lint comparison to bool (e.g. y == true)

Addresses #630

src/lib.rs

@@ -105,6 +105,7 @@ pub fn plugin_registrar(reg: &mut Registry) {
     reg.register_late_lint_pass(box bit_mask::BitMask);
     reg.register_late_lint_pass(box ptr_arg::PtrArg);
     reg.register_late_lint_pass(box needless_bool::NeedlessBool);
+    reg.register_late_lint_pass(box needless_bool::BoolComparison);
     reg.register_late_lint_pass(box approx_const::ApproxConstant);
     reg.register_late_lint_pass(box misc::FloatCmp);
     reg.register_early_lint_pass(box precedence::Precedence);
@@ -253,6 +254,7 @@ pub fn plugin_registrar(reg: &mut Registry) {
         mut_reference::UNNECESSARY_MUT_PASSED,
         mutex_atomic::MUTEX_ATOMIC,
         needless_bool::NEEDLESS_BOOL,
+        needless_bool::BOOL_COMPARISON,
         needless_features::UNSTABLE_AS_MUT_SLICE,
         needless_features::UNSTABLE_AS_SLICE,
         needless_update::NEEDLESS_UPDATE,

src/needless_bool.rs

@@ -6,6 +6,7 @@ use rustc::lint::*;
 use rustc_front::hir::*;
 
 use syntax::ast::Lit_;
+use syntax::codemap::Spanned;
 
 use utils::{span_lint, snippet};
 
@@ -23,6 +24,20 @@ declare_lint! {
      `if p { true } else { false }`"
 }
 
+/// **What it does:** This lint checks for expressions of the form `x == true` (or vice versa) and suggest using the variable directly.
+///
+/// **Why is this bad?** Unnecessary code.
+///
+/// **Known problems:** None.
+///
+/// **Example:** `if x == true { }` could be `if x { }`
+declare_lint! {
+    pub BOOL_COMPARISON,
+    Warn,
+    "comparing a variable to a boolean, e.g. \
+     `if x == true`"
+}
+
 #[derive(Copy,Clone)]
 pub struct NeedlessBool;
 
@@ -78,6 +93,57 @@ impl LateLintPass for NeedlessBool {
     }
 }
 
+#[derive(Copy,Clone)]
+pub struct BoolComparison;
+
+impl LintPass for BoolComparison {
+    fn get_lints(&self) -> LintArray {
+        lint_array!(BOOL_COMPARISON)
+    }
+}
+
+impl LateLintPass for BoolComparison {
+    fn check_expr(&mut self, cx: &LateContext, e: &Expr) {
+        if let ExprBinary(Spanned{ node: BiEq, .. }, ref left_side, ref right_side) = e.node {
+            match (fetch_bool_expr(left_side), fetch_bool_expr(right_side)) {
+                (Some(true), None) => {
+                    let side_snip = snippet(cx, right_side.span, "..");
+                    let hint = format!("`{}`", side_snip);
+                    span_lint(cx,
+                              BOOL_COMPARISON,
+                              e.span,
+                              &format!("you can simplify this boolean comparison to {}", hint));
+                }
+                (None, Some(true)) => {
+                    let side_snip = snippet(cx, left_side.span, "..");
+                    let hint = format!("`{}`", side_snip);
+                    span_lint(cx,
+                              BOOL_COMPARISON,
+                              e.span,
+                              &format!("you can simplify this boolean comparison to {}", hint));
+                }
+                (Some(false), None) => {
+                    let side_snip = snippet(cx, right_side.span, "..");
+                    let hint = format!("`!{}`", side_snip);
+                    span_lint(cx,
+                              BOOL_COMPARISON,
+                              e.span,
+                              &format!("you can simplify this boolean comparison to {}", hint));
+                }
+                (None, Some(false)) => {
+                    let side_snip = snippet(cx, left_side.span, "..");
+                    let hint = format!("`!{}`", side_snip);
+                    span_lint(cx,
+                              BOOL_COMPARISON,
+                              e.span,
+                              &format!("you can simplify this boolean comparison to {}", hint));
+                }
+                _ => (),
+            }
+        }
+    }
+}
+
 fn fetch_bool_block(block: &Block) -> Option<bool> {
     if block.stmts.is_empty() {
         block.expr.as_ref().and_then(|e| fetch_bool_expr(e))

tests/compile-fail/bool_comparison.rs

@@ -0,0 +1,12 @@
+#![feature(plugin)]
+#![plugin(clippy)]
+
+#[allow(needless_bool)]
+#[deny(bool_comparison)]
+fn main() {
+    let x = true;
+    if x == true { true } else { false }; //~ERROR you can simplify this boolean comparison to `x`
+    if x == false { true } else { false }; //~ERROR you can simplify this boolean comparison to `!x`
+    if true == x { true } else { false }; //~ERROR you can simplify this boolean comparison to `x`
+    if false == x { true } else { false }; //~ERROR you can simplify this boolean comparison to `!x`
+}