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::{in_macro, span_lint, span_lint_and_sugg};
   7 use rustc::hir::*;
   8 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
   9 use rustc::{declare_tool_lint, lint_array};
  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
  28     /// if x {
  29     ///     false
  30     /// } else {
  31     ///     true
  32     /// }
  33     /// ```
  34     pub NEEDLESS_BOOL,
  35     complexity,
  36     "if-statements with plain booleans in the then- and else-clause, e.g. `if p { true } else { false }`"
  37 }
  38
  39 declare_clippy_lint! {
  40     /// **What it does:** Checks for expressions of the form `x == true`,
  41     /// `x != true` and order comparisons such as `x < true` (or vice versa) and
  42     /// suggest using the variable directly.
  43     ///
  44     /// **Why is this bad?** Unnecessary code.
  45     ///
  46     /// **Known problems:** None.
  47     ///
  48     /// **Example:**
  49     /// ```rust
  50     /// if x == true {} // could be `if x { }`
  51     /// ```
  52     pub BOOL_COMPARISON,
  53     complexity,
  54     "comparing a variable to a boolean, e.g. `if x == true` or `if x != true`"
  55 }
  56
  57 #[derive(Copy, Clone)]
  58 pub struct NeedlessBool;
  59
  60 impl LintPass for NeedlessBool {
  61     fn get_lints(&self) -> LintArray {
  62         lint_array!(NEEDLESS_BOOL)
  63     }
  64
  65     fn name(&self) -> &'static str {
  66         "NeedlessBool"
  67     }
  68 }
  69
  70 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NeedlessBool {
  71     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, e: &'tcx Expr) {
  72         use self::Expression::*;
  73         if let ExprKind::If(ref pred, ref then_block, Some(ref else_expr)) = e.node {
  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.node {
  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 fn parent_node_is_if_expr<'a, 'b>(expr: &Expr, cx: &LateContext<'a, 'b>) -> bool {
 129     let parent_id = cx.tcx.hir().get_parent_node_by_hir_id(expr.hir_id);
 130     let parent_node = cx.tcx.hir().get_by_hir_id(parent_id);
 131
 132     if let rustc::hir::Node::Expr(e) = parent_node {
 133         if let ExprKind::If(_, _, _) = e.node {
 134             return true;
 135         }
 136     }
 137
 138     false
 139 }
 140
 141 #[derive(Copy, Clone)]
 142 pub struct BoolComparison;
 143
 144 impl LintPass for BoolComparison {
 145     fn get_lints(&self) -> LintArray {
 146         lint_array!(BOOL_COMPARISON)
 147     }
 148
 149     fn name(&self) -> &'static str {
 150         "BoolComparison"
 151     }
 152 }
 153
 154 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for BoolComparison {
 155     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, e: &'tcx Expr) {
 156         if in_macro(e.span) {
 157             return;
 158         }
 159
 160         if let ExprKind::Binary(Spanned { node, .. }, ..) = e.node {
 161             let ignore_case = None::<(fn(_) -> _, &str)>;
 162             let ignore_no_literal = None::<(fn(_, _) -> _, &str)>;
 163             match node {
 164                 BinOpKind::Eq => {
 165                     let true_case = Some((|h| h, "equality checks against true are unnecessary"));
 166                     let false_case = Some((
 167                         |h: Sugg<'_>| !h,
 168                         "equality checks against false can be replaced by a negation",
 169                     ));
 170                     check_comparison(cx, e, true_case, false_case, true_case, false_case, ignore_no_literal)
 171                 },
 172                 BinOpKind::Ne => {
 173                     let true_case = Some((
 174                         |h: Sugg<'_>| !h,
 175                         "inequality checks against true can be replaced by a negation",
 176                     ));
 177                     let false_case = Some((|h| h, "inequality checks against false are unnecessary"));
 178                     check_comparison(cx, e, true_case, false_case, true_case, false_case, ignore_no_literal)
 179                 },
 180                 BinOpKind::Lt => check_comparison(
 181                     cx,
 182                     e,
 183                     ignore_case,
 184                     Some((|h| h, "greater than checks against false are unnecessary")),
 185                     Some((
 186                         |h: Sugg<'_>| !h,
 187                         "less than comparison against true can be replaced by a negation",
 188                     )),
 189                     ignore_case,
 190                     Some((
 191                         |l: Sugg<'_>, r: Sugg<'_>| (!l).bit_and(&r),
 192                         "order comparisons between booleans can be simplified",
 193                     )),
 194                 ),
 195                 BinOpKind::Gt => check_comparison(
 196                     cx,
 197                     e,
 198                     Some((
 199                         |h: Sugg<'_>| !h,
 200                         "less than comparison against true can be replaced by a negation",
 201                     )),
 202                     ignore_case,
 203                     ignore_case,
 204                     Some((|h| h, "greater than checks against false are unnecessary")),
 205                     Some((
 206                         |l: Sugg<'_>, r: Sugg<'_>| l.bit_and(&(!r)),
 207                         "order comparisons between booleans can be simplified",
 208                     )),
 209                 ),
 210                 _ => (),
 211             }
 212         }
 213     }
 214 }
 215
 216 fn check_comparison<'a, 'tcx>(
 217     cx: &LateContext<'a, 'tcx>,
 218     e: &'tcx Expr,
 219     left_true: Option<(impl FnOnce(Sugg<'a>) -> Sugg<'a>, &str)>,
 220     left_false: Option<(impl FnOnce(Sugg<'a>) -> Sugg<'a>, &str)>,
 221     right_true: Option<(impl FnOnce(Sugg<'a>) -> Sugg<'a>, &str)>,
 222     right_false: Option<(impl FnOnce(Sugg<'a>) -> Sugg<'a>, &str)>,
 223     no_literal: Option<(impl FnOnce(Sugg<'a>, Sugg<'a>) -> Sugg<'a>, &str)>,
 224 ) {
 225     use self::Expression::*;
 226
 227     if let ExprKind::Binary(_, ref left_side, ref right_side) = e.node {
 228         let (l_ty, r_ty) = (cx.tables.expr_ty(left_side), cx.tables.expr_ty(right_side));
 229         if l_ty.is_bool() && r_ty.is_bool() {
 230             let mut applicability = Applicability::MachineApplicable;
 231             match (fetch_bool_expr(left_side), fetch_bool_expr(right_side)) {
 232                 (Bool(true), Other) => left_true.map_or((), |(h, m)| {
 233                     suggest_bool_comparison(cx, e, right_side, applicability, m, h)
 234                 }),
 235                 (Other, Bool(true)) => right_true.map_or((), |(h, m)| {
 236                     suggest_bool_comparison(cx, e, left_side, applicability, m, h)
 237                 }),
 238                 (Bool(false), Other) => left_false.map_or((), |(h, m)| {
 239                     suggest_bool_comparison(cx, e, right_side, applicability, m, h)
 240                 }),
 241                 (Other, Bool(false)) => right_false.map_or((), |(h, m)| {
 242                     suggest_bool_comparison(cx, e, left_side, applicability, m, h)
 243                 }),
 244                 (Other, Other) => no_literal.map_or((), |(h, m)| {
 245                     let left_side = Sugg::hir_with_applicability(cx, left_side, "..", &mut applicability);
 246                     let right_side = Sugg::hir_with_applicability(cx, right_side, "..", &mut applicability);
 247                     span_lint_and_sugg(
 248                         cx,
 249                         BOOL_COMPARISON,
 250                         e.span,
 251                         m,
 252                         "try simplifying it as shown",
 253                         h(left_side, right_side).to_string(),
 254                         applicability,
 255                     )
 256                 }),
 257                 _ => (),
 258             }
 259         }
 260     }
 261 }
 262
 263 fn suggest_bool_comparison<'a, 'tcx>(
 264     cx: &LateContext<'a, 'tcx>,
 265     e: &'tcx Expr,
 266     expr: &Expr,
 267     mut applicability: Applicability,
 268     message: &str,
 269     conv_hint: impl FnOnce(Sugg<'a>) -> Sugg<'a>,
 270 ) {
 271     let hint = Sugg::hir_with_applicability(cx, expr, "..", &mut applicability);
 272     span_lint_and_sugg(
 273         cx,
 274         BOOL_COMPARISON,
 275         e.span,
 276         message,
 277         "try simplifying it as shown",
 278         conv_hint(hint).to_string(),
 279         applicability,
 280     );
 281 }
 282
 283 enum Expression {
 284     Bool(bool),
 285     RetBool(bool),
 286     Other,
 287 }
 288
 289 fn fetch_bool_block(block: &Block) -> Expression {
 290     match (&*block.stmts, block.expr.as_ref()) {
 291         (&[], Some(e)) => fetch_bool_expr(&**e),
 292         (&[ref e], None) => {
 293             if let StmtKind::Semi(ref e) = e.node {
 294                 if let ExprKind::Ret(_) = e.node {
 295                     fetch_bool_expr(&**e)
 296                 } else {
 297                     Expression::Other
 298                 }
 299             } else {
 300                 Expression::Other
 301             }
 302         },
 303         _ => Expression::Other,
 304     }
 305 }
 306
 307 fn fetch_bool_expr(expr: &Expr) -> Expression {
 308     match expr.node {
 309         ExprKind::Block(ref block, _) => fetch_bool_block(block),
 310         ExprKind::Lit(ref lit_ptr) => {
 311             if let LitKind::Bool(value) = lit_ptr.node {
 312                 Expression::Bool(value)
 313             } else {
 314                 Expression::Other
 315             }
 316         },
 317         ExprKind::Ret(Some(ref expr)) => match fetch_bool_expr(expr) {
 318             Expression::Bool(value) => Expression::RetBool(value),
 319             _ => Expression::Other,
 320         },
 321         _ => Expression::Other,
 322     }
 323 }