[SymbolGraphGen] Allow documenting API using modernized syntax even when ABI prevents an update to the code

[glessard]

Feature Name

Documentation with Modernized Syntax for old API

Description

Some older API in the standard library are defined using classic generics syntax, and cannot be changed for historical (ABI) reasons. It would be nice to be able to display these using a source-compatible modernized syntax in documentation.

Motivation

Some API are declared using old generics syntax, and would read better using modernized syntax. For example:

public func initializeMemory<C: Collection>(as: C.Element.Type, fromContentsOf source: C) -> UnsafeMutableBufferPointer<C.Element>

would read better as:

public func initializeMemory<Element>(as: Element.type, fromContentsOf source: some Collection<Element>) -> UnsafeMutableBufferPointer<Element>

The second version names the important type (Element), while making the incidental type (the collection) anonymous. The original version, using classic syntax, names the incidental type while obscuring the important one.

This would not be an ABI-compatible change, though it is source-compatible.

Importance

It would allow our documentation to use modern syntax even in cases where the function definition requires the old syntax for historical reasons (e.g. ABI stability).

Alternatives Considered

No response

[d-ronnqvist]

DocC itself isn't tied to any specific language so it wouldn't rewrite function signatures. Instead the symbol information that's displayed in DocC comes from the source language's compilers.

This has previously been reported here #70588