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 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 }` (or vice versa) and suggests using the condition directly.
  19     ///
  20     /// **Why is this bad?** Redundant code.
  21     ///
  22     /// **Known problems:** Maybe false positives: Sometimes, the two branches are
  23     /// painstakingly documented (which we, of course, do not detect), so they *may*
  24     /// have some value. Even then, the documentation can be rewritten to match the
  25     /// shorter code.
  26     ///
  27     /// **Example:**
  28     /// ```rust,ignore
  29     /// if x {
  30     ///     false
  31     /// } else {
  32     ///     true
  33     /// }
  34     /// ```
  35     /// Could be written as
  36     /// ```rust,ignore
  37     /// !x
  38     /// ```
  39     pub NEEDLESS_BOOL,
  40     complexity,
  41     "if-statements with plain booleans in the then- and else-clause, e.g., `if p { true } else { false }`"
  42 }
  43
  44 declare_clippy_lint! {
  45     /// **What it does:** Checks for expressions of the form `x == true`,
  46     /// `x != true` and order comparisons such as `x < true` (or vice versa) and
  47     /// suggest using the variable directly.
  48     ///
  49     /// **Why is this bad?** Unnecessary code.
  50     ///
  51     /// **Known problems:** None.
  52     ///
  53     /// **Example:**
  54     /// ```rust,ignore
  55     /// if x == true {}
  56     /// if y == false {}
  57     /// ```
  58     /// use `x` directly:
  59     /// ```rust,ignore
  60     /// if x {}
  61     /// if !y {}
  62     /// ```
  63     pub BOOL_COMPARISON,
  64     complexity,
  65     "comparing a variable to a boolean, e.g., `if x == true` or `if x != true`"
  66 }
  67
  68 declare_lint_pass!(NeedlessBool => [NEEDLESS_BOOL]);
  69
  70 impl<'tcx> LateLintPass<'tcx> for NeedlessBool {
  71     fn check_expr(&mut self, cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) {
  72         use self::Expression::{Bool, RetBool};
  73         if let Some((ref pred, ref then_block, Some(ref else_expr))) = higher::if_block(&e) {
  74             let reduce = |ret, not| {
  75                 let mut applicability = Applicability::MachineApplicable;
  76                 let snip = Sugg::hir_with_applicability(cx, pred, "<predicate>", &mut applicability);
  77                 let mut snip = if not { !snip } else { snip };
  78
  79                 if ret {
  80                     snip = snip.make_return();
  81                 }
  82
  83                 if parent_node_is_if_expr(&e, &cx) {
  84                     snip = snip.blockify()
  85                 }
  86
  87                 span_lint_and_sugg(
  88                     cx,
  89                     NEEDLESS_BOOL,
  90                     e.span,
  91                     "this if-then-else expression returns a bool literal",
  92                     "you can reduce it to",
  93                     snip.to_string(),
  94                     applicability,
  95                 );
  96             };
  97             if let ExprKind::Block(ref then_block, _) = then_block.kind {
  98                 match (fetch_bool_block(then_block), fetch_bool_expr(else_expr)) {
  99                     (RetBool(true), RetBool(true)) | (Bool(true), Bool(true)) => {
 100                         span_lint(
 101                             cx,
 102                             NEEDLESS_BOOL,
 103                             e.span,
 104                             "this if-then-else expression will always return true",
 105                         );
 106                     },
 107                     (RetBool(false), RetBool(false)) | (Bool(false), Bool(false)) => {
 108                         span_lint(
 109                             cx,
 110                             NEEDLESS_BOOL,
 111                             e.span,
 112                             "this if-then-else expression will always return false",
 113                         );
 114                     },
 115                     (RetBool(true), RetBool(false)) => reduce(true, false),
 116                     (Bool(true), Bool(false)) => reduce(false, false),
 117                     (RetBool(false), RetBool(true)) => reduce(true, true),
 118                     (Bool(false), Bool(true)) => reduce(false, true),
 119                     _ => (),
 120                 }
 121             } else {
 122                 panic!("IfExpr `then` node is not an `ExprKind::Block`");
 123             }
 124         }
 125     }
 126 }
 127
 128 declare_lint_pass!(BoolComparison => [BOOL_COMPARISON]);
 129
 130 impl<'tcx> LateLintPass<'tcx> for BoolComparison {
 131     fn check_expr(&mut self, cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) {
 132         if e.span.from_expansion() {
 133             return;
 134         }
 135
 136         if let ExprKind::Binary(Spanned { node, .. }, ..) = e.kind {
 137             let ignore_case = None::<(fn(_) -> _, &str)>;
 138             let ignore_no_literal = None::<(fn(_, _) -> _, &str)>;
 139             match node {
 140                 BinOpKind::Eq => {
 141                     let true_case = Some((|h| h, "equality checks against true are unnecessary"));
 142                     let false_case = Some((
 143                         |h: Sugg<'_>| !h,
 144                         "equality checks against false can be replaced by a negation",
 145                     ));
 146                     check_comparison(cx, e, true_case, false_case, true_case, false_case, ignore_no_literal)
 147                 },
 148                 BinOpKind::Ne => {
 149                     let true_case = Some((
 150                         |h: Sugg<'_>| !h,
 151                         "inequality checks against true can be replaced by a negation",
 152                     ));
 153                     let false_case = Some((|h| h, "inequality checks against false are unnecessary"));
 154                     check_comparison(cx, e, true_case, false_case, true_case, false_case, ignore_no_literal)
 155                 },
 156                 BinOpKind::Lt => check_comparison(
 157                     cx,
 158                     e,
 159                     ignore_case,
 160                     Some((|h| h, "greater than checks against false are unnecessary")),
 161                     Some((
 162                         |h: Sugg<'_>| !h,
 163                         "less than comparison against true can be replaced by a negation",
 164                     )),
 165                     ignore_case,
 166                     Some((
 167                         |l: Sugg<'_>, r: Sugg<'_>| (!l).bit_and(&r),
 168                         "order comparisons between booleans can be simplified",
 169                     )),
 170                 ),
 171                 BinOpKind::Gt => check_comparison(
 172                     cx,
 173                     e,
 174                     Some((
 175                         |h: Sugg<'_>| !h,
 176                         "less than comparison against true can be replaced by a negation",
 177                     )),
 178                     ignore_case,
 179                     ignore_case,
 180                     Some((|h| h, "greater than checks against false are unnecessary")),
 181                     Some((
 182                         |l: Sugg<'_>, r: Sugg<'_>| l.bit_and(&(!r)),
 183                         "order comparisons between booleans can be simplified",
 184                     )),
 185                 ),
 186                 _ => (),
 187             }
 188         }
 189     }
 190 }
 191
 192 struct ExpressionInfoWithSpan {
 193     one_side_is_unary_not: bool,
 194     left_span: Span,
 195     right_span: Span,
 196 }
 197
 198 fn is_unary_not(e: &Expr<'_>) -> (bool, Span) {
 199     if_chain! {
 200         if let ExprKind::Unary(unop, operand) = e.kind;
 201         if let UnOp::UnNot = unop;
 202         then {
 203             return (true, operand.span);
 204         }
 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, ref left_side, ref 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 l_ty.is_bool() && r_ty.is_bool() {
 237             let mut applicability = Applicability::MachineApplicable;
 238
 239             if let BinOpKind::Eq = op.node {
 240                 let expression_info = one_side_is_unary_not(&left_side, &right_side);
 241                 if expression_info.one_side_is_unary_not {
 242                     span_lint_and_sugg(
 243                         cx,
 244                         BOOL_COMPARISON,
 245                         e.span,
 246                         "this comparison might be written more concisely",
 247                         "try simplifying it as shown",
 248                         format!(
 249                             "{} != {}",
 250                             snippet_with_applicability(cx, expression_info.left_span, "..", &mut applicability),
 251                             snippet_with_applicability(cx, expression_info.right_span, "..", &mut applicability)
 252                         ),
 253                         applicability,
 254                     )
 255                 }
 256             }
 257
 258             match (fetch_bool_expr(left_side), fetch_bool_expr(right_side)) {
 259                 (Bool(true), Other) => left_true.map_or((), |(h, m)| {
 260                     suggest_bool_comparison(cx, e, right_side, applicability, m, h)
 261                 }),
 262                 (Other, Bool(true)) => right_true.map_or((), |(h, m)| {
 263                     suggest_bool_comparison(cx, e, left_side, applicability, m, h)
 264                 }),
 265                 (Bool(false), Other) => left_false.map_or((), |(h, m)| {
 266                     suggest_bool_comparison(cx, e, right_side, applicability, m, h)
 267                 }),
 268                 (Other, Bool(false)) => right_false.map_or((), |(h, m)| {
 269                     suggest_bool_comparison(cx, e, left_side, applicability, m, h)
 270                 }),
 271                 (Other, Other) => no_literal.map_or((), |(h, m)| {
 272                     let left_side = Sugg::hir_with_applicability(cx, left_side, "..", &mut applicability);
 273                     let right_side = Sugg::hir_with_applicability(cx, right_side, "..", &mut applicability);
 274                     span_lint_and_sugg(
 275                         cx,
 276                         BOOL_COMPARISON,
 277                         e.span,
 278                         m,
 279                         "try simplifying it as shown",
 280                         h(left_side, right_side).to_string(),
 281                         applicability,
 282                     )
 283                 }),
 284                 _ => (),
 285             }
 286         }
 287     }
 288 }
 289
 290 fn suggest_bool_comparison<'a, 'tcx>(
 291     cx: &LateContext<'tcx>,
 292     e: &'tcx Expr<'_>,
 293     expr: &Expr<'_>,
 294     mut applicability: Applicability,
 295     message: &str,
 296     conv_hint: impl FnOnce(Sugg<'a>) -> Sugg<'a>,
 297 ) {
 298     let hint = Sugg::hir_with_applicability(cx, expr, "..", &mut applicability);
 299     span_lint_and_sugg(
 300         cx,
 301         BOOL_COMPARISON,
 302         e.span,
 303         message,
 304         "try simplifying it as shown",
 305         conv_hint(hint).to_string(),
 306         applicability,
 307     );
 308 }
 309
 310 enum Expression {
 311     Bool(bool),
 312     RetBool(bool),
 313     Other,
 314 }
 315
 316 fn fetch_bool_block(block: &Block<'_>) -> Expression {
 317     match (&*block.stmts, block.expr.as_ref()) {
 318         (&[], Some(e)) => fetch_bool_expr(&**e),
 319         (&[ref e], None) => {
 320             if let StmtKind::Semi(ref e) = e.kind {
 321                 if let ExprKind::Ret(_) = e.kind {
 322                     fetch_bool_expr(&**e)
 323                 } else {
 324                     Expression::Other
 325                 }
 326             } else {
 327                 Expression::Other
 328             }
 329         },
 330         _ => Expression::Other,
 331     }
 332 }
 333
 334 fn fetch_bool_expr(expr: &Expr<'_>) -> Expression {
 335     match expr.kind {
 336         ExprKind::Block(ref block, _) => fetch_bool_block(block),
 337         ExprKind::Lit(ref lit_ptr) => {
 338             if let LitKind::Bool(value) = lit_ptr.node {
 339                 Expression::Bool(value)
 340             } else {
 341                 Expression::Other
 342             }
 343         },
 344         ExprKind::Ret(Some(ref expr)) => match fetch_bool_expr(expr) {
 345             Expression::Bool(value) => Expression::RetBool(value),
 346             _ => Expression::Other,
 347         },
 348         _ => Expression::Other,
 349     }
 350 }