cargo-rail: feat(config): add cargo rail config sync for config upgrades; add a new command that safely updates rail.toml when upgrading cargo-rail versions.

The command:
  - Adds missing config fields with sensible defaults (27 fields)
  - Union-merges targets from TOML and GitHub workflow YAML files
  - Preserves all existing user values, comments, and formatting
  - Supports --check mode for CI/pre-commit hooks

Schema coverage:
  - [unify]: 16 fields (including new msrv_source, preserve_features)
  - [release]: 10 fields
  - [change-detection]: 1 field (infrastructure)

This replaces the deprecated `cargo rail unify sync` command which only handled target detection. Users upgrading from v0.1.0 to v0.4.0 can now run `cargo rail config sync` to see all available configuration options.

Exit codes: 0 = up to date, 1 = changes needed (--check), 2 = error

Author: loadingalias

.config/rail.toml

@@ -21,6 +21,9 @@ targets = [
   "aarch64-unknown-linux-gnu",
   "x86_64-pc-windows-msvc",
   "x86_64-unknown-linux-gnu",
+  # Tier 2 (Common)
+  "aarch64-unknown-linux-musl",
+  "x86_64-unknown-linux-musl",
   # Other
   "aarch64-pc-windows-msvc",
 ]
@@ -40,6 +43,8 @@ detect_unused = true  # Detect unused dependencies (default: true)
 remove_unused = true  # Auto-remove unused deps when applying (default: true)
 max_backups = 2  # Number of backup files to keep (default: 3)
 prune_dead_features = true  # Remove features never enabled in resolved graph (default: true)
+msrv_source = "max"  # How to compute MSRV: deps, workspace, max (default: max)
+preserve_features = []  # Features to preserve from pruning (glob patterns)
 
 
 [release]

docs/commands.md

@@ -151,19 +151,18 @@ Usage: cargo rail unify [OPTIONS] [COMMAND]
 
 Commands:
   undo  Restore manifests from a previous backup
-  sync  Re-detect and sync targets to rail.toml
   help  Print this message or the help of the given subcommand(s)
 
 Options:
   -q, --quiet
           Suppress progress messages (for CI/automation)
 
-      --json
-          Output in JSON format (shorthand for -f json)
-
   -c, --check
           Dry-run mode: preview changes without modifying files
 
+      --json
+          Output in JSON format (shorthand for -f json)
+
   -f, --format <FORMAT>
           Output format
 
@@ -203,8 +202,6 @@ Examples:
   cargo rail unify --show-diff            # Show manifest changes
   cargo rail unify undo                   # Restore from backup
   cargo rail unify undo --list            # List available backups
-  cargo rail unify sync --check           # Preview target sync
-  cargo rail unify sync                   # Re-detect and merge targets into rail.toml
 ```
 
 ---
@@ -227,34 +224,6 @@ Options:
 
 ---
 
-### cargo rail unify sync
-
-```
-Re-detect and sync targets to rail.toml
-
-Scans workspace for target triples (rust-toolchain.toml, .cargo/config.toml, etc.) and merges any new targets into your rail.toml configuration. Preserves existing targets and all other settings.
-
-Usage: cargo rail unify sync [OPTIONS]
-
-Options:
-  -c, --check
-          Preview changes without modifying rail.toml
-
-  -q, --quiet
-          Suppress progress messages (for CI/automation)
-
-      --json
-          Output in JSON format (shorthand for -f json)
-
-  -h, --help
-          Print help (see a summary with '-h')
-
-  -V, --version
-          Print version
-```
-
----
-
 ## cargo rail init
 
 ```
@@ -704,6 +673,7 @@ Usage: cargo rail config [OPTIONS] <COMMAND>
 
 Commands:
   validate  Validate the configuration file
+  sync      Sync configuration: add missing fields and update targets
   help      Print this message or the help of the given subcommand(s)
 
 Options:
@@ -722,6 +692,8 @@ Options:
 Examples:
   cargo rail config validate            # Validate rail.toml
   cargo rail config validate -f json    # JSON output for CI
+  cargo rail config sync --check        # Preview config updates
+  cargo rail config sync                # Add missing fields, sync targets
 ```
 
 ---
@@ -760,3 +732,47 @@ Options:
   -V, --version
           Print version
 ```
+
+---
+
+### cargo rail config sync
+
+```
+Sync configuration: add missing fields and update targets
+
+Scans the workspace for target triples and adds any missing config fields with their default values. Preserves all existing settings, comments, and formatting.
+
+Use this after upgrading cargo-rail to get new configuration options.
+
+Usage: cargo rail config sync [OPTIONS]
+
+Options:
+  -c, --check
+          Preview changes without modifying rail.toml
+
+  -q, --quiet
+          Suppress progress messages (for CI/automation)
+
+  -f, --format <FORMAT>
+          Output format
+
+          Possible values:
+          - text:          Human-readable text output (default)
+          - json:          Machine-readable JSON output
+          - names-only:    Names only, one per line
+          - cargo-args:    Cargo -p flag format: -p crate1 -p crate2
+          - github:        GitHub Actions output format for $GITHUB_OUTPUT
+          - github-matrix: GitHub Actions matrix format for strategy.matrix
+          - jsonl:         JSON Lines format (one object per line)
+          
+          [default: text]
+
+      --json
+          Output in JSON format (shorthand for -f json)
+
+  -h, --help
+          Print help (see a summary with '-h')
+
+  -V, --version
+          Print version
+```

src/commands/cli.rs

@@ -78,9 +78,7 @@ Examples:
   cargo rail unify --backup               # Apply with backup
   cargo rail unify --show-diff            # Show manifest changes
   cargo rail unify undo                   # Restore from backup
-  cargo rail unify undo --list            # List available backups
-  cargo rail unify sync --check           # Preview target sync
-  cargo rail unify sync                   # Re-detect and merge targets into rail.toml";
+  cargo rail unify undo --list            # List available backups";
 
 const SPLIT_HELP: &str = "\
 Examples:
@@ -128,7 +126,9 @@ Examples:
 const CONFIG_HELP: &str = "\
 Examples:
   cargo rail config validate            # Validate rail.toml
-  cargo rail config validate -f json    # JSON output for CI";
+  cargo rail config validate -f json    # JSON output for CI
+  cargo rail config sync --check        # Preview config updates
+  cargo rail config sync                # Add missing fields, sync targets";
 
 /// Available subcommands
 #[derive(Subcommand)]
@@ -303,6 +303,21 @@ pub enum ConfigCommand {
     #[arg(long, short = 'f', default_value_t, value_enum)]
     format: OutputFormat,
   },
+  /// Sync configuration: add missing fields and update targets
+  ///
+  /// Scans the workspace for target triples and adds any missing config
+  /// fields with their default values. Preserves all existing settings,
+  /// comments, and formatting.
+  ///
+  /// Use this after upgrading cargo-rail to get new configuration options.
+  Sync {
+    /// Preview changes without modifying rail.toml
+    #[arg(long, short = 'c')]
+    check: bool,
+    /// Output format
+    #[arg(long, short = 'f', default_value_t, value_enum)]
+    format: OutputFormat,
+  },
 }
 
 /// Subcommands for `cargo rail unify`
@@ -317,16 +332,6 @@ pub enum UnifyCommand {
     #[arg(long = "backup-id")]
     backup_id: Option<String>,
   },
-  /// Re-detect and sync targets to rail.toml
-  ///
-  /// Scans workspace for target triples (rust-toolchain.toml, .cargo/config.toml, etc.)
-  /// and merges any new targets into your rail.toml configuration.
-  /// Preserves existing targets and all other settings.
-  Sync {
-    /// Preview changes without modifying rail.toml
-    #[arg(long, short = 'c')]
-    check: bool,
-  },
 }
 
 /// Subcommands for `cargo rail split`
@@ -436,7 +441,7 @@ impl Commands {
         ReleaseCommand::Run { format, .. } | ReleaseCommand::Check { format, .. } => format.is_json_like(),
       },
       Commands::Config { command } => match command {
-        ConfigCommand::Validate { format } => format.is_json_like(),
+        ConfigCommand::Validate { format } | ConfigCommand::Sync { format, .. } => format.is_json_like(),
       },
       _ => false,
     }
@@ -458,7 +463,7 @@ impl Commands {
       } => *format = OutputFormat::Json,
       Commands::Release { .. } => {}
       Commands::Config { command } => match command {
-        ConfigCommand::Validate { format } => *format = OutputFormat::Json,
+        ConfigCommand::Validate { format } | ConfigCommand::Sync { format, .. } => *format = OutputFormat::Json,
       },
       _ => {}
     }