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