dotnet/roslyn

EnC: disallow editing async methods with incompatible constraints on their method builders

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#51,234 opened on Feb 15, 2021

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Area-InteractiveConcept-Continuous ImprovementInteractive-EnChelp wanted

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Description

Follow up on discussion: https://github.com/dotnet/roslyn/discussions/51189

In debug builds we emit async state machine types as classes so that we can add fields to them as needed during EnC. Emitting the state machine type as reference type instead of a value type might however conflict with a constraint:

If

  1. the return type of an async method specifies a custom builder B via AsyncMethodBuilder, and
  2. B.Start<TStateMachine>(ref TStateMachine), B.AwaitOnCompleted<TAwaiter, TStateMachine>(ref TAwaiter awaiter, ref TStateMachine stateMachine), or B.AwaitUnsafeOnCompleted<TAwaiter, TStateMachine>(ref TAwaiter awaiter, ref TStateMachine stateMachine) have a constraint on TStateMachine that disallows it to be a reference type.

Fix: The compiler should identify these cases and keep the state machine type as struct. The IDE should disallow editing such async methods.

Contributor guide

Research direction
Examine the linked discussion and identify the constraints on TStateMachine that prevent struct usage. Then determine where in the compiler to enforce the check.
Tech stack
csharp
Domain
compilers
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
3-5 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Mostly clear
Prerequisites
C#.NET compiler internals
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
20