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