Enable go 1.27 support

[jprieto-temporal]

Adds support for Go 1.27. Follows the pattern of changes in #50.

[cockroach-teamcity]

This change is 

[RaduBerinde]

Thanks! Did you verify that the aliased structures in the runtime are unchanged (at least the prefix that we're using)?

[RaduBerinde]

Claude checked:

⏺ Verified — and I ran it, not just read it.

Source check (github.com/golang/go @ tag go1.27.0)

internal/abi.Type — field-for-field identical to swiss's rtType: Size_, PtrBytes uintptr; Hash uint32; TFlag, Align_, FieldAlign_, Kind_ uint8; Equal func; GCData
*byte; Str NameOff; PtrToThis TypeOff. (TFlag/NameOff/TypeOff are still uint8/int32/int32; Kind_'s declared type is now abi.Kind, which is uint8 — same size, no
layout effect.)

internal/abi.MapType (src/internal/abi/map.go) — the aliased prefix is unchanged:

type MapType struct {
Type
Key *Type
Elem *Type
Group *Type // swiss calls this Bucket — same position, same type
Hasher func(unsafe.Pointer, uintptr) uintptr
... // GroupSize, KeysOff, KeyStride, ElemsOff, ElemStride, ElemOff, Flags
}

Everything past Hasher was reworked in 1.27 for the mapsplitgroup experiment, but that's all beyond the prefix swiss reads. Note internal/abi/map_swiss.go and the
old OldMapType are gone — there's now just one MapType — so the "prefix of both abi.SwissMapType and abi.OldMapType" comment in swiss is stale.

runtime.eface — still {_type *_type; data unsafe.Pointer}.

runtime.fastrand64 — still exists as a push-linknamed compat shim (src/runtime/rand.go:341, //go:linkname legacy_fastrand64 runtime.fastrand64), so swiss's pull
linkname remains legal under the 1.23+ linkname restrictions.

Empirical check

Pulled the real go1.27.0 toolchain from the module proxy and ran swiss against it with -tags untested_go_version:

• Full suite: ok (plain, and -race -tags invariants).
• A layout test I added (/tmp/claude-503/swiss/runtime_layout_check_test.go) that cross-checks the aliased fields against reflect ground truth for 8 key types:
  rtEface.typ == the *abi.Type reflect reports for the map type; Size_/Align_/FieldAlign_/Kind_ match reflect.Type; Equal == nil, GCData != nil; mt.Key/mt.Elem are
  pointer-identical to typePtr(rt.Key())/typePtr(rt.Elem()) — which pins every offset up to and including Elem; Bucket/Group is a plausible non-nil type; Hasher is
  callable, deterministic, seed-sensitive, and well-distributed. All pass.
• Passes under both GOEXPERIMENT=mapsplitgroup and nomapsplitgroup.

Conclusion

Safe to widen the build tag to (go1.20 && !go1.28) || untested_go_version. This was tested on darwin/arm64 only, but since the struct definitions are textually
identical to swiss's copies, the layouts agree on every architecture. Worth also fixing the abi.SwissMapType/abi.OldMapType comment while you're in there.

[jprieto-temporal]

Hi, thanks for taking a look. Yes, I can confirm that the runtime aliases all still match:

Type: https://github.com/golang/go/blob/8af21751f066eced273ca3ce49506b366847c623/src/internal/abi/type.go#L21
MapType: https://github.com/golang/go/blob/8af21751f066eced273ca3ce49506b366847c623/src/internal/abi/map.go#L32
eface: https://github.com/golang/go/blob/8af21751f066eced273ca3ce49506b366847c623/src/runtime/runtime2.go#L189
fastrand64 unchanged: https://github.com/golang/go/blob/8af21751f066eced273ca3ce49506b366847c623/src/runtime/rand.go#L341

[RaduBerinde]

Could you update these matrices to use only 1.24+? Older macos CI builds fail. Also, add fail-fast so all jobs run even if some fail. Here's a diff:

--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -10,14 +10,12 @@ on:
 jobs:
   test:
     strategy:
+      fail-fast: false
       matrix:
         os:
           - ubuntu-latest
           - macos-latest
         go:
-          - '1.21'
-          - '1.22'
-          - '1.23'
           - '1.24'
           - '1.25'
           - '>=1.26.0-rc.1'
@@ -36,6 +34,7 @@ jobs:
 
   linux-noswissmap:
     strategy:
+      fail-fast: false
       matrix:
         go:
           - '1.24'
@@ -54,6 +53,7 @@ jobs:
   linux-race:
     runs-on: ubuntu-latest
     strategy:
+      fail-fast: false
       matrix:
         go:
           - '1.24'
@@ -73,6 +73,7 @@ jobs:
   linux-32bit:
     runs-on: ubuntu-latest
     strategy:
+      fail-fast: false
       matrix:
         go:
           - '1.24'
@@ -91,6 +92,7 @@ jobs:
   linux-qemu-s390x:
     runs-on: ubuntu-latest
     strategy:
+      fail-fast: false
       matrix:
         go:
           - '1.24.4'

[jprieto-temporal]

Done. Thank you!

[RaduBerinde]

Thank you!