serpent_serializer/

serialize.rs

//! The serializer core.
//!
//! Translates a [`Value`] graph to Lua source. This follows serpent's `val2str`
//! recursion and top-level assembly: array-first key ordering, short vs bracket
//! key notation, shared and cyclic reference wiring through a self-reference
//! section, and the `do ... return name end` wrapper when a name is set.

use crate::numfmt;
use crate::options::Options;
use crate::quote::quote;
use crate::value::{Ident, Key, Table, Value};
use std::collections::HashMap;

/// The 22 Lua reserved words. String keys that are keywords never use short
/// notation.
const KEYWORDS: &[&str] = &[
    "and", "break", "do", "else", "elseif", "end", "false", "for", "function", "goto", "if", "in",
    "local", "nil", "not", "or", "repeat", "return", "then", "true", "until", "while",
];

fn is_keyword(s: &str) -> bool {
    KEYWORDS.contains(&s)
}

/// Serializer state carried through the recursion.
struct Serializer<'o> {
    opts: &'o Options,
    space: &'static str,
    huge: bool,
    maxl: usize,
    maxlen: Option<i64>,
    numformat: String,
    iname: String,
    comm: Option<usize>,
    /// Map from a reference identity to the path expression where it was placed.
    seen: HashMap<Ident, String>,
    /// Self-reference section statements. Index 0 is the helper-table decl.
    sref: Vec<String>,
    /// Symbol table for gensym: matched substring to stable counter.
    syms: HashMap<String, usize>,
    symn: usize,
    /// tostring of the first non-serializable value reached under `fatal`.
    /// serpent raises `error("Can't serialize "..tostring(s))` at that point.
    /// Recording the text lets the top level build the same message.
    fatal_value: Option<String>,
}

/// The recursion context passed to [`Serializer::val2str`].
///
/// Grouping these together keeps the call sites readable. Transposing two
/// borrowed strings in a positional list would compile and emit wrong output,
/// so the names carry the meaning here instead of the argument order.
#[derive(Clone, Copy)]
struct Frame<'a> {
    /// The key this value sits under, if any.
    name: Option<&'a Key>,
    /// The indentation unit, when output is multi-line.
    indent: Option<&'a str>,
    /// The path to register in `seen` for this value, overriding the computed
    /// path. serpent's `insref`.
    insref: Option<&'a str>,
    /// The access path prefix this value hangs off, such as `_.a`.
    path: Option<&'a str>,
    /// Whether this element uses array notation with no `key =` prefix.
    plainindex: bool,
    /// The nesting depth, starting at 0.
    level: usize,
}

impl<'o> Serializer<'o> {
    fn new(opts: &'o Options) -> Self {
        let name = opts.name.clone().unwrap_or_default();
        let iname = format!("_{name}");
        let space = if opts.compact { "" } else { " " };
        Serializer {
            space,
            huge: !opts.nohuge,
            maxl: opts.maxlevel.unwrap_or(usize::MAX),
            maxlen: opts.maxlength,
            numformat: opts
                .numformat
                .clone()
                .unwrap_or_else(|| "%.17g".to_string()),
            sref: vec![format!("local {iname}={{}}")],
            iname,
            comm: opts.comment,
            seen: HashMap::new(),
            syms: HashMap::new(),
            symn: 0,
            fatal_value: None,
            opts,
        }
    }

    /// serpent's `gensym`: build a stable identifier for a reference used as a
    /// key. Strip non-alphanumerics from the identity text, then remap each
    /// `[0-9][A-Za-z0-9]+` run to a stable counter, and prefix `_`.
    fn gensym(&mut self, raw: &str) -> String {
        let stripped: String = raw.chars().filter(|c| c.is_ascii_alphanumeric()).collect();
        let remapped = self.remap_runs(&stripped);
        format!("_{remapped}")
    }

    /// Replace each maximal `[0-9][A-Za-z0-9]+` run with a stable 1-based
    /// counter assigned in first-seen order.
    fn remap_runs(&mut self, s: &str) -> String {
        let b = s.as_bytes();
        let mut out = String::new();
        let mut i = 0;
        while i < b.len() {
            if b[i].is_ascii_digit() {
                // A run must be a digit followed by one or more word chars.
                let start = i;
                i += 1;
                let mut count = 0;
                while i < b.len() && b[i].is_ascii_alphanumeric() {
                    i += 1;
                    count += 1;
                }
                let run = &s[start..i];
                if count >= 1 {
                    let n = *self.syms.entry(run.to_string()).or_insert_with(|| {
                        self.symn += 1;
                        self.symn
                    });
                    out.push_str(&n.to_string());
                } else {
                    out.push_str(run);
                }
            } else {
                out.push(b[i] as char);
                i += 1;
            }
        }
        out
    }

    /// serpent's `safestr`: a scalar value as a Lua literal.
    fn safestr(&self, v: &Value) -> String {
        match v {
            Value::Number(n) => {
                if self.huge {
                    if n.is_infinite() {
                        return if *n > 0.0 {
                            "1/0 --[[math.huge]]".to_string()
                        } else {
                            "-1/0 --[[-math.huge]]".to_string()
                        };
                    }
                    if n.is_nan() {
                        return "0/0".to_string();
                    }
                }
                numfmt::format(&self.numformat, *n)
            }
            Value::Str(s) => quote(s),
            Value::Bool(b) => b.to_string(),
            Value::Nil => "nil".to_string(),
            // Non-scalar reaching safestr means a fallback tostring; use a
            // placeholder path text.
            other => quote(tostring(other).as_bytes()),
        }
    }

    /// serpent's `comment`: an inline `--[[...]]` comment, when enabled and the
    /// level is under the threshold.
    fn comment(&self, text: &str, level: usize) -> String {
        match self.comm {
            Some(threshold) if level < threshold => format!(" --[[{text}]]"),
            _ => String::new(),
        }
    }

    /// serpent's `safename`: build an access path and return `(path, safe)`.
    fn safename(&self, path: Option<&str>, name: Option<&Key>) -> (String, String) {
        let plain_name = match name {
            Some(Key::Str(s)) => std::str::from_utf8(s)
                .ok()
                .filter(|n| is_plain_ident(n) && !is_keyword(n))
                .map(str::to_string),
            _ => None,
        };
        let (safe, is_plain) = match &plain_name {
            Some(n) => (n.clone(), true),
            None => {
                let key_val = name.map_or(Value::Str(Vec::new()), Key::to_value);
                (format!("[{}]", self.safestr(&key_val)), false)
            }
        };
        let joiner = if is_plain && path.is_some() { "." } else { "" };
        let full = format!("{}{}{}", path.unwrap_or(""), joiner, safe);
        (full, safe)
    }

    /// serpent's `val2str`. Returns the element text and appends any needed
    /// self-reference statements.
    fn val2str(&mut self, t: &Value, frame: Frame) -> String {
        let level = frame.level;
        let (spath, sname) = self.safename(frame.path, frame.name);
        let tag = self.build_tag(frame.name, &sname, frame.plainindex);

        // Already seen: emit nil and wire the reference in the self-ref section.
        if let Some(id) = t.ident() {
            if let Some(prev) = self.seen.get(&id).cloned() {
                self.sref
                    .push(format!("{spath}{}={}{prev}", self.space, self.space));
                return format!("{tag}nil{}", self.comment("ref", level));
            }
        }

        // Metamethods. When a table has __serialize or __tostring and
        // metatostring is not false, the successful result replaces the table.
        // __serialize wins over __tostring. A raising metamethod is ignored.
        let replaced = self.apply_metamethods(t, frame.insref.unwrap_or(&spath));
        let t = replaced.as_ref().unwrap_or(t);

        match t {
            Value::Table(tab) => self.table_to_str(tab, &tag, &spath, frame),
            Value::Global(_) => {
                if let Some(id) = t.ident() {
                    self.seen
                        .insert(id, frame.insref.unwrap_or(&spath).to_string());
                }
                format!("{tag}{}", self.globerr(t, level))
            }
            Value::Function(f) => {
                if let Some(id) = t.ident() {
                    self.seen
                        .insert(id, frame.insref.unwrap_or(&spath).to_string());
                }
                if self.opts.nocode {
                    return format!(
                        "{tag}function() --[[..skipped..]] end{}",
                        self.comment("", level)
                    );
                }
                match &f.bytecode {
                    Some(code) => {
                        let quoted = quote(code);
                        format!(
                            "{tag}((loadstring or load)({quoted},'@serialized')){}",
                            self.comment("", level)
                        )
                    }
                    None => format!("{tag}{}", self.globerr(t, level)),
                }
            }
            _ => format!("{tag}{}", self.safestr(t)),
        }
    }

    /// Build the `key =` prefix for an element.
    fn build_tag(&self, name: Option<&Key>, sname: &str, plainindex: bool) -> String {
        if plainindex {
            match name {
                Some(Key::Number(_)) => String::new(),
                Some(k) => {
                    let raw = tostring(&k.to_value());
                    format!("{raw}{}={}", self.space, self.space)
                }
                None => String::new(),
            }
        } else if name.is_some() {
            format!("{sname}{}={}", self.space, self.space)
        } else {
            String::new()
        }
    }

    /// Try the table's metamethods. Returns the replacement value when one
    /// succeeds. `__serialize` is preferred over `__tostring`. A metamethod that
    /// errors is skipped. The table is marked seen at `seen_path` when a
    /// metamethod applies, matching serpent's `seen[t] = insref or spath`.
    fn apply_metamethods(&mut self, t: &Value, seen_path: &str) -> Option<Value> {
        let Value::Table(tab) = t else {
            return None;
        };
        if self.opts.metatostring == Some(false) {
            return None;
        }
        let ser = tab.serialize_meta();
        let tos = tab.tostring_meta();
        if ser.is_none() && tos.is_none() {
            return None;
        }
        // __serialize wins when it succeeds, else __tostring.
        let result = ser
            .and_then(|f| f().ok())
            .or_else(|| tos.and_then(|f| f().ok()))?;
        if let Some(id) = t.ident() {
            self.seen.insert(id, seen_path.to_string());
        }
        Some(result)
    }

    /// serpent's `globerr`: a global name, a fallback string, or an error.
    ///
    /// A `Global` with a non-empty name is a known global and emits its name.
    /// An empty name means the value has no global mapping, matching an unknown
    /// userdata: it falls back to a quoted string, or records a fatal value when
    /// `fatal` is set so the top level can raise an error.
    fn globerr(&mut self, v: &Value, level: usize) -> String {
        if let Value::Global(g) = v {
            if !g.name.is_empty() {
                return format!("{}{}", g.name, self.comment(&g.name, level));
            }
        }
        // A value with no global mapping stands in for unknown userdata, whose
        // Lua tostring is `userdata: 0x...`. Build that address form for both
        // the fallback string and the fatal message.
        let text = match v {
            Value::Global(g) if g.name.is_empty() => format!("userdata: 0x{:012x}", g.id),
            _ => tostring(v),
        };
        if !self.opts.fatal {
            return self.safestr(&Value::Str(text.into_bytes()));
        }
        // fatal: record the value text for the error message. Keep the first one.
        if self.fatal_value.is_none() {
            self.fatal_value = Some(text.clone());
        }
        // The output is discarded once a fatal value is recorded, so the return
        // value here does not reach the caller.
        self.safestr(&Value::Str(text.into_bytes()))
    }

    fn table_to_str(&mut self, tab: &Table, tag: &str, spath: &str, frame: Frame) -> String {
        let Frame {
            indent,
            insref,
            level,
            ..
        } = frame;
        if level >= self.maxl {
            return format!("{tag}{{}}{}", self.comment("maxlvl", level));
        }
        let id = Ident::Table(tab.id());
        self.seen.insert(id, insref.unwrap_or(spath).to_string());
        if tab.is_empty() {
            let text = tab_tostring(tab);
            return format!("{tag}{{}}{}", self.comment(&text, level));
        }
        if let Some(ml) = self.maxlen {
            if ml < 0 {
                return format!("{tag}{{}}{}", self.comment("maxlen", level));
            }
        }

        let entries = tab.entries();
        if let Some((key, _)) = entries
            .iter()
            .find(|(key, _)| matches!(key, Key::Number(n) if n.is_nan()))
        {
            self.fatal_value = Some(tostring(&key.to_value()));
            return String::new();
        }
        let border = tab.border();
        let maxn = match self.opts.maxnum {
            Some(m) => border.min(m),
            None => border,
        };

        // Index the entries by key once for O(1) value lookup. Keys are unique
        // under `Key::same`, so each projection maps to a single slot.
        let index: HashMap<KeyId, usize> = entries
            .iter()
            .enumerate()
            .filter_map(|(i, (k, _))| key_id(k).map(|id| (id, i)))
            .collect();
        let lookup = |key: &Key| -> Option<Value> {
            key_id(key)
                .and_then(|id| index.get(&id))
                .map(|&i| entries[i].1.clone())
        };

        // Ordered key list: array indices 1..maxn first, then remaining keys.
        let mut o: Vec<Key> = (1..=maxn).map(|i| Key::Number(i as f64)).collect();
        if self.opts.maxnum.is_none() || o.len() < self.opts.maxnum.unwrap() {
            for (k, _) in &entries {
                if !is_array_index(k, maxn) {
                    o.push(k.clone());
                }
            }
        }
        if let Some(m) = self.opts.maxnum {
            if o.len() > m {
                o.truncate(m);
            }
        }
        if (self.opts.sortkeys || self.opts.sortfn.is_some()) && o.len() > maxn {
            self.sort_keys(&mut o, &entries);
        }
        let sparse = self.opts.sparse && o.len() > maxn;

        // Compute the table's seen-path once, not per element.
        let seen_t = self.seen.get(&Ident::Table(tab.id())).cloned();

        let mut out: Vec<String> = Vec::new();
        for (n, key) in o.iter().enumerate() {
            let idx = n + 1;
            let value = lookup(key);
            let plainindex = idx <= maxn && !sparse;

            if self.should_skip(key, value.as_ref(), sparse) {
                continue;
            }

            let is_complex = matches!(key, Key::Table(_) | Key::Function(_) | Key::Global(_));
            if is_complex {
                self.emit_complex_key(tab, key, value, indent);
            } else {
                let v = value.unwrap_or(Value::Nil);
                let text = self.val2str(
                    &v,
                    Frame {
                        name: Some(key),
                        indent,
                        insref: None,
                        path: seen_t.as_deref(),
                        plainindex,
                        level: level + 1,
                    },
                );
                let len = text.len() as i64;
                out.push(text);
                if let Some(ml) = self.maxlen.as_mut() {
                    *ml -= len;
                    if *ml < 0 {
                        break;
                    }
                }
            }
        }

        self.assemble_table(tab, tag, indent, &out, level)
    }

    /// A complex key (table/function/global). Emit its assignment into the
    /// self-reference section, declaring a helper local first if needed.
    fn emit_complex_key(
        &mut self,
        tab: &Table,
        key: &Key,
        value: Option<Value>,
        indent: Option<&str>,
    ) {
        let key_val = key.to_value();
        let key_id = key_val.ident();
        let key_seen = key_id.as_ref().and_then(|i| self.seen.get(i)).cloned();
        let key_global = match key {
            Key::Global(g) => Some(g.name.clone()),
            _ => None,
        };

        if key_seen.is_none() && key_global.is_none() {
            self.sref.push("placeholder".to_string());
            let idx = self.sref.len() - 1;
            let sym = self.gensym(&tostring(&key_val));
            let iname = self.iname.clone();
            let (sname, _) = self.safename(Some(&iname), Some(&Key::Str(sym.into_bytes())));
            let stmt = self.val2str(
                &key_val,
                Frame {
                    name: Some(&Key::Str(sname.clone().into_bytes())),
                    indent,
                    insref: Some(&sname),
                    path: Some(&iname),
                    plainindex: true,
                    level: 0,
                },
            );
            self.sref[idx] = stmt;
        }

        self.sref.push("placeholder".to_string());
        let idx = self.sref.len() - 1;
        let seen_t = self
            .seen
            .get(&Ident::Table(tab.id()))
            .cloned()
            .unwrap_or_default();
        // Recompute seen[key] here: the helper serialization above may have just
        // registered the key at its helper-table path, matching serpent's
        // `seen[key] or globals[key] or gensym(key)` at LHS-building time.
        let key_seen_now = key_id.as_ref().and_then(|i| self.seen.get(i)).cloned();
        let key_ref = key_seen_now
            .or(key_global)
            .unwrap_or_else(|| self.gensym(&tostring(&key_val)));
        let path = format!("{seen_t}[{key_ref}]");
        let value = value.unwrap_or(Value::Nil);
        let val_seen = value.ident().and_then(|i| self.seen.get(&i)).cloned();
        let rhs = match val_seen {
            Some(p) => p,
            // serpent calls val2str(value, nil, indent, path) so `insref` is the
            // path and the fifth path argument is nil. That makes the value
            // register at `path`, not at `path[""]`.
            None => self.val2str(
                &value,
                Frame {
                    name: None,
                    indent,
                    insref: Some(&path),
                    path: None,
                    plainindex: false,
                    level: 0,
                },
            ),
        };
        self.sref[idx] = format!("{path}{}={}{rhs}", self.space, self.space);
    }

    /// Assemble the final table text from the emitted element list.
    fn assemble_table(
        &self,
        tab: &Table,
        tag: &str,
        indent: Option<&str>,
        out: &[String],
        level: usize,
    ) -> String {
        let prefix = indent.map_or(String::new(), |ind| ind.repeat(level));
        let head = match indent {
            Some(ind) => format!("{{\n{prefix}{ind}"),
            None => "{".to_string(),
        };
        let sep = match indent {
            Some(ind) => format!(",\n{prefix}{ind}"),
            None => format!(",{}", self.space),
        };
        let body = out.join(&sep);
        let tail = match indent {
            Some(_) => format!("\n{prefix}}}"),
            None => "}".to_string(),
        };
        let table_text = match &self.opts.custom {
            Some(f) => f(tag, &head, &body, &tail, level),
            None => format!("{tag}{head}{body}{tail}"),
        };
        let text = tab_tostring(tab);
        format!("{table_text}{}", self.comment(&text, level))
    }

    /// Whether an element is skipped by the ignore filters or sparse-nil rule.
    fn should_skip(&self, key: &Key, value: Option<&Value>, sparse: bool) -> bool {
        if let Some(v) = value {
            if let Some(ig) = &self.opts.valignore {
                if let Some(id) = v.ident() {
                    if ig.contains(&id) {
                        return true;
                    }
                }
            }
            if let Some(ig) = &self.opts.valignore_scalar {
                if ig.iter().any(|x| x == v) {
                    return true;
                }
            }
        }
        if let Some(allow) = &self.opts.keyallow {
            if !allow.iter().any(|k| k == key) {
                return true;
            }
        }
        if let Some(ignore) = &self.opts.keyignore {
            if ignore.iter().any(|k| k == key) {
                return true;
            }
        }
        if let Some(tignore) = &self.opts.valtypeignore {
            let tn = value.map_or("nil", type_name);
            if tignore.contains(tn) {
                return true;
            }
        }
        if sparse && value.is_none() {
            return true;
        }
        false
    }

    /// serpent's `alphanumsort`, or the custom sort when set.
    fn sort_keys(&self, o: &mut Vec<Key>, entries: &[(Key, Value)]) {
        if let Some(f) = &self.opts.sortfn {
            // The closure reorders the keys directly and can read values from
            // `entries`, so no value round-trip and no key remap is needed.
            f(o, entries);
            return;
        }
        // Default: sort by a computed string. A numeric key whose value is a
        // positive integer landing on a filled slot of the ordered list ranks
        // first. serpent tests `o[key] ~= nil`, which holds for any integer key
        // in 1..#o, not only the array indices 1..maxn.
        let len = o.len();
        o.sort_by_key(|a| sortkey(a, len));
    }
}

/// Format a table like Lua's default `tostring`: `table: 0x...`.
fn tab_tostring(tab: &Table) -> String {
    format!("table: 0x{:012x}", tab.id())
}

/// Lua-style default `tostring` for non-string values used in comments and
/// fallbacks.
fn tostring(v: &Value) -> String {
    match v {
        Value::Nil => "nil".to_string(),
        Value::Bool(b) => b.to_string(),
        Value::Number(n) => {
            if n.is_nan() {
                "nan".to_string()
            } else if n.is_infinite() {
                if *n > 0.0 {
                    "inf".to_string()
                } else {
                    "-inf".to_string()
                }
            } else {
                numfmt::format("%.14g", *n)
            }
        }
        Value::Str(s) => String::from_utf8_lossy(s).into_owned(),
        Value::Table(t) => tab_tostring(t),
        Value::Function(f) => format!("function: 0x{:012x}", f.id),
        Value::Global(g) => g.name.clone(),
    }
}

/// The Lua type name of a value.
fn type_name(v: &Value) -> &'static str {
    match v {
        Value::Nil => "nil",
        Value::Bool(_) => "boolean",
        Value::Number(_) => "number",
        Value::Str(_) => "string",
        Value::Table(_) => "table",
        Value::Function(_) => "function",
        Value::Global(_) => "userdata",
    }
}

/// A valid Lua identifier: starts with a letter or underscore, then word chars.
fn is_plain_ident(s: &str) -> bool {
    let mut chars = s.chars();
    match chars.next() {
        Some(c) if c.is_ascii_alphabetic() || c == '_' => {}
        _ => return false,
    }
    chars.all(|c| c.is_ascii_alphanumeric() || c == '_')
}

/// Whether a key is one of the array indices 1..maxn.
fn is_array_index(k: &Key, maxn: usize) -> bool {
    match k {
        Key::Number(x) => {
            let f = *x;
            f.fract() == 0.0 && f >= 1.0 && f <= maxn as f64
        }
        _ => false,
    }
}

/// A hashable projection of a key, used to index a table's entries for O(1)
/// value lookup.
///
/// Numbers use their bit pattern after normalizing `-0.0` to `0.0`, which keeps
/// the projection consistent with [`Key::same`] (where `-0.0 == 0.0`). A `NaN`
/// key returns `None`, since Lua forbids `NaN` keys and [`Key::same`] never
/// matches two `NaN` values.
#[derive(PartialEq, Eq, Hash)]
enum KeyId {
    Bool(bool),
    Number(u64),
    Str(Vec<u8>),
    Table(usize),
    Function(usize),
    Global(usize),
}

fn key_id(k: &Key) -> Option<KeyId> {
    Some(match k {
        Key::Bool(b) => KeyId::Bool(*b),
        Key::Number(n) if n.is_nan() => return None,
        Key::Number(n) => KeyId::Number(if *n == 0.0 { 0.0_f64 } else { *n }.to_bits()),
        Key::Str(s) => KeyId::Str(s.clone()),
        Key::Table(t) => KeyId::Table(t.id()),
        Key::Function(f) => KeyId::Function(f.id),
        Key::Global(g) => KeyId::Global(g.id),
    })
}

/// Build serpent's sort string for a key.
///
/// `list_len` is the length of the ordered key list. A numeric key whose value
/// is a positive integer at or below `list_len` gets the `"0"` prefix, matching
/// serpent's `o[key] ~= nil` test over the ordered list. Every other key falls
/// back to the type rank.
fn sortkey(k: &Key, list_len: usize) -> String {
    let on_filled_slot = match k {
        Key::Number(x) => x.fract() == 0.0 && *x >= 1.0 && *x <= list_len as f64,
        _ => false,
    };
    let prefix = if on_filled_slot {
        "0"
    } else {
        match k {
            Key::Number(_) => "a",
            Key::Str(_) => "b",
            _ => "z",
        }
    };
    let body = pad_numbers(&key_tostring(k));
    format!("{prefix}{body}")
}

/// serpent's `padnum`: zero-pad each digit run to 12 places.
fn pad_numbers(s: &str) -> String {
    let mut out = String::new();
    let b = s.as_bytes();
    let mut i = 0;
    while i < b.len() {
        if b[i].is_ascii_digit() {
            let start = i;
            while i < b.len() && b[i].is_ascii_digit() {
                i += 1;
            }
            let num = &s[start..i];
            let val: u64 = num.parse().unwrap_or(0);
            out.push_str(&format!("{val:012}"));
        } else {
            out.push(b[i] as char);
            i += 1;
        }
    }
    out
}

/// `tostring` of a key for sort-string building.
fn key_tostring(k: &Key) -> String {
    tostring(&k.to_value())
}

/// Serialize a value graph with the given options.
///
/// This is the raw entry point. Returns the Lua source, or an error when
/// `fatal` is set and a non-serializable value is reached.
///
/// # Errors
///
/// Returns [`SerError`] when `opts.fatal` is set and a value cannot be
/// serialized as data.
pub fn serialize(t: &Value, opts: &Options) -> Result<String, SerError> {
    let mut s = Serializer::new(opts);
    let name = opts.name.clone();
    let indent = opts.indent.clone();
    let sepr = match &indent {
        Some(_) => "\n".to_string(),
        None => format!(";{}", s.space),
    };
    let top_name = name.as_ref().map(|n| Key::Str(n.clone().into_bytes()));
    let body = s.val2str(
        t,
        Frame {
            name: top_name.as_ref(),
            indent: indent.as_deref(),
            insref: None,
            path: None,
            plainindex: false,
            level: 0,
        },
    );

    if let Some(value) = s.fatal_value.take() {
        return Err(SerError::NotSerializable(value));
    }

    let tail = if s.sref.len() > 1 {
        format!("{}{sepr}", s.sref.join(&sepr))
    } else {
        String::new()
    };
    let warn = if opts.comment.is_some() && s.sref.len() > 1 {
        format!(
            "{}--[[incomplete output with shared/self-references skipped]]",
            s.space
        )
    } else {
        String::new()
    };

    let result = match name {
        None => format!("{body}{warn}"),
        Some(n) => format!("do local {body}{sepr}{tail}return {n}{sepr}end"),
    };
    Ok(result)
}

/// A serialization error.
#[derive(Debug, PartialEq, Eq)]
pub enum SerError {
    /// A value could not be serialized and `fatal` was set. Carries the value's
    /// `tostring` text, so the message names the value that failed.
    NotSerializable(String),
}

impl std::fmt::Display for SerError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            SerError::NotSerializable(value) => write!(f, "Can't serialize {value}"),
        }
    }
}

impl std::error::Error for SerError {}

#[cfg(test)]
mod tests {
    use super::SerError;
    use crate::value::{Key, Table, Value};
    use crate::{dump, line};

    #[test]
    fn nan_table_keys_return_error() {
        let table = Table::new();
        table.set(Key::Number(f64::NAN), Value::Number(7.0));

        assert_eq!(
            line(&Value::Table(table.clone())).unwrap_err(),
            SerError::NotSerializable("nan".to_string())
        );
        assert_eq!(
            dump(&Value::Table(table)).unwrap_err(),
            SerError::NotSerializable("nan".to_string())
        );
    }
}