configs: Fix check block configuration diagnostics

When validating self-references for resource and data source
preconditions and postconditions, we previously did not nil-check the
block's condition field, which caused a panic when the block had no
condition.

While fixing this I noticed that we were not validating that there are
no self-references in the error message, so fixed that.

internal/configs/checks.go

@@ -40,27 +40,36 @@ type CheckRule struct {
 // is found.
 func (cr *CheckRule) validateSelfReferences(checkType string, addr addrs.Resource) hcl.Diagnostics {
 	var diags hcl.Diagnostics
-	refs, _ := lang.References(cr.Condition.Variables())
-	for _, ref := range refs {
-		var refAddr addrs.Resource
-
-		switch rs := ref.Subject.(type) {
-		case addrs.Resource:
-			refAddr = rs
-		case addrs.ResourceInstance:
-			refAddr = rs.Resource
-		default:
+	exprs := []hcl.Expression{
+		cr.Condition,
+		cr.ErrorMessage,
+	}
+	for _, expr := range exprs {
+		if expr == nil {
 			continue
 		}
-
-		if refAddr.Equal(addr) {
-			diags = diags.Append(&hcl.Diagnostic{
-				Severity: hcl.DiagError,
-				Summary:  fmt.Sprintf("Invalid reference in %s", checkType),
-				Detail:   fmt.Sprintf("Configuration for %s may not refer to itself.", addr.String()),
-				Subject:  cr.Condition.Range().Ptr(),
-			})
-			break
+		refs, _ := lang.References(expr.Variables())
+		for _, ref := range refs {
+			var refAddr addrs.Resource
+
+			switch rs := ref.Subject.(type) {
+			case addrs.Resource:
+				refAddr = rs
+			case addrs.ResourceInstance:
+				refAddr = rs.Resource
+			default:
+				continue
+			}
+
+			if refAddr.Equal(addr) {
+				diags = diags.Append(&hcl.Diagnostic{
+					Severity: hcl.DiagError,
+					Summary:  fmt.Sprintf("Invalid reference in %s", checkType),
+					Detail:   fmt.Sprintf("Configuration for %s may not refer to itself.", addr.String()),
+					Subject:  expr.Range().Ptr(),
+				})
+				break
+			}
 		}
 	}
 	return diags

internal/configs/testdata/invalid-files/precondition-postcondition-badref.tf

@@ -0,0 +1,29 @@
+data "example" "example" {
+  foo = 5
+
+  lifecycle {
+    precondition {
+      condition     = data.example.example.foo == 5 # ERROR: Invalid reference in precondition
+      error_message = "Must be five."
+    }
+    postcondition {
+      condition     = self.foo == 5
+      error_message = "Must be five, but is ${data.example.example.foo}." # ERROR: Invalid reference in postcondition
+    }
+  }
+}
+
+resource "example" "example" {
+  foo = 5
+
+  lifecycle {
+    precondition {
+      condition     = example.example.foo == 5 # ERROR: Invalid reference in precondition
+      error_message = "Must be five."
+    }
+    postcondition {
+      condition     = self.foo == 5
+      error_message = "Must be five, but is ${example.example.foo}." # ERROR: Invalid reference in postcondition
+    }
+  }
+}

internal/configs/testdata/invalid-files/precondition-postcondition-missing-condition.tf

@@ -0,0 +1,12 @@
+resource "example" "example" {
+  foo = 5
+
+  lifecycle {
+    precondition { # ERROR: Missing required argument
+      error_message = "Can a check block fail without a condition?"
+    }
+    postcondition { # ERROR: Missing required argument
+      error_message = "Do not try to pass the check; only realize that there is no check."
+    }
+  }
+}