[dev.typeparams] all: merge master (4711bf3) into dev.typeparams

Conflicts:

- src/cmd/compile/internal/walk/builtin.go

  On dev.typeparams, CL 330194 changed OCHECKNIL to not require manual
  SetTypecheck(1) anymore; while on master, CL 331070 got rid of the
  OCHECKNIL altogether by moving the check into the runtime support
  functions.

- src/internal/buildcfg/exp.go

  On master, CL 331109 refactored the logic for parsing the
  GOEXPERIMENT string, so that it could be more easily reused by
  cmd/go; while on dev.typeparams, several CLs tweaked the regabi
  experiment defaults.

Merge List:

+ 2021-06-30 4711bf30e5 doc/go1.17: linkify "language changes" in the runtime section
+ 2021-06-30 ed56ea73e8 path/filepath: deflake TestEvalSymlinksAboveRoot on darwin
+ 2021-06-30 c080d0323b cmd/dist: pass -Wno-unknown-warning-option in swig_callback_lto
+ 2021-06-30 7d0e9e6e74 image/gif: fix typo in the comment (io.ReadByte -> io.ByteReader)
+ 2021-06-30 0fa3265fe1 os: change example to avoid deprecated function
+ 2021-06-30 d19a53338f image: add Uniform.RGBA64At and Rectangle.RGBA64At
+ 2021-06-30 c45e800e0c crypto/x509: don't fail on optional auth key id fields
+ 2021-06-29 f9d50953b9 net: fix failure of TestCVE202133195
+ 2021-06-29 e294b8a49e doc/go1.17: fix typo "MacOS" -> "macOS"
+ 2021-06-29 3463852b76 math/big: fix typo of comment (`BytesScanner` to `ByteScanner`)
+ 2021-06-29 fd4b587da3 cmd/compile: suppress details error for invalid variadic argument type
+ 2021-06-29 e2e05af6e1 cmd/internal/obj/arm64: fix an encoding error of CMPW instruction
+ 2021-06-28 4bb0847b08 cmd/compile,runtime: change unsafe.Slice((*T)(nil), 0) to return []T(nil)
+ 2021-06-28 1519271a93 spec: change unsafe.Slice((*T)(nil), 0) to return []T(nil)
+ 2021-06-28 5385e2386b runtime/internal/atomic: drop Cas64 pointer indirection in comments
+ 2021-06-28 956c81bfe6 cmd/go: add GOEXPERIMENT to `go env` output
+ 2021-06-28 a1d27269d6 cmd/go: prep for 'go env' refactoring
+ 2021-06-28 901510ed4e cmd/link/internal/ld: skip the windows ASLR test when CGO_ENABLED=0
+ 2021-06-28 361159c055 cmd/cgo: fix 'see gmp.go' to 'see doc.go'
+ 2021-06-27 c95464f0ea internal/buildcfg: refactor GOEXPERIMENT parsing code somewhat
+ 2021-06-25 ed01ceaf48 runtime/race: use race build tag on syso_test.go
+ 2021-06-25 d1916e5e84 go/types: in TestCheck/issues.src, import regexp/syntax instead of cmd/compile/internal/syntax
+ 2021-06-25 5160896c69 go/types: in TestStdlib, import from source instead of export data
+ 2021-06-25 d01bc571f7 runtime: make ncgocall a global counter

Change-Id: I1ce4a3b3ff7c824c67ad66dd27d9d5f1d25c0023

45 files changed, 476 insertions, 220 deletions

diff --git a/src/cmd/asm/internal/asm/testdata/arm64.s b/src/cmd/asm/internal/asm/testdata/arm64.s
index 5f1e68545b..d8a20edfc1 100644
--- a/src/cmd/asm/internal/asm/testdata/arm64.s
+++ b/src/cmd/asm/internal/asm/testdata/arm64.s
@@ -89,7 +89,7 @@ TEXT	foo(SB), DUPOK|NOSPLIT, $-8
 	CMP	R1<<33, R2
 	CMP	R22.SXTX, RSP                    // ffe336eb
 	CMP	$0x22220000, RSP                 // CMP $572653568, RSP   // 5b44a4d2ff633beb
-	CMPW	$0x22220000, RSP                 // CMPW $572653568, RSP  // 5b44a452ff633b6b
+	CMPW	$0x22220000, RSP                 // CMPW $572653568, RSP  // 5b44a452ff433b6b
 	CCMN	MI, ZR, R1, $4	                 // e44341ba
 	// MADD Rn,Rm,Ra,Rd
 	MADD	R1, R2, R3, R4                   // 6408019b
@@ -377,6 +377,7 @@ TEXT	foo(SB), DUPOK|NOSPLIT, $-8
 	MOVD	$0x1000100010001000, RSP      // MOVD	$1152939097061330944, RSP   // ff8304b2
 	MOVW	$0x10001000, RSP              // MOVW	$268439552, RSP             // ff830432
 	ADDW	$0x10001000, R1               // ADDW	$268439552, R1              // fb83043221001b0b
+	ADDW	$0x22220000, RSP, R3          // ADDW	$572653568, RSP, R3         // 5b44a452e3433b0b
 
 // move a large constant to a Vd.
 	VMOVS	$0x80402010, V11                                      // VMOVS	$2151686160, V11
diff --git a/src/cmd/cgo/main.go b/src/cmd/cgo/main.go
index 03a662e689..c6a0c525e6 100644
--- a/src/cmd/cgo/main.go
+++ b/src/cmd/cgo/main.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Cgo; see gmp.go for an overview.
+// Cgo; see doc.go for an overview.
 
 // TODO(rsc):
 //	Emit correct line number annotations.
diff --git a/src/cmd/compile/internal/typecheck/builtin.go b/src/cmd/compile/internal/typecheck/builtin.go
index 67a894c7ed..833b17b414 100644
--- a/src/cmd/compile/internal/typecheck/builtin.go
+++ b/src/cmd/compile/internal/typecheck/builtin.go
@@ -138,6 +138,7 @@ var runtimeDecls = [...]struct {
 	{"growslice", funcTag, 116},
 	{"unsafeslice", funcTag, 117},
 	{"unsafeslice64", funcTag, 118},
+	{"unsafeslicecheckptr", funcTag, 118},
 	{"memmove", funcTag, 119},
 	{"memclrNoHeapPointers", funcTag, 120},
 	{"memclrHasPointers", funcTag, 120},
@@ -341,8 +342,8 @@ func runtimeTypes() []*types.Type {
 	typs[114] = newSig(params(typs[1], typs[15], typs[15], typs[7]), params(typs[7]))
 	typs[115] = types.NewSlice(typs[2])
 	typs[116] = newSig(params(typs[1], typs[115], typs[15]), params(typs[115]))
-	typs[117] = newSig(params(typs[1], typs[15]), nil)
-	typs[118] = newSig(params(typs[1], typs[22]), nil)
+	typs[117] = newSig(params(typs[1], typs[7], typs[15]), nil)
+	typs[118] = newSig(params(typs[1], typs[7], typs[22]), nil)
 	typs[119] = newSig(params(typs[3], typs[3], typs[5]), nil)
 	typs[120] = newSig(params(typs[7], typs[5]), nil)
 	typs[121] = newSig(params(typs[3], typs[3], typs[5]), params(typs[6]))
diff --git a/src/cmd/compile/internal/typecheck/builtin/runtime.go b/src/cmd/compile/internal/typecheck/builtin/runtime.go
index ebeaeae79e..2b29ea3c08 100644
--- a/src/cmd/compile/internal/typecheck/builtin/runtime.go
+++ b/src/cmd/compile/internal/typecheck/builtin/runtime.go
@@ -183,8 +183,9 @@ func makeslice(typ *byte, len int, cap int) unsafe.Pointer
 func makeslice64(typ *byte, len int64, cap int64) unsafe.Pointer
 func makeslicecopy(typ *byte, tolen int, fromlen int, from unsafe.Pointer) unsafe.Pointer
 func growslice(typ *byte, old []any, cap int) (ary []any)
-func unsafeslice(typ *byte, len int)
-func unsafeslice64(typ *byte, len int64)
+func unsafeslice(typ *byte, ptr unsafe.Pointer, len int)
+func unsafeslice64(typ *byte, ptr unsafe.Pointer, len int64)
+func unsafeslicecheckptr(typ *byte, ptr unsafe.Pointer, len int64)
 
 func memmove(to *any, frm *any, length uintptr)
 func memclrNoHeapPointers(ptr unsafe.Pointer, n uintptr)
diff --git a/src/cmd/compile/internal/typecheck/func.go b/src/cmd/compile/internal/typecheck/func.go
index 5f9f8c6ebd..d0aad5ac07 100644
--- a/src/cmd/compile/internal/typecheck/func.go
+++ b/src/cmd/compile/internal/typecheck/func.go
@@ -961,7 +961,14 @@ func tcUnsafeSlice(n *ir.BinaryExpr) *ir.BinaryExpr {
 	t := n.X.Type()
 	if !t.IsPtr() {
 		base.Errorf("first argument to unsafe.Slice must be pointer; have %L", t)
+	} else if t.Elem().NotInHeap() {
+		// TODO(mdempsky): This can be relaxed, but should only affect the
+		// Go runtime itself. End users should only see //go:notinheap
+		// types due to incomplete C structs in cgo, and those types don't
+		// have a meaningful size anyway.
+		base.Errorf("unsafe.Slice of incomplete (or unallocatable) type not allowed")
 	}
+
 	if !checkunsafeslice(&n.Y) {
 		n.SetType(nil)
 		return n
diff --git a/src/cmd/compile/internal/typecheck/typecheck.go b/src/cmd/compile/internal/typecheck/typecheck.go
index 232c0e66ef..21d3100f66 100644
--- a/src/cmd/compile/internal/typecheck/typecheck.go
+++ b/src/cmd/compile/internal/typecheck/typecheck.go
@@ -1471,15 +1471,22 @@ toomany:
 }
 
 func errorDetails(nl ir.Nodes, tstruct *types.Type, isddd bool) string {
-	// If we don't know any type at a call site, let's suppress any return
-	// message signatures. See Issue https://golang.org/issues/19012.
+	// Suppress any return message signatures if:
+	//
+	// (1) We don't know any type at a call site (see #19012).
+	// (2) Any node has an unknown type.
+	// (3) Invalid type for variadic parameter (see #46957).
 	if tstruct == nil {
-		return ""
+		return "" // case 1
 	}
-	// If any node has an unknown type, suppress it as well
+
+	if isddd && !nl[len(nl)-1].Type().IsSlice() {
+		return "" // case 3
+	}
+
 	for _, n := range nl {
 		if n.Type() == nil {
-			return ""
+			return "" // case 2
 		}
 	}
 	return fmt.Sprintf("\n\thave %s\n\twant %v", fmtSignature(nl, isddd), tstruct)
diff --git a/src/cmd/compile/internal/walk/builtin.go b/src/cmd/compile/internal/walk/builtin.go
index 135eaee6bc..e1a2319e14 100644
--- a/src/cmd/compile/internal/walk/builtin.go
+++ b/src/cmd/compile/internal/walk/builtin.go
@@ -647,35 +647,28 @@ func walkRecoverFP(nn *ir.CallExpr, init *ir.Nodes) ir.Node {
 }
 
 func walkUnsafeSlice(n *ir.BinaryExpr, init *ir.Nodes) ir.Node {
+	ptr := safeExpr(n.X, init)
 	len := safeExpr(n.Y, init)
 
 	fnname := "unsafeslice64"
-	argtype := types.Types[types.TINT64]
+	lenType := types.Types[types.TINT64]
 
 	// Type checking guarantees that TIDEAL len/cap are positive and fit in an int.
 	// The case of len or cap overflow when converting TUINT or TUINTPTR to TINT
 	// will be handled by the negative range checks in unsafeslice during runtime.
-	if len.Type().IsKind(types.TIDEAL) || len.Type().Size() <= types.Types[types.TUINT].Size() {
+	if ir.ShouldCheckPtr(ir.CurFunc, 1) {
+		fnname = "unsafeslicecheckptr"
+		// for simplicity, unsafeslicecheckptr always uses int64
+	} else if len.Type().IsKind(types.TIDEAL) || len.Type().Size() <= types.Types[types.TUINT].Size() {
 		fnname = "unsafeslice"
-		argtype = types.Types[types.TINT]
+		lenType = types.Types[types.TINT]
 	}
 
 	t := n.Type()
 
-	// Call runtime.unsafeslice[64] to check that the length argument is
-	// non-negative and smaller than the max length allowed for the
-	// element type.
+	// Call runtime.unsafeslice{,64,checkptr} to check ptr and len.
 	fn := typecheck.LookupRuntime(fnname)
-	init.Append(mkcall1(fn, nil, init, reflectdata.TypePtr(t.Elem()), typecheck.Conv(len, argtype)))
-
-	ptr := walkExpr(n.X, init)
-
-	check := ir.NewUnaryExpr(n.Pos(), ir.OCHECKNIL, ptr)
-	init.Append(typecheck.Stmt(check))
-
-	// TODO(mdempsky): checkptr instrumentation. Maybe merge into length
-	// check above, along with nil check? Need to be careful about
-	// notinheap pointers though: can't pass them as unsafe.Pointer.
+	init.Append(mkcall1(fn, nil, init, reflectdata.TypePtr(t.Elem()), typecheck.Conv(ptr, types.Types[types.TUNSAFEPTR]), typecheck.Conv(len, lenType)))
 
 	h := ir.NewSliceHeaderExpr(n.Pos(), t,
 		typecheck.Conv(ptr, types.Types[types.TUNSAFEPTR]),
diff --git a/src/cmd/dist/test.go b/src/cmd/dist/test.go
index f2c4cf0b43..4acd357974 100644
--- a/src/cmd/dist/test.go
+++ b/src/cmd/dist/test.go
@@ -737,9 +737,9 @@ func (t *tester) registerTests() {
 						fn: func(dt *distTest) error {
 							cmd := t.addCmd(dt, "misc/swig/callback", t.goTest())
 							cmd.Env = append(os.Environ(),
-								"CGO_CFLAGS=-flto -Wno-lto-type-mismatch",
-								"CGO_CXXFLAGS=-flto -Wno-lto-type-mismatch",
-								"CGO_LDFLAGS=-flto -Wno-lto-type-mismatch",
+								"CGO_CFLAGS=-flto -Wno-lto-type-mismatch -Wno-unknown-warning-option",
+								"CGO_CXXFLAGS=-flto -Wno-lto-type-mismatch -Wno-unknown-warning-option",
+								"CGO_LDFLAGS=-flto -Wno-lto-type-mismatch -Wno-unknown-warning-option",
 							)
 							return nil
 						},
diff --git a/src/cmd/go/alldocs.go b/src/cmd/go/alldocs.go
index fd95da23eb..90eb3e2a00 100644
--- a/src/cmd/go/alldocs.go
+++ b/src/cmd/go/alldocs.go
@@ -1903,6 +1903,12 @@
 // 	GCCGOTOOLDIR
 // 		If set, where to find gccgo tools, such as cgo.
 // 		The default is based on how gccgo was configured.
+// 	GOEXPERIMENT
+// 		Comma-separated list of toolchain experiments to enable or disable.
+// 		The list of available experiments may change arbitrarily over time.
+// 		See src/internal/goexperiment/flags.go for currently valid values.
+// 		Warning: This variable is provided for the development and testing
+// 		of the Go toolchain itself. Use beyond that purpose is unsupported.
 // 	GOROOT_FINAL
 // 		The root of the installed Go tree, when it is
 // 		installed in a location other than where it is built.
diff --git a/src/cmd/go/internal/cfg/cfg.go b/src/cmd/go/internal/cfg/cfg.go
index fc6989097e..57a3c1ff6f 100644
--- a/src/cmd/go/internal/cfg/cfg.go
+++ b/src/cmd/go/internal/cfg/cfg.go
@@ -79,7 +79,7 @@ func defaultContext() build.Context {
 
 	// The experiments flags are based on GOARCH, so they may
 	// need to change.  TODO: This should be cleaned up.
-	buildcfg.UpdateExperiments(ctxt.GOARCH)
+	buildcfg.UpdateExperiments(ctxt.GOOS, ctxt.GOARCH, envOr("GOEXPERIMENT", buildcfg.DefaultGOEXPERIMENT))
 	ctxt.ToolTags = nil
 	for _, exp := range buildcfg.EnabledExperiments() {
 		ctxt.ToolTags = append(ctxt.ToolTags, "goexperiment."+exp)
diff --git a/src/cmd/go/internal/envcmd/env.go b/src/cmd/go/internal/envcmd/env.go
index b30c37ab27..1553d26391 100644
--- a/src/cmd/go/internal/envcmd/env.go
+++ b/src/cmd/go/internal/envcmd/env.go
@@ -10,6 +10,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"go/build"
+	"internal/buildcfg"
 	"io"
 	"os"
 	"path/filepath"
@@ -72,6 +73,7 @@ func MkEnv() []cfg.EnvVar {
 		{Name: "GOCACHE", Value: cache.DefaultDir()},
 		{Name: "GOENV", Value: envFile},
 		{Name: "GOEXE", Value: cfg.ExeSuffix},
+		{Name: "GOEXPERIMENT", Value: buildcfg.GOEXPERIMENT()},
 		{Name: "GOFLAGS", Value: cfg.Getenv("GOFLAGS")},
 		{Name: "GOHOSTARCH", Value: runtime.GOARCH},
 		{Name: "GOHOSTOS", Value: runtime.GOOS},
@@ -197,6 +199,21 @@ func runEnv(ctx context.Context, cmd *base.Command, args []string) {
 	if *envU && *envW {
 		base.Fatalf("go env: cannot use -u with -w")
 	}
+
+	// Handle 'go env -w' and 'go env -u' before calling buildcfg.Check,
+	// so they can be used to recover from an invalid configuration.
+	if *envW {
+		runEnvW(args)
+		return
+	}
+
+	if *envU {
+		runEnvU(args)
+		return
+	}
+
+	buildcfg.Check()
+
 	env := cfg.CmdEnv
 	env = append(env, ExtraEnvVars()...)
 
@@ -206,14 +223,7 @@ func runEnv(ctx context.Context, cmd *base.Command, args []string) {
 
 	// Do we need to call ExtraEnvVarsCostly, which is a bit expensive?
 	needCostly := false
-	if *envU || *envW {
-		// We're overwriting or removing default settings,
-		// so it doesn't really matter what the existing settings are.
-		//
-		// Moreover, we haven't validated the new settings yet, so it is
-		// important that we NOT perform any actions based on them,
-		// such as initializing the builder to compute other variables.
-	} else if len(args) == 0 {
+	if len(args) == 0 {
 		// We're listing all environment variables ("go env"),
 		// including the expensive ones.
 		needCostly = true
@@ -238,95 +248,6 @@ func runEnv(ctx context.Context, cmd *base.Command, args []string) {
 		env = append(env, ExtraEnvVarsCostly()...)
 	}
 
-	if *envW {
-		// Process and sanity-check command line.
-		if len(args) == 0 {
-			base.Fatalf("go env -w: no KEY=VALUE arguments given")
-		}
-		osEnv := make(map[string]string)
-		for _, e := range cfg.OrigEnv {
-			if i := strings.Index(e, "="); i >= 0 {
-				osEnv[e[:i]] = e[i+1:]
-			}
-		}
-		add := make(map[string]string)
-		for _, arg := range args {
-			i := strings.Index(arg, "=")
-			if i < 0 {
-				base.Fatalf("go env -w: arguments must be KEY=VALUE: invalid argument: %s", arg)
-			}
-			key, val := arg[:i], arg[i+1:]
-			if err := checkEnvWrite(key, val); err != nil {
-				base.Fatalf("go env -w: %v", err)
-			}
-			if _, ok := add[key]; ok {
-				base.Fatalf("go env -w: multiple values for key: %s", key)
-			}
-			add[key] = val
-			if osVal := osEnv[key]; osVal != "" && osVal != val {
-				fmt.Fprintf(os.Stderr, "warning: go env -w %s=... does not override conflicting OS environment variable\n", key)
-			}
-		}
-
-		goos, okGOOS := add["GOOS"]
-		goarch, okGOARCH := add["GOARCH"]
-		if okGOOS || okGOARCH {
-			if !okGOOS {
-				goos = cfg.Goos
-			}
-			if !okGOARCH {
-				goarch = cfg.Goarch
-			}
-			if err := work.CheckGOOSARCHPair(goos, goarch); err != nil {
-				base.Fatalf("go env -w: %v", err)
-			}
-		}
-
-		gotmp, okGOTMP := add["GOTMPDIR"]
-		if okGOTMP {
-			if !filepath.IsAbs(gotmp) && gotmp != "" {
-				base.Fatalf("go env -w: GOTMPDIR must be an absolute path")
-			}
-		}
-
-		updateEnvFile(add, nil)
-		return
-	}
-
-	if *envU {
-		// Process and sanity-check command line.
-		if len(args) == 0 {
-			base.Fatalf("go env -u: no arguments given")
-		}
-		del := make(map[string]bool)
-		for _, arg := range args {
-			if err := checkEnvWrite(arg, ""); err != nil {
-				base.Fatalf("go env -u: %v", err)
-			}
-			del[arg] = true
-		}
-		if del["GOOS"] || del["GOARCH"] {
-			goos, goarch := cfg.Goos, cfg.Goarch
-			if del["GOOS"] {
-				goos = getOrigEnv("GOOS")
-				if goos == "" {
-					goos = build.Default.GOOS
-				}
-			}
-			if del["GOARCH"] {
-				goarch = getOrigEnv("GOARCH")
-				if goarch == "" {
-					goarch = build.Default.GOARCH
-				}
-			}
-			if err := work.CheckGOOSARCHPair(goos, goarch); err != nil {
-				base.Fatalf("go env -u: %v", err)
-			}
-		}
-		updateEnvFile(nil, del)
-		return
-	}
-
 	if len(args) > 0 {
 		if *envJson {
 			var es []cfg.EnvVar
@@ -351,6 +272,109 @@ func runEnv(ctx context.Context, cmd *base.Command, args []string) {
 	PrintEnv(os.Stdout, env)
 }
 
+func runEnvW(args []string) {
+	// Process and sanity-check command line.
+	if len(args) == 0 {
+		base.Fatalf("go env -w: no KEY=VALUE arguments given")
+	}
+	osEnv := make(map[string]string)
+	for _, e := range cfg.OrigEnv {
+		if i := strings.Index(e, "="); i >= 0 {
+			osEnv[e[:i]] = e[i+1:]
+		}
+	}
+	add := make(map[string]string)
+	for _, arg := range args {
+		i := strings.Index(arg, "=")
+		if i < 0 {
+			base.Fatalf("go env -w: arguments must be KEY=VALUE: invalid argument: %s", arg)
+		}
+		key, val := arg[:i], arg[i+1:]
+		if err := checkEnvWrite(key, val); err != nil {
+			base.Fatalf("go env -w: %v", err)
+		}
+		if _, ok := add[key]; ok {
+			base.Fatalf("go env -w: multiple values for key: %s", key)
+		}
+		add[key] = val
+		if osVal := osEnv[key]; osVal != "" && osVal != val {
+			fmt.Fprintf(os.Stderr, "warning: go env -w %s=... does not override conflicting OS environment variable\n", key)
+		}
+	}
+
+	if err := checkBuildConfig(add, nil); err != nil {
+		base.Fatalf("go env -w: %v", err)
+	}
+
+	gotmp, okGOTMP := add["GOTMPDIR"]
+	if okGOTMP {
+		if !filepath.IsAbs(gotmp) && gotmp != "" {
+			base.Fatalf("go env -w: GOTMPDIR must be an absolute path")
+		}
+	}
+
+	updateEnvFile(add, nil)
+}
+
+func runEnvU(args []string) {
+	// Process and sanity-check command line.
+	if len(args) == 0 {
+		base.Fatalf("go env -u: no arguments given")
+	}
+	del := make(map[string]bool)
+	for _, arg := range args {
+		if err := checkEnvWrite(arg, ""); err != nil {
+			base.Fatalf("go env -u: %v", err)
+		}
+		del[arg] = true
+	}
+
+	if err := checkBuildConfig(nil, del); err != nil {
+		base.Fatalf("go env -u: %v", err)
+	}
+
+	updateEnvFile(nil, del)
+}
+
+// checkBuildConfig checks whether the build configuration is valid
+// after the specified configuration environment changes are applied.
+func checkBuildConfig(add map[string]string, del map[string]bool) error {
+	// get returns the value for key after applying add and del and
+	// reports whether it changed. cur should be the current value
+	// (i.e., before applying changes) and def should be the default
+	// value (i.e., when no environment variables are provided at all).
+	get := func(key, cur, def string) (string, bool) {
+		if val, ok := add[key]; ok {
+			return val, true
+		}
+		if del[key] {
+			val := getOrigEnv(key)
+			if val == "" {
+				val = def
+			}
+			return val, true
+		}
+		return cur, false
+	}
+
+	goos, okGOOS := get("GOOS", cfg.Goos, build.Default.GOOS)
+	goarch, okGOARCH := get("GOARCH", cfg.Goarch, build.Default.GOARCH)
+	if okGOOS || okGOARCH {
+		if err := work.CheckGOOSARCHPair(goos, goarch); err != nil {
+			return err
+		}
+	}
+
+	goexperiment, okGOEXPERIMENT := get("GOEXPERIMENT", buildcfg.GOEXPERIMENT(), "")
+	if okGOEXPERIMENT {
+		if _, _, err := buildcfg.ParseGOEXPERIMENT(goos, goarch, goexperiment); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
 // PrintEnv prints the environment variables to w.
 func PrintEnv(w io.Writer, env []cfg.EnvVar) {
 	for _, e := range env {
diff --git a/src/cmd/go/internal/help/helpdoc.go b/src/cmd/go/internal/help/helpdoc.go
index b552777e3e..490ff1fb7c 100644
--- a/src/cmd/go/internal/help/helpdoc.go
+++ b/src/cmd/go/internal/help/helpdoc.go
@@ -610,6 +610,12 @@ Special-purpose environment variables:
 	GCCGOTOOLDIR
 		If set, where to find gccgo tools, such as cgo.
 		The default is based on how gccgo was configured.
+	GOEXPERIMENT
+		Comma-separated list of toolchain experiments to enable or disable.
+		The list of available experiments may change arbitrarily over time.
+		See src/internal/goexperiment/flags.go for currently valid values.
+		Warning: This variable is provided for the development and testing
+		of the Go toolchain itself. Use beyond that purpose is unsupported.
 	GOROOT_FINAL
 		The root of the installed Go tree, when it is
 		installed in a location other than where it is built.
diff --git a/src/cmd/go/main.go b/src/cmd/go/main.go
index 02174a56ff..16361e02ca 100644
--- a/src/cmd/go/main.go
+++ b/src/cmd/go/main.go
@@ -145,24 +145,6 @@ func main() {
 		os.Exit(2)
 	}
 
-	if err := buildcfg.Error; err != nil {
-		fmt.Fprintf(os.Stderr, "go: %v\n", buildcfg.Error)
-		os.Exit(2)
-	}
-
-	// Set environment (GOOS, GOARCH, etc) explicitly.
-	// In theory all the commands we invoke should have
-	// the same default computation of these as we do,
-	// but in practice there might be skew
-	// This makes sure we all agree.
-	cfg.OrigEnv = os.Environ()
-	cfg.CmdEnv = envcmd.MkEnv()
-	for _, env := range cfg.CmdEnv {
-		if os.Getenv(env.Name) != env.Value {
-			os.Setenv(env.Name, env.Value)
-		}
-	}
-
 BigCmdLoop:
 	for bigCmd := base.Go; ; {
 		for _, cmd := range bigCmd.Comm