DI Source Generator

Demo.csproj

<Project Sdk="Microsoft.NET.Sdk">
    <PropertyGroup>
        <OutputType>Exe</OutputType>
        <TargetFramework>net5.0</TargetFramework>
        <LangVersion>preview</LangVersion>
    </PropertyGroup>
    <ItemGroup>
        <PackageReference Include="Microsoft.Extensions.DependencyInjection" Version="5.0.0-preview.3.20215.2" />
    </ItemGroup>
    <ItemGroup>
        <ProjectReference Include="..\SourceGenerators\SourceGenerators.csproj" OutputItemType="Analyzer" ReferenceOutputAssembly="false" />
    </ItemGroup>
</Project>

DependencyInjectionGenerator.cs

using System.Collections.Generic;
using System.Text;
using System.Linq;
using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.CSharp.Syntax;
using Microsoft.CodeAnalysis.Text;
using Microsoft.Extensions.DependencyInjection;

namespace SourceGenerators
{
    [Generator]
    public class DependencyInjectionGenerator : ISourceGenerator
    {
        public void Execute(SourceGeneratorContext context)
        {
            var body = new StringBuilder(@"
using System;

namespace Generated
{
    public static class DependencyResolver
    {
");
            var usings = new HashSet<string>();

            var syntaxTrees = context.Compilation.SyntaxTrees;

            foreach (SyntaxTree tree in syntaxTrees)
            {
                var nodes = tree.GetRoot().DescendantNodes();
                var model = context.Compilation.GetSemanticModel(tree);

                IEnumerable<InvocationExpressionSyntax> invocations = nodes.OfType<InvocationExpressionSyntax>();

                foreach (var invocation in invocations)
                {
                    if (model.GetSymbolInfo(invocation.Expression).Symbol is IMethodSymbol method)
                    {
                        if (method.Name == nameof(ServiceCollectionServiceExtensions.AddTransient))
                        {
                            //
                            // .AddTransient<ContractType, ImplementationType>();
                            //
                            if (method.TypeArguments.Length == 2)
                            {
                                var contract = method.TypeArguments[0];
                                var concrete = method.TypeArguments[1];

                                usings.Add(contract.ContainingNamespace.Name);
                                usings.Add(concrete.ContainingNamespace.Name);

                                body.AppendLine($"public static {contract.ContainingNamespace.Name}.{contract.Name} Resolve<{contract.Name}>() ");
                                body.AppendLine("{");                                
                                body.AppendLine($"    return new {concrete.ContainingNamespace.Name}.{concrete.Name}();");
                                body.AppendLine("}");
                            }
                        }
                    }
                }
            }

            body.Append(@"
    } // DependencyResolver
} // Generated");

            var code = new StringBuilder();

            foreach (var @using in usings)
                code.AppendLine($"using {@using};");    
            
            code.AppendLine(body.ToString());

            context.AddSource(nameof(DependencyInjectionGenerator), SourceText.From(code.ToString(), Encoding.UTF8));
        }

        public void Initialize(InitializationContext context) { }
    }
}

Program.cs

using System;
using Microsoft.Extensions.DependencyInjection;

namespace Demo
{
    static class Program
    {
        static void Main(string[] args)
        {
            IFoo foo = Generated.DependencyResolver.Resolve<IFoo>();

            foo.Bar();
        }

        static void Dependencies(this IServiceCollection services)
        {
            services.AddTransient<IFoo, Foo>();
        }
    }

    public interface IFoo
    {
        void Bar();
    }

    public class Foo : IFoo
    {
        public void Bar()
        {
            Console.WriteLine("Bar!");
        }
    }
}

SourceGenerators.csproj

<Project Sdk="Microsoft.NET.Sdk">

    <PropertyGroup>
        <TargetFramework>netstandard2.0</TargetFramework>
        <LangVersion>preview</LangVersion>
    </PropertyGroup>

    <PropertyGroup>
        <RestoreAdditionalProjectSources>https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet5/nuget/v3/index.json ;$(RestoreAdditionalProjectSources)</RestoreAdditionalProjectSources>
    </PropertyGroup>

    <ItemGroup>
        <PackageReference Include="Microsoft.CodeAnalysis.CSharp.Workspaces" Version="3.6.0-3.20207.2" PrivateAssets="all" />
        <PackageReference Include="Microsoft.CodeAnalysis.Analyzers" Version="3.0.0-beta2.final" PrivateAssets="all" />
        <PackageReference Include="Microsoft.Extensions.DependencyInjection" Version="5.0.0-preview.3.20215.2" />
    </ItemGroup>

</Project>