Trait TaskScheduler 

pub trait TaskScheduler:
    Send
    + Sync
    + Debug {
    // Required method
    fn acquire_execution_slot<'life0, 'async_trait>(
        &'life0 self,
        cancel_token: CancellationToken,
    ) -> Pin<Box<dyn Future<Output = SchedulingResult<Box<dyn ResourceGuard>>> + Send + 'async_trait>>
       where Self: 'async_trait,
             'life0: 'async_trait;
}

Trait for implementing task scheduling policies

This trait enforces proper cancellation semantics by splitting resource acquisition (which can be cancelled) from task execution (which cannot).

Design Philosophy

Tasks may or may not support cancellation (depending on whether they were created with spawn_cancellable or regular spawn). This split design ensures:

• Resource acquisition: Can respect cancellation tokens to avoid unnecessary allocation
• Task execution: Always runs to completion; tasks handle their own cancellation

This makes it impossible to accidentally interrupt task execution with tokio::select!.

Required Methods

fn acquire_execution_slot<'life0, 'async_trait>( &'life0 self, cancel_token: CancellationToken, ) -> Pin<Box<dyn Future<Output = SchedulingResult<Box<dyn ResourceGuard>>> + Send + 'async_trait>>

where Self: 'async_trait, 'life0: 'async_trait,

Acquire resources needed for task execution and return a guard

This method handles resource allocation (permits, queue slots, etc.) and can respect cancellation tokens to avoid unnecessary resource consumption.

Cancellation Behavior

The cancel_token is used for scheduler-level cancellation (e.g., “don’t start new work”). If cancellation is requested before or during resource acquisition, this method should return SchedulingResult::Cancelled.

Arguments

• cancel_token - CancellationToken for scheduler-level cancellation

Returns

• SchedulingResult::Execute(guard) - Resources acquired, ready to execute
• SchedulingResult::Cancelled - Cancelled before or during resource acquisition
• SchedulingResult::Rejected(reason) - Resources unavailable or policy violation

Implementors

impl TaskScheduler for SemaphoreScheduler

impl TaskScheduler for UnlimitedScheduler