purego: support structs on Windows amd64/arm64

[hajimehoshi]

What issue is this addressing?

Closes #237

What type of issue is this addressing?

feature

What this PR does | solves

Adds support for passing and returning C structs by value on Windows
for forward calls (Go calling C), on both amd64 and arm64.
Previously RegisterFunc/RegisterLibFunc panicked with
struct arguments/returns are only supported on darwin and linux on
Windows, so callers had to manually decompose structs into uintptrs
or pass pointers.

amd64 (Win64 ABI)

Windows amd64 goes through the standard library syscall.SyscallN,
which only carries positional uintptr arguments. This is sufficient
because the Win64 struct ABI never uses XMM registers for struct
fields:

• Arguments: aggregates of exactly 1, 2, 4, or 8 bytes are passed
  by value in a single integer slot; all other sizes are passed as a
  pointer to a caller-allocated copy.
• Returns: 1/2/4/8-byte aggregates come back in RAX; every other
  size is returned through a caller-allocated hidden pointer (which the
  callee also returns in RAX).

This is implemented as a small Win64-specific branch in
struct_amd64.go plus an amd64StructReturnInMemory helper, since the
existing System V eightbyte-classification path cannot be reused.

arm64 (AAPCS64)

Windows arm64 already runs through purego's own assembly
(sys_arm64.s includes windows and loads X0–X7/F0–F7/R8/stack),
and the AAPCS64 placement logic in struct_arm64.go has no GOOS
constraint. Windows arm64 follows AAPCS64 for structs in non-variadic
functions, so only the support gate needed to be opened — no new ABI
code.

Not supported: structs in callbacks

Passing or returning structs in callbacks created with NewCallback
remains unsupported on Windows, because Windows callbacks go through
the standard library syscall.NewCallback, which cannot carry struct
values. This is documented in the RegisterFunc godoc and as a README
support note.

Testing

• struct_test.go now builds on Windows (amd64/arm64); the callback
  sub-implementation is skipped there, exercising only forward calls.
• The CI matrix already runs go test ./... on windows-latest
  (amd64, with gcc), so the forward-call struct tests are exercised
  there automatically. Windows arm64 is compile-checked only (no arm64
  Windows runner).
• Verified cross-compilation for windows amd64/arm64/386 and all
  existing targets, and that the existing struct tests still pass on
  darwin/arm64.

Notes for reviewers

• Win64 requires by-pointer struct copies to be 16-byte aligned;
  reflect.New aligns to the type (often 8). This is fine for
  scalar-field structs in practice but is a candidate follow-up if a
  vector-type case ever appears.
• The README support-note footnotes were renumbered into reading order
  to accommodate the two new struct notes.
• iOS and Android are documented as not supporting structs (their
  runtime.GOOS is ios/android, not darwin/linux); enabling
  them is left as a separate change.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds Windows support for passing/returning C structs by value for forward calls (Go → C) on amd64 and arm64, and updates tests/docs accordingly.

Changes:

• Enable struct argument/return support gating for Windows in RegisterFunc and add Win64-specific struct ABI handling on amd64.
• Update struct tests to run on Windows and use a cross-platform library loader helper.
• Update README/platform notes and public docs to reflect Windows struct support limitations (no struct callbacks on Windows).

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
func.go	Extends struct support gating to Windows; adds amd64StructReturnInMemory helper and adjusts empty-struct arg handling on Windows.
struct_amd64.go	Implements Win64-specific struct argument passing and struct return reconstruction logic.
struct_test.go	Enables Windows builds for struct tests; switches to internal/load and skips callback-based struct testing on Windows.
testdata/structtest/struct_test.c	Normalizes integer/pointer types for consistent ABI sizing across platforms (incl. Windows).
README.md	Updates support matrix/footnotes to document Windows struct support for forward calls and callback limitation.

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[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

[hajimehoshi]

@TotallyGamerJet PTAL

[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

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

[gdams]

FYI if you add windows-11-arm to the GitHub actions testing matrix you can test on native Windows arm64

[hajimehoshi]

FYI if you add windows-11-arm to the GitHub actions testing matrix you can test on native Windows arm64

Thanks, done

[hajimehoshi]

Unfortunately, the msys tools on GitHub Windows Arm are not for Arm64 but for x86_64, then they don't work. I'll make a following PR later after this PR. I'll revert adding Windows Arm from this PR.