internal/aliases: expose Enabled

The predicate that determines whether the type checker
creates types.Alias nodes is complex: it depends on an
environment variable (GODEBUG) that is somewhat tricky
to parse correctly, since it is a rightmost-wins list.

Critically, however, its default value is a function of
the go directive in the application's go.mod file, which
is inaccessible to logic in x/tools (per-file build tags
can detect the toolchain version, but not the go.mod version).
Equally critically, the current effective value of the
gotypesalias variable changes when os.Setenv(GODEBUG) is
called, which happens in tests, and there is no way to
detect those events; therefore any attempt to cache the
value is wrong.

This change exposes a simplified version of the Enabled
function from aliases, which always computes the ground
truth by invoking the type checker, regardless of cost.
It also adds an 'enabled' parameter to NewAlias so that
callers can amortize this cost.

Also, various minor cleanups related to aliases.

Updates golang/go#64581

Change-Id: If926edefb8e1c1f63c17e4fad0a808e27bac6d5b
Reviewed-on: https://go-review.googlesource.com/c/tools/+/580455
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Robert Findley <[email protected]>

Author: adonovan

go.mod

@@ -1,18 +1,14 @@
 module github.com/apstndb/gotoolsdiff
 
-go 1.19
+go 1.19 // => default GODEBUG has gotypesalias=0
 
 require (
+	github.com/google/go-cmp v0.6.0
 	github.com/yuin/goldmark v1.4.13
 	golang.org/x/mod v0.17.0
 	golang.org/x/net v0.24.0
-)
-
-require golang.org/x/sync v0.7.0
-
-require github.com/google/go-cmp v0.6.0 // indirect
-
-require (
-	golang.org/x/sys v0.19.0 // indirect
+	golang.org/x/sync v0.7.0
 	golang.org/x/telemetry v0.0.0-20240228155512-f48c80bd79b2
 )
+
+require golang.org/x/sys v0.19.0 // indirect

internal/testenv/exec.go

@@ -12,7 +12,6 @@ import (
 	"reflect"
 	"runtime"
 	"strconv"
-	"strings"
 	"sync"
 	"testing"
 	"time"
@@ -191,22 +190,3 @@ func Command(t testing.TB, name string, args ...string) *exec.Cmd {
 	t.Helper()
 	return CommandContext(t, context.Background(), name, args...)
 }
-
-// SkipMaterializedAliases skips the test if go/types would create
-// instances of types.Alias, which some tests do not yet handle
-// correctly.
-func SkipMaterializedAliases(t *testing.T, message string) {
-	if hasMaterializedAliases(Go1Point()) {
-		t.Skipf("%s", message)
-	}
-}
-
-func hasMaterializedAliases(minor int) bool {
-	if minor >= 23 && !strings.Contains(os.Getenv("GODEBUG"), "gotypesalias=0") {
-		return true // gotypesalias=1 became the default in go1.23
-	}
-	if minor == 22 && strings.Contains(os.Getenv("GODEBUG"), "gotypesalias=1") {
-		return true // gotypesalias=0 was the default in go1.22
-	}
-	return false // types.Alias didn't exist in go1.21
-}