internal/goexperiment,cmd: consolidate GOEXPERIMENTs into a new package

Currently there's knowledge about the list of GOEXPERIMENTs in a few
different places. This CL introduces a new package and consolidates
the list into one place: the internal/goexperiment.Flags struct type.

This package gives us a central place to document the experiments as
well as the GOEXPERIMENT environment variable itself. It will also
give us a place to put built-time constants derived from the enabled
experiments.

Now the objabi package constructs experiment names by reflecting over
this struct type rather than having a separate list of these names
(this is similar to how the compiler handles command-line flags and
debug options). We also expose a better-typed API to the toolchain for
propagating enabled experiments.

Change-Id: I06e026712b59fe2bd7cd11a869aedb48ffe5a4b7
Reviewed-on: https://go-review.googlesource.com/c/go/+/307817
Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

1 files changed, 5 insertions, 18 deletions

diff --git a/src/cmd/asm/internal/lex/input.go b/src/cmd/asm/internal/lex/input.go
index 8aa6becf55..aa03759c7d 100644
--- a/src/cmd/asm/internal/lex/input.go
+++ b/src/cmd/asm/internal/lex/input.go
@@ -46,31 +46,18 @@ func NewInput(name string) *Input {
 func predefine(defines flags.MultiFlag) map[string]*Macro {
 	macros := make(map[string]*Macro)
 
-	// Set macros for various GOEXPERIMENTs so we can easily
-	// switch runtime assembly code based on them.
+	// Set macros for GOEXPERIMENTs so we can easily switch
+	// runtime assembly code based on them.
 	if *flags.CompilingRuntime {
-		set := func(name string) {
+		for _, exp := range objabi.EnabledExperiments() {
+			// Define macro.
+			name := "GOEXPERIMENT_" + exp
 			macros[name] = &Macro{
 				name:   name,
 				args:   nil,
 				tokens: Tokenize("1"),
 			}
 		}
-		if objabi.Experiment.RegabiWrappers {
-			set("GOEXPERIMENT_regabiwrappers")
-		}
-		if objabi.Experiment.RegabiG {
-			set("GOEXPERIMENT_regabig")
-		}
-		if objabi.Experiment.RegabiReflect {
-			set("GOEXPERIMENT_regabireflect")
-		}
-		if objabi.Experiment.RegabiDefer {
-			set("GOEXPERIMENT_regabidefer")
-		}
-		if objabi.Experiment.RegabiArgs {
-			set("GOEXPERIMENT_regabiargs")
-		}
 	}
 
 	for _, name := range defines {