Compile time only attribute for source generators

[thargol1]

Compile time only attribute for source generators

I've been experimenting with source generators and it works great. Only I'm not happy with how to trigger a source generator.

Most examples use an attribute to indicate where source should be generated.

In my experiments I also used comments:

//# ValueStruct: double,json,comparable,plusminus
public partial struct Celcius
{
	private const string PostFix = "°C";
	public bool IsValid => Value >= -273.15d;
}

And the source generator than generates the code for a single value struct with json conversion, validation, + and - operators, comparers and string parsing.

Using comments is technically easy using GetLeadingTrivia() but the big disavantage is that you have to parse the comment yourself and there is no validation during edit time.

I don't want to use attributes because they get emitted in to the resulting binary which results in unnecessary bytes spend and leaks some implementation details.

Therefor I would like to propose attributes that are available during compile time to source generators but don't get emmitted in the resulting binary.

Possible syntaxes:

[[MyAttribute]]  // using double brackets to indicate a non emitting attribute

public MyAttribute : NoEmitAttribute // using a special base class

[NoEmit] public MyAttribute : Attribute // using an attribute on the attribute

Every usage of the attribute in actual code, outside the source generator, should generate a compiler error, like when calling GetCustomAttribute<MyAttribute>.

Or perhaps a different implementation is possible like:

sourcegenerator(sourcegeneratoridentifier, param1, param2, .... paramx)
{
	public partial struct Celcius
	{
		private const string PostFix = "°C";
		public bool IsValid => Value >= -273.15d;
	}
}

Where sourcegenerator is a new keyword and calls the specified source generator with the embedded syntaxtree and specified parameters.

[chrisoverzero]

I don't want to use attributes because they get emitted in to the resulting binary […]

This has been discussed elsewhere (#4715), and the solution is to apply ConditionalAttribute with a value of "DEBUG" to the custom attributes.

[ufcpp]

This was also discussed in 2015.

[thargol1]

Hmm, I have to work on my search skills, I did try to find and existing issue, but didn't find it.

Thanks for the tip with ConditionalAttribute.

This issue can be closed.