[SR-2608] Enhance default arguments printing in module interface

[rintaro]

Previous ID	SR-2608
Radar	rdar://problem/18675831
Original Reporter	@rintaro
Type	Improvement
Status	Resolved
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s	Compiler, Source Tooling
Labels	Improvement
Assignee	@belkadan
Priority	Medium

md5: 66d899d56bcca613cc9bf6ca9ead7b52

relates to:

• SR-8501 Properly syntax-color default argument expressions in generated interfaces

Issue Description:

It would be great if we can see the value of any default arguments in module interface printing.
(as long as it doesn't include non-public elements)

Currently, only very limited special literals are printed:
#file, #line, #column, #function, and #dsohandle

e.g.

public func fatalError(_ message: @autoclosure () -> String = default, file: StaticString = #file, line: UInt = #line) -> Never

public func print(_ items: Any..., separator: String = default, terminator: String = default)

should be:

public func fatalError(_ message: @autoclosure () -> String = String(), file: StaticString = #file, line: UInt = #line) -> Never

public func print(_ items: Any..., separator: String = " ", terminator: String = "\n")

[belkadan]

This has some complexity if we want to do it right, in that you can reference things in default arguments that aren't part of the public interface of the module. But I'm fine with us doing this wrong for now.

We don't serialize expressions in modules, but we can certainly store a stringified version of the expression.

[belkadan]

If anyone wants to pick this up I'd be happy to answer questions about our current module format and how this might fit in.

[belkadan]

#18579