Enforce AttributeTargets properly.

[edgarfgp]

I propose we Enforce the correct use of AttributeTargets in the language and write a specification to document it.

The existing way of approaching this problem in F# is

In most cases just use interchangeably custom attributes with AttributeTargets and the compiler will not raise any errors.

open System

[<AttributeUsage(AttributeTargets.Method)>]
type MethodOnlyAttribute() = 
  inherit System.Attribute()

[<MethodOnly>]
let someFunction () = "abc"

[<MethodOnly>] // Should error  with: This attribute is not valid for use on this language element
let someValue =  "abc"

[<AttributeUsage(AttributeTargets.Field)>]
type FieldOnlyAttribute() = 
  inherit System.Attribute()

[<FieldOnly>] // Should error  with: This attribute is not valid for use on this language element
let someFunction () = "abc"

[<FieldOnly>]
let someValue =  "abc"

Note: The above snippet is just one of the many issues with AttributeTargets.

Pros and Cons

The advantages of making this adjustment to F# are

• The compiler will raise errors when using an incorrect AttributeTargets
• We will have a language specification about where and how this AttributeTargets are meant to used.

The disadvantages of making this adjustment to F# are

• Any existing custom defined attribute that uses and incorrect AttributeTarget will get an error.

Extra information

**Estimated cost (XS, S, M, L, XL, XXL): M

Related suggestions: (put links to related suggestions here)

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Please tick all that apply:

• This is not a breaking change to the F# language design
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[edgarfgp]

I have already done most of the job for this and now I would like to create a RFC for this :). cc @vzarytovskii