Inequality

The != (not equal to) operator is a binary comparison operator that evaluates to true if its two operands are not equivalent, and false otherwise. It serves as syntactic sugar for the ne method defined in the std::cmp::PartialEq trait.

fn main() {
    let a = 5;
    let b = 10;
    let is_not_equal = a != b; // Evaluates to true
}

Trait Resolution and Desugaring

In Rust, operators are backed by the trait system. The != operator requires the left-hand operand’s type to implement PartialEq<Rhs>, where Rhs is the type of the right-hand operand.

When the compiler encounters the != operator, it implicitly borrows both operands and desugars the expression into a method call:

fn main() {
    let a = 5;
    let b = 10;

    // Standard syntax
    let result_standard = a != b;

    // Desugared compiler equivalent
    let result_desugared = std::cmp::PartialEq::ne(&a, &b);
}

The PartialEq Trait Mechanics

The behavior of != is dictated by the PartialEq trait definition in the standard library:

pub trait PartialEq<Rhs = Self>
where
    Rhs: ?Sized,
{
    fn eq(&self, other: &Rhs) -> bool;

    // Default implementation for !=
    fn ne(&self, other: &Rhs) -> bool {
        !self.eq(other)
    }
}

Because ne provides a default implementation that strictly returns the logical negation of eq, custom types typically only need to implement the eq method. The != operator will then automatically utilize this default ne method implementation without requiring any procedural macro derivation or manual implementation of the inequality logic.

Technical Characteristics

• Ownership and Borrowing: The != operator evaluates operands by reference (&self and &other). It does not consume, move, or mutate the underlying values.
• Type Coercion: Rust does not perform implicit type coercion during comparison. Attempting to use != on variables of different types will result in a compile-time type error, unless a specific PartialEq<TypeB> implementation exists for TypeA.
• Floating-Point Behavior: Because floating-point numbers (f32, f64) implement PartialEq rather than Eq (due to IEEE 754 standards), comparing f32::NAN != f32::NAN evaluates to true. This occurs because NaN lacks the reflexive property (f32::NAN == f32::NAN is false), and the ne method strictly negates the result of eq.

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