backend/local: Allow experiments when loading config from snapshot

If a saved plan is being applied using a Terraform CLI/Core build that
allows use of language experiments then that should be allowed when the
configuration is being loaded from the snapshot saved in a plan file, in
the same way as it would've been handled when preparing to generate that
plan file.

The local backend doesn't get told directly whether it should allow
experiments, and instead its operation comes with a pre-configured
configuration loader that may have the experiment flag set internally.

When we load from snapshot we can't use the operation's own config loader
because that loads from the real filesystem, and so we'll copy over the
experiments-allowed flag from the operation's config loader to achieve
consistency while still loading the configuration from the snapshot in
the plan file.

Without this a plan successfully created with experiments would fail with
confusing errors during apply, because the snapshot config loader would
not accept the same experiments that the plan-time config loader did.

internal/backend/local/backend_local.go

@@ -239,6 +239,7 @@ func (b *Local) localRunForPlanFile(op *backendrun.Operation, pf *planfile.Reade
 		return nil, snap, diags
 	}
 	loader := configload.NewLoaderFromSnapshot(snap)
+	loader.AllowLanguageExperiments(op.ConfigLoader.AllowsLanguageExperiments())
 	config, configDiags := loader.LoadConfig(snap.Modules[""].Dir)
 	diags = diags.Append(configDiags)
 	if configDiags.HasErrors() {

internal/configs/configload/loader.go

@@ -164,3 +164,10 @@ func (l *Loader) ImportSourcesFromSnapshot(snap *Snapshot) {
 func (l *Loader) AllowLanguageExperiments(allowed bool) {
 	l.parser.AllowLanguageExperiments(allowed)
 }
+
+// AllowsLanguageExperiments returns the value most recently passed to
+// [Loader.AllowLanguageExperiments], or false if that method has not been
+// called on this object.
+func (l *Loader) AllowsLanguageExperiments() bool {
+	return l.parser.AllowsLanguageExperiments()
+}

internal/configs/parser.go

@@ -121,3 +121,10 @@ func (p *Parser) ForceFileSource(filename string, src []byte) {
 func (p *Parser) AllowLanguageExperiments(allowed bool) {
 	p.allowExperiments = allowed
 }
+
+// AllowsLanguageExperiments returns the value most recently passed to
+// [Parser.AllowLanguageExperiments], or false if that method has not been
+// called on this object.
+func (p *Parser) AllowsLanguageExperiments() bool {
+	return p.allowExperiments
+}