Add "constant" to DurationKnownEncoding

[pshao25]

It starts from Azure/typespec-azure#1901. To represent "x-ms-format": "duration-constant", we are expecting something like

model Test {
  @encode(DurationKnownEncoding.constant)
  input: duration;
}

What's more, we might need to change in TypeSpec-Azure to emit "x-ms-format": "duration-constant" in the swagger.

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[timotheeguerin]

can we get more detail about what this format is, if its not something defined by an rfc it shouldn't be added here

[pshao25]

In the description of this issue, Azure/typespec-azure#1901, we are converting Compute swagger to TypeSpec. They have defined "x-ms-format": "duration-constant", like below:

"maximumDuration": {
  "type": "string",
  "x-ms-format": "duration-constant"
},

which impacts the deserialization, meaning SDK does different serialization in the payload for this specific property.

In the description of that issue, I have tried two ways to represent it but failed. Therefore, we propose to add a "constant", so that we can do

model Test {
  @encode(DurationKnownEncoding.constant)
  input: duration;
}

[pshao25]

I've no idea whether it is RFC, but we need a solution if we don't add "constant".

[timotheeguerin]

X-Ms-format though wasn't an autorest core thing though?

What is the format of serialization where does it come from?

This seems like quite an edge case to consider this level of support

[pshao25]

Maybe. Then how about we enable

model Test {
  @format("duration-constant")
  input: duration;
}

to make it pass compilation.

[ArcturusZhang]

why not we just simply use

model Test {
	@encode("constant") // or duration-constant
	input: duration;
}

encode decorator could accept any string as values, and if when you do this, TCGC is not able to handle them properly, this is a bug in TCGC.
We do not really have to introduce such a value into this enum.

And FYI @format might work fine on swaggers, but the SDK generators now ignore format therefore it will not work for SDKs.

On the other hand, if you could find this format in RFC, we could have it here.

[pshao25]

I updated the description of that issue with trial 3. We need an "x-ms-format": "duration-constant", not an "format": "duration-constant".

[timotheeguerin]

Yeah, @ArcturusZhang is right, encore doesn't require the value to be defined there is is open. You can support whatever additional custom encoding you want without the standard library defining it.

Format is incorrect as you saw for 2 reason it's for format not x-ms-format and it's about giving a known pattern to a string so will never work with anything else