luby/crates/luaify/src/generate.rs

use crate::utils::{syn_assert, syn_error, wrap_expr_block};
use proc_macro2::TokenStream;
use quote::quote;
use std::fmt::Display;
use syn::{ext::*, punctuated::*, spanned::*, *};

pub fn generate(expr: &Expr) -> Result<TokenStream> {
    let mut f = Formatter::default();
    match expr {
        Expr::Block(block) => generate_block(&mut f, &block.block, Context::stmt(true))?,
        _ => generate_expr(&mut f, expr, Context::expr(true))?,
    }

    Ok(f.done())
}

#[derive(Default)]
struct Formatter {
    buf: String,
    space: bool,
    format_args: Vec<Expr>,
}

impl Formatter {
    fn write(&mut self, s: impl Display) -> &mut Self {
        fn sep(c: char) -> bool {
            match c {
                '(' | ')' | '[' | ']' | '{' | '}' | '+' | '-' | '*' | '/' | '%' | '^' | '#'
                | '=' | '~' | '<' | '>' | ':' | ';' | '.' | ',' | '\'' | '"' | ' ' => true,
                _ => false,
            }
        }

        // we are inside a format string, so we need to escape braces
        let s = format!("{s}").replace("{", "{{").replace("}", "}}");
        if !s.is_empty() {
            if self.space && !sep(s.chars().next().unwrap()) {
                self.buf.push(' ');
            }
            self.buf.push_str(&s);
            self.space = !sep(s.chars().last().unwrap());
        }

        self
    }

    fn interpolate(&mut self, arg: Expr) -> &mut Self {
        self.space.then(|| self.buf.push(' '));
        self.buf.push_str("{}");
        self.format_args.push(arg);
        self.space = true;
        self
    }

    fn done(self) -> TokenStream {
        let fmt = self.buf;
        let args = self.format_args;
        if args.is_empty() {
            let fmt = fmt.replace("{{", "{").replace("}}", "}");
            quote!(#fmt)
        } else {
            quote!(::std::format!(#fmt, #(#args),*))
        }
    }
}

macro_rules! assert_no_attrs {
    ($src:expr) => {{ syn_assert!($src.attrs.is_empty(), $src, "unsupported attribute") }};
}

macro_rules! assert_no_suffix {
    ($src:expr) => {{ syn_assert!($src.suffix() == "", $src, "cannot have suffix") }};
}

#[derive(Debug, Clone, Copy)]
enum Context {
    Stmt { ret: bool },
    Expr { multi: bool },
}

impl Context {
    fn stmt(ret: bool) -> Self {
        Self::Stmt { ret }
    }

    fn expr(multi: bool) -> Self {
        Self::Expr { multi }
    }

    fn is_stmt(&self) -> bool {
        matches!(self, Self::Stmt { .. })
    }

    fn is_ret(&self) -> bool {
        matches!(self, Self::Stmt { ret: true, .. })
    }

    fn is_expr(&self) -> bool {
        matches!(self, Self::Expr { .. })
    }

    fn is_multi_expr(&self) -> bool {
        matches!(self, Self::Expr { multi: true, .. })
    }

    fn is_value(&self) -> bool {
        self.is_expr() | self.is_ret()
    }
}

fn generate_expr(f: &mut Formatter, expr: &Expr, cx: Context) -> Result<()> {
    match expr {
        Expr::Assign(ass) => generate_expr_assign(f, ass, cx),
        Expr::Binary(bin) => generate_expr_binary(f, bin, cx),
        Expr::Block(block) => generate_expr_block(f, block, cx),
        Expr::Break(brk) => generate_expr_break(f, brk, cx),
        Expr::Call(call) => generate_expr_call(f, call, cx),
        Expr::Closure(clo) => generate_expr_closure(f, clo, cx),
        Expr::Continue(cont) => generate_expr_continue(f, cont, cx),
        Expr::Field(field) => generate_expr_field(f, field, cx),
        Expr::ForLoop(fo) => generate_expr_forloop(f, fo, cx),
        Expr::If(xif) => generate_expr_if(f, xif, cx),
        Expr::Index(index) => generate_expr_index(f, index, cx),
        Expr::Infer(infer) => generate_expr_infer(f, infer, cx),
        Expr::Lit(lit) => generate_expr_lit(f, lit, cx),
        Expr::Loop(lo) => generate_expr_loop(f, lo, cx),
        Expr::Macro(mac) => generate_expr_macro(f, mac, cx),
        Expr::MethodCall(call) => generate_expr_method_call(f, call, cx),
        Expr::Paren(paren) => generate_expr_paren(f, paren, cx),
        Expr::Path(path) => generate_expr_path(f, path, cx),
        Expr::Return(ret) => generate_expr_return(f, ret, cx),
        Expr::Tuple(tuple) => generate_expr_tuple(f, tuple, cx),
        Expr::Unary(un) => generate_expr_unary(f, un, cx),
        Expr::While(whil) => generate_expr_while(f, whil, cx),
        expr => syn_error!(expr, "unsupported expression"),
    }
}

fn generate_expr_assign(f: &mut Formatter, ass: &ExprAssign, cx: Context) -> Result<()> {
    assert_no_attrs!(ass);
    syn_assert!(
        cx.is_stmt(),
        ass,
        "assignment must be in statement position"
    );
    generate_expr(f, &ass.left, Context::expr(true))?;
    f.write("=");
    generate_expr(f, &ass.right, Context::expr(true))?;
    Ok(())
}

fn generate_expr_binary(f: &mut Formatter, bin: &ExprBinary, cx: Context) -> Result<()> {
    assert_no_attrs!(bin);
    match bin.op {
        BinOp::Add(_)
        | BinOp::Sub(_)
        | BinOp::Mul(_)
        | BinOp::Div(_)
        | BinOp::Rem(_)
        | BinOp::BitAnd(_)
        | BinOp::BitOr(_)
        | BinOp::BitXor(_)
        | BinOp::Shl(_)
        | BinOp::Shr(_)
        | BinOp::Eq(_)
        | BinOp::Ne(_)
        | BinOp::Lt(_)
        | BinOp::Le(_)
        | BinOp::Ge(_)
        | BinOp::Gt(_)
        | BinOp::And(_)
        | BinOp::Or(_) => {
            syn_assert!(cx.is_value(), bin, "must be in expression position");
            cx.is_ret().then(|| f.write("return"));
        }
        BinOp::AddAssign(_)
        | BinOp::SubAssign(_)
        | BinOp::MulAssign(_)
        | BinOp::DivAssign(_)
        | BinOp::RemAssign(_)
        | BinOp::BitXorAssign(_)
        | BinOp::BitAndAssign(_)
        | BinOp::BitOrAssign(_)
        | BinOp::ShlAssign(_)
        | BinOp::ShrAssign(_) => {
            syn_assert!(cx.is_stmt(), bin, "must be in statement position");
        }
        op => syn_error!(op, "unsupported binary operator"),
    }

    let cx = Context::expr(false);

    macro_rules! bin_op {
        ($op:expr) => {{
            generate_expr(f, &bin.left, cx)?;
            f.write($op);
            generate_expr(f, &bin.right, cx)?;
        }};
    }

    macro_rules! call_op {
        ($name:expr) => {{
            f.write(format_args!("{}(", $name));
            generate_expr(f, &bin.left, cx)?;
            f.write(",");
            generate_expr(f, &bin.right, cx)?;
            f.write(")");
        }};
    }

    macro_rules! assign_bin_op {
        ($op:expr) => {{
            generate_expr(f, &bin.left, cx)?;
            f.write("=");
            bin_op!($op);
        }};
    }

    macro_rules! assign_call_op {
        ($name:expr) => {{
            generate_expr(f, &bin.left, cx)?;
            f.write("=");
            call_op!($name);
        }};
    }

    match bin.op {
        BinOp::Add(_) => bin_op!("+"),
        BinOp::AddAssign(_) => assign_bin_op!("+"),
        BinOp::Sub(_) => bin_op!("-"),
        BinOp::SubAssign(_) => assign_bin_op!("-"),
        BinOp::Mul(_) => bin_op!("*"),
        BinOp::MulAssign(_) => assign_bin_op!("*"),
        BinOp::Div(_) => bin_op!("/"),
        BinOp::DivAssign(_) => assign_bin_op!("/"),
        BinOp::Rem(_) => call_op!("__fmod"),
        BinOp::RemAssign(_) => assign_call_op!("__fmod"),
        BinOp::BitAnd(_) => call_op!("__band"),
        BinOp::BitAndAssign(_) => assign_call_op!("__band"),
        BinOp::BitOr(_) => call_op!("__bor"),
        BinOp::BitOrAssign(_) => assign_call_op!("__bor"),
        BinOp::BitXor(_) => call_op!("__bxor"),
        BinOp::BitXorAssign(_) => assign_call_op!("__bxor"),
        BinOp::Shl(_) => call_op!("__blshift"),
        BinOp::ShlAssign(_) => assign_call_op!("__blshift"),
        BinOp::Shr(_) => call_op!("__barshift"),
        BinOp::ShrAssign(_) => assign_call_op!("__barshift"),
        BinOp::Eq(_) => bin_op!("=="),
        BinOp::Ne(_) => bin_op!("~="),
        BinOp::Lt(_) => bin_op!("<"),
        BinOp::Le(_) => bin_op!("<="),
        BinOp::Ge(_) => bin_op!(">="),
        BinOp::Gt(_) => bin_op!(">"),
        BinOp::And(_) => bin_op!("and"),
        BinOp::Or(_) => bin_op!("or"),
        op => syn_error!(op, "unsupported binary operator"),
    };

    Ok(())
}

fn generate_expr_block(f: &mut Formatter, block: &ExprBlock, cx: Context) -> Result<()> {
    assert_no_attrs!(block);
    syn_assert!(cx.is_stmt(), block, "block must be in statement position");
    f.write("do");
    generate_block(f, &block.block, cx)?;
    if let Some(ref label) = block.label {
        generate_label_continue(f, label)?;
    }
    f.write("end");
    if let Some(ref label) = block.label {
        generate_label_break(f, label)?;
    }
    Ok(())
}

fn generate_expr_break(f: &mut Formatter, brk: &ExprBreak, cx: Context) -> Result<()> {
    assert_no_attrs!(brk);
    syn_assert!(cx.is_stmt(), brk, "break must be in statement position");
    syn_assert!(brk.expr.is_none(), brk, "use return instead");
    match brk.label {
        Some(ref label) => f.write(format_args!("goto {}_brk", label.ident.unraw())),
        None => f.write("break"),
    };
    Ok(())
}

fn generate_expr_call(f: &mut Formatter, call: &ExprCall, cx: Context) -> Result<()> {
    assert_no_attrs!(call);
    cx.is_ret().then(|| f.write("return"));
    generate_expr(f, &call.func, Context::expr(false))?;
    f.write("(");
    generate_punctuated_expr(f, &call.args)?;
    f.write(")");
    Ok(())
}

fn generate_expr_closure(f: &mut Formatter, clo: &ExprClosure, cx: Context) -> Result<()> {
    assert_no_attrs!(clo);
    syn_assert!(cx.is_value(), clo, "closure must be in expression position");

    if let Some(ref bounds) = clo.lifetimes {
        syn_error!(bounds, "cannot have lifetime bindings");
    } else if let Some(ref stat) = clo.movability {
        syn_error!(stat, "cannot be static");
    } else if let Some(ref cons) = clo.constness {
        syn_error!(cons, "cannot be const");
    } else if let Some(ref asyn) = clo.asyncness {
        syn_error!(asyn, "cannot be async");
    } else if let Some(ref capt) = clo.capture {
        syn_error!(capt, "cannot be move");
    } else if let ReturnType::Type(_, ref ty) = clo.output {
        syn_error!(ty, "cannot have return type");
    }

    cx.is_ret().then(|| f.write("return"));
    f.write("function(");
    generate_punctuated_pat(f, &clo.inputs)?;
    f.write(")");
    generate_block(f, &wrap_expr_block(&clo.body), Context::stmt(true))?;
    f.write("end");
    Ok(())
}

fn generate_expr_continue(f: &mut Formatter, cont: &ExprContinue, cx: Context) -> Result<()> {
    assert_no_attrs!(cont);
    syn_assert!(cx.is_stmt(), cont, "continue must be in statement position");
    match cont.label {
        Some(ref label) => f.write(format_args!("goto {}_cnt", label.ident.unraw())),
        None => syn_error!(cont, "continue requires a label"),
    };
    Ok(())
}

fn generate_expr_field(f: &mut Formatter, field: &ExprField, cx: Context) -> Result<()> {
    assert_no_attrs!(field);
    syn_assert!(cx.is_value(), field, "must be in expression position");
    cx.is_ret().then(|| f.write("return"));
    generate_expr(f, &field.base, Context::expr(false))?;
    match field.member {
        Member::Named(ref ident) => {
            f.write(".");
            generate_ident(f, ident)?;
        }
        Member::Unnamed(ref index) => {
            f.write("[");
            generate_index(f, index)?;
            f.write("]");
        }
    }
    Ok(())
}

fn generate_expr_forloop(f: &mut Formatter, fo: &ExprForLoop, cx: Context) -> Result<()> {
    assert_no_attrs!(fo);
    syn_assert!(cx.is_stmt(), fo, "for loop must be in statement position");
    f.write("for");
    match *fo.expr {
        Expr::Range(ref range) => {
            assert_no_attrs!(range);
            // TODO: reverse ranges
            let start = match range.start {
                Some(ref start) => (**start).clone(),
                None => parse_quote!(0),
            };
            let end = match range.end {
                Some(ref end) => match range.limits {
                    RangeLimits::HalfOpen(_) => parse_quote!(#end - 1),
                    RangeLimits::Closed(_) => (**end).clone(),
                },
                None => syn_error!(range, "end of range must be specified"),
            };
            generate_pat(f, &fo.pat, PatContext::Single)?;
            f.write("=");
            generate_expr(f, &start, Context::expr(false))?;
            f.write(",");
            generate_expr(f, &end, Context::expr(false))?;
        }
        ref expr => {
            generate_pat(f, &fo.pat, PatContext::Multi)?;
            f.write("in");
            generate_expr(f, expr, Context::expr(true))?;
        }
    }
    f.write("do");
    generate_block(f, &fo.body, Context::stmt(false))?;
    if let Some(ref label) = fo.label {
        generate_label_continue(f, label)?;
    }
    f.write("end");
    if let Some(ref label) = fo.label {
        generate_label_break(f, label)?;
    }
    Ok(())
}

fn generate_expr_if(f: &mut Formatter, mut xif: &ExprIf, cx: Context) -> Result<()> {
    assert_no_attrs!(xif);
    syn_assert!(cx.is_stmt(), xif, "if must be in statement position");
    f.write("if");
    loop {
        generate_expr(f, &xif.cond, Context::expr(false))?;
        f.write("then");
        generate_block(f, &xif.then_branch, cx)?;
        if let Some((_, ref expr)) = xif.else_branch {
            match **expr {
                Expr::If(ref elseif) => {
                    f.write("elseif");
                    xif = elseif;
                    continue;
                }
                ref els => {
                    f.write("else");
                    generate_block(f, &wrap_expr_block(els), cx)?;
                }
            }
        }
        break;
    }
    f.write("end");
    Ok(())
}

fn generate_expr_index(f: &mut Formatter, index: &ExprIndex, cx: Context) -> Result<()> {
    assert_no_attrs!(index);
    syn_assert!(cx.is_value(), index, "must be in expression position");
    cx.is_ret().then(|| f.write("return"));
    generate_expr(f, &index.expr, Context::expr(false))?;
    f.write("[");
    generate_expr(f, &index.index, Context::expr(false))?;
    f.write("]");
    Ok(())
}

fn generate_expr_infer(f: &mut Formatter, infer: &ExprInfer, cx: Context) -> Result<()> {
    assert_no_attrs!(infer);
    syn_assert!(cx.is_value(), infer, "must be in expression position");
    cx.is_ret().then(|| f.write("return"));
    f.write("_");
    Ok(())
}

fn generate_expr_lit(f: &mut Formatter, lit: &ExprLit, cx: Context) -> Result<()> {
    assert_no_attrs!(lit);
    syn_assert!(cx.is_value(), lit, "literal must be in expression position");
    cx.is_ret().then(|| f.write("return"));
    generate_lit(f, &lit.lit)
}

fn generate_expr_loop(f: &mut Formatter, lo: &ExprLoop, cx: Context) -> Result<()> {
    assert_no_attrs!(lo);
    syn_assert!(cx.is_stmt(), lo, "loop must be in statement position");
    f.write("while true do");
    generate_block(f, &lo.body, Context::stmt(false))?;
    if let Some(ref label) = lo.label {
        generate_label_continue(f, label)?;
    }
    f.write("end");
    if let Some(ref label) = lo.label {
        generate_label_break(f, label)?;
    }
    Ok(())
}

fn generate_expr_macro(f: &mut Formatter, mac: &ExprMacro, cx: Context) -> Result<()> {
    assert_no_attrs!(mac);
    generate_macro(f, &mac.mac, cx)
}

fn generate_expr_method_call(f: &mut Formatter, call: &ExprMethodCall, cx: Context) -> Result<()> {
    assert_no_attrs!(call);

    if let Some(ref fish) = call.turbofish {
        syn_error!(fish, "cannot be generic");
    }

    cx.is_ret().then(|| f.write("return"));
    generate_expr(f, &call.receiver, Context::expr(false))?;
    f.write(":");
    generate_ident(f, &call.method)?;
    f.write("(");
    generate_punctuated_expr(f, &call.args)?;
    f.write(")");
    Ok(())
}

fn generate_expr_paren(f: &mut Formatter, paren: &ExprParen, cx: Context) -> Result<()> {
    assert_no_attrs!(paren);
    syn_assert!(cx.is_value(), paren, "must be in expression position");
    cx.is_ret().then(|| f.write("return"));
    f.write("(");
    generate_expr(f, &paren.expr, Context::expr(false))?;
    f.write(")");
    Ok(())
}

fn generate_expr_path(f: &mut Formatter, path: &ExprPath, cx: Context) -> Result<()> {
    assert_no_attrs!(path);
    syn_assert!(cx.is_value(), path, "must be in expression position");
    cx.is_ret().then(|| f.write("return"));
    match path.qself {
        Some(ref qself) => syn_error!(qself, "cannot be generic"),
        None => generate_path(f, &path.path),
    }
}

fn generate_expr_return(f: &mut Formatter, ret: &ExprReturn, cx: Context) -> Result<()> {
    assert_no_attrs!(ret);
    syn_assert!(cx.is_stmt(), ret, "return must be in statement position");
    f.write("return");
    match ret.expr {
        Some(ref value) => generate_expr(f, value, Context::expr(true)),
        None => Ok(()),
    }
}

fn generate_expr_tuple(f: &mut Formatter, tuple: &ExprTuple, cx: Context) -> Result<()> {
    assert_no_attrs!(tuple);
    syn_assert!(cx.is_value(), tuple, "tuple must be in expression position");
    cx.is_ret().then(|| f.write("return"));
    match tuple.elems.len() {
        0 => {
            f.write("nil");
            Ok(())
        }
        _ if cx.is_ret() || cx.is_multi_expr() => generate_punctuated_expr(f, &tuple.elems),
        _ => syn_error!(tuple, "expected single-valued expression"),
    }
}

fn generate_expr_unary(f: &mut Formatter, un: &ExprUnary, cx: Context) -> Result<()> {
    assert_no_attrs!(un);
    syn_assert!(cx.is_value(), un, "must be in expression position");
    cx.is_ret().then(|| f.write("return"));

    let cx = Context::expr(false);

    macro_rules! un_op {
        ($op:expr) => {{
            f.write($op);
            generate_expr(f, &un.expr, cx)?;
        }};
    }

    match un.op {
        UnOp::Not(_) => un_op!("not"),
        UnOp::Neg(_) => un_op!("-"),
        op => syn_error!(op, "unsupported unary operator"),
    }

    Ok(())
}

fn generate_expr_while(f: &mut Formatter, whil: &ExprWhile, cx: Context) -> Result<()> {
    assert_no_attrs!(whil);
    syn_assert!(cx.is_stmt(), whil, "while must be in statement position");
    f.write("while");
    generate_expr(f, &whil.cond, Context::expr(false))?;
    f.write("do");
    generate_block(f, &whil.body, Context::stmt(false))?;
    if let Some(ref label) = whil.label {
        generate_label_continue(f, label)?;
    }
    f.write("end");
    if let Some(ref label) = whil.label {
        generate_label_break(f, label)?;
    }
    Ok(())
}

fn generate_punctuated_expr(f: &mut Formatter, exprs: &Punctuated<Expr, Token![,]>) -> Result<()> {
    let len = exprs.len();
    for (i, expr) in exprs.iter().enumerate() {
        (i != 0).then(|| f.write(","));
        generate_expr(f, expr, Context::expr(i == len - 1))?;
    }
    Ok(())
}

fn generate_ident(f: &mut Formatter, ident: &Ident) -> Result<()> {
    // https://www.lua.org/manual/5.2/manual.html#3.1
    let name = format!("{}", ident.unraw());
    match name.as_str() {
        "and" | "break" | "do" | "else" | "elseif" | "end" | "false" | "for" | "function"
        | "goto" | "if" | "in" | "local" | "nil" | "not" | "or" | "repeat" | "return" | "then"
        | "true" | "until" | "while" => {
            syn_error!(ident, "'{name}' cannot be used as an identifier")
        }
        s => f.write(s),
    };
    Ok(())
}

fn generate_index(f: &mut Formatter, index: &Index) -> Result<()> {
    f.write(index.index);
    Ok(())
}

fn generate_label_continue(f: &mut Formatter, label: &Label) -> Result<()> {
    f.write(format_args!("::{}_cnt::", label.name.ident.unraw()));
    Ok(())
}

fn generate_label_break(f: &mut Formatter, label: &Label) -> Result<()> {
    f.write(format_args!("::{}_brk::", label.name.ident.unraw()));
    Ok(())
}

fn generate_lit(f: &mut Formatter, lit: &Lit) -> Result<()> {
    match lit {
        Lit::Bool(b) => generate_lit_bool(f, b),
        Lit::Byte(b) => generate_lit_byte(f, b),
        Lit::Int(n) => generate_lit_int(f, n),
        Lit::Float(n) => generate_lit_float(f, n),
        Lit::Str(s) => generate_lit_str(f, s),
        Lit::ByteStr(s) => generate_lit_byte_str(f, s),
        Lit::CStr(s) => generate_lit_cstr(f, s),
        lit => syn_error!(lit, "unsupported literal"),
    }
}

fn generate_lit_bool(f: &mut Formatter, b: &LitBool) -> Result<()> {
    f.write(b.value());
    Ok(())
}

fn generate_lit_byte(f: &mut Formatter, b: &LitByte) -> Result<()> {
    assert_no_suffix!(b);
    f.write(b.value());
    Ok(())
}

fn generate_lit_int(f: &mut Formatter, n: &LitInt) -> Result<()> {
    assert_no_suffix!(n);
    f.write(n.base10_parse::<f64>()?);
    Ok(())
}

fn generate_lit_float(f: &mut Formatter, n: &LitFloat) -> Result<()> {
    assert_no_suffix!(n);
    f.write(n.base10_parse::<f64>()?);
    Ok(())
}

fn generate_lit_str(f: &mut Formatter, s: &LitStr) -> Result<()> {
    assert_no_suffix!(s);
    f.write(format!(r#""{}""#, escape_str(s.value())));
    Ok(())
}

fn generate_lit_byte_str(f: &mut Formatter, s: &LitByteStr) -> Result<()> {
    assert_no_suffix!(s);
    f.write(format!(r#""{}""#, escape_str(s.value())));
    Ok(())
}

fn generate_lit_cstr(f: &mut Formatter, s: &LitCStr) -> Result<()> {
    assert_no_suffix!(s);
    f.write(format!(r#""{}\0""#, escape_str(s.value().as_bytes())));
    Ok(())
}

fn escape_str(s: impl AsRef<[u8]>) -> String {
    // this produces an escaped string with \xNN hexadecimal notation which lua 5.1 normally
    // wouldn't understand, but luajit supports this unconditionally as an extension
    // https://docs.rs/bstr/latest/bstr/trait.ByteSlice.html#method.escape_bytes
    // https://luajit.org/extensions.html#lua52
    String::from_utf8(
        s.as_ref()
            .iter()
            .flat_map(|b| std::ascii::escape_default(*b))
            .collect(),
    )
    .unwrap()
}

fn generate_path(f: &mut Formatter, path: &Path) -> Result<()> {
    for (i, segment) in path.segments.iter().enumerate() {
        (i != 0).then(|| f.write("."));
        generate_path_segment(f, segment)?;
    }
    Ok(())
}

fn generate_path_segment(f: &mut Formatter, seg: &PathSegment) -> Result<()> {
    match seg.arguments {
        PathArguments::AngleBracketed(ref arg) => syn_error!(arg, "cannot be generic"),
        PathArguments::Parenthesized(ref arg) => syn_error!(arg, "cannot be generic"),
        PathArguments::None => generate_ident(f, &seg.ident),
    }
}

fn generate_block(f: &mut Formatter, block: &Block, cx: Context) -> Result<()> {
    let len = block.stmts.len();
    for (i, stmt) in block.stmts.iter().enumerate() {
        match stmt {
            Stmt::Local(local) => generate_local(f, local),
            Stmt::Item(item) => generate_item(f, item),
            Stmt::Expr(expr, semi) => match semi {
                None if i == len - 1 && cx.is_ret() => generate_expr(f, expr, Context::stmt(true)),
                _ => generate_expr(f, expr, Context::stmt(false)),
            },
            Stmt::Macro(smac) => generate_macro(f, &smac.mac, cx),
        }?;
        f.write(";");
    }
    Ok(())
}

fn generate_local(f: &mut Formatter, local: &Local) -> Result<()> {
    assert_no_attrs!(local);
    f.write("local");
    generate_pat(f, &local.pat, PatContext::Multi)?;
    match local.init {
        Some(ref init) => generate_local_init(f, init),
        None => Ok(()),
    }
}

fn generate_local_init(f: &mut Formatter, init: &LocalInit) -> Result<()> {
    match init.diverge {
        Some((ref token, _)) => syn_error!(token, "let-else is not supported"),
        None => {
            f.write("=");
            generate_expr(f, &init.expr, Context::expr(true))
        }
    }
}

fn generate_item(f: &mut Formatter, item: &Item) -> Result<()> {
    match item {
        Item::Fn(func) => generate_item_fn(f, func),
        item => syn_error!(item, "unsupported item"),
    }
}

fn generate_item_fn(f: &mut Formatter, func: &ItemFn) -> Result<()> {
    assert_no_attrs!(func);
    let Visibility::Inherited = func.vis else {
        syn_error!(func, "cannot have visibility");
    };
    f.write("local");
    generate_signature(f, &func.sig)?;
    generate_block(f, &func.block, Context::stmt(true))?;
    f.write("end");
    Ok(())
}

fn generate_signature(f: &mut Formatter, sig: &Signature) -> Result<()> {
    if let Some(ref cons) = sig.constness {
        syn_error!(cons, "cannot be const");
    } else if let Some(ref asyn) = sig.asyncness {
        syn_error!(asyn, "cannot be async");
    } else if let Some(ref uns) = sig.unsafety {
        syn_error!(uns, "cannot be unsafe");
    } else if let Some(ref abi) = sig.abi {
        syn_error!(abi, "cannot be extern");
    } else if !sig.generics.params.is_empty() {
        syn_error!(sig.generics.params, "cannot be generic");
    } else if let Some(ref wher) = sig.generics.where_clause {
        syn_error!(wher, "cannot have where clause");
    } else if let ReturnType::Type(_, ref ty) = sig.output {
        syn_error!(ty, "cannot have return type");
    }

    f.write("function");
    generate_ident(f, &sig.ident)?;
    f.write("(");
    for (i, param) in sig.inputs.iter().enumerate() {
        (i != 0).then(|| f.write(","));
        generate_fn_arg(f, param)?;
    }
    f.write(")");
    Ok(())
}

fn generate_fn_arg(f: &mut Formatter, param: &FnArg) -> Result<()> {
    match param {
        FnArg::Receiver(recv) => generate_receiver(f, recv),
        FnArg::Typed(typed) => generate_pat_typed(f, typed, PatContext::Single),
    }
}

fn generate_receiver(f: &mut Formatter, recv: &Receiver) -> Result<()> {
    assert_no_attrs!(recv);
    syn_assert!(recv.colon_token.is_none(), recv, "must be `self`");
    if let Some(ref l) = recv.lifetime() {
        syn_error!(l, "cannot have lifetimes");
    } else if let Some(ref m) = recv.mutability {
        syn_error!(m, "cannot be mut (implicitly mutable)");
    }
    f.write("self");
    Ok(())
}

fn generate_macro(f: &mut Formatter, mac: &Macro, cx: Context) -> Result<()> {
    match format!("{}", mac.path.require_ident()?).as_str() {
        "concat" => generate_macro_concat(f, mac, cx),
        "len" => generate_macro_len(f, mac, cx),
        "variadic" => generate_macro_variadic(f, mac, cx),
        "embed" => generate_macro_embed(f, mac, cx),
        "raw" => generate_macro_raw(f, mac, cx),
        name => syn_error!(mac.path, "unknown macro '{name}'"),
    }
}

fn generate_macro_concat(f: &mut Formatter, mac: &Macro, cx: Context) -> Result<()> {
    syn_assert!(cx.is_value(), mac, "concat! must be in expression position");
    cx.is_ret().then(|| f.write("return"));
    let args = mac.parse_body_with(<Punctuated<Expr, Token![,]>>::parse_terminated)?;
    syn_assert!(!args.is_empty(), mac, "expected at least one argument");
    for (i, arg) in args.iter().enumerate() {
        (i != 0).then(|| f.write(".."));
        generate_expr(f, arg, Context::expr(false))?;
    }
    Ok(())
}

fn generate_macro_len(f: &mut Formatter, mac: &Macro, cx: Context) -> Result<()> {
    syn_assert!(cx.is_value(), mac, "len! must be in expression position");
    cx.is_ret().then(|| f.write("return"));
    f.write("#");
    generate_expr(f, &mac.parse_body()?, Context::expr(false))
}

fn generate_macro_variadic(f: &mut Formatter, mac: &Macro, cx: Context) -> Result<()> {
    syn_assert!(
        cx.is_multi_expr(),
        mac,
        "variadic! must be in multi-value expression position"
    );
    cx.is_ret().then(|| f.write("return"));
    f.write("...");
    Ok(())
}

fn generate_macro_embed(f: &mut Formatter, mac: &Macro, cx: Context) -> Result<()> {
    syn_assert!(cx.is_value(), mac, "embed! must be in expression position");
    cx.is_ret().then(|| f.write("return"));
    let arg = mac.parse_body::<Expr>()?;
    f.write("\"");
    f.interpolate(parse_quote! {
        str::replace(&str::replace(&::std::format!("{}", #arg), "\\", "\\\\"), "\"", "\\\"")
    });
    f.write("\"");
    Ok(())
}

fn generate_macro_raw(f: &mut Formatter, mac: &Macro, _cx: Context) -> Result<()> {
    f.interpolate(mac.parse_body()?);
    Ok(())
}

#[derive(Debug, Clone, Copy)]
enum PatContext {
    Single,
    Multi,
}

impl PatContext {
    fn _is_single(&self) -> bool {
        matches!(self, Self::Single)
    }

    fn is_multi(&self) -> bool {
        matches!(self, Self::Multi)
    }
}

fn generate_pat(f: &mut Formatter, pat: &Pat, cx: PatContext) -> Result<()> {
    match pat {
        Pat::Ident(ident) => generate_pat_ident(f, ident, cx),
        Pat::Macro(mac) => generate_pat_macro(f, mac, cx),
        Pat::Tuple(tuple) => generate_pat_tuple(f, tuple, cx),
        Pat::Type(typed) => generate_pat_typed(f, typed, cx),
        Pat::Wild(wild) => generate_pat_wild(f, wild, cx),
        pat => syn_error!(pat, "unsupported pattern"),
    }
}

fn generate_pat_ident(f: &mut Formatter, ident: &PatIdent, _cx: PatContext) -> Result<()> {
    assert_no_attrs!(ident);

    if let Some(ref r) = ident.by_ref {
        syn_error!(r, "cannot be ref");
    } else if let Some(ref m) = ident.mutability {
        syn_error!(m, "cannot be mut (implicitly mutable)");
    } else if let Some((_, ref pat)) = ident.subpat {
        syn_error!(pat, "subpatterns are not supported");
    }

    generate_ident(f, &ident.ident)
}

fn generate_pat_macro(f: &mut Formatter, mac: &PatMacro, cx: PatContext) -> Result<()> {
    assert_no_attrs!(mac);
    generate_macro(f, &mac.mac, Context::expr(cx.is_multi()))
}

fn generate_pat_tuple(f: &mut Formatter, tuple: &PatTuple, cx: PatContext) -> Result<()> {
    assert_no_attrs!(tuple);
    match tuple.elems.len() {
        0 => syn_error!(tuple, "must have at least one element"),
        _ if cx.is_multi() => generate_punctuated_pat(f, &tuple.elems),
        _ => syn_error!(tuple, "expected single-valued pattern"),
    }
}

fn generate_pat_typed(f: &mut Formatter, typed: &PatType, cx: PatContext) -> Result<()> {
    assert_no_attrs!(typed);
    match *typed.ty {
        Type::Infer(_) => generate_pat(f, &typed.pat, cx),
        ref ty => syn_error!(ty, "cannot have type"),
    }
}

fn generate_pat_wild(f: &mut Formatter, wild: &PatWild, _cx: PatContext) -> Result<()> {
    assert_no_attrs!(wild);
    f.write("_");
    Ok(())
}

fn generate_punctuated_pat(f: &mut Formatter, pats: &Punctuated<Pat, Token![,]>) -> Result<()> {
    let len = pats.len();
    for (i, pat) in pats.iter().enumerate() {
        (i != 0).then(|| f.write(","));
        let cx = if i == len - 1 {
            PatContext::Multi
        } else {
            PatContext::Single
        };
        generate_pat(f, pat, cx)?;
    }
    Ok(())
}