'_>) -> (bool, Span) {
 251     if let ExprKind::Unary(UnOp::Not, operand) = e.kind {
 252         return (true, operand.span);
 253     }
 254     (false, e.span)
 255 }
 256
 257 fn one_side_is_unary_not<'tcx>(left_side: &'tcx Expr<'_>, right_side: &'tcx Expr<'_>) -> ExpressionInfoWithSpan {
 258     let left = is_unary_not(left_side);
 259     let right = is_unary_not(right_side);
 260
 261     ExpressionInfoWithSpan {
 262         one_side_is_unary_not: left.0 != right.0,
 263         left_span: left.1,
 264         right_span: right.1,
 265     }
 266 }
 267
 268 fn check_comparison<'a, 'tcx>(
 269     cx: &LateContext<'tcx>,
 270     e: &'tcx Expr<'_>,
 271     left_true: Option<(impl FnOnce(Sugg<'a>) -> Sugg<'a>, &str)>,
 272     left_false: Option<(impl FnOnce(Sugg<'a>) -> Sugg<'a>, &str)>,
 273     right_true: Option<(impl FnOnce(Sugg<'a>) -> Sugg<'a>, &str)>,
 274     right_false: Option<(impl FnOnce(Sugg<'a>) -> Sugg<'a>, &str)>,
 275     no_literal: Option<(impl FnOnce(Sugg<'a>, Sugg<'a>) -> Sugg<'a>, &str)>,
 276 ) {
 277     if let ExprKind::Binary(op, left_side, right_side) = e.kind {
 278         let (l_ty, r_ty) = (
 279             cx.typeck_results().expr_ty(left_side),
 280             cx.typeck_results().expr_ty(right_side),
 281         );
 282         if is_expn_of(left_side.span, "cfg").is_some() || is_expn_of(right_side.span, "cfg").is_some() {
 283             return;
 284         }
 285         if l_ty.is_bool() && r_ty.is_bool() {
 286             let mut applicability = Applicability::MachineApplicable;
 287
 288             if op.node == BinOpKind::Eq {
 289                 let expression_info = one_side_is_unary_not(left_side, right_side);
 290                 if expression_info.one_side_is_unary_not {
 291                     span_lint_and_sugg(
 292                         cx,
 293                         BOOL_COMPARISON,
 294                         e.span,
 295                         "this comparison might be written more concisely",
 296                         "try simplifying it as shown",
 297                         format!(
 298                             "{} != {}",
 299                             snippet_with_applicability(cx, expression_info.left_span, "..", &mut applicability),
 300                             snippet_with_applicability(cx, expression_info.right_span, "..", &mut applicability)
 301                         ),
 302                         applicability,
 303                     );
 304                 }
 305             }
 306
 307             match (fetch_bool_expr(left_side), fetch_bool_expr(right_side)) {
 308                 (Some(true), None) => left_true.map_or((), |(h, m)| {
 309                     suggest_bool_comparison(cx, e, right_side, applicability, m, h);
 310                 }),
 311                 (None, Some(true)) => right_true.map_or((), |(h, m)| {
 312                     suggest_bool_comparison(cx, e, left_side, applicability, m, h);
 313                 }),
 314                 (Some(false), None) => left_false.map_or((), |(h, m)| {
 315                     suggest_bool_comparison(cx, e, right_side, applicability, m, h);
 316                 }),
 317                 (None, Some(false)) => right_false.map_or((), |(h, m)| {
 318                     suggest_bool_comparison(cx, e, left_side, applicability, m, h);
 319                 }),
 320                 (None, None) => no_literal.map_or((), |(h, m)| {
 321                     let left_side = Sugg::hir_with_applicability(cx, left_side, "..", &mut applicability);
 322                     let right_side = Sugg::hir_with_applicability(cx, right_side, "..", &mut applicability);
 323                     span_lint_and_sugg(
 324                         cx,
 325                         BOOL_COMPARISON,
 326                         e.span,
 327                         m,
 328                         "try simplifying it as shown",
 329                         h(left_side, right_side).to_string(),
 330                         applicability,
 331                     );
 332                 }),
 333                 _ => (),
 334             }
 335         }
 336     }
 337 }
 338
 339 fn suggest_bool_comparison<'a, 'tcx>(
 340     cx: &LateContext<'tcx>,
 341     e: &'tcx Expr<'_>,
 342     expr: &Expr<'_>,
 343     mut applicability: Applicability,
 344     message: &str,
 345     conv_hint: impl FnOnce(Sugg<'a>) -> Sugg<'a>,
 346 ) {
 347     let hint = if expr.span.from_expansion() {
 348         if applicability != Applicability::Unspecified {
 349             applicability = Applicability::MaybeIncorrect;
 350         }
 351         Sugg::hir_with_macro_callsite(cx, expr, "..")
 352     } else {
 353         Sugg::hir_with_applicability(cx, expr, "..", &mut applicability)
 354     };
 355     span_lint_and_sugg(
 356         cx,
 357         BOOL_COMPARISON,
 358         e.span,
 359         message,
 360         "try simplifying it as shown",
 361         conv_hint(hint).to_string(),
 362         applicability,
 363     );
 364 }
 365
 366 enum Expression {
 367     Bool(bool),
 368     RetBool(bool),
 369 }
 370
 371 fn fetch_bool_block(expr: &Expr<'_>) -> Option<Expression> {
 372     match peel_blocks_with_stmt(expr).kind {
 373         ExprKind::Ret(Some(ret)) => Some(Expression::RetBool(fetch_bool_expr(ret)?)),
 374         _ => Some(Expression::Bool(fetch_bool_expr(expr)?)),
 375     }
 376 }
 377
 378 fn fetch_bool_expr(expr: &Expr<'_>) -> Option<bool> {
 379     if let ExprKind::Lit(ref lit_ptr) = peel_blocks(expr).kind {
 380         if let LitKind::Bool(value) = lit_ptr.node {
 381             return Some(value);
 382         }
 383     }
 384     None
 385 }