cli: add +explain-config

This is a new CLI action that prints an option or keybind's help
documentation to stdout.

    ghostty +explain-config font-size
    ghostty +explain-config copy_to_clipboard
    ghostty +explain-config --option=font-size
    ghostty +explain-config --keybind=copy_to_clipboard

The --option and --keybind flags perform a specific lookup. A string
passed as a positional argument attempts to look up the name first as an
option and then as a keybind.

Our vim plugin uses this with &keywordprg, which allows you to look up
the documentation for the config option or keybind under the cursor (K).

src/cli/explain_config.zig

@@ -0,0 +1,130 @@
+const std = @import("std");
+const args = @import("args.zig");
+const Allocator = std.mem.Allocator;
+const Action = @import("ghostty.zig").Action;
+const help_strings = @import("help_strings");
+const Config = @import("../config/Config.zig");
+const ConfigKey = @import("../config/key.zig").Key;
+const KeybindAction = @import("../input/Binding.zig").Action;
+
+pub const Options = struct {
+    /// The config option to explain. For example:
+    ///
+    ///   ghostty +explain-config --option=font-size
+    option: ?[]const u8 = null,
+
+    /// The keybind action to explain. For example:
+    ///
+    ///   ghostty +explain-config --keybind=copy_to_clipboard
+    keybind: ?[]const u8 = null,
+
+    pub fn deinit(self: Options) void {
+        _ = self;
+    }
+
+    /// Enables `-h` and `--help` to work.
+    pub fn help(self: Options) !void {
+        _ = self;
+        return Action.help_error;
+    }
+};
+
+/// The `explain-config` command prints the documentation for a single
+/// Ghostty configuration option or keybind action.
+///
+/// Examples:
+///
+///   ghostty +explain-config font-size
+///   ghostty +explain-config copy_to_clipboard
+///   ghostty +explain-config --option=font-size
+///   ghostty +explain-config --keybind=copy_to_clipboard
+///
+/// Flags:
+///
+///   * `--option`: The name of the configuration option to explain.
+///   * `--keybind`: The name of the keybind action to explain.
+pub fn run(alloc: Allocator) !u8 {
+    var option_name: ?[]const u8 = null;
+    var keybind_name: ?[]const u8 = null;
+    var positional: ?[]const u8 = null;
+    var iter = try args.argsIterator(alloc);
+    defer iter.deinit();
+
+    while (iter.next()) |arg| {
+        if (std.mem.startsWith(u8, arg, "--option=")) {
+            option_name = arg["--option=".len..];
+        } else if (std.mem.startsWith(u8, arg, "--keybind=")) {
+            keybind_name = arg["--keybind=".len..];
+        } else if (std.mem.eql(u8, arg, "--help") or std.mem.eql(u8, arg, "-h")) {
+            return Action.help_error;
+        } else if (!std.mem.startsWith(u8, arg, "-")) {
+            positional = arg;
+        }
+    }
+
+    // Resolve what to look up. Explicit flags go directly to their
+    // respective lookup. A bare positional argument tries config
+    // options first, then keybind actions as a fallback.
+    const name = keybind_name orelse option_name orelse positional orelse {
+        var stderr: std.fs.File = .stderr();
+        var buffer: [4096]u8 = undefined;
+        var stderr_writer = stderr.writer(&buffer);
+        try stderr_writer.interface.writeAll("Usage: ghostty +explain-config <option>\n");
+        try stderr_writer.interface.writeAll("       ghostty +explain-config --option=<option>\n");
+        try stderr_writer.interface.writeAll("       ghostty +explain-config --keybind=<action>\n");
+        try stderr_writer.end();
+        return 1;
+    };
+
+    const text = if (keybind_name != null)
+        explainKeybind(name)
+    else if (option_name != null)
+        explainOption(name)
+    else
+        explainOption(name) orelse explainKeybind(name);
+
+    var stdout: std.fs.File = .stdout();
+    var buffer: [4096]u8 = undefined;
+    var stdout_writer = stdout.writer(&buffer);
+    const writer = &stdout_writer.interface;
+
+    if (text) |t| {
+        try writer.writeAll(t);
+        try writer.writeAll("\n");
+    } else {
+        try writer.writeAll("Unknown: '");
+        try writer.writeAll(name);
+        try writer.writeAll("'.\n");
+        try stdout_writer.end();
+        return 1;
+    }
+
+    try stdout_writer.end();
+    return 0;
+}
+
+fn explainOption(name: []const u8) ?[]const u8 {
+    const key = std.meta.stringToEnum(ConfigKey, name) orelse return null;
+    return switch (key) {
+        inline else => |tag| {
+            const field_name = @tagName(tag);
+            return if (@hasDecl(help_strings.Config, field_name))
+                @field(help_strings.Config, field_name)
+            else
+                null;
+        },
+    };
+}
+
+fn explainKeybind(name: []const u8) ?[]const u8 {
+    const tag = std.meta.stringToEnum(std.meta.Tag(KeybindAction), name) orelse return null;
+    return switch (tag) {
+        inline else => |t| {
+            const field_name = @tagName(t);
+            return if (@hasDecl(help_strings.KeybindAction, field_name))
+                @field(help_strings.KeybindAction, field_name)
+            else
+                null;
+        },
+    };
+}

src/cli/ghostty.zig

@@ -14,6 +14,7 @@ const list_actions = @import("list_actions.zig");
 const ssh_cache = @import("ssh_cache.zig");
 const edit_config = @import("edit_config.zig");
 const show_config = @import("show_config.zig");
+const explain_config = @import("explain_config.zig");
 const validate_config = @import("validate_config.zig");
 const crash_report = @import("crash_report.zig");
 const show_face = @import("show_face.zig");
@@ -54,6 +55,9 @@ pub const Action = enum {
     /// Dump the config to stdout
     @"show-config",
 
+    /// Explain a single config option
+    @"explain-config",
+
     // Validate passed config file
     @"validate-config",
 
@@ -142,6 +146,7 @@ pub const Action = enum {
             .@"ssh-cache" => try ssh_cache.run(alloc),
             .@"edit-config" => try edit_config.run(alloc),
             .@"show-config" => try show_config.run(alloc),
+            .@"explain-config" => try explain_config.run(alloc),
             .@"validate-config" => try validate_config.run(alloc),
             .@"crash-report" => try crash_report.run(alloc),
             .@"show-face" => try show_face.run(alloc),
@@ -181,6 +186,7 @@ pub const Action = enum {
                 .@"ssh-cache" => ssh_cache.Options,
                 .@"edit-config" => edit_config.Options,
                 .@"show-config" => show_config.Options,
+                .@"explain-config" => explain_config.Options,
                 .@"validate-config" => validate_config.Options,
                 .@"crash-report" => crash_report.Options,
                 .@"show-face" => show_face.Options,

src/extra/vim.zig

@@ -31,7 +31,10 @@ pub const ftplugin =
     \\" Use syntax keywords for completion
     \\setlocal omnifunc=syntaxcomplete#Complete
     \\
-    \\let b:undo_ftplugin = 'setl cms< isk< ofu<'
+    \\" Ask ghostty to explain config keywords
+    \\setlocal keywordprg=ghostty\ +explain-config
+    \\
+    \\let b:undo_ftplugin = 'setl cms< isk< ofu< kp<'
     \\
     \\if !exists('current_compiler')
     \\  compiler ghostty