Improve diagnostics when encountering TypeRef in metadata

[MarijnS95]

Note that code gen from TypeRef is not a good idea as it implies the TypeDef is not available and thus you have no idea what kind of type you're actually generating code for. Usually this implies you're missing metadata. I should have better diagnostics for this in riddle soon.

TypeRef currently raises an unimplemented!() assuming this i something that could be implemented at some point:

windows-rs/crates/libs/bindgen/src/rust/writer.rs

Line 173 in 831b99b

rest => unimplemented!("{rest:?}"),

This is showing when generating (custom) metadata on (top of) win32metadata main (currently at microsoft/win32metadata@3d65d37):

     Running `target\debug\riddle.exe --etc crates/tools/windows/bindings.txt`
thread '<unnamed>' panicked at crates\libs\bindgen\src\rust\writer.rs:173:21:
not implemented: TypeRef(TypeName { namespace: "Windows.UI.Composition", name: "ICompositionTexture" })

This is caused by an upstream issue, where metadata is missing for a type that is used by other functions (microsoft/win32metadata#1760). riddle needs to understand the type here, and for that the generator should emit a more descriptive diagnostic upon encountering TypeRef.

Originally posted by @MarijnS95 in #2539 (comment)

[kennykerr]

I'm working on some winmd validation. It will be some time before this is available, but its goal is to catch stuff like this.

[MarijnS95]

As it turns out the proper fix for microsoft/win32metadata#1760 was simply that the new Win32 winmd I was generating was referencing newer types from the "API contracts" that windows-rs was using. Updating them here (#2745) allows me to once again regenerate windows-rs from newer Win32 winmd.

This means somewhere TypeRef gets replaced with TypeDef when multiple winmds are loaded, maybe the diagnostics could build off of that process and report any unresolved TypeRefs (where required)?

[kennykerr]

Yes, I intentionally use TypeRefs in this case so that the panic message gives you a clue of what's missing.

[MarijnS95]

Clarifying that precedent in the panic message might be useful, then :)