refactor: compare foreign function declarations by resolved type - #1936
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Regression test: `test_reorganize_foreign_fn_type_identity`
(`tests/snapshots/reorganize_foreign_fn_type_identity.rs`). Each translation
unit has its own `buf`, and the two differ in field visibility, so the
transform correctly keeps them apart and renames the second to `buf_1`. Both
units then declare `fill(b: *mut buf)` — spelled identically, but naming a
different type in each unit.
`find_foreign_item` compares the two with `compatible_fn_prototypes`, which
resolves the parameter types through `opt_node_type`. A foreign item has no
body and so no typeck results to resolve against, and the comparison falls
through to `unnamed_equiv`, a purely syntactic check that sees `*mut buf`
against `*mut buf` and reports a match. The second declaration is dropped
and both call sites are pointed at the first, so the caller written against
`buf_1` now passes it to a function taking `*mut buf`:
error[E0308]: mismatched types
This is the same defect that makes the output of `test_reorganize_definitions`
fail to compile, reduced to the two types and one function that cause it.
Comparing foreign functions by resolved type routed the comparison through `structural_eq_tys_impl`, whose array arm lets a zero-length array match an array of any length. That leniency is there for an `extern` array declaration, which C lets omit the length that the definition gives, and which we translate to a zero length; it does not hold inside a function signature. `*mut [c_int; 0]` and `*mut [c_int; 4]` are distinct Rust types with no coercion between them, so two declarations that differ only there are not interchangeable at a call site. `test_reorganize_foreign_fn_rename` regressed on exactly that: its two `compute` declarations were collapsed into one and `b_call` was rewritten to call a function it cannot pass its argument to, an `E0308` in the output. The syntactic path this replaced happened to reject the pair because it compared the written-out lengths. `TypeCompare` now carries an `exact_array_lens` flag, off by default so that the `extern` array declaration it exists for keeps matching its definition, and on for `compatible_fn_sigs` and `compatible_fn_prototypes`. An array length reached through a signature, including one nested in a pointed-to struct, has to match exactly. No snapshot changed: the `statfs64` unification and the `statvfs` separation from the previous commit both still hold.
`find_foreign_item` decided whether two foreign functions were the same by handing their `FnDecl`s to `compatible_fn_prototypes`. That resolves each parameter through `opt_node_type`, but a foreign item has no body and so no typeck results, so every lookup came back empty and the comparison fell through to `unnamed_equiv` — a syntactic check with no notion of what a name resolves to. Two declarations from different modules whose parameters were merely spelled alike were reported identical, one was dropped, and its callers were rewritten to a function taking a different type. `compatible_foreign_fns` now asks `tcx` for each declaration's signature and compares those with `compatible_fn_sigs`, which resolves the types properly. `match_exports` already compared foreign functions against external ones this way; this brings the crate-local comparison in line with it. The syntactic path is kept as a fallback for a signature that will not resolve, which is also why the arity check added earlier still matters. Structurally identical types from different modules still compare equal, so this only separates declarations that genuinely differ: the `statfs64` declarations in `test_reorganize_definitions` are still unified with libc's, while its two `statvfs` declarations are now kept apart, and its output loses the two `E0308` errors. The remaining `E0603` there is a separate defect, fixed next.
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