feat: experimental Rust runtime (#1924)

This Rust runtime implementation needs a fair amount of work to be
stable.

Features:
- Verb service and client.
- Can answer verb call requests.
- Can call out to verbs in other runtimes and process responses.
- FTL can build and restart the runtime automatically.
- Generates schema protos for verbs and args/return types recursively.

Limitations:
- Very limited functionality: verbs only, no generics.
- No error handling (unwraps everywhere).
- No scaffolding generation.
- No external module code generation.
- Only struct types referenced by verb arguments and returns in the same
module will correctly work.
- Hardcoded paths to crate dependencies in Cargo.toml

backend/controller/scaling/localscaling/local_scaling.go

@@ -92,7 +92,7 @@ func (l *LocalScaling) SetReplicas(ctx context.Context, replicas int, idleRunner
 		simpleName := fmt.Sprintf("runner%d", keySuffix)
 		if err := kong.ApplyDefaults(&config, kong.Vars{
 			"deploymentdir": filepath.Join(l.cacheDir, "ftl-runner", simpleName, "deployments"),
-			"language":      "go,kotlin",
+			"language":      "go,kotlin,rust",
 		}); err != nil {
 			return err
 		}

backend/runner/runner.go

@@ -46,7 +46,7 @@ type Config struct {
 	TemplateDir           string          `help:"Template directory to copy into each deployment, if any." type:"existingdir"`
 	DeploymentDir         string          `help:"Directory to store deployments in." default:"${deploymentdir}"`
 	DeploymentKeepHistory int             `help:"Number of deployments to keep history for." default:"3"`
-	Language              []string        `short:"l" help:"Languages the runner supports." env:"FTL_LANGUAGE" default:"go,kotlin"`
+	Language              []string        `short:"l" help:"Languages the runner supports." env:"FTL_LANGUAGE" default:"go,kotlin,rust"`
 	HeartbeatPeriod       time.Duration   `help:"Minimum period between heartbeats." default:"3s"`
 	HeartbeatJitter       time.Duration   `help:"Jitter to add to heartbeat period." default:"2s"`
 }

buildengine/build.go

@@ -46,6 +46,8 @@ func buildModule(ctx context.Context, sch *schema.Schema, module Module, filesTr
 		err = buildGoModule(ctx, sch, module, filesTransaction)
 	case "kotlin":
 		err = buildKotlinModule(ctx, sch, module)
+	case "rust":
+		err = buildRustModule(ctx, sch, module)
 	default:
 		return fmt.Errorf("unknown language %q", module.Config.Language)
 	}

buildengine/build_rust.go

@@ -0,0 +1,21 @@
+package buildengine
+
+import (
+	"context"
+	"fmt"
+	"github.com/TBD54566975/ftl/backend/schema"
+	"github.com/TBD54566975/ftl/internal/exec"
+	"github.com/TBD54566975/ftl/internal/log"
+)
+
+func buildRustModule(ctx context.Context, _ *schema.Schema, module Module) error {
+	logger := log.FromContext(ctx)
+
+	logger.Debugf("Using build command '%s'", module.Config.Build)
+	err := exec.Command(ctx, log.Debug, module.Config.Dir+"/_ftl", "bash", "-c", module.Config.Build).RunBuffered(ctx)
+	if err != nil {
+		return fmt.Errorf("failed to build module %q: %w", module.Config.Module, err)
+	}
+
+	return nil
+}

buildengine/deps.go

@@ -51,6 +51,9 @@ func extractDependencies(module Module) ([]string, error) {
 	case "kotlin":
 		return extractKotlinFTLImports(module.Config.Module, module.Config.Dir)
 
+	case "rust":
+		return extractRustFTLImports(module.Config.Module, module.Config.Dir)
+
 	default:
 		return nil, fmt.Errorf("unsupported language: %s", module.Config.Language)
 	}
@@ -136,3 +139,9 @@ func extractKotlinFTLImports(self, dir string) ([]string, error) {
 	sort.Strings(modules)
 	return modules, nil
 }
+
+func extractRustFTLImports(self, dir string) ([]string, error) {
+	fmt.Fprintf(os.Stderr, "RUST TODO extractRustFTLImports\n")
+
+	return nil, nil
+}

common/moduleconfig/moduleconfig.go

@@ -146,6 +146,20 @@ func setConfigDefaults(moduleDir string, config *ModuleConfig) error {
 			}
 			config.Watch = append(config.Watch, watches...)
 		}
+
+	case "rust":
+		if config.Build == "" {
+			config.Build = "cargo build"
+		}
+		if config.DeployDir == "" {
+			config.DeployDir = "_ftl/target/debug"
+		}
+		if len(config.Deploy) == 0 {
+			config.Deploy = []string{"main"}
+		}
+		if len(config.Watch) == 0 {
+			config.Watch = []string{"**/*.rs", "Cargo.toml", "Cargo.lock"}
+		}
 	}
 
 	// Do some validation.