Add main attributes proposal

proposals/global-attributes-for-main.md

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+# Attribute target for Main
+
+## Summary
+
+C# should allow an attribute to target the main method / program entrypoint
+without the need for it to be syntactically placed next to it.
+
+## Motivation
+
+Today, top level (simple) programs have no way to place attributes on the main
+method, as there is no syntactic place for them to attach to. This means that
+application models, such as Windows Forms, cannot take advantage of the
+simplified entry point as they require additional attributes to be placed on
+main (see
+<https://github.com/dotnet/designs/blob/main/accepted/2021/winforms/streamline-application-bootstrap.md>).
+
+Further if a source generator wishes to add an attribute to the main method, the
+user of the generator must not use top level statements and explicitly make
+their main method `partial`.
+
+## Detailed design
+
+A new global attribute target `main` will be recognized alongside `assembly` and
+`module`, that allows an attribute to specify it should be attached to the
+entrypoint of the program.
+
+For example:
+
+```csharp
+[main: STAThread]
+```
+
+Can be placed in any syntax tree of the compilation, and the `[STAThread]`
+attribute will be attached the entrypoint of the program.
+
+Both top level statements and regular entrypoints are supported by the attribute
+target. In the case of multiple entrypoints the attribute will only apply to the
+first chosen location.
+
+### Assemblies with no Entry Point
+
+It is an error to use the `main` target location for an attribute in an assembly
+with no entry point (e.g. a class library).
+
+### Attribute locations
+
+There is no new
+[`AttributeTargets`](https://docs.microsoft.com/en-us/dotnet/api/system.attributetargets?view=net-6.0)
+enum value defined as part of this proposal, and it is an error to prefix an
+attribute with `[main:` that has any target other than `All` or `Method`.
+
+The
+[`AllowMultiple`](https://docs.microsoft.com/en-us/dotnet/api/system.attributeusageattribute.allowmultiple?view=net-6.0)
+field of `AttributeUsage` is respected, and it is an error to prefix an
+attribute with `[main:` multiple times unless `AllowMultiple` is set to true.
+This applies across all syntax trees, only one prefixed attribute is allowed
+regardless of location defined.
+
+```csharp
+public class SingleAttribute : Attribute { }
+
+[AttributeUsage(AttributeTargets.All, AllowMultiple = true)]
+public class MultiAttribute : Attribute { }
+
+[AttributeUsage(AttributeTargets.Class)]
+public class ClassOnlyAttribute : Attribute { }
+
+/// file1.cs
+[main: Single]
+[main: Single] // error CS0579: Duplicate 'Single' attribute
+[main: Multi]
+[main: Multi]
+
+/// file2.cs
+
+[main: Single] // error CS0579: Duplicate 'Single' attribute
+[main: Multi]
+[main:ClassOnly] //error CS0592: Attribute 'ClassOnly' is not valid on this declaration type. It is only valid on 'class' declarations.
+```
+
+## Open Questions
+
+### Targeting non simple entry points
+
+There is some complexity in identifying exactly which method is the entrypoint
+for regular (non-simple) entrypoints in the compiler. It currently happens
+fairly late and we will need to bring it forward in order just to bind for this
+to work.
+
+However, if we don't do it, then it pushes that complexity onto the generator
+author to identify which kind of entrypoint the user is using and change
+generation strategies accordingly.
+
+That might be considered a feature: source generators can't edit user code and
+one could argue that adding attributes to a regular entrypoint without a
+`partial` modifier is effectively editing it. The same argument then applies to
+simple programs.
+
+If we decide *not* to support source generation scenarios, then we could limit
+the syntactic location to be required to be in the top level syntax tree itself.
+
+A further alternative would be to loosen the restrictions on source generators
+and allow them to apply attributes to *any* method, regardless of its partial
+status. This would need some level of language design not considered here.
+
+### Naming
+
+This proposal is using `main` as straw man syntax for the real value. We should
+decide exactly what to call it. Suggestions include `main` or `entrypoint`.
+
+### Attribute Target
+
+Should we introduce a corresponding `AttributeTargets.Main` that allows an
+attribute to specify that it only applies to main methods? Currently we don't,
+and users can apply any regular Method targeted attribute (or All) to the main
+method.