GOEXPERIMENT Catalog

GOEXPERIMENT Catalog

Overview

GOEXPERIMENT enables incomplete or alternate runtime/compiler features. Experiments graduate, change, or vanish — always pair with release notes.

GOEXPERIMENT=name go test ./...
go doc go/token  # not helpful; use:
go tool dist list
# list experiments for your version:
go doc internal/goexperiment   # may be unavailable; see src/internal/goexperiment

From user-facing docs / release notes (illustrative — verify for your version):

Experiment	Theme
greenteagc / nogreenteagc	Green Tea GC default since 1.26; opt-out remains
simd	Experimental portable simd + simd/archsimd (1.26–1.27)
nosizespecializedmalloc	Opt out of 1.27 size-specialized small allocs (expected gone in 1.28)
nojsonv2	Restore pre-1.27 encoding/json engine (temporary)
arenas	Historical arena experiment (do not bet new code without checking status)
loopvar	Historical loop var semantics (graduated)
rangefunc	Historical range-over-func (graduated)
newinliner	Inliner experiments

Diagram: Experiment lifecycle

flow:
  [Test]
       |
       v
  [Grad]

  [Test]
       |
       v
  [Drop]

Rules

1. Never enable random experiments in production without a rollback plan.
2. Test performance and correctness under the experiment and default.
3. Prefer graduated language features over experiments for library APIs.

JSON v2 and Friends

encoding/json/v2 graduated in Go 1.27. The v1 package uses the v2 engine; GOEXPERIMENT=nojsonv2 is an emergency opt-out. goroutineleakprofile was deleted when the leak profile became generally available.

Experiment

go version
GOEXPERIMENT=nogreenteagc go test ./...   # only if valid on your version
# or simply read:
go help build | less

Document in your runbook which experiments you intentionally ship (ideally none).

Try next: Skim the Go 1.27 release notes “Runtime” + “Compiler” sections and map each experiment bullet to a chapter in this part.