feat: add --opaque-type-alias-as-c-void option

[dhmztr]

Summary

Fixes #1874.

Adds an option to emit opaque type aliases as c_void instead of a sized byte blob, e.g.:

// before
pub type WINDOW = [u64; 11usize];

// after, with --opaque-type-alias-as-c-void
pub type WINDOW = ::core::ffi::c_void;

This is useful for types that are only ever handled behind a pointer (e.g. WINDOW in curses), where communicating "this can't be read from or written to directly" is more valuable than preserving the type's size and alignment through a same-sized blob.

Implementation

• New bool option opaque_type_alias_as_c_void, following the exact same shape as the existing impl_debug option (bindgen/options/mod.rs), including CLI wiring in bindgen/options/cli.rs (--opaque-type-alias-as-c-void).
• Wired into the opaque type-alias codegen branch in bindgen/codegen/mod.rs (Type::codegen's TypeKind::Alias/TemplateAlias handling): when the option is set and the alias target is opaque, emits helpers::ast_ty::c_void(ctx) instead of the usual self.to_opaque(ctx, item) blob.
• Reuses the pre-existing helpers::ast_ty::c_void helper (already used elsewhere for TypeKind::Void/TypeKind::NullPtr), so --ctypes-prefix and --use-core are automatically respected with no extra code.

Scope

This only affects the top-level opaque type-alias path (typedef struct Foo Foo;-style declarations), matching the issue's own example exactly. It does not affect:

• Opaque struct/union declared without a typedef (a separate codegen path in CompInfo::codegen).
• Opaque struct fields (which go through the unconditional ToOpaque blanket impl used for padding/layout elsewhere, and must stay sized for the containing type's layout to be correct).

Test plan

• New regression test issue-1874-opaque-type-alias-as-c-void.h/.rs, using a WINDOW-style opaque typedef (the issue's own motivating example).
• Manually confirmed via the built CLI: flag off → unchanged pub type WINDOW = u8; blob output (no regression); flag on → pub type WINDOW = ::std::os::raw::c_void;; flag on + --use-core → pub type WINDOW = ::core::ffi::c_void;.
• Confirmed the generated output actually compiles (rustc --crate-type lib).
• Full bindgen-tests corpus passes aside from 2 pre-existing, unrelated failures also present on unmodified main (a toolchain-version-dependent const-generics codegen difference, and a dump_preprocessed_input test needing the clang CLI binary rather than just libclang).
• cargo clippy -p bindgen -p bindgen-cli --all-targets -- -D warnings clean.

[Copilot]

Pull request overview

Adds a new opt-in bindgen configuration/CLI flag to render opaque type aliases (typedef-style opaques) as c_void rather than a sized opaque byte-blob, aligning with the use case where the type is only intended to be handled behind pointers.

Changes:

• Introduces new Builder/options boolean: opaque_type_alias_as_c_void and wires it to CLI as --opaque-type-alias-as-c-void.
• Updates alias codegen so that when an alias is considered opaque and the option is enabled, it emits helpers::ast_ty::c_void(ctx) instead of to_opaque(...).
• Adds a regression test case for issue #1874 ensuring the alias becomes c_void under the new flag.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
bindgen/options/mod.rs	Adds a new boolean option and builder method for opaque_type_alias_as_c_void.
bindgen/options/cli.rs	Wires the new option into the CLI and applies it onto the Builder.
bindgen/codegen/mod.rs	Switches opaque type-alias emission to c_void when the option is enabled.
bindgen-tests/tests/headers/issue-1874-opaque-type-alias-as-c-void.h	Adds a header-based regression test using the new flag.
bindgen-tests/tests/expectations/tests/issue-1874-opaque-type-alias-as-c-void.rs	Adds expected generated output validating WINDOW aliases to c_void.

---

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.