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, span_lint, span_lint_and_sugg};
   7 use rustc::hir::*;
   8 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
   9 use rustc::{declare_lint_pass, declare_tool_lint};
  10 use rustc_errors::Applicability;
  11 use syntax::ast::LitKind;
  12 use syntax::source_map::Spanned;
  13
  14 declare_clippy_lint! {
  15     /// **What it does:** Checks for expressions of the form `if c { true } else {
  16     /// false }`
  17     /// (or vice versa) and suggest 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 {} // could be `if x { }`
  55     /// ```
  56     pub BOOL_COMPARISON,
  57     complexity,
  58     "comparing a variable to a boolean, e.g., `if x == true` or `if x != true`"
  59 }
  60
  61 declare_lint_pass!(NeedlessBool => [NEEDLESS_BOOL]);
  62
  63 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NeedlessBool {
  64     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, e: &'tcx Expr) {
  65         use self::Expression::*;
  66         if let Some((ref pred, ref then_block, Some(ref else_expr))) = higher::if_block(&e) {
  67             let reduce = |ret, not| {
  68                 let mut applicability = Applicability::MachineApplicable;
  69                 let snip = Sugg::hir_with_applicability(cx, pred, "<predicate>", &mut applicability);
  70                 let mut snip = if not { !snip } else { snip };
  71
  72                 if ret {
  73                     snip = snip.make_return();
  74                 }
  75
  76                 if parent_node_is_if_expr(&e, &cx) {
  77                     snip = snip.blockify()
  78                 }
  79
  80                 span_lint_and_sugg(
  81                     cx,
  82                     NEEDLESS_BOOL,
  83                     e.span,
  84                     "this if-then-else expression returns a bool literal",
  85                     "you can reduce it to",
  86                     snip.to_string(),
  87                     applicability,
  88                 );
  89             };
  90             if let ExprKind::Block(ref then_block, _) = then_block.node {
  91                 match (fetch_bool_block(then_block), fetch_bool_expr(else_expr)) {
  92                     (RetBool(true), RetBool(true)) | (Bool(true), Bool(true)) => {
  93                         span_lint(
  94                             cx,
  95                             NEEDLESS_BOOL,
  96                             e.span,
  97                             "this if-then-else expression will always return true",
  98                         );
  99                     },
 100                     (RetBool(false), RetBool(false)) | (Bool(false), Bool(false)) => {
 101                         span_lint(
 102                             cx,
 103                             NEEDLESS_BOOL,
 104                             e.span,
 105                             "this if-then-else expression will always return false",
 106                         );
 107                     },
 108                     (RetBool(true), RetBool(false)) => reduce(true, false),
 109                     (Bool(true), Bool(false)) => reduce(false, false),
 110                     (RetBool(false), RetBool(true)) => reduce(true, true),
 111                     (Bool(false), Bool(true)) => reduce(false, true),
 112                     _ => (),
 113                 }
 114             } else {
 115                 panic!("IfExpr 'then' node is not an ExprKind::Block");
 116             }
 117         }
 118     }
 119 }
 120
 121 fn parent_node_is_if_expr<'a, 'b>(expr: &Expr, cx: &LateContext<'a, 'b>) -> bool {
 122     let parent_id = cx.tcx.hir().get_parent_node(expr.hir_id);
 123     let parent_node = cx.tcx.hir().get(parent_id);
 124
 125     match parent_node {
 126         rustc::hir::Node::Expr(e) => higher::if_block(&e).is_some(),
 127         rustc::hir::Node::Arm(e) => higher::if_block(&e.body).is_some(),
 128         _ => false,
 129     }
 130 }
 131
 132 declare_lint_pass!(BoolComparison => [BOOL_COMPARISON]);
 133
 134 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for BoolComparison {
 135     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, e: &'tcx Expr) {
 136         if e.span.from_expansion() {
 137             return;
 138         }
 139
 140         if let ExprKind::Binary(Spanned { node, .. }, ..) = e.node {
 141             let ignore_case = None::<(fn(_) -> _, &str)>;
 142             let ignore_no_literal = None::<(fn(_, _) -> _, &str)>;
 143             match node {
 144                 BinOpKind::Eq => {
 145                     let true_case = Some((|h| h, "equality checks against true are unnecessary"));
 146                     let false_case = Some((
 147                         |h: Sugg<'_>| !h,
 148                         "equality checks against false can be replaced by a negation",
 149                     ));
 150                     check_comparison(cx, e, true_case, false_case, true_case, false_case, ignore_no_literal)
 151                 },
 152                 BinOpKind::Ne => {
 153                     let true_case = Some((
 154                         |h: Sugg<'_>| !h,
 155                         "inequality checks against true can be replaced by a negation",
 156                     ));
 157                     let false_case = Some((|h| h, "inequality checks against false are unnecessary"));
 158                     check_comparison(cx, e, true_case, false_case, true_case, false_case, ignore_no_literal)
 159                 },
 160                 BinOpKind::Lt => check_comparison(
 161                     cx,
 162                     e,
 163                     ignore_case,
 164                     Some((|h| h, "greater than checks against false are unnecessary")),
 165                     Some((
 166                         |h: Sugg<'_>| !h,
 167                         "less than comparison against true can be replaced by a negation",
 168                     )),
 169                     ignore_case,
 170                     Some((
 171                         |l: Sugg<'_>, r: Sugg<'_>| (!l).bit_and(&r),
 172                         "order comparisons between booleans can be simplified",
 173                     )),
 174                 ),
 175                 BinOpKind::Gt => check_comparison(
 176                     cx,
 177                     e,
 178                     Some((
 179                         |h: Sugg<'_>| !h,
 180                         "less than comparison against true can be replaced by a negation",
 181                     )),
 182                     ignore_case,
 183                     ignore_case,
 184                     Some((|h| h, "greater than checks against false are unnecessary")),
 185                     Some((
 186                         |l: Sugg<'_>, r: Sugg<'_>| l.bit_and(&(!r)),
 187                         "order comparisons between booleans can be simplified",
 188                     )),
 189                 ),
 190                 _ => (),
 191             }
 192         }
 193     }
 194 }
 195
 196 fn check_comparison<'a, 'tcx>(
 197     cx: &LateContext<'a, 'tcx>,
 198     e: &'tcx Expr,
 199     left_true: Option<(impl FnOnce(Sugg<'a>) -> Sugg<'a>, &str)>,
 200     left_false: Option<(impl FnOnce(Sugg<'a>) -> Sugg<'a>, &str)>,
 201     right_true: Option<(impl FnOnce(Sugg<'a>) -> Sugg<'a>, &str)>,
 202     right_false: Option<(impl FnOnce(Sugg<'a>) -> Sugg<'a>, &str)>,
 203     no_literal: Option<(impl FnOnce(Sugg<'a>, Sugg<'a>) -> Sugg<'a>, &str)>,
 204 ) {
 205     use self::Expression::*;
 206
 207     if let ExprKind::Binary(_, ref left_side, ref right_side) = e.node {
 208         let (l_ty, r_ty) = (cx.tables.expr_ty(left_side), cx.tables.expr_ty(right_side));
 209         if l_ty.is_bool() && r_ty.is_bool() {
 210             let mut applicability = Applicability::MachineApplicable;
 211             match (fetch_bool_expr(left_side), fetch_bool_expr(right_side)) {
 212                 (Bool(true), Other) => left_true.map_or((), |(h, m)| {
 213                     suggest_bool_comparison(cx, e, right_side, applicability, m, h)
 214                 }),
 215                 (Other, Bool(true)) => right_true.map_or((), |(h, m)| {
 216                     suggest_bool_comparison(cx, e, left_side, applicability, m, h)
 217                 }),
 218                 (Bool(false), Other) => left_false.map_or((), |(h, m)| {
 219                     suggest_bool_comparison(cx, e, right_side, applicability, m, h)
 220                 }),
 221                 (Other, Bool(false)) => right_false.map_or((), |(h, m)| {
 222                     suggest_bool_comparison(cx, e, left_side, applicability, m, h)
 223                 }),
 224                 (Other, Other) => no_literal.map_or((), |(h, m)| {
 225                     let left_side = Sugg::hir_with_applicability(cx, left_side, "..", &mut applicability);
 226                     let right_side = Sugg::hir_with_applicability(cx, right_side, "..", &mut applicability);
 227                     span_lint_and_sugg(
 228                         cx,
 229                         BOOL_COMPARISON,
 230                         e.span,
 231                         m,
 232                         "try simplifying it as shown",
 233                         h(left_side, right_side).to_string(),
 234                         applicability,
 235                     )
 236                 }),
 237                 _ => (),
 238             }
 239         }
 240     }
 241 }
 242
 243 fn suggest_bool_comparison<'a, 'tcx>(
 244     cx: &LateContext<'a, 'tcx>,
 245     e: &'tcx Expr,
 246     expr: &Expr,
 247     mut applicability: Applicability,
 248     message: &str,
 249     conv_hint: impl FnOnce(Sugg<'a>) -> Sugg<'a>,
 250 ) {
 251     let hint = Sugg::hir_with_applicability(cx, expr, "..", &mut applicability);
 252     span_lint_and_sugg(
 253         cx,
 254         BOOL_COMPARISON,
 255         e.span,
 256         message,
 257         "try simplifying it as shown",
 258         conv_hint(hint).to_string(),
 259         applicability,
 260     );
 261 }
 262
 263 enum Expression {
 264     Bool(bool),
 265     RetBool(bool),
 266     Other,
 267 }
 268
 269 fn fetch_bool_block(block: &Block) -> Expression {
 270     match (&*block.stmts, block.expr.as_ref()) {
 271         (&[], Some(e)) => fetch_bool_expr(&**e),
 272         (&[ref e], None) => {
 273             if let StmtKind::Semi(ref e) = e.node {
 274                 if let ExprKind::Ret(_) = e.node {
 275                     fetch_bool_expr(&**e)
 276                 } else {
 277                     Expression::Other
 278                 }
 279             } else {
 280                 Expression::Other
 281             }
 282         },
 283         _ => Expression::Other,
 284     }
 285 }
 286
 287 fn fetch_bool_expr(expr: &Expr) -> Expression {
 288     match expr.node {
 289         ExprKind::Block(ref block, _) => fetch_bool_block(block),
 290         ExprKind::Lit(ref lit_ptr) => {
 291             if let LitKind::Bool(value) = lit_ptr.node {
 292                 Expression::Bool(value)
 293             } else {
 294                 Expression::Other
 295             }
 296         },
 297         ExprKind::Ret(Some(ref expr)) => match fetch_bool_expr(expr) {
 298             Expression::Bool(value) => Expression::RetBool(value),
 299             _ => Expression::Other,
 300         },
 301         _ => Expression::Other,
 302     }
 303 }