Add SemIR instruction to track that a conversion was performed.

[zygoloid]

Instead of ad-hoc conversion tracking on some kinds of nodes that conversion creates, consolidate tracking into a single node kind. This frees up an operand on Init instructions that can be used to store the destination.

[jonmeow]

Looks good!

[jonmeow]

Maybe "convert" or "convert_to" instead of "conversion"? Partly shorter, partly a better match for "initialize_from" versus "initialization_from" (word form, not as focused on "from" versus "to", although maybe "to" would help you on name collisions)

[zygoloid]

Hm, I've tried this out, and to me it gives the suggestion that the convert node is actually performing a conversion itself, rather than just tracking that one was performed. By contrast, initialize_from actually performs initialization itself.

Maybe something like converted_from would work? But that is definitely not shorter any more :-)

[jonmeow]

I'd be fine with converted, converted_from, convert_from, etc.

Noting the example IR:

// CHECK:STDOUT:   %.loc13_26.1: {.a: i32, .b: i32} = struct_literal (%.loc13_17, %.loc13_25)
// CHECK:STDOUT:   %Class.ref: type = name_reference "Class", file.%Class
// CHECK:STDOUT:   %.loc13_26.2: ref Class = temporary_storage
// CHECK:STDOUT:   %.loc13_26.3: ref i32 = class_field_access %.loc13_26.2, member0
// CHECK:STDOUT:   %.loc13_26.4: init i32 = initialize_from %.loc13_17 to %.loc13_26.3
// CHECK:STDOUT:   %.loc13_26.5: ref i32 = class_field_access %.loc13_26.2, member1
// CHECK:STDOUT:   %.loc13_26.6: init i32 = initialize_from %.loc13_25 to %.loc13_26.5
// CHECK:STDOUT:   %.loc13_26.7: init Class = class_init (%.loc13_26.4, %.loc13_26.6), %.loc13_26.2
// CHECK:STDOUT:   %.loc13_26.8: ref Class = temporary %.loc13_26.2, %.loc13_26.7
// CHECK:STDOUT:   %.loc13_26.9: ref Class = conversion %.loc13_26.1, %.loc13_26.8

initialize_from has to, whereas conversion doesn't. But the second argument is still kind of a "to", isn't it?

Maybe:

// CHECK:STDOUT:   %.loc13_26.9: ref Class = convert_from %.loc13_26.1 to %.loc13_26.8

I think "to" or something like that might make sense here? But feel free to do something you think makes sense here, which could include just merging as-is.

[zygoloid]

to is used to indicate the thing being initialized by an initializing expression, which doesn't fit here. In the interest of brevity and avoiding the link with to, I'll go with converted.