[API Proposal]: Add native support for exhaustive enum-based switch expressions

[skashtanoff]

Background and motivation

Now (.NET 7), if we have a enum:

enum State
{
    Off,
    On
}

Then when we use it in switch-expression:

var nextState = currentState switch
{
    State.Off => State.On,
    State.On  => State.Off 
};

The compiler will produce a warning/error despite of the fact the pattern is exhaustive.

API Proposal

Any of the following would be acceptable

• allow the compiler to generate the default branch if the pattern is exhaustive. for example it could be throw UnreachableException by default
• add an attribute like [ExhaustivePattern] that turns the warning off. For example, we are sure that the value comes to us are West or East, and never equals to North, South, how about we tell about that to the compiler? maybe the JIT will manage to produce more optimal code based on this assumption

API Usage

// no warning here
var nextState = currentState switch
{
    State.Off => State.On,
    State.On  => State.Off 
};

Alternative Designs

No response

Risks

backward compatibility

[huoyaoyuan]

This is a C# language proposal, not API proposal. See #3179

[huoyaoyuan]

Some additions:

• allow the compiler to generate the default branch if the pattern is exhaustive. for example it could be throw UnreachableException by default

The compiler already generates a branch throws SwitchExpressionException, with a warning about it.

• add an attribute like [ExhaustivePattern] that turns the warning off.

Currently attributes can only be applied to constructs with compiled form in IL, like methods and fields. It can't be applied to statements inside a method.

[HaloFour]

See: #3179