lints: constant_bool_expr: add support for rudimentary const eval

Author: poliorcetics

clippy_lints/src/operators/constant_bool_expr.rs

@@ -1,3 +1,4 @@
+use clippy_utils::consts::{ConstEvalCtxt, Constant};
 use clippy_utils::diagnostics::span_lint;
 use clippy_utils::eq_expr_value;
 use rustc_hir::{BinOpKind, Expr, ExprKind};
@@ -68,11 +69,27 @@ fn detect_variants<'tcx>(
 ) -> bool {
     let ctxt = e.span.ctxt();
 
-    let detect_variant = |left_a, right_a, left_literal: &Expr<'_>, right_literal: &Expr<'_>| {
-        matches!(left_literal.kind, ExprKind::Lit(_))
-            && matches!(right_literal.kind, ExprKind::Lit(_))
-            && !eq_expr_value(cx, ctxt, left_literal, right_literal)
-            && eq_expr_value(cx, ctxt, left_a, right_a)
+    let detect_variant = |left_a, right_a, left_cmp: &Expr<'_>, right_cmp: &Expr<'_>| {
+        // If the `a` expression is not the same on both sides, there is no need to go further
+        if !eq_expr_value(cx, ctxt, left_a, right_a) {
+            return false;
+        }
+
+        let const_ctx = ConstEvalCtxt::new(cx);
+
+        // We try to look into constants and const blocks, but if that fails we fall back to handling
+        // literals only
+        if let Some(left_evaluated) = const_ctx.eval(left_cmp)
+            && let Some(right_evaluated) = const_ctx.eval(right_cmp)
+            && left_evaluated != Constant::Err
+            && right_evaluated != Constant::Err
+        {
+            left_evaluated != right_evaluated
+        } else {
+            matches!(left_cmp.kind, ExprKind::Lit(_))
+                && matches!(right_cmp.kind, ExprKind::Lit(_))
+                && !eq_expr_value(cx, ctxt, left_cmp, right_cmp)
+        }
     };
 
     // (a CMP_OP _) BIN_OP (a CMP_OP _)

clippy_lints/src/operators/mod.rs

@@ -215,7 +215,8 @@ declare_clippy_lint! {
     /// This is most likely a logic bug.
     ///
     /// ### Limitations
-    /// This lint only checks against literals.
+    /// This lint only checks against literals and constant expressions since it cannot determine
+    /// if `a == bar() && a == foo()` will actually be different or not at runtime.
     ///
     /// ### Example
     /// ```no_run

tests/ui/constant_bool_expr.rs

@@ -17,5 +17,25 @@ fn main() {
     const A: i32 = 100;
 
     let _b = a != A || a != 101;
+    //~^ constant_bool_expr
     let _b = a == A && a == 101;
+    //~^ constant_bool_expr
+
+    let _b = a != A || a != const { 100 + 1 };
+    //~^ constant_bool_expr
+    let _b = a == A && a == const { 100 + 1 };
+    //~^ constant_bool_expr
 }
+
+const B: i32 = 100;
+const C: i32 = 99;
+
+const _: bool = C != B || C != const { 100 + 1 };
+//~^ constant_bool_expr
+const _: bool = C == B && C == const { 100 + 1 };
+//~^ constant_bool_expr
+
+static D: bool = C != B || C != const { 100 + 1 };
+//~^ constant_bool_expr
+static E: bool = C == B && C == const { 100 + 1 };
+//~^ constant_bool_expr

tests/ui/constant_bool_expr.stderr

@@ -25,5 +25,53 @@ error: expression always evaluates to `false`
 LL |     let _b = a == 100 && a == 101;
    |              ^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 4 previous errors
+error: expression always evaluates to `true`
+  --> tests/ui/constant_bool_expr.rs:19:14
+   |
+LL |     let _b = a != A || a != 101;
+   |              ^^^^^^^^^^^^^^^^^^
+
+error: expression always evaluates to `false`
+  --> tests/ui/constant_bool_expr.rs:21:14
+   |
+LL |     let _b = a == A && a == 101;
+   |              ^^^^^^^^^^^^^^^^^^
+
+error: expression always evaluates to `true`
+  --> tests/ui/constant_bool_expr.rs:24:14
+   |
+LL |     let _b = a != A || a != const { 100 + 1 };
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: expression always evaluates to `false`
+  --> tests/ui/constant_bool_expr.rs:26:14
+   |
+LL |     let _b = a == A && a == const { 100 + 1 };
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: expression always evaluates to `true`
+  --> tests/ui/constant_bool_expr.rs:33:17
+   |
+LL | const _: bool = C != B || C != const { 100 + 1 };
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: expression always evaluates to `false`
+  --> tests/ui/constant_bool_expr.rs:35:17
+   |
+LL | const _: bool = C == B && C == const { 100 + 1 };
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: expression always evaluates to `true`
+  --> tests/ui/constant_bool_expr.rs:38:18
+   |
+LL | static D: bool = C != B || C != const { 100 + 1 };
+   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: expression always evaluates to `false`
+  --> tests/ui/constant_bool_expr.rs:40:18
+   |
+LL | static E: bool = C == B && C == const { 100 + 1 };
+   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 12 previous errors