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Dana Jansens
e682a6660d
When deducing arguments for generic parameters of an `impl`, the deduction calls `Convert` on the input arguments. Often, the input argument is a facet, and needs to be converted to a type via FacetAccessType in order to produce a different facet. These instructions end up being added to the semir, but only their constant values are needed for the resulting specific returned from Deduce. In the best case, these extra instructions are just noise in the semir, or they just cause instruction names to get differentiated with larger suffixes. In the worst case, these extra instructions contain references to instructions from a generic context, and leak them out of that generic context and into another. In particular, when importing a LookupImplWitness instruction, the re-evaluation of it can do deduce (when the lookup is against a generic `impl`). The instructions created in Deduce are not part of the import, and end up referring to imported instructions from the local context, which leads to confusion in the toolchain, and can crash. The `import_self_specific.carbon` test demonstrates this. It causes the `I.F` function to be imported from the `I` interface when building the witness table for the `impl`. Doing so imports the specific of `C` which includes a LookupImplWitness for `Self.Accoc` in `I`. The `Self` is a BindSymbolicName with generic binding index 0, in `I`. When Convert creates instructions in the generic `impl forall D`, however, they end up referencing and including this BindSymbolicName into its eval block. But the generic binding 0 in the `impl` is a very different thing (a value of type `E`). This confusion leads to crashes.