// The fish parser. Contains functions for parsing and evaluating code.
#include "config.h"  // IWYU pragma: keep

#include "parser.h"

#include <fcntl.h>
#include <stdio.h>

#include <algorithm>
#include <cwchar>
#include <memory>
#include <utility>

#include "ast.h"
#include "common.h"
#include "env.h"
#include "event.h"
#include "expand.h"
#include "fallback.h"  // IWYU pragma: keep
#include "flog.h"
#include "function.h"
#include "intern.h"
#include "job_group.h"
#include "parse_constants.h"
#include "parse_execution.h"
#include "parse_util.h"
#include "proc.h"
#include "reader.h"
#include "sanity.h"
#include "signal.h"
#include "wutil.h"  // IWYU pragma: keep

class io_chain_t;

/// Error for evaluating in illegal scope.
#define INVALID_SCOPE_ERR_MSG _(L"Tried to evaluate commands using invalid block type '%ls'")

/// While block description.
#define WHILE_BLOCK N_(L"'while' block")

/// For block description.
#define FOR_BLOCK N_(L"'for' block")

/// Breakpoint block.
#define BREAKPOINT_BLOCK N_(L"block created by breakpoint")

/// Variable assignment block.
#define VARIABLE_ASSIGNMENT_BLOCK N_(L"block created by variable assignment prefixing a command")

/// If block description.
#define IF_BLOCK N_(L"'if' conditional block")

/// Function invocation block description.
#define FUNCTION_CALL_BLOCK N_(L"function invocation block")

/// Function invocation block description.
#define FUNCTION_CALL_NO_SHADOW_BLOCK N_(L"function invocation block with no variable shadowing")

/// Switch block description.
#define SWITCH_BLOCK N_(L"'switch' block")

/// Top block description.
#define TOP_BLOCK N_(L"global root block")

/// Command substitution block description.
#define SUBST_BLOCK N_(L"command substitution block")

/// Begin block description.
#define BEGIN_BLOCK N_(L"'begin' unconditional block")

/// Source block description.
#define SOURCE_BLOCK N_(L"block created by the . builtin")

/// Source block description.
#define EVENT_BLOCK N_(L"event handler block")

/// Unknown block description.
#define UNKNOWN_BLOCK N_(L"unknown/invalid block")

// Given a file path, return something nicer. Currently we just "unexpand" tildes.
static wcstring user_presentable_path(const wcstring &path, const environment_t &vars) {
    return replace_home_directory_with_tilde(path, vars);
}

parser_t::parser_t(std::shared_ptr<env_stack_t> vars) : variables(std::move(vars)) {
    assert(variables.get() && "Null variables in parser initializer");
    int cwd = open_cloexec(".", O_RDONLY);
    if (cwd < 0) {
        perror("Unable to open the current working directory");
        return;
    }
    libdata().cwd_fd = std::make_shared<const autoclose_fd_t>(cwd);
}

parser_t::parser_t() : parser_t(env_stack_t::principal_ref()) {}

// Out of line destructor to enable forward declaration of parse_execution_context_t
parser_t::~parser_t() = default;

std::shared_ptr<parser_t> parser_t::principal{new parser_t()};

parser_t &parser_t::principal_parser() {
    ASSERT_IS_MAIN_THREAD();
    return *principal;
}

int parser_t::set_var_and_fire(const wcstring &key, env_mode_flags_t mode, wcstring_list_t vals) {
    std::vector<event_t> events;
    int res = vars().set(key, mode, std::move(vals), &events);
    for (const auto &evt : events) {
        event_fire(*this, evt);
    }
    return res;
}

int parser_t::set_var_and_fire(const wcstring &key, env_mode_flags_t mode, wcstring val) {
    wcstring_list_t vals;
    vals.push_back(std::move(val));
    return set_var_and_fire(key, mode, std::move(vals));
}

int parser_t::set_empty_var_and_fire(const wcstring &key, env_mode_flags_t mode) {
    return set_var_and_fire(key, mode, wcstring_list_t{});
}

// Given a new-allocated block, push it onto our block list, acquiring ownership.
block_t *parser_t::push_block(block_t &&block) {
    block_t new_current{block};
    const enum block_type_t type = new_current.type();
    new_current.src_lineno = parser_t::get_lineno();

    wcstring func = new_current.function_name;

    const wchar_t *filename = parser_t::current_filename();
    if (filename != nullptr) {
        new_current.src_filename = intern(filename);
    }

    // Types top and subst are not considered blocks for the purposes of `status is-block`.
    if (type != block_type_t::top && type != block_type_t::subst) {
        libdata().is_block = true;
    }

    if (type == block_type_t::breakpoint) {
        libdata().is_breakpoint = true;
    }

    if (new_current.type() != block_type_t::top) {
        bool shadow = (type == block_type_t::function_call);
        vars().push(shadow);
        new_current.wants_pop_env = true;
    }

    // Push it onto our list and return a pointer to it.
    // Note that deques do not move their contents so this is safe.
    this->block_list.push_front(std::move(new_current));
    return &this->block_list.front();
}

void parser_t::pop_block(const block_t *expected) {
    assert(expected == this->current_block());
    assert(!block_list.empty() && "empty block list");

    // Acquire ownership out of the block list.
    block_t old = block_list.front();
    block_list.pop_front();

    if (old.wants_pop_env) vars().pop();

    // Figure out if `status is-block` should consider us to be in a block now.
    bool new_is_block = false;
    for (const auto &b : block_list) {
        if (b.type() != block_type_t::top && b.type() != block_type_t::subst) {
            new_is_block = true;
            break;
        }
    }
    libdata().is_block = new_is_block;

    // Are we still in a breakpoint?
    bool new_is_breakpoint = false;
    for (const auto &b : block_list) {
        if (b.type() == block_type_t::breakpoint) {
            new_is_breakpoint = true;
            break;
        }
    }
    libdata().is_breakpoint = new_is_breakpoint;
}

const wchar_t *parser_t::get_block_desc(block_type_t block) {
    switch (block) {
        case block_type_t::while_block:
            return WHILE_BLOCK;
        case block_type_t::for_block:
            return FOR_BLOCK;
        case block_type_t::if_block:
            return IF_BLOCK;
        case block_type_t::function_call:
            return FUNCTION_CALL_BLOCK;

        case block_type_t::function_call_no_shadow:
            return FUNCTION_CALL_NO_SHADOW_BLOCK;
        case block_type_t::switch_block:
            return SWITCH_BLOCK;
        case block_type_t::subst:
            return SUBST_BLOCK;
        case block_type_t::top:
            return TOP_BLOCK;
        case block_type_t::begin:
            return BEGIN_BLOCK;
        case block_type_t::source:
            return SOURCE_BLOCK;
        case block_type_t::event:
            return EVENT_BLOCK;
        case block_type_t::breakpoint:
            return BREAKPOINT_BLOCK;
        case block_type_t::variable_assignment:
            return VARIABLE_ASSIGNMENT_BLOCK;
    }
    return _(UNKNOWN_BLOCK);
}

#if 0
// TODO: Lint says this isn't used (which is true). Should this be removed?
wcstring parser_t::block_stack_description() const {
    wcstring result;
    size_t idx = this->block_count();
    size_t spaces = 0;
    while (idx--) {
        if (spaces > 0) {
            result.push_back(L'\n');
        }
        for (size_t j = 0; j < spaces; j++) {
            result.push_back(L' ');
        }
        result.append(this->block_at_index(idx)->description());
        spaces++;
    }
    return result;
}
#endif

const block_t *parser_t::block_at_index(size_t idx) const {
    return idx < block_list.size() ? &block_list[idx] : nullptr;
}

block_t *parser_t::block_at_index(size_t idx) {
    return idx < block_list.size() ? &block_list[idx] : nullptr;
}

block_t *parser_t::current_block() { return block_at_index(0); }

/// Print profiling information to the specified stream.
static void print_profile(const std::deque<profile_item_t> &items, FILE *out) {
    for (size_t idx = 0; idx < items.size(); idx++) {
        const profile_item_t &item = items.at(idx);
        if (item.skipped || item.cmd.empty()) continue;

        long long total_time = item.duration;

        // Compute the self time as the total time, minus the total time consumed by subsequent
        // items exactly one eval level deeper.
        long long self_time = item.duration;
        for (size_t i = idx + 1; i < items.size(); i++) {
            const profile_item_t &nested_item = items.at(i);
            if (nested_item.skipped) continue;

            // If the eval level is not larger, then we have exhausted nested items.
            if (nested_item.level <= item.level) break;

            // If the eval level is exactly one more than our level, it is a directly nested item.
            if (nested_item.level == item.level + 1) self_time -= nested_item.duration;
        }

        if (std::fwprintf(out, L"%lld\t%lld\t", self_time, total_time) < 0) {
            wperror(L"fwprintf");
            return;
        }

        for (size_t i = 0; i < item.level; i++) {
            if (std::fwprintf(out, L"-") < 0) {
                wperror(L"fwprintf");
                return;
            }
        }

        if (std::fwprintf(out, L"> %ls\n", item.cmd.c_str()) < 0) {
            wperror(L"fwprintf");
            return;
        }
    }
}

void parser_t::emit_profiling(const char *path) const {
    // Save profiling information. OK to not use CLO_EXEC here because this is called while fish is
    // exiting (and hence will not fork).
    FILE *f = fopen(path, "w");
    if (!f) {
        FLOGF(warning, _(L"Could not write profiling information to file '%s'"), path);
    } else {
        if (std::fwprintf(f, _(L"Time\tSum\tCommand\n"), profile_items.size()) < 0) {
            wperror(L"fwprintf");
        } else {
            print_profile(profile_items, f);
        }

        if (fclose(f)) {
            wperror(L"fclose");
        }
    }
}

completion_list_t parser_t::expand_argument_list(const wcstring &arg_list_src,
                                                 expand_flags_t eflags,
                                                 const operation_context_t &ctx) {
    // Parse the string as an argument list.
    auto ast = ast::ast_t::parse_argument_list(arg_list_src);
    if (ast.errored()) {
        // Failed to parse. Here we expect to have reported any errors in test_args.
        return {};
    }

    // Get the root argument list and extract arguments from it.
    completion_list_t result;
    const ast::freestanding_argument_list_t *list =
        ast.top()->as<ast::freestanding_argument_list_t>();
    for (const ast::argument_t &arg : list->arguments) {
        wcstring arg_src = arg.source(arg_list_src);
        if (expand_string(arg_src, &result, eflags, ctx) == expand_result_t::error) {
            break;  // failed to expand a string
        }
    }
    return result;
}

std::shared_ptr<parser_t> parser_t::shared() { return shared_from_this(); }

cancel_checker_t parser_t::cancel_checker() const {
    return [] { return signal_check_cancel() != 0; };
}

operation_context_t parser_t::context() {
    return operation_context_t{this->shared(), this->vars(), this->cancel_checker()};
}

/// Append stack trace info for the block \p b to \p trace.
static void append_block_description_to_stack_trace(const parser_t &parser, const block_t &b,
                                                    wcstring &trace) {
    bool print_call_site = false;
    switch (b.type()) {
        case block_type_t::function_call:
        case block_type_t::function_call_no_shadow: {
            append_format(trace, _(L"in function '%ls'"), b.function_name.c_str());
            // Print arguments on the same line.
            wcstring args_str;
            for (const wcstring &arg : b.function_args) {
                if (!args_str.empty()) args_str.push_back(L' ');
                // We can't quote the arguments because we print this in quotes.
                // As a special-case, add the empty argument as "".
                if (!arg.empty()) {
                    args_str.append(escape_string(arg, ESCAPE_ALL | ESCAPE_NO_QUOTED));
                } else {
                    args_str.append(L"\"\"");
                }
            }
            if (!args_str.empty()) {
                // TODO: Escape these.
                append_format(trace, _(L" with arguments '%ls'"), args_str.c_str());
            }
            trace.push_back('\n');
            print_call_site = true;
            break;
        }
        case block_type_t::subst: {
            append_format(trace, _(L"in command substitution\n"));
            print_call_site = true;
            break;
        }
        case block_type_t::source: {
            const wchar_t *source_dest = b.sourced_file;
            append_format(trace, _(L"from sourcing file %ls\n"),
                          user_presentable_path(source_dest, parser.vars()).c_str());
            print_call_site = true;
            break;
        }
        case block_type_t::event: {
            assert(b.event && "Should have an event");
            wcstring description = event_get_desc(parser, *b.event);
            append_format(trace, _(L"in event handler: %ls\n"), description.c_str());
            print_call_site = true;
            break;
        }

        case block_type_t::top:
        case block_type_t::begin:
        case block_type_t::switch_block:
        case block_type_t::while_block:
        case block_type_t::for_block:
        case block_type_t::if_block:
        case block_type_t::breakpoint:
        case block_type_t::variable_assignment:
            break;
    }

    if (print_call_site) {
        // Print where the function is called.
        const wchar_t *file = b.src_filename;
        if (file) {
            append_format(trace, _(L"\tcalled on line %d of file %ls\n"), b.src_lineno,
                          user_presentable_path(file, parser.vars()).c_str());
        } else if (is_within_fish_initialization()) {
            append_format(trace, _(L"\tcalled during startup\n"));
        }
    }
}

wcstring parser_t::stack_trace() const {
    wcstring trace;
    for (const auto &b : blocks()) {
        append_block_description_to_stack_trace(*this, b, trace);

        // Stop at event handler. No reason to believe that any other code is relevant.
        //
        // It might make sense in the future to continue printing the stack trace of the code
        // that invoked the event, if this is a programmatic event, but we can't currently
        // detect that.
        if (b.type() == block_type_t::event) break;
    }
    return trace;
}

/// Returns the name of the currently evaluated function if we are currently evaluating a function,
/// NULL otherwise. This is tested by moving down the block-scope-stack, checking every block if it
/// is of type FUNCTION_CALL. If the caller doesn't specify a starting position in the stack we
/// begin with the current block.
const wchar_t *parser_t::is_function(size_t idx) const {
    // PCA: Have to make this a string somehow.
    ASSERT_IS_MAIN_THREAD();

    for (size_t block_idx = idx; block_idx < block_list.size(); block_idx++) {
        const block_t &b = block_list[block_idx];
        if (b.is_function_call()) {
            return b.function_name.c_str();
        } else if (b.type() == block_type_t::source) {
            // If a function sources a file, obviously that function's offset doesn't
            // contribute.
            break;
        }
    }
    return nullptr;
}

/// Return the function name for the specified stack frame. Default is zero (current frame).
/// The special value zero means the function frame immediately above the closest breakpoint frame.
const wchar_t *parser_t::get_function_name(int level) {
    if (level == 0) {
        // Return the function name for the level preceding the most recent breakpoint. If there
        // isn't one return the function name for the current level.
        // Walk until we find a breakpoint, then take the next function.
        bool found_breakpoint = false;
        for (const auto &b : block_list) {
            if (b.type() == block_type_t::breakpoint) {
                found_breakpoint = true;
            } else if (found_breakpoint && b.is_function_call()) {
                return b.function_name.c_str();
            }
        }
        return nullptr;  // couldn't find a breakpoint frame
    } else if (level == 1) {
        // Return the function name for the current level.
        return this->is_function();
    }

    // Level 1 is the topmost function call. Level 2 is its caller. Etc.
    int funcs_seen = 0;
    for (const auto &b : block_list) {
        if (b.is_function_call()) {
            funcs_seen++;
            if (funcs_seen == level) {
                return b.function_name.c_str();
            }
        }
    }
    return nullptr;  // couldn't find that function level
}

int parser_t::get_lineno() const {
    int lineno = -1;
    if (execution_context) {
        lineno = execution_context->get_current_line_number();
    }
    return lineno;
}

const wchar_t *parser_t::current_filename() const {
    ASSERT_IS_MAIN_THREAD();

    for (const auto &b : block_list) {
        if (b.is_function_call()) {
            return function_get_definition_file(b.function_name);
        } else if (b.type() == block_type_t::source) {
            return b.sourced_file;
        }
    }
    // Fall back to the file being sourced.
    return libdata().current_filename;
}

bool parser_t::function_stack_is_overflowing() const {
    // We are interested in whether the count of functions on the stack exceeds
    // FISH_MAX_STACK_DEPTH. We don't separately track the number of functions, but we can have a
    // fast path through the eval_level. If the eval_level is in bounds, so must be the stack depth.
    if (eval_level <= FISH_MAX_STACK_DEPTH) {
        return false;
    }
    // Count the functions.
    int depth = 0;
    for (const auto &b : block_list) {
        depth += b.is_function_call();
    }
    return depth > FISH_MAX_STACK_DEPTH;
}

wcstring parser_t::current_line() {
    if (!execution_context) {
        return wcstring();
    }
    int source_offset = execution_context->get_current_source_offset();
    if (source_offset < 0) {
        return wcstring();
    }

    const int lineno = this->get_lineno();
    const wchar_t *file = this->current_filename();

    wcstring prefix;

    // If we are not going to print a stack trace, at least print the line number and filename.
    if (!is_interactive() || is_function()) {
        if (file) {
            append_format(prefix, _(L"%ls (line %d): "),
                          user_presentable_path(file, vars()).c_str(), lineno);
        } else if (is_within_fish_initialization()) {
            append_format(prefix, L"%ls (line %d): ", _(L"Startup"), lineno);
        } else {
            append_format(prefix, L"%ls (line %d): ", _(L"Standard input"), lineno);
        }
    }

    bool skip_caret = is_interactive() && !is_function();

    // Use an error with empty text.
    assert(source_offset >= 0);
    parse_error_t empty_error = {};
    empty_error.source_start = source_offset;

    wcstring line_info = empty_error.describe_with_prefix(execution_context->get_source(), prefix,
                                                          is_interactive(), skip_caret);
    if (!line_info.empty()) {
        line_info.push_back(L'\n');
    }

    line_info.append(this->stack_trace());
    return line_info;
}

void parser_t::job_add(shared_ptr<job_t> job) {
    assert(job != nullptr);
    assert(!job->processes.empty());
    job_list.push_front(std::move(job));
}

void parser_t::job_promote(job_t *job) {
    job_list_t::iterator loc;
    for (loc = job_list.begin(); loc != job_list.end(); ++loc) {
        if (loc->get() == job) {
            break;
        }
    }
    assert(loc != job_list.end());

    // Move the job to the beginning.
    std::rotate(job_list.begin(), loc, std::next(loc));
}

job_t *parser_t::job_get(job_id_t id) {
    for (const auto &job : job_list) {
        if (id <= 0 || job->job_id() == id) return job.get();
    }
    return nullptr;
}

const job_t *parser_t::job_get(job_id_t id) const {
    for (const auto &job : job_list) {
        if (id <= 0 || job->job_id() == id) return job.get();
    }
    return nullptr;
}

job_t *parser_t::job_get_from_pid(pid_t pid) const {
    for (const auto &job : jobs()) {
        for (const process_ptr_t &p : job->processes) {
            if (p->pid == pid) {
                return job.get();
            }
        }
    }
    return nullptr;
}

profile_item_t *parser_t::create_profile_item() {
    if (g_profiling_active) {
        profile_items.emplace_back();
        return &profile_items.back();
    }
    return nullptr;
}

eval_res_t parser_t::eval(const wcstring &cmd, const io_chain_t &io,
                          const job_group_ref_t &job_group, enum block_type_t block_type) {
    // Parse the source into a tree, if we can.
    parse_error_list_t error_list;
    if (parsed_source_ref_t ps = parse_source(wcstring{cmd}, parse_flag_none, &error_list)) {
        return this->eval(ps, io, job_group, block_type);
    } else {
        // Get a backtrace. This includes the message.
        wcstring backtrace_and_desc;
        this->get_backtrace(cmd, error_list, backtrace_and_desc);

        // Print it.
        std::fwprintf(stderr, L"%ls\n", backtrace_and_desc.c_str());

        // Set a valid status.
        this->set_last_statuses(statuses_t::just(STATUS_ILLEGAL_CMD));
        bool break_expand = true;
        return eval_res_t{proc_status_t::from_exit_code(STATUS_ILLEGAL_CMD), break_expand};
    }
}

eval_res_t parser_t::eval(const parsed_source_ref_t &ps, const io_chain_t &io,
                          const job_group_ref_t &job_group, enum block_type_t block_type) {
    assert(block_type == block_type_t::top || block_type == block_type_t::subst);
    const auto *job_list = ps->ast.top()->as<ast::job_list_t>();
    if (!job_list->empty()) {
        // Execute the top job list.
        return this->eval_node(ps, *job_list, io, job_group, block_type);
    } else {
        auto status = proc_status_t::from_exit_code(get_last_status());
        bool break_expand = false;
        bool was_empty = true;
        return eval_res_t{status, break_expand, was_empty};
    }
}

template <typename T>
eval_res_t parser_t::eval_node(const parsed_source_ref_t &ps, const T &node,
                               const io_chain_t &block_io, const job_group_ref_t &job_group,
                               block_type_t block_type) {
    static_assert(
        std::is_same<T, ast::statement_t>::value || std::is_same<T, ast::job_list_t>::value,
        "Unexpected node type");

    // Only certain blocks are allowed.
    assert((block_type == block_type_t::top || block_type == block_type_t::subst) &&
           "Invalid block type");

    // If fish itself got a cancel signal, then we want to unwind back to the principal parser.
    // If we are the principal parser and our block stack is empty, then we want to clear the
    // signal.
    // Note this only happens in interactive sessions. In non-interactive sessions, SIGINT will
    // cause fish to exit.
    if (int sig = signal_check_cancel()) {
        if (this == principal.get() && block_list.empty()) {
            signal_clear_cancel();
        } else {
            return proc_status_t::from_signal(sig);
        }
    }

    // A helper to detect if we got a signal.
    // This includes both signals sent to fish (user hit control-C while fish is foreground) and
    // signals from the job group (e.g. some external job terminated with SIGQUIT).
    auto check_cancel_signal = [=] {
        // Did fish itself get a signal?
        int sig = signal_check_cancel();
        // Has this job group been cancelled?
        if (!sig && job_group) sig = job_group->get_cancel_signal();
        return sig;
    };

    // If we have a job group which is cancelled, then do nothing.
    if (int sig = check_cancel_signal()) {
        return proc_status_t::from_signal(sig);
    }

    job_reap(*this, false);  // not sure why we reap jobs here

    // Start it up
    operation_context_t op_ctx = this->context();
    block_t *scope_block = this->push_block(block_t::scope_block(block_type));

    // Propogate our job group.
    op_ctx.job_group = job_group;

    // Replace the context's cancel checker with one that checks the job group's signal.
    op_ctx.cancel_checker = [=] { return check_cancel_signal() != 0; };

    // Create and set a new execution context.
    using exc_ctx_ref_t = std::unique_ptr<parse_execution_context_t>;
    scoped_push<exc_ctx_ref_t> exc(&execution_context,
                                   make_unique<parse_execution_context_t>(ps, op_ctx, block_io));

    // Check the exec count so we know if anything got executed.
    const size_t prev_exec_count = libdata().exec_count;
    end_execution_reason_t reason = execution_context->eval_node(node, scope_block);
    const size_t new_exec_count = libdata().exec_count;

    exc.restore();
    this->pop_block(scope_block);

    job_reap(*this, false);  // reap again

    if (int sig = check_cancel_signal()) {
        // We were signalled.
        return proc_status_t::from_signal(sig);
    } else {
        auto status = proc_status_t::from_exit_code(this->get_last_status());
        bool break_expand = (reason == end_execution_reason_t::error);
        bool was_empty = !break_expand && prev_exec_count == new_exec_count;
        return eval_res_t{status, break_expand, was_empty};
    }
}

// Explicit instantiations. TODO: use overloads instead?
template eval_res_t parser_t::eval_node(const parsed_source_ref_t &, const ast::statement_t &,
                                        const io_chain_t &, const job_group_ref_t &, block_type_t);
template eval_res_t parser_t::eval_node(const parsed_source_ref_t &, const ast::job_list_t &,
                                        const io_chain_t &, const job_group_ref_t &, block_type_t);

void parser_t::get_backtrace(const wcstring &src, const parse_error_list_t &errors,
                             wcstring &output) const {
    if (!errors.empty()) {
        const parse_error_t &err = errors.at(0);

        // Determine if we want to try to print a caret to point at the source error. The
        // err.source_start <= src.size() check is due to the nasty way that slices work, which is
        // by rewriting the source.
        size_t which_line = 0;
        bool skip_caret = true;
        if (err.source_start != SOURCE_LOCATION_UNKNOWN && err.source_start <= src.size()) {
            // Determine which line we're on.
            which_line = 1 + std::count(src.begin(), src.begin() + err.source_start, L'\n');

            // Don't include the caret if we're interactive, this is the first line of text, and our
            // source is at its beginning, because then it's obvious.
            skip_caret = (is_interactive() && which_line == 1 && err.source_start == 0);
        }

        wcstring prefix;
        const wchar_t *filename = this->current_filename();
        if (filename) {
            if (which_line > 0) {
                prefix = format_string(_(L"%ls (line %lu): "),
                                       user_presentable_path(filename, vars()).c_str(), which_line);
            } else {
                prefix =
                    format_string(_(L"%ls: "), user_presentable_path(filename, vars()).c_str());
            }
        } else {
            prefix = L"fish: ";
        }

        const wcstring description =
            err.describe_with_prefix(src, prefix, is_interactive(), skip_caret);
        if (!description.empty()) {
            output.append(description);
            output.push_back(L'\n');
        }
        output.append(this->stack_trace());
    }
}

block_t::block_t(block_type_t t) : block_type(t) {}

block_t::~block_t() = default;

wcstring block_t::description() const {
    wcstring result;
    switch (this->type()) {
        case block_type_t::while_block: {
            result.append(L"while");
            break;
        }
        case block_type_t::for_block: {
            result.append(L"for");
            break;
        }
        case block_type_t::if_block: {
            result.append(L"if");
            break;
        }
        case block_type_t::function_call: {
            result.append(L"function_call");
            break;
        }
        case block_type_t::function_call_no_shadow: {
            result.append(L"function_call_no_shadow");
            break;
        }
        case block_type_t::switch_block: {
            result.append(L"switch");
            break;
        }
        case block_type_t::subst: {
            result.append(L"substitution");
            break;
        }
        case block_type_t::top: {
            result.append(L"top");
            break;
        }
        case block_type_t::begin: {
            result.append(L"begin");
            break;
        }
        case block_type_t::source: {
            result.append(L"source");
            break;
        }
        case block_type_t::event: {
            result.append(L"event");
            break;
        }
        case block_type_t::breakpoint: {
            result.append(L"breakpoint");
            break;
        }
        case block_type_t::variable_assignment: {
            result.append(L"variable_assignment");
            break;
        }
    }

    if (this->src_lineno >= 0) {
        append_format(result, L" (line %d)", this->src_lineno);
    }
    if (this->src_filename != nullptr) {
        append_format(result, L" (file %ls)", this->src_filename);
    }
    return result;
}

// Various block constructors.

block_t block_t::if_block() { return block_t(block_type_t::if_block); }

block_t block_t::event_block(event_t evt) {
    block_t b{block_type_t::event};
    b.event = std::move(evt);
    return b;
}

block_t block_t::function_block(wcstring name, wcstring_list_t args, bool shadows) {
    block_t b{shadows ? block_type_t::function_call : block_type_t::function_call_no_shadow};
    b.function_name = std::move(name);
    b.function_args = std::move(args);
    return b;
}

block_t block_t::source_block(const wchar_t *src) {
    block_t b{block_type_t::source};
    b.sourced_file = src;
    return b;
}

block_t block_t::for_block() { return block_t{block_type_t::for_block}; }
block_t block_t::while_block() { return block_t{block_type_t::while_block}; }
block_t block_t::switch_block() { return block_t{block_type_t::switch_block}; }
block_t block_t::scope_block(block_type_t type) {
    assert(
        (type == block_type_t::begin || type == block_type_t::top || type == block_type_t::subst) &&
        "Invalid scope type");
    return block_t(type);
}
block_t block_t::breakpoint_block() { return block_t(block_type_t::breakpoint); }
block_t block_t::variable_assignment_block() { return block_t(block_type_t::variable_assignment); }