configs: Parser.LoadConfigDir

This method wraps LoadConfigFile to load all of the .tf and .tf.json files
in a given directory and then bundle them together into a Module object.

This function also deals with the distinction between primary and override
files, first appending together the primary files in lexicographic order
by filename, and then merging in override files in the same order.

The merging behavior is not fully implemented as of this commit, and so
will be expanded in future commits.

configs/module.go

@@ -1,6 +1,8 @@
 package configs
 
 import (
+	"fmt"
+
 	"github.com/hashicorp/hcl2/hcl"
 )
 
@@ -23,20 +25,6 @@ type Module struct {
 	DataResources    map[string]*DataResource
 }
 
-// NewModule takes a list of primary files and a list of override files and
-// produces a *Module by combining the files together.
-//
-// If there are any conflicting declarations in the given files -- for example,
-// if the same variable name is defined twice -- then the resulting module
-// will be incomplete and error diagnostics will be returned. Careful static
-// analysis of the returned Module is still possible in this case, but the
-// module will probably not be semantically valid.
-func NewModule(primaryFiles, overrideFiles []*File) (*Module, hcl.Diagnostics) {
-	// TODO: process each file in turn, combining and merging as necessary
-	// to produce a single flat *Module.
-	panic("NewModule not yet implemented")
-}
-
 // File describes the contents of a single configuration file.
 //
 // Individual files are not usually used alone, but rather combined together
@@ -64,3 +52,325 @@ type File struct {
 	ManagedResources []*ManagedResource
 	DataResources    []*DataResource
 }
+
+// NewModule takes a list of primary files and a list of override files and
+// produces a *Module by combining the files together.
+//
+// If there are any conflicting declarations in the given files -- for example,
+// if the same variable name is defined twice -- then the resulting module
+// will be incomplete and error diagnostics will be returned. Careful static
+// analysis of the returned Module is still possible in this case, but the
+// module will probably not be semantically valid.
+func NewModule(primaryFiles, overrideFiles []*File) (*Module, hcl.Diagnostics) {
+	var diags hcl.Diagnostics
+	mod := &Module{
+		ProviderConfigs:      map[string]*Provider{},
+		ProviderRequirements: map[string][]VersionConstraint{},
+		Variables:            map[string]*Variable{},
+		Locals:               map[string]*Local{},
+		Outputs:              map[string]*Output{},
+		ModuleCalls:          map[string]*ModuleCall{},
+		ManagedResources:     map[string]*ManagedResource{},
+		DataResources:        map[string]*DataResource{},
+	}
+
+	for _, file := range primaryFiles {
+		fileDiags := mod.appendFile(file)
+		diags = append(diags, fileDiags...)
+	}
+
+	for _, file := range overrideFiles {
+		fileDiags := mod.mergeFile(file)
+		diags = append(diags, fileDiags...)
+	}
+
+	return mod, diags
+}
+
+func (m *Module) appendFile(file *File) hcl.Diagnostics {
+	var diags hcl.Diagnostics
+
+	for _, constraint := range file.CoreVersionConstraints {
+		// If there are any conflicting requirements then we'll catch them
+		// when we actually check these constraints.
+		m.CoreVersionConstraints = append(m.CoreVersionConstraints, constraint)
+	}
+
+	for _, b := range file.Backends {
+		if m.Backend != nil {
+			diags = append(diags, &hcl.Diagnostic{
+				Severity: hcl.DiagError,
+				Summary:  "Duplicate backend configuration",
+				Detail:   fmt.Sprintf("A module may have only one backend configuration. The backend was previously configured at %s.", m.Backend.DeclRange),
+				Subject:  &b.DeclRange,
+			})
+			continue
+		}
+		m.Backend = b
+	}
+
+	for _, pc := range file.ProviderConfigs {
+		key := pc.moduleUniqueKey()
+		if existing, exists := m.ProviderConfigs[key]; exists {
+			if existing.Alias == "" {
+				diags = append(diags, &hcl.Diagnostic{
+					Severity: hcl.DiagError,
+					Summary:  "Duplicate provider configuration",
+					Detail:   fmt.Sprintf("A default (non-aliased) provider configuration for %q was already given at %s. If multiple configurations are required, set the \"alias\" argument for alternative configurations.", existing.Name, existing.DeclRange),
+					Subject:  &pc.DeclRange,
+				})
+			} else {
+				diags = append(diags, &hcl.Diagnostic{
+					Severity: hcl.DiagError,
+					Summary:  "Duplicate provider configuration",
+					Detail:   fmt.Sprintf("A provider configuration for %q with alias %q was already given at %s. Each configuration for the same provider must have a distinct alias.", existing.Name, existing.Alias, existing.DeclRange),
+					Subject:  &pc.DeclRange,
+				})
+			}
+			continue
+		}
+		m.ProviderConfigs[key] = pc
+	}
+
+	for _, reqd := range file.ProviderRequirements {
+		m.ProviderRequirements[reqd.Name] = append(m.ProviderRequirements[reqd.Name], reqd.Requirement)
+	}
+
+	for _, v := range file.Variables {
+		if existing, exists := m.Variables[v.Name]; exists {
+			diags = append(diags, &hcl.Diagnostic{
+				Severity: hcl.DiagError,
+				Summary:  "Duplicate variable declaration",
+				Detail:   fmt.Sprintf("A variable named %q was already declared at %s. Variable names must be unique within a module.", existing.Name, existing.DeclRange),
+				Subject:  &v.DeclRange,
+			})
+		}
+		m.Variables[v.Name] = v
+	}
+
+	for _, l := range file.Locals {
+		if existing, exists := m.Locals[l.Name]; exists {
+			diags = append(diags, &hcl.Diagnostic{
+				Severity: hcl.DiagError,
+				Summary:  "Duplicate local value definition",
+				Detail:   fmt.Sprintf("A local value named %q was already defined at %s. Local value names must be unique within a module.", existing.Name, existing.DeclRange),
+				Subject:  &l.DeclRange,
+			})
+		}
+		m.Locals[l.Name] = l
+	}
+
+	for _, o := range file.Outputs {
+		if existing, exists := m.Outputs[o.Name]; exists {
+			diags = append(diags, &hcl.Diagnostic{
+				Severity: hcl.DiagError,
+				Summary:  "Duplicate output definition",
+				Detail:   fmt.Sprintf("An output named %q was already defined at %s. Output names must be unique within a module.", existing.Name, existing.DeclRange),
+				Subject:  &o.DeclRange,
+			})
+		}
+		m.Outputs[o.Name] = o
+	}
+
+	for _, mc := range file.ModuleCalls {
+		if existing, exists := m.ModuleCalls[mc.Name]; exists {
+			diags = append(diags, &hcl.Diagnostic{
+				Severity: hcl.DiagError,
+				Summary:  "Duplicate module call",
+				Detail:   fmt.Sprintf("An module call named %q was already defined at %s. Module calls must have unique names within a module.", existing.Name, existing.DeclRange),
+				Subject:  &mc.DeclRange,
+			})
+		}
+		m.ModuleCalls[mc.Name] = mc
+	}
+
+	for _, r := range file.ManagedResources {
+		key := r.moduleUniqueKey()
+		if existing, exists := m.ManagedResources[key]; exists {
+			diags = append(diags, &hcl.Diagnostic{
+				Severity: hcl.DiagError,
+				Summary:  fmt.Sprintf("Duplicate resource %q configuration", existing.Type),
+				Detail:   fmt.Sprintf("A %s resource named %q was already declared at %s. Resource names must be unique per type in each module.", existing.Type, existing.Name, existing.DeclRange),
+				Subject:  &r.DeclRange,
+			})
+			continue
+		}
+		m.ManagedResources[key] = r
+	}
+
+	for _, r := range file.DataResources {
+		key := r.moduleUniqueKey()
+		if existing, exists := m.DataResources[key]; exists {
+			diags = append(diags, &hcl.Diagnostic{
+				Severity: hcl.DiagError,
+				Summary:  fmt.Sprintf("Duplicate data %q configuration", existing.Type),
+				Detail:   fmt.Sprintf("A %s data resource named %q was already declared at %s. Resource names must be unique per type in each module.", existing.Type, existing.Name, existing.DeclRange),
+				Subject:  &r.DeclRange,
+			})
+			continue
+		}
+		m.DataResources[key] = r
+	}
+
+	return diags
+}
+
+func (m *Module) mergeFile(file *File) hcl.Diagnostics {
+	var diags hcl.Diagnostics
+
+	if len(file.CoreVersionConstraints) != 0 {
+		// This is a bit of a strange case for overriding since we normally
+		// would union together across multiple files anyway, but we'll
+		// allow it and have each override file clobber any existing list.
+		m.CoreVersionConstraints = nil
+		for _, constraint := range file.CoreVersionConstraints {
+			m.CoreVersionConstraints = append(m.CoreVersionConstraints, constraint)
+		}
+	}
+
+	if len(file.Backends) != 0 {
+		switch len(file.Backends) {
+		case 1:
+			m.Backend = file.Backends[0]
+		default:
+			// An override file with multiple backends is still invalid, even
+			// though it can override backends from _other_ files.
+			diags = append(diags, &hcl.Diagnostic{
+				Severity: hcl.DiagError,
+				Summary:  "Duplicate backend configuration",
+				Detail:   fmt.Sprintf("Each override file may have only one backend configuration. A backend was previously configured at %s.", file.Backends[0].DeclRange),
+				Subject:  &file.Backends[1].DeclRange,
+			})
+		}
+	}
+
+	for _, pc := range file.ProviderConfigs {
+		key := pc.moduleUniqueKey()
+		existing, exists := m.ProviderConfigs[key]
+		if pc.Alias == "" {
+			// We allow overriding a non-existing _default_ provider configuration
+			// because the user model is that an absent provider configuration
+			// implies an empty provider configuration, which is what the user
+			// is therefore overriding here.
+			if exists {
+				mergeDiags := existing.merge(pc)
+				diags = append(diags, mergeDiags...)
+			} else {
+				m.ProviderConfigs[key] = pc
+			}
+		} else {
+			// For aliased providers, there must be a base configuration to
+			// override. This allows us to detect and report alias typos
+			// that might otherwise cause the override to not apply.
+			if !exists {
+				diags = append(diags, &hcl.Diagnostic{
+					Severity: hcl.DiagError,
+					Summary:  "Missing base provider configuration for override",
+					Detail:   fmt.Sprintf("There is no %s provider configuration with the alias %q. An override file can only override an aliased provider configuration that was already defined in a primary configuration file.", pc.Name, pc.Alias),
+					Subject:  &pc.DeclRange,
+				})
+				continue
+			}
+			mergeDiags := existing.merge(pc)
+			diags = append(diags, mergeDiags...)
+		}
+	}
+
+	if len(file.ProviderRequirements) != 0 {
+		mergeProviderVersionConstraints(m.ProviderRequirements, file.ProviderRequirements)
+	}
+
+	for _, v := range file.Variables {
+		existing, exists := m.Variables[v.Name]
+		if !exists {
+			diags = append(diags, &hcl.Diagnostic{
+				Severity: hcl.DiagError,
+				Summary:  "Missing base variable declaration to override",
+				Detail:   fmt.Sprintf("There is no variable named %q. An override file can only override a variable that was already declared in a primary configuration file.", v.Name),
+				Subject:  &v.DeclRange,
+			})
+			continue
+		}
+		mergeDiags := existing.merge(v)
+		diags = append(diags, mergeDiags...)
+	}
+
+	for _, l := range file.Locals {
+		existing, exists := m.Locals[l.Name]
+		if !exists {
+			diags = append(diags, &hcl.Diagnostic{
+				Severity: hcl.DiagError,
+				Summary:  "Missing base local value definition to override",
+				Detail:   fmt.Sprintf("There is no local value named %q. An override file can only override a local value that was already defined in a primary configuration file.", l.Name),
+				Subject:  &l.DeclRange,
+			})
+			continue
+		}
+		mergeDiags := existing.merge(l)
+		diags = append(diags, mergeDiags...)
+	}
+
+	for _, o := range file.Outputs {
+		existing, exists := m.Outputs[o.Name]
+		if !exists {
+			diags = append(diags, &hcl.Diagnostic{
+				Severity: hcl.DiagError,
+				Summary:  "Missing base output definition to override",
+				Detail:   fmt.Sprintf("There is no output named %q. An override file can only override an output that was already defined in a primary configuration file.", o.Name),
+				Subject:  &o.DeclRange,
+			})
+			continue
+		}
+		mergeDiags := existing.merge(o)
+		diags = append(diags, mergeDiags...)
+	}
+
+	for _, mc := range file.ModuleCalls {
+		existing, exists := m.ModuleCalls[mc.Name]
+		if !exists {
+			diags = append(diags, &hcl.Diagnostic{
+				Severity: hcl.DiagError,
+				Summary:  "Missing module call to override",
+				Detail:   fmt.Sprintf("There is no module call named %q. An override file can only override a module call that was defined in a primary configuration file.", mc.Name),
+				Subject:  &mc.DeclRange,
+			})
+			continue
+		}
+		mergeDiags := existing.merge(mc)
+		diags = append(diags, mergeDiags...)
+	}
+
+	for _, r := range file.ManagedResources {
+		key := r.moduleUniqueKey()
+		existing, exists := m.ManagedResources[key]
+		if !exists {
+			diags = append(diags, &hcl.Diagnostic{
+				Severity: hcl.DiagError,
+				Summary:  "Missing resource to override",
+				Detail:   fmt.Sprintf("There is no %s resource named %q. An override file can only override a resource block defined in a primary configuration file.", r.Type, r.Name),
+				Subject:  &r.DeclRange,
+			})
+			continue
+		}
+		mergeDiags := existing.merge(r)
+		diags = append(diags, mergeDiags...)
+	}
+
+	for _, r := range file.DataResources {
+		key := r.moduleUniqueKey()
+		existing, exists := m.DataResources[key]
+		if !exists {
+			diags = append(diags, &hcl.Diagnostic{
+				Severity: hcl.DiagError,
+				Summary:  "Missing data resource to override",
+				Detail:   fmt.Sprintf("There is no %s data resource named %q. An override file can only override a data block defined in a primary configuration file.", r.Type, r.Name),
+				Subject:  &r.DeclRange,
+			})
+			continue
+		}
+		mergeDiags := existing.merge(r)
+		diags = append(diags, mergeDiags...)
+	}
+
+	return diags
+}

configs/module_merge.go

@@ -0,0 +1,212 @@
+package configs
+
+import (
+	"github.com/hashicorp/hcl2/hcl"
+	"github.com/zclconf/go-cty/cty"
+)
+
+// The methods in this file are used by Module.mergeFile to apply overrides
+// to our different configuration elements. These methods all follow the
+// pattern of mutating the receiver to incorporate settings from the parameter,
+// returning error diagnostics if any aspect of the parameter cannot be merged
+// into the receiver for some reason.
+//
+// User expectation is that anything _explicitly_ set in the given object
+// should take precedence over the corresponding settings in the receiver,
+// but that anything omitted in the given object should be left unchanged.
+// In some cases it may be reasonable to do a "deep merge" of certain nested
+// features, if it is possible to unambiguously correlate the nested elements
+// and their behaviors are orthogonal to each other.
+
+func (p *Provider) merge(op *Provider) hcl.Diagnostics {
+	var diags hcl.Diagnostics
+
+	if op.Version.Required != nil {
+		p.Version = op.Version
+	}
+
+	p.Config = mergeBodies(p.Config, op.Config)
+
+	return diags
+}
+
+func mergeProviderVersionConstraints(recv map[string][]VersionConstraint, ovrd []*ProviderRequirement) {
+	// Any provider name that's mentioned in the override gets nilled out in
+	// our map so that we'll rebuild it below. Any provider not mentioned is
+	// left unchanged.
+	for _, reqd := range ovrd {
+		delete(recv, reqd.Name)
+	}
+	for _, reqd := range ovrd {
+		recv[reqd.Name] = append(recv[reqd.Name], reqd.Requirement)
+	}
+}
+
+func (v *Variable) merge(ov *Variable) hcl.Diagnostics {
+	var diags hcl.Diagnostics
+
+	if ov.Description != "" {
+		v.Description = ov.Description
+	}
+	if ov.Default != cty.NilVal {
+		v.Default = ov.Default
+	}
+	if ov.TypeHint != TypeHintNone {
+		v.TypeHint = ov.TypeHint
+	}
+
+	return diags
+}
+
+func (l *Local) merge(ol *Local) hcl.Diagnostics {
+	var diags hcl.Diagnostics
+
+	// Since a local is just a single expression in configuration, the
+	// override definition entirely replaces the base definition, including
+	// the source range so that we'll send the user to the right place if
+	// there is an error.
+	l.Expr = ol.Expr
+	l.DeclRange = ol.DeclRange
+
+	return diags
+}
+
+func (o *Output) merge(oo *Output) hcl.Diagnostics {
+	var diags hcl.Diagnostics
+
+	if oo.Description != "" {
+		o.Description = oo.Description
+	}
+	if oo.Expr != nil {
+		o.Expr = oo.Expr
+	}
+	if oo.Sensitive {
+		// Since this is just a bool, we can't distinguish false from unset
+		// and so the override can only make the output _more_ sensitive.
+		o.Sensitive = oo.Sensitive
+	}
+
+	// We don't allow depends_on to be overridden because that is likely to
+	// cause confusing misbehavior.
+	if len(oo.DependsOn) != 0 {
+		diags = append(diags, &hcl.Diagnostic{
+			Severity: hcl.DiagError,
+			Summary:  "Unsupported override",
+			Detail:   "The depends_on argument may not be overridden.",
+			Subject:  oo.DependsOn[0].SourceRange().Ptr(), // the first item is the closest range we have
+		})
+	}
+
+	return diags
+}
+
+func (mc *ModuleCall) merge(omc *ModuleCall) hcl.Diagnostics {
+	var diags hcl.Diagnostics
+
+	if omc.SourceAddr != "" {
+		mc.SourceAddr = omc.SourceAddr
+		mc.SourceAddrRange = omc.SourceAddrRange
+	}
+
+	if omc.Count != nil {
+		mc.Count = omc.Count
+	}
+
+	if omc.ForEach != nil {
+		mc.ForEach = omc.ForEach
+	}
+
+	if len(omc.Version.Required) != 0 {
+		mc.Version = omc.Version
+	}
+
+	mc.Config = mergeBodies(mc.Config, omc.Config)
+
+	// We don't allow depends_on to be overridden because that is likely to
+	// cause confusing misbehavior.
+	if len(mc.DependsOn) != 0 {
+		diags = append(diags, &hcl.Diagnostic{
+			Severity: hcl.DiagError,
+			Summary:  "Unsupported override",
+			Detail:   "The depends_on argument may not be overridden.",
+			Subject:  mc.DependsOn[0].SourceRange().Ptr(), // the first item is the closest range we have
+		})
+	}
+
+	return diags
+}
+
+func (r *ManagedResource) merge(or *ManagedResource) hcl.Diagnostics {
+	var diags hcl.Diagnostics
+
+	if or.Connection != nil {
+		r.Connection = or.Connection
+	}
+	if or.Count != nil {
+		r.Count = or.Count
+	}
+	if or.CreateBeforeDestroy {
+		// We can't distinguish false from unset here
+		r.CreateBeforeDestroy = or.CreateBeforeDestroy
+	}
+	if or.ForEach != nil {
+		r.ForEach = or.ForEach
+	}
+	if len(or.IgnoreChanges) != 0 {
+		r.IgnoreChanges = or.IgnoreChanges
+	}
+	if or.PreventDestroy {
+		// We can't distinguish false from unset here
+		r.PreventDestroy = or.PreventDestroy
+	}
+	if or.ProviderConfigRef != nil {
+		r.ProviderConfigRef = or.ProviderConfigRef
+	}
+	if len(or.Provisioners) != 0 {
+		r.Provisioners = or.Provisioners
+	}
+
+	r.Config = mergeBodies(r.Config, or.Config)
+
+	// We don't allow depends_on to be overridden because that is likely to
+	// cause confusing misbehavior.
+	if len(r.DependsOn) != 0 {
+		diags = append(diags, &hcl.Diagnostic{
+			Severity: hcl.DiagError,
+			Summary:  "Unsupported override",
+			Detail:   "The depends_on argument may not be overridden.",
+			Subject:  r.DependsOn[0].SourceRange().Ptr(), // the first item is the closest range we have
+		})
+	}
+
+	return diags
+}
+
+func (r *DataResource) merge(or *DataResource) hcl.Diagnostics {
+	var diags hcl.Diagnostics
+
+	if or.Count != nil {
+		r.Count = or.Count
+	}
+	if or.ForEach != nil {
+		r.ForEach = or.ForEach
+	}
+	if or.ProviderConfigRef != nil {
+		r.ProviderConfigRef = or.ProviderConfigRef
+	}
+
+	r.Config = mergeBodies(r.Config, or.Config)
+
+	// We don't allow depends_on to be overridden because that is likely to
+	// cause confusing misbehavior.
+	if len(r.DependsOn) != 0 {
+		diags = append(diags, &hcl.Diagnostic{
+			Severity: hcl.DiagError,
+			Summary:  "Unsupported override",
+			Detail:   "The depends_on argument may not be overridden.",
+			Subject:  r.DependsOn[0].SourceRange().Ptr(), // the first item is the closest range we have
+		})
+	}
+
+	return diags
+}

configs/module_merge_body.go

@@ -0,0 +1,47 @@
+package configs
+
+import (
+	"github.com/hashicorp/hcl2/hcl"
+)
+
+func mergeBodies(base, override hcl.Body) hcl.Body {
+	return mergeBody{
+		Base:     base,
+		Override: override,
+	}
+}
+
+// mergeBody is a hcl.Body implementation that wraps a pair of other bodies
+// and allows attributes and blocks within the override to take precedence
+// over those defined in the base body.
+//
+// This is used to deal with dynamically-processed bodies in Module.mergeFile.
+// It uses a shallow-only merging strategy where direct attributes defined
+// in Override will override attributes of the same name in Base, while any
+// blocks defined in Override will hide all blocks of the same type in Base.
+//
+// This cannot possibly "do the right thing" in all cases, because we don't
+// have enough information about user intent. However, this behavior is intended
+// to be reasonable for simple overriding use-cases.
+type mergeBody struct {
+	Base     hcl.Body
+	Override hcl.Body
+}
+
+var _ hcl.Body = mergeBody{}
+
+func (b mergeBody) Content(schema *hcl.BodySchema) (*hcl.BodyContent, hcl.Diagnostics) {
+	panic("mergeBody.Content not yet implemented")
+}
+
+func (b mergeBody) PartialContent(schema *hcl.BodySchema) (*hcl.BodyContent, hcl.Body, hcl.Diagnostics) {
+	panic("mergeBody.Content not yet implemented")
+}
+
+func (b mergeBody) JustAttributes() (hcl.Attributes, hcl.Diagnostics) {
+	panic("mergeBody.JustAttributes not yet implemented")
+}
+
+func (b mergeBody) MissingItemRange() hcl.Range {
+	return b.Base.MissingItemRange()
+}

configs/parser_config_dir.go

@@ -0,0 +1,125 @@
+package configs
+
+import (
+	"fmt"
+	"path/filepath"
+	"strings"
+
+	"github.com/hashicorp/hcl2/hcl"
+)
+
+// LoadConfigDir reads the .tf and .tf.json files in the given directory
+// as config files (using LoadConfigFile) and then combines these files into
+// a single Module.
+//
+// If this method returns nil, that indicates that the given directory does not
+// exist at all or could not be opened for some reason. Callers may wish to
+// detect this case and ignore the returned diagnostics so that they can
+// produce a more context-aware error message in that case.
+//
+// If this method returns a non-nil module while error diagnostics are returned
+// then the module may be incomplete but can be used carefully for static
+// analysis.
+//
+// This file does not consider a directory with no files to be an error, and
+// will simply return an empty module in that case. Callers should first call
+// Parser.IsConfigDir if they wish to recognize that situation.
+//
+// .tf files are parsed using the HCL native syntax while .tf.json files are
+// parsed using the HCL JSON syntax.
+func (p *Parser) LoadConfigDir(path string) (*Module, hcl.Diagnostics) {
+	primaryPaths, overridePaths, diags := p.dirFiles(path)
+	if diags.HasErrors() {
+		return nil, diags
+	}
+
+	primary, fDiags := p.loadFiles(primaryPaths)
+	diags = append(diags, fDiags...)
+	override, fDiags := p.loadFiles(overridePaths)
+	diags = append(diags, fDiags...)
+
+	mod, modDiags := NewModule(primary, override)
+	diags = append(diags, modDiags...)
+
+	return mod, diags
+}
+
+// IsConfigDir determines whether the given path refers to a directory that
+// exists and contains at least one Terraform config file (with a .tf or
+// .tf.json extension.)
+func (p *Parser) IsConfigDir(path string) bool {
+	primaryPaths, overridePaths, _ := p.dirFiles(path)
+	return (len(primaryPaths) + len(overridePaths)) > 0
+}
+
+func (p *Parser) loadFiles(paths []string) ([]*File, hcl.Diagnostics) {
+	var files []*File
+	var diags hcl.Diagnostics
+
+	for _, path := range paths {
+		f, fDiags := p.LoadConfigFile(path)
+		diags = append(diags, fDiags...)
+		if f != nil {
+			files = append(files, f)
+		}
+	}
+
+	return files, diags
+}
+
+func (p *Parser) dirFiles(dir string) (primary, override []string, diags hcl.Diagnostics) {
+	infos, err := p.fs.ReadDir(dir)
+	if err != nil {
+		diags = append(diags, &hcl.Diagnostic{
+			Severity: hcl.DiagError,
+			Summary:  "Failed to read module directory",
+			Detail:   fmt.Sprintf("Module directory %s does not exist or cannot be read.", dir),
+		})
+		return
+	}
+
+	for _, info := range infos {
+		if info.IsDir() {
+			// We only care about files
+			continue
+		}
+
+		name := info.Name()
+		ext := fileExt(name)
+		if ext == "" || IsIgnoredFile(name) {
+			continue
+		}
+
+		baseName := name[:len(name)-len(ext)] // strip extension
+		isOverride := baseName == "override" || strings.HasSuffix(baseName, "_override")
+
+		fullPath := filepath.Join(dir, name)
+		if isOverride {
+			override = append(override, fullPath)
+		} else {
+			primary = append(primary, fullPath)
+		}
+	}
+
+	return
+}
+
+// fileExt returns the Terraform configuration extension of the given
+// path, or a blank string if it is not a recognized extension.
+func fileExt(path string) string {
+	if strings.HasSuffix(path, ".tf") {
+		return ".tf"
+	} else if strings.HasSuffix(path, ".tf.json") {
+		return ".tf.json"
+	} else {
+		return ""
+	}
+}
+
+// IsIgnoredFile returns true if the given filename (which must not have a
+// directory path ahead of it) should be ignored as e.g. an editor swap file.
+func IsIgnoredFile(name string) bool {
+	return strings.HasPrefix(name, ".") || // Unix-like hidden files
+		strings.HasSuffix(name, "~") || // vim
+		strings.HasPrefix(name, "#") && strings.HasSuffix(name, "#") // emacs
+}

configs/parser_config_dir_test.go

@@ -0,0 +1,136 @@
+package configs
+
+import (
+	"fmt"
+	"io/ioutil"
+	"path/filepath"
+	"testing"
+)
+
+// TestParseLoadConfigDirSuccess is a simple test that just verifies that
+// a number of test configuration directories (in test-fixtures/valid-modules)
+// can be parsed without raising any diagnostics.
+//
+// It also re-tests the individual files in test-fixtures/valid-files as if
+// they were single-file modules, to ensure that they can be bundled into
+// modules correctly.
+//
+// This test does not verify that reading these modules produces the correct
+// module element contents. More detailed assertions may be made on some subset
+// of these configuration files in other tests.
+func TestParserLoadConfigDirSuccess(t *testing.T) {
+	dirs, err := ioutil.ReadDir("test-fixtures/valid-modules")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	for _, info := range dirs {
+		name := info.Name()
+		t.Run(name, func(t *testing.T) {
+			parser := NewParser(nil)
+			path := filepath.Join("test-fixtures/valid-modules", name)
+
+			_, diags := parser.LoadConfigDir(path)
+			if len(diags) != 0 {
+				t.Errorf("unexpected diagnostics")
+				for _, diag := range diags {
+					t.Logf("- %s", diag)
+				}
+			}
+		})
+	}
+
+	// The individual files in test-fixtures/valid-files should also work
+	// when loaded as modules.
+	files, err := ioutil.ReadDir("test-fixtures/valid-files")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	for _, info := range files {
+		name := info.Name()
+		t.Run(fmt.Sprintf("%s as module", name), func(t *testing.T) {
+			src, err := ioutil.ReadFile(filepath.Join("test-fixtures/valid-files", name))
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			parser := testParser(map[string]string{
+				"mod/" + name: string(src),
+			})
+
+			_, diags := parser.LoadConfigDir("mod")
+			if len(diags) != 0 {
+				t.Errorf("unexpected diagnostics")
+				for _, diag := range diags {
+					t.Logf("- %s", diag)
+				}
+			}
+		})
+	}
+
+}
+
+// TestParseLoadConfigDirFailure is a simple test that just verifies that
+// a number of test configuration directories (in test-fixtures/invalid-modules)
+// produce diagnostics when parsed.
+//
+// It also re-tests the individual files in test-fixtures/invalid-files as if
+// they were single-file modules, to ensure that their errors are still
+// detected when loading as part of a module.
+//
+// This test does not verify that reading these modules produces any
+// diagnostics in particular. More detailed assertions may be made on some subset
+// of these configuration files in other tests.
+func TestParserLoadConfigDirFailure(t *testing.T) {
+	dirs, err := ioutil.ReadDir("test-fixtures/invalid-modules")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	for _, info := range dirs {
+		name := info.Name()
+		t.Run(name, func(t *testing.T) {
+			parser := NewParser(nil)
+			path := filepath.Join("test-fixtures/invalid-modules", name)
+
+			_, diags := parser.LoadConfigDir(path)
+			if !diags.HasErrors() {
+				t.Errorf("no errors; want at least one")
+				for _, diag := range diags {
+					t.Logf("- %s", diag)
+				}
+			}
+		})
+	}
+
+	// The individual files in test-fixtures/valid-files should also work
+	// when loaded as modules.
+	files, err := ioutil.ReadDir("test-fixtures/invalid-files")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	for _, info := range files {
+		name := info.Name()
+		t.Run(fmt.Sprintf("%s as module", name), func(t *testing.T) {
+			src, err := ioutil.ReadFile(filepath.Join("test-fixtures/invalid-files", name))
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			parser := testParser(map[string]string{
+				"mod/" + name: string(src),
+			})
+
+			_, diags := parser.LoadConfigDir("mod")
+			if !diags.HasErrors() {
+				t.Errorf("no errors; want at least one")
+				for _, diag := range diags {
+					t.Logf("- %s", diag)
+				}
+			}
+		})
+	}
+
+}

configs/provider.go

@@ -57,6 +57,13 @@ func decodeProviderBlock(block *hcl.Block) (*Provider, hcl.Diagnostics) {
 	return provider, diags
 }
 
+func (p *Provider) moduleUniqueKey() string {
+	if p.Alias != "" {
+		return fmt.Sprintf("%s.%s", p.Name, p.Alias)
+	}
+	return p.Name
+}
+
 // ProviderRequirement represents a declaration of a dependency on a particular
 // provider version without actually configuring that provider. This is used in
 // child modules that expect a provider to be passed in from their parent.

configs/resource.go

@@ -31,6 +31,10 @@ type ManagedResource struct {
 	TypeRange hcl.Range
 }
 
+func (r *ManagedResource) moduleUniqueKey() string {
+	return fmt.Sprintf("%s.%s", r.Name, r.Type)
+}
+
 func decodeResourceBlock(block *hcl.Block) (*ManagedResource, hcl.Diagnostics) {
 	r := &ManagedResource{
 		Type:      block.Labels[0],
@@ -170,6 +174,10 @@ type DataResource struct {
 	TypeRange hcl.Range
 }
 
+func (r *DataResource) moduleUniqueKey() string {
+	return fmt.Sprintf("data.%s.%s", r.Name, r.Type)
+}
+
 func decodeDataBlock(block *hcl.Block) (*DataResource, hcl.Diagnostics) {
 	r := &DataResource{
 		Type:      block.Labels[0],

configs/test-fixtures/invalid-files/json-as-native-syntax.tf

@@ -0,0 +1,3 @@
+{
+  "terraform": {}
+}

configs/test-fixtures/invalid-files/native-syntax-as-json.tf.json

@@ -0,0 +1,2 @@
+terraform {
+}

configs/test-fixtures/invalid-modules/override-nonexist-variable/override.tf

@@ -0,0 +1,3 @@
+variable "foo" {
+  description = "overridden"
+}

configs/test-fixtures/valid-modules/empty/README

@@ -0,0 +1,2 @@
+This directory is intentionally empty, to test what happens when we load
+a module that contains no configuration files.

configs/test-fixtures/valid-modules/override-variable/a_override.tf

@@ -0,0 +1,9 @@
+variable "fully_overridden" {
+  default = "a_override"
+  description = "a_override description"
+  type = string
+}
+
+variable "partially_overridden" {
+  default = "a_override partial"
+}

configs/test-fixtures/valid-modules/override-variable/b_override.tf

@@ -0,0 +1,9 @@
+variable "fully_overridden" {
+  default = "b_override"
+  description = "b_override description"
+  type = string
+}
+
+variable "partially_overridden" {
+  default = "b_override partial"
+}

configs/test-fixtures/valid-modules/override-variable/primary.tf

@@ -0,0 +1,11 @@
+variable "fully_overridden" {
+  default = "base"
+  description = "base description"
+  type = string
+}
+
+variable "partially_overridden" {
+  default = "base"
+  description = "base description"
+  type = string
+}