tfdiags: Expose expression evaluation information from diagnostics

HCL diagnostics relating to expression evaluation include these, so this
is really just to pass that information through our Terraform-specific
diagnostics interface.

tfdiags/diagnostic.go

@@ -1,9 +1,18 @@
 package tfdiags
 
+import (
+	"github.com/hashicorp/hcl2/hcl"
+)
+
 type Diagnostic interface {
 	Severity() Severity
 	Description() Description
 	Source() Source
+
+	// FromExpr returns the expression-related context for the diagnostic, if
+	// available. Returns nil if the diagnostic is not related to an
+	// expression evaluation.
+	FromExpr() *FromExpr
 }
 
 type Severity rune
@@ -24,3 +33,8 @@ type Source struct {
 	Subject *SourceRange
 	Context *SourceRange
 }
+
+type FromExpr struct {
+	Expression  hcl.Expression
+	EvalContext *hcl.EvalContext
+}

tfdiags/diagnostic_base.go

@@ -2,9 +2,9 @@ package tfdiags
 
 // diagnosticBase can be embedded in other diagnostic structs to get
 // default implementations of Severity and Description. This type also
-// has a default implementation of Source that returns no source location
-// information, so embedders should generally override that method to
-// return more useful results.
+// has default implementations of Source and FromExpr that return no source
+// location or expression-related information, so embedders should generally
+// override those method to return more useful results where possible.
 type diagnosticBase struct {
 	severity Severity
 	summary  string
@@ -25,3 +25,7 @@ func (d diagnosticBase) Description() Description {
 func (d diagnosticBase) Source() Source {
 	return Source{}
 }
+
+func (d diagnosticBase) FromExpr() *FromExpr {
+	return nil
+}

tfdiags/error.go

@@ -21,3 +21,8 @@ func (e nativeError) Source() Source {
 	// No source information available for a native error
 	return Source{}
 }
+
+func (e nativeError) FromExpr() *FromExpr {
+	// Native errors are not expression-related
+	return nil
+}

tfdiags/hcl.go

@@ -40,6 +40,16 @@ func (d hclDiagnostic) Source() Source {
 	return ret
 }
 
+func (d hclDiagnostic) FromExpr() *FromExpr {
+	if d.diag.Expression == nil || d.diag.EvalContext == nil {
+		return nil
+	}
+	return &FromExpr{
+		Expression:  d.diag.Expression,
+		EvalContext: d.diag.EvalContext,
+	}
+}
+
 // SourceRangeFromHCL constructs a SourceRange from the corresponding range
 // type within the HCL package.
 func SourceRangeFromHCL(hclRange hcl.Range) SourceRange {

tfdiags/rpc_friendly.go

@@ -48,6 +48,12 @@ func (d *rpcFriendlyDiag) Source() Source {
 	}
 }
 
+func (d rpcFriendlyDiag) FromExpr() *FromExpr {
+	// RPC-friendly diagnostics cannot preserve expression information because
+	// expressions themselves are not RPC-friendly.
+	return nil
+}
+
 func init() {
 	gob.Register((*rpcFriendlyDiag)(nil))
 }

tfdiags/simple_warning.go

@@ -20,6 +20,11 @@ func (e simpleWarning) Description() Description {
 }
 
 func (e simpleWarning) Source() Source {
-	// No source information available for a native error
+	// No source information available for a simple warning
 	return Source{}
 }
+
+func (e simpleWarning) FromExpr() *FromExpr {
+	// Simple warnings are not expression-related
+	return nil
+}