xconfig: Add shell expression support and fix choice default selection

[Copilot]

The Kconfig parser failed on shell expressions in default values and didn't apply choice defaults, breaking configuration for multi-architecture builds.

Changes

Shell Expression Evaluation

• Added Expr::ShellExpr variant to AST for $(if condition,then,else) and $(VAR) syntax
• Implemented recursive evaluator in shell_expr.rs handling nested conditionals
• Updated parser to detect and preserve shell expressions in string literals
• Proper escape sequence handling for quoted strings

Choice Default Selection

• Modified extract_symbols_from_entries to process Choice entries recursively
• Apply explicit defaults or select first option per Kconfig spec
• Integrated into both menuconfig TUI and saveconfig command

Example

This now parses and evaluates correctly:

choice
    prompt "Target Architecture"
    default ARCH_AARCH64

config ARCH_AARCH64
    bool "AArch64"

config ARCH_X86_64
    bool "x86_64"
endchoice

config ARCH_NAME
    string
    default "$(if $(ARCH_AARCH64),aarch64,$(if $(ARCH_X86_64),x86_64,unknown))"

Generated config:

ARCH_AARCH64=y
ARCH_NAME="aarch64"

Original prompt

Fix xconfig: Support shell expressions in default values and Choice default selection

Problem 1: Shell Expression Support in Default Values

Currently, the xconfig parser cannot handle Kconfig default values with shell-style conditional expressions like:

config ARCH_NAME
    string
    default "$(if $(ARCH_AARCH64),aarch64,$(if $(ARCH_RISCV64),riscv64,$(if $(ARCH_X86_64),x86_64,$(if $(ARCH_LOONGARCH64),loongarch64,unknown))))"

Root Cause:

• The parser in xtask/xconfig/src/kconfig/parser.rs only supports simple expressions (symbols, constants, numbers)
• It lacks support for $() shell command substitution and nested $(if ...) conditionals
• The lexer doesn't recognize shell expression syntax

Solution:

1. Add shell expression parsing support to handle $(if condition,then,else) syntax
2. Create a shell expression evaluator that can:
   • Parse nested if conditions
   • Evaluate conditions based on symbol table values
   • Recursively expand nested expressions

Problem 2: Choice Default Values Not Applied in TUI

When a Kconfig choice block has a default option specified, the TUI (menuconfig) doesn't correctly select that option by default.

Root Cause:

• In xtask/xconfig/src/ui/app.rs, the MenuConfigApp::new() method initializes values for Config and MenuConfig items
• However, it does not process Choice items to set their default selections
• The ConfigState::build_from_entries() method creates Choice items but doesn't apply the default value

Solution:

1. Add Choice initialization logic in MenuConfigApp::new() after building the config state
2. When a Choice has a default field, set the corresponding option as selected
3. If no default is specified, select the first visible option (standard Kconfig behavior)

Implementation Details

Files to Modify:

1. xtask/xconfig/src/kconfig/shell_expr.rs (NEW FILE)

   • Create shell expression parser and evaluator
   • Support $(if condition, then_value, else_value) syntax
   • Support variable references like $(VARIABLE_NAME)
   • Handle nested expressions recursively

2. xtask/xconfig/src/kconfig/lexer.rs

   • Add support for recognizing shell expressions in string literals
   • Keep shell expressions as-is for later evaluation

3. xtask/xconfig/src/kconfig/parser.rs

   • When parsing default values, check if the string contains shell expressions
   • Store shell expressions in AST for later evaluation

4. xtask/xconfig/src/kconfig/expr.rs

   • Add Expr::ShellExpr(String) variant to the expression enum
   • Update expression evaluator to handle shell expressions

5. xtask/xconfig/src/ui/app.rs

   • Add Choice initialization in MenuConfigApp::new() method
   • After the existing initialization loops (around line 107), add:

// Initialize Choice default selections
for item in &mut config_state.all_items {
    if let MenuItemKind::Choice { options } = &item.kind {
        // Find the Choice AST entry to get its default value
        if let Some(default_option) = find_choice_default(&entries, &item.id) {
            // Set the default option as selected in symbol_table
            symbol_table.set_value(&default_option, "y".to_string());
            
            // Update all choice options visibility/state
            for opt_name in options {
                if opt_name == &default_option {
                    // Mark this as selected
                    if let Some(opt_item) = config_state.all_items.iter_mut().find(|i| i.id == *opt_name) {
                        opt_item.value = Some(ConfigValue::Bool(true));
                    }
                } else {
                    // Mark others as unselected
                    if let Some(opt_item) = config_state.all_items.iter_mut().find(|i| i.id == *opt_name) {
                        opt_item.value = Some(ConfigValue::Bool(false));
                    }
                }
            }
        } else if let Some(first_option) = options.first() {
            // No default specified, select first option
            symbol_table.set_value(first_option, "y".to_string());
            if let Some(opt_item) = config_state.all_items.iter_mut().find(|i| &i.id == first_option) {
                opt_item.value = Some(ConfigValue::Bool(true));
            }
        }
    }
}

// Also update menu_tree
for (_, items) in config_state.menu_tree.iter_mut() {
    for item in items {
        if let MenuItemKind::Choice { options } = &item.kind {
            // Similar logic for menu_tree items
            // ... (duplicate the logic above for menu_tree)
        }
    }
}

6. xtask/xconfig/src/kconfig/mod.rs

   • Export the new shell_expr module

7. xtask/xconfig/src/cli/saveconfig.rs

   • Update default value evaluation to use shell expression evaluator when applicable

Testing Requirements

1. Shell Expression Tests:
   • Test simple $(if SYMBOL,yes,no) expressions
   • Test neste...

This pull request was created from Copilot chat.

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