Module predicates 

Parse predicates for compile-time parser control.

This module provides zero-cost abstractions for controlling parser behavior at compile time. All predicate types are zero-sized and have no runtime overhead.

Base Values

• Enable - Always succeeds without consuming tokens
• Disable - Always fails without consuming tokens
• TokensRemain - Succeeds only when tokens remain in the stream

Logical Operators

• AllOf - Logical AND (all must succeed, 2-4 operands)
• AnyOf - Logical OR (at least one must succeed, 2-4 operands)
• OneOf - Logical XOR (exactly one must succeed, 2-4 operands)
• Not - Logical NOT (succeeds if inner fails)

Example

The predicateflag macro creates newtype wrappers that can implement custom traits for compile-time context validation:

use unsynn::*;
use unsynn::predicates::*;


unsynn! {
   // Define a custom context predicate marker-trait
    trait ExpressionContext: PredicateOp;

    // Creates newtype implementing ExpressionContext
    predicateflag InExpression = Enable for ExpressionContext;

    // Only accepts ExpressionContext predicates
    struct StructLiteral<P: ExpressionContext> {
        _guard: P,
        name: Ident,
    }
}

See the Parse Predicates chapter in the Cookbook for detailed usage examples.

Structs

AllOf

Logical AND: 2-4 predicates must all succeed.

AnyOf

Logical OR: 2-4 predicates, at least one must succeed.

Disable

Always fails without consuming tokens.

Enable

Always succeeds without consuming tokens.

Not

Logical NOT: succeeds if inner predicate fails.

OneOf

Logical XOR: exactly one of 2-4 predicates must succeed.

PredicateCmp

Predicate that compares type A with type B at runtime.

TokensRemain

Succeeds only when tokens remain in the stream.

Traits

PredicateOp

Marker trait for compile-time parser predicates.