fix(internal/semver): add Rust pre-GA change downgrade option (#3217)

Author: noahdietz

internal/librarian/internal/rust/release.go

@@ -83,9 +83,10 @@ func release(cfg *config.Config, name string) (*config.Config, error) {
 		}
 
 		found = true
-		// Only ever take a minor version bump.
-		// TODO(https://github.com/googleapis/librarian/issues/3182): Implement desired pre-1.0.0 semantics.
-		newVersion, err := semver.DeriveNextOptions{BumpVersionCore: true}.DeriveNext(semver.Minor, manifest.Package.Version)
+		newVersion, err := semver.DeriveNextOptions{
+			BumpVersionCore:       true,
+			DowngradePreGAChanges: true,
+		}.DeriveNext(semver.Minor, manifest.Package.Version)
 		if err != nil {
 			return err
 		}

internal/semver/semver.go

@@ -189,6 +189,15 @@ type DeriveNextOptions struct {
 	// Default behavior is to prefer bumping the prerelease number or adding one
 	// when the version is a prerelease without a number.
 	BumpVersionCore bool
+
+	// DowngradePreGAChanges specifically forces [Minor] changes to be treated
+	// as [Patch] changes when the current version is pre-1.0.0. [Major] changes
+	// are always downgraded to [Minor] changes when the current version is
+	// pre-1.0.0 regardless of if this is enabled. This is primarily for Rust.
+	//
+	// This has no effect on prerelease versions unless BumpVersionCore is also
+	// enabled.
+	DowngradePreGAChanges bool
 }
 
 // DeriveNext determines the appropriate SemVer version bump based on the
@@ -223,9 +232,15 @@ func (o DeriveNextOptions) DeriveNext(highestChange ChangeLevel, currentVersion
 		*v.PrereleaseNumber = 1
 	}
 
-	// Breaking changes result in a minor bump for pre-1.0.0 versions.
-	if highestChange == Major && v.Major == 0 {
-		highestChange = Minor
+	// Breaking changes result in a minor bump for pre-1.0.0 versions across
+	// all languages. Some languages, however, prefer to downgrade all pre-1.0.0
+	// changes e.g. Rust.
+	if v.Major == 0 {
+		if highestChange == Major {
+			highestChange = Minor
+		} else if highestChange == Minor && o.DowngradePreGAChanges {
+			highestChange = Patch
+		}
 	}
 
 	// Bump the version core.

internal/semver/semver_test.go

@@ -282,11 +282,18 @@ func TestDeriveNextOptions_DeriveNext(t *testing.T) {
 			expectedVersion: "1.2.4",
 		},
 		{
-			name:            "pre-1.0.0 feat is patch bump",
-			highestChange:   Minor, // feat is minor
+			name:            "pre-1.0.0 feat is minor bump",
+			highestChange:   Minor,
 			currentVersion:  "0.2.3",
 			expectedVersion: "0.3.0",
 		},
+		{
+			name:            "pre-1.0.0 feat with downgrade is patch bump",
+			highestChange:   Minor,
+			currentVersion:  "0.2.3",
+			expectedVersion: "0.2.4",
+			opts:            DeriveNextOptions{DowngradePreGAChanges: true},
+		},
 		{
 			name:            "pre-1.0.0 fix is patch bump",
 			highestChange:   Patch,
@@ -337,6 +344,27 @@ func TestDeriveNextOptions_DeriveNext(t *testing.T) {
 			expectedVersion: "1.3.0-alpha.1",
 			opts:            DeriveNextOptions{BumpVersionCore: true},
 		},
+		{
+			name:            "pre-1.0.0 prerelease with numeric trailer and bump core option",
+			highestChange:   Minor,
+			currentVersion:  "0.2.3-alpha.2",
+			expectedVersion: "0.3.0-alpha.1",
+			opts:            DeriveNextOptions{BumpVersionCore: true},
+		},
+		{
+			name:            "pre-1.0.0 prerelease with numeric trailer, bump core and downgrade options",
+			highestChange:   Minor,
+			currentVersion:  "0.2.3-alpha.2",
+			expectedVersion: "0.2.4-alpha.1",
+			opts:            DeriveNextOptions{BumpVersionCore: true, DowngradePreGAChanges: true},
+		},
+		{
+			name:            "pre-1.0.0 prerelease feat with downgrade, no bump core, is still prerelease bump",
+			highestChange:   Minor,
+			currentVersion:  "0.2.3-alpha.2",
+			expectedVersion: "0.2.3-alpha.3",
+			opts:            DeriveNextOptions{DowngradePreGAChanges: true},
+		},
 	} {
 		t.Run(test.name, func(t *testing.T) {
 			nextVersion, err := test.opts.DeriveNext(test.highestChange, test.currentVersion)

internal/sidekick/rust_release/update_manifest.go

@@ -60,9 +60,10 @@ func updateManifest(config *config.Release, lastTag, manifest string) ([]string,
 	if !info.Package.Publish {
 		return nil, nil
 	}
-	// Only ever take a minor version bump.
-	// TODO(https://github.com/googleapis/librarian/issues/3182): Implement desired pre-1.0.0 semantics.
-	newVersion, err := semver.DeriveNextOptions{BumpVersionCore: true}.DeriveNext(semver.Minor, info.Package.Version)
+	newVersion, err := semver.DeriveNextOptions{
+		BumpVersionCore:       true,
+		DowngradePreGAChanges: true,
+	}.DeriveNext(semver.Minor, info.Package.Version)
 	if err != nil {
 		return nil, err
 	}