Do not emit unused code with Request Delegate Generator

[martincostello]

Is there an existing issue for this?

• I have searched the existing issues

Is your feature request related to a problem? Please describe the problem.

While looking at the code for RequestDelegateGeneratorSources to see if I could see the source of the bug for #48379 I noticed that private static constants are emitted for all HTTP methods, whether or not they are actually used:

aspnetcore/src/Http/Http.Extensions/gen/RequestDelegateGeneratorSources.cs

Lines 648 to 652 in 39564d5

	private static readonly string[] GetVerb = new[] { global::Microsoft.AspNetCore.Http.HttpMethods.Get };
	private static readonly string[] PostVerb = new[] { global::Microsoft.AspNetCore.Http.HttpMethods.Post };
	private static readonly string[] PutVerb = new[] { global::Microsoft.AspNetCore.Http.HttpMethods.Put };
	private static readonly string[] DeleteVerb = new[] { global::Microsoft.AspNetCore.Http.HttpMethods.Delete };
	private static readonly string[] PatchVerb = new[] { global::Microsoft.AspNetCore.Http.HttpMethods.Patch };

In the following generated code, only GetVerb and PostVerb are used:

[System.CodeDom.Compiler.GeneratedCodeAttribute("Microsoft.AspNetCore.Http.RequestDelegateGenerator, Version=8.0.0.0, Culture=neutral, PublicKeyToken=adb9793829ddae60", "8.0.0.0")]
internal static class GenerateRouteBuilderEndpoints
{
    private static readonly string[] GetVerb = new[] { global::Microsoft.AspNetCore.Http.HttpMethods.Get };
    private static readonly string[] PostVerb = new[] { global::Microsoft.AspNetCore.Http.HttpMethods.Post };
    private static readonly string[] PutVerb = new[]  { global::Microsoft.AspNetCore.Http.HttpMethods.Put };
    private static readonly string[] DeleteVerb = new[] { global::Microsoft.AspNetCore.Http.HttpMethods.Delete };
    private static readonly string[] PatchVerb = new[] { global::Microsoft.AspNetCore.Http.HttpMethods.Patch };

    internal static global::Microsoft.AspNetCore.Builder.RouteHandlerBuilder MapGet(
        this global::Microsoft.AspNetCore.Routing.IEndpointRouteBuilder endpoints,
        [global::System.Diagnostics.CodeAnalysis.StringSyntax("Route")] string pattern,
        global::System.Func<global::System.Collections.Generic.IEnumerable<global::MartinCostello.AdventOfCode.PuzzleAttribute>, global::Microsoft.AspNetCore.Http.IResult> handler,
        [global::System.Runtime.CompilerServices.CallerFilePath] string filePath = "",
        [global::System.Runtime.CompilerServices.CallerLineNumber]int lineNumber = 0)
    {
        return global::Microsoft.AspNetCore.Http.Generated.GeneratedRouteBuilderExtensionsCore.MapCore(
            endpoints,
            pattern,
            handler,
            GetVerb,
            filePath,
            lineNumber);
    }
    internal static global::Microsoft.AspNetCore.Builder.RouteHandlerBuilder MapPost(
        this global::Microsoft.AspNetCore.Routing.IEndpointRouteBuilder endpoints,
        [global::System.Diagnostics.CodeAnalysis.StringSyntax("Route")] string pattern,
        global::System.Func<global::System.Int32, global::System.Int32, global::Microsoft.AspNetCore.Http.HttpRequest, global::Microsoft.AspNetCore.Http.IFormFile?, global::MartinCostello.AdventOfCode.PuzzleFactory, global::Microsoft.Extensions.Logging.ILogger<global::MartinCostello.AdventOfCode.Puzzle>, global::System.Threading.CancellationToken, global::System.Threading.Tasks.Task<global::Microsoft.AspNetCore.Http.IResult>> handler,
        [global::System.Runtime.CompilerServices.CallerFilePath] string filePath = "",
        [global::System.Runtime.CompilerServices.CallerLineNumber]int lineNumber = 0)
    {
        return global::Microsoft.AspNetCore.Http.Generated.GeneratedRouteBuilderExtensionsCore.MapCore(
            endpoints,
            pattern,
            handler,
            PostVerb,
            filePath,
            lineNumber);
    }
    internal static global::Microsoft.AspNetCore.Builder.RouteHandlerBuilder MapGet(
        this global::Microsoft.AspNetCore.Routing.IEndpointRouteBuilder endpoints,
        [global::System.Diagnostics.CodeAnalysis.StringSyntax("Route")] string pattern,
        global::System.Func<global::Microsoft.AspNetCore.Http.IResult> handler,
        [global::System.Runtime.CompilerServices.CallerFilePath] string filePath = "",
        [global::System.Runtime.CompilerServices.CallerLineNumber]int lineNumber = 0)
    {
        return global::Microsoft.AspNetCore.Http.Generated.GeneratedRouteBuilderExtensionsCore.MapCore(
            endpoints,
            pattern,
            handler,
            GetVerb,
            filePath,
            lineNumber);
    }
}

The memory and IL size would be ever so slightly smaller if the fields weren't emitted (assuming the compiler isn't removing them anyway). This is of course a micro-optimisation, so feel free to close this suggestion if the complexity of the generator code isn't worth changing it to detect which of the arrays are actually needed.

Describe the solution you'd like

The source generator only emits the string arrays that are needed by the endpoints used in the compilation.

Additional context

No response

[mitchdenny]

It is a complexity trade off but we've definitely got logic already where we omit certain types entirely based on the endpoints they are defined.

[ghost]

We've moved this issue to the Backlog milestone. This means that it is not going to be worked on for the coming release. We will reassess the backlog following the current release and consider this item at that time. To learn more about our issue management process and to have better expectation regarding different types of issues you can read our Triage Process.

[danmoseley]

@mitchdenny is this the sort of issue where we'd take a community PR? I'm hoping we can apply the 'help wanted' label whenever we'd take a community PR and believe we would have time to review and accept one (which of course is not always the case, eg., if something would likely need a lot of collaboration)

[mitchdenny]

I think we would absolutely take a PR on this.

[martincostello]

I'll see if I can come up with a PR to address this that doesn't make things too complicated.

[martincostello]

Proposed fix in #48426.