Better error message for panic on "failed to find alias" during composition #1147
Description
Given this input as root.wat:
(component
(import (interface "foo:bar/a") (instance $types
(type $t u32)
(export "t" (type (eq $t)))
))
(alias export $types "t" (type $t'))
(import (interface "foo:bar/b") (instance
(export $t "t" (type (eq $t')))
(export "f" (func (result $t)))
))
(import (interface "foo:bar/c") (instance
(export $t "t" (type (eq $t')))
(export "f" (func (result $t)))
))
)and this input as dep.wat:
(component
(type $t u32)
(instance $types (export (interface "foo:bar/a"))
(export "t" (type $t))
)
(alias export $types "t" (type $t'))
(core module $m
(func (export "f") (result i32) unreachable)
)
(core instance $i (instantiate $m))
(func $f (result $t) (canon lift (core func $i "f")))
(instance (export (interface "foo:bar/b"))
(export "t" (type $t'))
(export "f" (func $f))
)
)then the panic is with:
$ wasm-tools compose root.wat -d dep.wat -o foo.wasm
[2023-07-26T20:54:37Z WARN ] instance `foo:bar/c` will be imported because a dependency named `foo:bar/c` could not be found
thread 'main' panicked at 'failed to find alias of TypeId { index: 0, type_size: 1, unique_id: 1 }', /Users/alex/.cargo/registry/src/index.crates.io-6f17d22bba15001f/wasm-compose-0.4.0/src/encoding.rs:660:17
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
The problem here is that despite A/B being virtualized the final component needs to import C which depends on A, which means that at least the type information from A needs to be imported. Currently though wasm-compose deduces that A/B are provided by the component instantiation, so only C is imported. This means that during the import of C it asserts that its type information was already imported, which isn't true in this case, thus the panic.
Fixing this will require updates to the dependency graph in wasm-compose and probably the encoding phase too because only the types from A need to be imported to import C.