re-validate data source config during read

Just like resources, early data soruce validation will contain many
unknown values. Re-validate once we have the config config.

terraform/context_refresh_test.go

@@ -1866,3 +1866,43 @@ test_thing.bar:
 		}
 	}
 }
+
+func TestContext2Refresh_dataValidation(t *testing.T) {
+	m := testModuleInline(t, map[string]string{
+		"main.tf": `
+data "aws_data_source" "foo" {
+  foo = "bar"
+}
+`,
+	})
+
+	p := testProvider("aws")
+	p.PlanResourceChangeFn = func(req providers.PlanResourceChangeRequest) (resp providers.PlanResourceChangeResponse) {
+		resp.PlannedState = req.ProposedNewState
+		return
+	}
+	p.ReadDataSourceFn = func(req providers.ReadDataSourceRequest) (resp providers.ReadDataSourceResponse) {
+		resp.State = req.Config
+		return
+	}
+
+	ctx := testContext2(t, &ContextOpts{
+		Config: m,
+		ProviderResolver: providers.ResolverFixed(
+			map[string]providers.Factory{
+				"aws": testProviderFuncFixed(p),
+			},
+		),
+	})
+
+	_, diags := ctx.Refresh()
+	if diags.HasErrors() {
+		// Should get this error:
+		// Unsupported attribute: This object does not have an attribute named "missing"
+		t.Fatal(diags.Err())
+	}
+
+	if !p.ValidateDataSourceConfigCalled {
+		t.Fatal("ValidateDataSourceConfig not called during plan")
+	}
+}

terraform/eval_read_data.go

@@ -179,6 +179,17 @@ func (n *EvalReadData) Eval(ctx EvalContext) (interface{}, error) {
 		)
 	}
 
+	log.Printf("[TRACE] Re-validating config for %s", absAddr)
+	validateResp := provider.ValidateDataSourceConfig(
+		providers.ValidateDataSourceConfigRequest{
+			TypeName: n.Addr.Resource.Type,
+			Config:   configVal,
+		},
+	)
+	if validateResp.Diagnostics.HasErrors() {
+		return nil, validateResp.Diagnostics.InConfigBody(n.Config.Config).Err()
+	}
+
 	// If we get down here then our configuration is complete and we're read
 	// to actually call the provider to read the data.
 	log.Printf("[TRACE] EvalReadData: %s configuration is complete, so reading from provider", absAddr)