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, snippet_with_applicability, span_lint, span_lint_and_sugg};
   7 use if_chain::if_chain;
   8 use rustc_ast::ast::LitKind;
   9 use rustc_errors::Applicability;
  10 use rustc_hir::{BinOpKind, Block, Expr, ExprKind, StmtKind, UnOp};
  11 use rustc_lint::{LateContext, LateLintPass};
  12 use rustc_session::{declare_lint_pass, declare_tool_lint};
  13 use rustc_span::source_map::Spanned;
  14 use rustc_span::Span;
  15
  16 declare_clippy_lint! {
  17     /// **What it does:** Checks for expressions of the form `if c { true } else {
  18     /// false }`
  19     /// (or vice versa) and suggest 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<'a, 'tcx> LateLintPass<'a, 'tcx> for NeedlessBool {
  72     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, e: &'tcx Expr<'_>) {
  73         use self::Expression::{Bool, RetBool};
  74         if let Some((ref pred, ref then_block, Some(ref else_expr))) = higher::if_block(&e) {
  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 parent_node_is_if_expr(&e, &cx) {
  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(ref 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<'a, 'tcx> LateLintPass<'a, 'tcx> for BoolComparison {
 132     fn check_expr(&mut self, cx: &LateContext<'a, '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_chain! {
 201         if let ExprKind::Unary(unop, operand) = e.kind;
 202         if let UnOp::UnNot = unop;
 203         then {
 204             return (true, operand.span);
 205         }
 206     };
 207     (false, e.span)
 208 }
 209
 210 fn one_side_is_unary_not<'tcx>(left_side: &'tcx Expr<'_>, right_side: &'tcx Expr<'_>) -> ExpressionInfoWithSpan {
 211     let left = is_unary_not(left_side);
 212     let right = is_unary_not(right_side);
 213
 214     ExpressionInfoWithSpan {
 215         one_side_is_unary_not: left.0 != right.0,
 216         left_span: left.1,
 217         right_span: right.1,
 218     }
 219 }
 220
 221 fn check_comparison<'a, 'tcx>(
 222     cx: &LateContext<'a, 'tcx>,
 223     e: &'tcx Expr<'_>,
 224     left_true: Option<(impl FnOnce(Sugg<'a>) -> Sugg<'a>, &str)>,
 225     left_false: Option<(impl FnOnce(Sugg<'a>) -> Sugg<'a>, &str)>,
 226     right_true: Option<(impl FnOnce(Sugg<'a>) -> Sugg<'a>, &str)>,
 227     right_false: Option<(impl FnOnce(Sugg<'a>) -> Sugg<'a>, &str)>,
 228     no_literal: Option<(impl FnOnce(Sugg<'a>, Sugg<'a>) -> Sugg<'a>, &str)>,
 229 ) {
 230     use self::Expression::{Bool, Other};
 231
 232     if let ExprKind::Binary(op, ref left_side, ref right_side) = e.kind {
 233         let (l_ty, r_ty) = (cx.tables.expr_ty(left_side), cx.tables.expr_ty(right_side));
 234         if l_ty.is_bool() && r_ty.is_bool() {
 235             let mut applicability = Applicability::MachineApplicable;
 236
 237             if let BinOpKind::Eq = op.node {
 238                 let expression_info = one_side_is_unary_not(&left_side, &right_side);
 239                 if expression_info.one_side_is_unary_not {
 240                     span_lint_and_sugg(
 241                         cx,
 242                         BOOL_COMPARISON,
 243                         e.span,
 244                         "This comparison might be written more concisely",
 245                         "try simplifying it as shown",
 246                         format!(
 247                             "{} != {}",
 248                             snippet_with_applicability(cx, expression_info.left_span, "..", &mut applicability),
 249                             snippet_with_applicability(cx, expression_info.right_span, "..", &mut applicability)
 250                         ),
 251                         applicability,
 252                     )
 253                 }
 254             }
 255
 256             match (fetch_bool_expr(left_side), fetch_bool_expr(right_side)) {
 257                 (Bool(true), Other) => left_true.map_or((), |(h, m)| {
 258                     suggest_bool_comparison(cx, e, right_side, applicability, m, h)
 259                 }),
 260                 (Other, Bool(true)) => right_true.map_or((), |(h, m)| {
 261                     suggest_bool_comparison(cx, e, left_side, applicability, m, h)
 262                 }),
 263                 (Bool(false), Other) => left_false.map_or((), |(h, m)| {
 264                     suggest_bool_comparison(cx, e, right_side, applicability, m, h)
 265                 }),
 266                 (Other, Bool(false)) => right_false.map_or((), |(h, m)| {
 267                     suggest_bool_comparison(cx, e, left_side, applicability, m, h)
 268                 }),
 269                 (Other, Other) => no_literal.map_or((), |(h, m)| {
 270                     let left_side = Sugg::hir_with_applicability(cx, left_side, "..", &mut applicability);
 271                     let right_side = Sugg::hir_with_applicability(cx, right_side, "..", &mut applicability);
 272                     span_lint_and_sugg(
 273                         cx,
 274                         BOOL_COMPARISON,
 275                         e.span,
 276                         m,
 277                         "try simplifying it as shown",
 278                         h(left_side, right_side).to_string(),
 279                         applicability,
 280                     )
 281                 }),
 282                 _ => (),
 283             }
 284         }
 285     }
 286 }
 287
 288 fn suggest_bool_comparison<'a, 'tcx>(
 289     cx: &LateContext<'a, 'tcx>,
 290     e: &'tcx Expr<'_>,
 291     expr: &Expr<'_>,
 292     mut applicability: Applicability,
 293     message: &str,
 294     conv_hint: impl FnOnce(Sugg<'a>) -> Sugg<'a>,
 295 ) {
 296     let hint = Sugg::hir_with_applicability(cx, expr, "..", &mut applicability);
 297     span_lint_and_sugg(
 298         cx,
 299         BOOL_COMPARISON,
 300         e.span,
 301         message,
 302         "try simplifying it as shown",
 303         conv_hint(hint).to_string(),
 304         applicability,
 305     );
 306 }
 307
 308 enum Expression {
 309     Bool(bool),
 310     RetBool(bool),
 311     Other,
 312 }
 313
 314 fn fetch_bool_block(block: &Block<'_>) -> Expression {
 315     match (&*block.stmts, block.expr.as_ref()) {
 316         (&[], Some(e)) => fetch_bool_expr(&**e),
 317         (&[ref e], None) => {
 318             if let StmtKind::Semi(ref e) = e.kind {
 319                 if let ExprKind::Ret(_) = e.kind {
 320                     fetch_bool_expr(&**e)
 321                 } else {
 322                     Expression::Other
 323                 }
 324             } else {
 325                 Expression::Other
 326             }
 327         },
 328         _ => Expression::Other,
 329     }
 330 }
 331
 332 fn fetch_bool_expr(expr: &Expr<'_>) -> Expression {
 333     match expr.kind {
 334         ExprKind::Block(ref block, _) => fetch_bool_block(block),
 335         ExprKind::Lit(ref lit_ptr) => {
 336             if let LitKind::Bool(value) = lit_ptr.node {
 337                 Expression::Bool(value)
 338             } else {
 339                 Expression::Other
 340             }
 341         },
 342         ExprKind::Ret(Some(ref expr)) => match fetch_bool_expr(expr) {
 343             Expression::Bool(value) => Expression::RetBool(value),
 344             _ => Expression::Other,
 345         },
 346         _ => Expression::Other,
 347     }
 348 }