Introduce hooks package and register first completion hook

internal/decoder/decoder.go

@@ -75,9 +75,11 @@ func varsPathContext(mod *state.Module) (*decoder.PathContext, error) {
 
 func DecoderContext(ctx context.Context) decoder.DecoderContext {
 	dCtx := decoder.DecoderContext{
-		UtmSource:     utm.UtmSource,
-		UtmMedium:     utm.UtmMedium(ctx),
-		UseUtmContent: true,
+		UtmSource:              utm.UtmSource,
+		UtmMedium:              utm.UtmMedium(ctx),
+		UseUtmContent:          true,
+		CompletionHooks:        make(decoder.CompletionFuncMap),
+		CompletionResolveHooks: make(decoder.CompletionResolveFuncMap),
 	}
 
 	cc, err := ilsp.ClientCapabilities(ctx)

internal/hooks/hooks.go

@@ -0,0 +1,21 @@
+// Package hooks enables the implementation of hooks for dynamic
+// autocompletion. Hooks should be added to this package and
+// registered in AppendCompletionHooks in completion_hooks.go.
+//
+// A hook must have the following signature:
+//  func (h *Hooks) Name(ctx context.Context, value cty.Value) ([]decoder.Candidate, error)
+// It receives the current value of the attribute and must return
+// a list of completion candidates.
+//
+// All hooks have access to path, filename and pos via context:
+//
+// 	path, ok := decoder.PathFromContext(ctx)
+// 	filename, ok := decoder.FilenameFromContext(ctx)
+// 	pos, ok := decoder.PosFromContext(ctx)
+package hooks
+
+import "github.com/hashicorp/terraform-ls/internal/state"
+
+type Hooks struct {
+	ModStore *state.ModuleStore
+}

internal/hooks/module_source_local.go

@@ -0,0 +1,18 @@
+package hooks
+
+import (
+	"context"
+
+	"github.com/hashicorp/hcl-lang/decoder"
+	"github.com/zclconf/go-cty/cty"
+)
+
+func (h *Hooks) LocalModuleSources(ctx context.Context, value cty.Value) ([]decoder.Candidate, error) {
+	candidates := make([]decoder.Candidate, 0)
+
+	// Obtain indexed modules via h.modStore.List()
+	// TODO filter modules inside .terraform
+	// TODO build candidates
+
+	return candidates, nil
+}

internal/langserver/handlers/completion_hooks.go

@@ -0,0 +1,15 @@
+package handlers
+
+import (
+	"github.com/hashicorp/hcl-lang/decoder"
+	"github.com/hashicorp/terraform-ls/internal/hooks"
+)
+
+func (s *service) AppendCompletionHooks(ctx decoder.DecoderContext) {
+	h := hooks.Hooks{
+		ModStore: s.modStore,
+	}
+
+	ctx.CompletionHooks["CompleteLocalModuleSources"] = h.LocalModuleSources
+
+}

internal/langserver/handlers/service.go

@@ -487,6 +487,7 @@ func (svc *service) configureSessionDependencies(ctx context.Context, cfgOpts *s
 		SchemaReader: svc.schemaStore,
 	})
 	decoderContext := idecoder.DecoderContext(ctx)
+	svc.AppendCompletionHooks(decoderContext)
 	svc.decoder.SetContext(decoderContext)
 
 	err = schemas.PreloadSchemasToStore(svc.stateStore.ProviderSchemas)