feat(internal/config): add tools section (#4981)

Adds config.Tools section to librarian.yaml to define language-specific
tool dependencies. `librarian install` support is added for Rust

For #4848

Author: julieqiu

doc/config-schema.md

@@ -10,6 +10,7 @@ This document describes the schema for the librarian.yaml.
 | `version` | string | Is the librarian tool version to use. |
 | `repo` | string | Is the repository name, such as "googleapis/google-cloud-python". It is used for:<br>- Providing to the Java GAPIC generator for observability features.<br>- Generating the .repo-metadata.json. |
 | `sources` | [Sources](#sources-configuration) (optional) | References external source repositories. |
+| `tools` | [Tools](#tools-configuration) (optional) | Defines required tools. |
 | `release` | [Release](#release-configuration) (optional) | Holds the configuration parameter for publishing and release subcommands. |
 | `default` | [Default](#default-configuration) (optional) | Contains default settings for all libraries. They apply to all libraries unless overridden. |
 | `libraries` | list of [Library](#library-configuration) (optional) | Contains configuration overrides for libraries that need special handling, and differ from default settings. |
@@ -49,6 +50,19 @@ This document describes the schema for the librarian.yaml.
 | `sha256` | string | Is the expected hash of the tarball for this commit. |
 | `subpath` | string | Is a directory inside the fetched archive that should be treated as the root for operations. |
 
+## Tools Configuration
+
+| Field | Type | Description |
+| :--- | :--- | :--- |
+| `cargo` | list of [CargoTool](#cargotool-configuration) (optional) | Defines tools to install via cargo. |
+
+## CargoTool Configuration
+
+| Field | Type | Description |
+| :--- | :--- | :--- |
+| `name` | string | Is the cargo package name. |
+| `version` | string | Is the version to install. |
+
 ## Default Configuration
 
 | Field | Type | Description |

internal/config/config.go

@@ -46,6 +46,9 @@ type Config struct {
 	// Sources references external source repositories.
 	Sources *Sources `yaml:"sources,omitempty"`
 
+	// Tools defines required tools.
+	Tools *Tools `yaml:"tools,omitempty"`
+
 	// Release holds the configuration parameter for publishing and release subcommands.
 	Release *Release `yaml:"release,omitempty"`
 
@@ -121,6 +124,21 @@ type Source struct {
 	Subpath string `yaml:"subpath,omitempty"`
 }
 
+// Tools defines required tools.
+type Tools struct {
+	// Cargo defines tools to install via cargo.
+	Cargo []*CargoTool `yaml:"cargo,omitempty"`
+}
+
+// CargoTool defines a tool to install via cargo.
+type CargoTool struct {
+	// Name is the cargo package name.
+	Name string `yaml:"name"`
+
+	// Version is the version to install.
+	Version string `yaml:"version"`
+}
+
 // Default contains default settings for all libraries.
 type Default struct {
 	// Keep lists files and directories to preserve during regeneration.

internal/config/config_test.go

@@ -52,6 +52,18 @@ func TestRustRead(t *testing.T) {
 				SHA256: "f572d396fae9206628714fb2ce00f72e94f2258f",
 			},
 		},
+		Tools: &Tools{
+			Cargo: []*CargoTool{
+				{
+					Name:    "cargo-semver-checks",
+					Version: "0.46.0",
+				},
+				{
+					Name:    "taplo-cli",
+					Version: "0.10.0",
+				},
+			},
+		},
 		Default: &Default{
 			Output:    "src/generated/",
 			TagFormat: "{name}/v{version}",

internal/config/testdata/rust/librarian.yaml

@@ -29,6 +29,12 @@ sources:
   conformance:
     commit: 1a2b3c4d5e6f7a8b9c0d1e2f3a4b5c6d7e8f9a0b
     sha256: f572d396fae9206628714fb2ce00f72e94f2258f
+tools:
+  cargo:
+    - name: cargo-semver-checks
+      version: "0.46.0"
+    - name: taplo-cli
+      version: "0.10.0"
 default:
   output: src/generated/
   release_level: stable

internal/librarian/librarian.go

@@ -24,6 +24,7 @@ import (
 	"github.com/googleapis/librarian/internal/command"
 	"github.com/googleapis/librarian/internal/config"
 	"github.com/googleapis/librarian/internal/librarian/golang"
+	"github.com/googleapis/librarian/internal/librarian/rust"
 	"github.com/googleapis/librarian/internal/yaml"
 	"github.com/urfave/cli/v3"
 )
@@ -76,6 +77,8 @@ func installCommand() *cli.Command {
 			switch cfg.Language {
 			case config.LanguageGo:
 				return golang.Install(ctx)
+			case config.LanguageRust:
+				return rust.Install(ctx, cfg.Tools)
 			default:
 				return fmt.Errorf("language %q does not support install", cfg.Language)
 			}

internal/librarian/rust/install.go

@@ -0,0 +1,44 @@
+// Copyright 2026 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package rust
+
+import (
+	"context"
+	"errors"
+	"fmt"
+
+	"github.com/googleapis/librarian/internal/command"
+	"github.com/googleapis/librarian/internal/config"
+)
+
+// ErrMissingToolVersion indicates a cargo tool entry is missing its version.
+var ErrMissingToolVersion = errors.New("cargo tool missing version")
+
+// Install installs cargo tool dependencies defined in the tools configuration.
+func Install(ctx context.Context, tools *config.Tools) error {
+	if len(tools.Cargo) == 0 {
+		return nil
+	}
+	for _, tool := range tools.Cargo {
+		if tool.Version == "" {
+			return fmt.Errorf("%w: %s", ErrMissingToolVersion, tool.Name)
+		}
+		t := fmt.Sprintf("%s@%s", tool.Name, tool.Version)
+		if err := command.Run(ctx, "cargo", "install", "--locked", t); err != nil {
+			return err
+		}
+	}
+	return nil
+}

internal/librarian/rust/install_test.go

@@ -0,0 +1,60 @@
+// Copyright 2026 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package rust
+
+import (
+	"errors"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/googleapis/librarian/internal/config"
+)
+
+func TestInstall(t *testing.T) {
+	bin := t.TempDir()
+	if err := os.WriteFile(filepath.Join(bin, "cargo"), []byte("#!/bin/sh\n"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	t.Setenv("PATH", bin+":"+os.Getenv("PATH"))
+
+	tools := &config.Tools{
+		Cargo: []*config.CargoTool{
+			{Name: "cargo-semver-checks", Version: "0.46.0"},
+			{Name: "taplo-cli", Version: "0.10.0"},
+		},
+	}
+	if err := Install(t.Context(), tools); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestInstall_MissingVersion(t *testing.T) {
+	tools := &config.Tools{
+		Cargo: []*config.CargoTool{
+			{Name: "some-tool"},
+		},
+	}
+	err := Install(t.Context(), tools)
+	if !errors.Is(err, ErrMissingToolVersion) {
+		t.Fatalf("got %v, want %v", err, ErrMissingToolVersion)
+	}
+}
+
+func TestInstall_NoCargoTools(t *testing.T) {
+	if err := Install(t.Context(), &config.Tools{}); err != nil {
+		t.Fatal(err)
+	}
+}

internal/librarian/tidy.go

@@ -25,6 +25,7 @@ import (
 	"github.com/googleapis/librarian/internal/config"
 	"github.com/googleapis/librarian/internal/librarian/golang"
 	"github.com/googleapis/librarian/internal/librarian/java"
+	"github.com/googleapis/librarian/internal/librarian/rust"
 	"github.com/googleapis/librarian/internal/serviceconfig"
 	"github.com/googleapis/librarian/internal/yaml"
 	"github.com/urfave/cli/v3"
@@ -54,6 +55,9 @@ func tidyCommand() *cli.Command {
 // RunTidyOnConfig formats and validates the provided librarian configuration
 // and writes it to disk, relative to the specified repository root directory.
 func RunTidyOnConfig(ctx context.Context, repoDir string, cfg *config.Config) error {
+	if err := validateTools(cfg); err != nil {
+		return err
+	}
 	if err := validateLibraries(cfg); err != nil {
 		return err
 	}
@@ -104,6 +108,18 @@ func isDerivableOutput(cfg *config.Config, lib *config.Library) bool {
 	return lib.Output == derivedOutput
 }
 
+func validateTools(cfg *config.Config) error {
+	if cfg.Tools == nil {
+		return nil
+	}
+	for _, tool := range cfg.Tools.Cargo {
+		if tool.Version == "" {
+			return fmt.Errorf("%w: %s", rust.ErrMissingToolVersion, tool.Name)
+		}
+	}
+	return nil
+}
+
 func validateLibraries(cfg *config.Config) error {
 	var (
 		errs      []error
@@ -175,6 +191,11 @@ func tidyRustConfig(lib *config.Library) *config.Library {
 }
 
 func formatConfig(cfg *config.Config) *config.Config {
+	if cfg.Tools != nil {
+		slices.SortFunc(cfg.Tools.Cargo, func(a, b *config.CargoTool) int {
+			return strings.Compare(a.Name, b.Name)
+		})
+	}
 	if cfg.Default != nil && cfg.Default.Rust != nil {
 		slices.SortFunc(cfg.Default.Rust.PackageDependencies, func(a, b *config.RustPackageDependency) int {
 			return strings.Compare(a.Name, b.Name)

internal/librarian/tidy_test.go

@@ -23,6 +23,7 @@ import (
 
 	"github.com/google/go-cmp/cmp"
 	"github.com/googleapis/librarian/internal/config"
+	"github.com/googleapis/librarian/internal/librarian/rust"
 	"github.com/googleapis/librarian/internal/sample"
 	"github.com/googleapis/librarian/internal/yaml"
 )
@@ -68,8 +69,47 @@ func TestValidateLibraries(t *testing.T) {
 	}
 }
 
+func TestValidateTools_NoTools(t *testing.T) {
+	if err := validateTools(&config.Config{}); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestValidateTools(t *testing.T) {
+	cfg := &config.Config{
+		Tools: &config.Tools{
+			Cargo: []*config.CargoTool{
+				{Name: "taplo-cli", Version: "0.10.0"},
+			},
+		},
+	}
+	if err := validateTools(cfg); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestValidateTools_MissingVersion(t *testing.T) {
+	cfg := &config.Config{
+		Tools: &config.Tools{
+			Cargo: []*config.CargoTool{
+				{Name: "taplo-cli"},
+			},
+		},
+	}
+	err := validateTools(cfg)
+	if !errors.Is(err, rust.ErrMissingToolVersion) {
+		t.Fatalf("got %v, want %v", err, rust.ErrMissingToolVersion)
+	}
+}
+
 func TestFormatConfig(t *testing.T) {
 	cfg := formatConfig(&config.Config{
+		Tools: &config.Tools{
+			Cargo: []*config.CargoTool{
+				{Name: "taplo-cli", Version: "0.10.0"},
+				{Name: "cargo-semver-checks", Version: "0.46.0"},
+			},
+		},
 		Default: &config.Default{
 			Rust: &config.RustDefault{
 				PackageDependencies: []*config.RustPackageDependency{
@@ -157,6 +197,17 @@ func TestFormatConfig(t *testing.T) {
 			t.Errorf("mismatch (-want +got):\n%s", diff)
 		}
 	})
+
+	t.Run("sorts cargo tools by name", func(t *testing.T) {
+		want := []string{"cargo-semver-checks", "taplo-cli"}
+		var got []string
+		for _, tool := range cfg.Tools.Cargo {
+			got = append(got, tool.Name)
+		}
+		if diff := cmp.Diff(want, got); diff != "" {
+			t.Errorf("mismatch (-want +got):\n%s", diff)
+		}
+	})
 }
 
 func TestTidyCommand(t *testing.T) {