TRLS009 — Use async method variant for async lambda

• Severity: Warning
• Category: Trellis

What it detects

Flags synchronous Trellis methods such as Map, Bind, Tap, Ensure, and TapOnFailure when any supplied lambda or method group does async work.

Why it matters

The sync method treats the returned Task or ValueTask as just another value. The async work does not get awaited by the Trellis pipeline.

Warning

This rule covers three cases: async lambdas, non-async lambdas whose converted return type is Task or ValueTask, and method groups returning Task or ValueTask.

Bad example

using System.Threading.Tasks;
using Trellis;

static class Example
{
    public static Result<Task<int>> Bad()
    {
        var result = Result.Ok("Ada");
        return result.Map(LookupLengthAsync);
    }

    static Task<int> LookupLengthAsync(string value) =>
        Task.FromResult(value.Length);
}

Good example

using System.Threading.Tasks;
using Trellis;

static class Example
{
    public static Task<Result<int>> Good()
    {
        var result = Result.Ok("Ada");
        return result.MapAsync(LookupLengthAsync);
    }

    static Task<int> LookupLengthAsync(string value) =>
        Task.FromResult(value.Length);
}

Code fix available

Yes, when the enclosing scope can safely become async. The fix renames the sync API to the matching async API, awaits the rewritten call, and adds async to Task/ValueTask-returning methods when it can do so without changing surrounding expressions or other returns.

No fix is offered for chained calls, nested wrapper expressions, non-async lambdas, used var locals, synchronous void methods, synchronous value-returning methods, direct returns from already-async scopes, methods/local functions with ref/out/in parameters, or Task/ValueTask-returning scopes with other task-returning return statements because those require manual return-type, delegate-shape, parameter-shape, or pipeline-flow changes first.

Configuration

Use standard Roslyn configuration if you need to suppress this rule in a specific scope.

dotnet_diagnostic.TRLS009.severity = none

#pragma warning disable TRLS009
// Intentional: documented exception or test-only pattern.
#pragma warning restore TRLS009

Tip

If the callback does async work, move to the matching async API immediately instead of returning a Task from the sync overload.