fix: substituted_component_type panic on guest-exported resources#13608
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alexcrichton merged 2 commits intoJun 10, 2026
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`Linker::substituted_component_type` builds a `types::Component` whose resource substitution map is `Some(..)` but only covers the component's *imported* resources. Introspecting an exported function that references an *exported* (non-imported) resource then panics with an index-out-of-bounds, because `InstanceType::resource_type` hard-indexes that partial map (`matching.rs`) instead of falling back to treating an absent resource as uninstantiated. This adds a test that builds a component exporting a resource plus a constructor returning `own<t>` and a method taking `borrow<t>`, then walks the introspected function types via `substituted_component_type`. It panics today (index out of bounds) and will pass once the resolver falls back gracefully. Assisted-by: claude:claude-opus-4-8
`Linker::substituted_component_type` constructs the introspection type with `resources: Some(&cx.imported_resources)`, a map that only contains the component's *imported* resources. `InstanceType::resource_type` previously hard-indexed that map (`self.resources.map(|t| t[ty])`), so resolving a resource that is not in the map -- e.g. a resource the component *exports* -- panicked with an index-out-of-bounds whenever an exported function's params/results referenced it. Fix this by using a fallible lookup and falling back to `ResourceType::uninstantiated` when the resource is absent from the map, exactly as the `resources: None` path already does. This is the semantically correct behavior: a resource that has no substitution is "not (yet) instantiated", which is also what `Component::component_type()` relies on (it passes `resources: None`). With this fix the same safe, public introspection API no longer panics depending on which (also public) API produced the `types::Component`. Assisted-by: claude:claude-opus-4-8
alexcrichton
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Thanks!
The only mild comment I'd have is that LLMs tend to be a bit too verbose in tests insofar as they try to document the exact state of the world before a test was added and when it was added, so the comment basically becomes irrelevant after a test lands. Not a huge thing though, so no worries.
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I've encountered a panic working with guest-exported resources and
Linker::substituted_component_type:I had an LLM debug and add a test, which fails on current
mainwith:Other than me changing the commit message titles (and opening this PR/writing the description), this contribution is entirely LLM-generated, but reviewed by me.
The fix itself is a one-line change, which seems correct to me.
The analysis added by the LLM in the commits seems valid.
Test seems to make sense - I guess we could make it shorter, but I'll leave this up to reviewers to decide.
This is my first LLM-assisted contribution to Wasmtime, I belive it's compliant with https://github.com/bytecodealliance/governance/blob/main/AI_TOOL_POLICY.md, but please let me know if I have missed something.