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