libexpr: rewrite the parser with pegtl instead of flex/bison

this gives about 20% performance improvements on pure parsing. obviously
it will be less on full eval, but depending on how much parsing is to be
done (e.g. including hackage-packages.nix or not) it's more like 4%-10%.

this has been tested (with thousands of core hours of fuzzing) to ensure
that the ASTs produced by the new parser are exactly the same as the old
one would have produced. error messages will change (sometimes by a lot)
and are not yet perfect, but we would rather leave this as is for later.

test results for running only the parser (excluding the variable binding
code) in a tight loop with inputs and parameters as given are promising:

  - 40% faster on lix's package.nix at 10000 iterations
  - 1.3% faster on nixpkgs all-packages.nix at 1000 iterations
  - equivalent on all of nixpkgs concatenated at 100 iterations
    (excluding invalid files, each file surrounded with parens)

more realistic benchmarks are somewhere in between the extremes, parsing
once again getting the largest uplift. other realistic workloads improve
by a few percentage points as well, notably system builds are 4% faster.

Benchmarks summary (from ./bench/summarize.jq bench/bench-*.json)
old/bin/nix --extra-experimental-features 'nix-command flakes' eval -f bench/nixpkgs/pkgs/development/haskell-modules/hackage-packages.nix
  mean:     0.408s ± 0.025s
            user: 0.355s | system: 0.033s
  median:   0.389s
  range:    0.388s ... 0.442s
  relative: 1

new/bin/nix --extra-experimental-features 'nix-command flakes' eval -f bench/nixpkgs/pkgs/development/haskell-modules/hackage-packages.nix
  mean:     0.332s ± 0.024s
            user: 0.279s | system: 0.033s
  median:   0.314s
  range:    0.313s ... 0.361s
  relative: 0.814

---

old/bin/nix --extra-experimental-features 'nix-command flakes' eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'
  mean:     6.133s ± 0.022s
            user: 5.395s | system: 0.437s
  median:   6.128s
  range:    6.099s ... 6.183s
  relative: 1

new/bin/nix --extra-experimental-features 'nix-command flakes' eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'
  mean:     5.925s ± 0.025s
            user: 5.176s | system: 0.456s
  median:   5.934s
  range:    5.861s ... 5.943s
  relative: 0.966

---

GC_INITIAL_HEAP_SIZE=10g old/bin/nix eval --extra-experimental-features 'nix-command flakes' --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'
  mean:     4.503s ± 0.027s
            user: 3.731s | system: 0.547s
  median:   4.499s
  range:    4.478s ... 4.541s
  relative: 1

GC_INITIAL_HEAP_SIZE=10g new/bin/nix eval --extra-experimental-features 'nix-command flakes' --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'
  mean:     4.285s ± 0.031s
            user: 3.504s | system: 0.571s
  median:   4.281s
  range:    4.221s ... 4.328s
  relative: 0.951

---

old/bin/nix --extra-experimental-features 'nix-command flakes' search --no-eval-cache github:nixos/nixpkgs/e1fa12d4f6c6fe19ccb59cac54b5b3f25e160870 hello
  mean:     16.475s ± 0.07s
            user: 14.088s | system: 1.572s
  median:   16.495s
  range:    16.351s ... 16.536s
  relative: 1

new/bin/nix --extra-experimental-features 'nix-command flakes' search --no-eval-cache github:nixos/nixpkgs/e1fa12d4f6c6fe19ccb59cac54b5b3f25e160870 hello
  mean:     15.973s ± 0.013s
            user: 13.558s | system: 1.615s
  median:   15.973s
  range:    15.946s ... 15.99s
  relative: 0.97

---

Change-Id: Ie66ec2d045dec964632c6541e25f8f0797319ee2

1 files changed, 862 insertions, 0 deletions

diff --git a/src/libexpr/parser/parser.cc b/src/libexpr/parser/parser.cc
new file mode 100644
index 000000000..850f1276e
--- /dev/null
+++ b/src/libexpr/parser/parser.cc
@@ -0,0 +1,862 @@
+#include "attr-set.hh"
+#include "error.hh"
+#include "eval-settings.hh"
+#include "eval.hh"
+#include "finally.hh"
+#include "nixexpr.hh"
+#include "symbol-table.hh"
+#include "users.hh"
+
+#include "change_head.hh"
+#include "grammar.hh"
+#include "state.hh"
+
+#include <charconv>
+#include <clocale>
+#include <memory>
+
+// flip this define when doing parser development to enable some g checks.
+#if 0
+#include <tao/pegtl/contrib/analyze.hpp>
+#define ANALYZE_GRAMMAR \
+    ([] { \
+        const std::size_t issues = tao::pegtl::analyze<grammar::root>(); \
+        assert(issues == 0); \
+    })()
+#else
+#define ANALYZE_GRAMMAR ((void) 0)
+#endif
+
+namespace p = tao::pegtl;
+
+namespace nix::parser {
+namespace {
+
+template<typename>
+inline constexpr const char * error_message = nullptr;
+
+#define error_message_for(...) \
+    template<> inline constexpr auto error_message<__VA_ARGS__>
+
+error_message_for(p::one<'{'>) = "expecting '{'";
+error_message_for(p::one<'}'>) = "expecting '}'";
+error_message_for(p::one<'"'>) = "expecting '\"'";
+error_message_for(p::one<';'>) = "expecting ';'";
+error_message_for(p::one<')'>) = "expecting ')'";
+error_message_for(p::one<'='>) = "expecting '='";
+error_message_for(p::one<']'>) = "expecting ']'";
+error_message_for(p::one<':'>) = "expecting ':'";
+error_message_for(p::string<'\'', '\''>) = "expecting \"''\"";
+error_message_for(p::any) = "expecting any character";
+error_message_for(grammar::eof) = "expecting end of file";
+error_message_for(grammar::seps) = "expecting separators";
+error_message_for(grammar::path::forbid_prefix_triple_slash) = "too many slashes in path";
+error_message_for(grammar::path::forbid_prefix_double_slash_no_interp) = "path has a trailing slash";
+error_message_for(grammar::expr) = "expecting expression";
+error_message_for(grammar::expr::unary) = "expecting expression";
+error_message_for(grammar::binding::equal) = "expecting '='";
+error_message_for(grammar::expr::lambda::arg) = "expecting identifier";
+error_message_for(grammar::formals) = "expecting formals";
+error_message_for(grammar::attrpath) = "expecting attribute path";
+error_message_for(grammar::expr::select) = "expecting selection expression";
+error_message_for(grammar::t::kw_then) = "expecting 'then'";
+error_message_for(grammar::t::kw_else) = "expecting 'else'";
+error_message_for(grammar::t::kw_in) = "expecting 'in'";
+
+struct SyntaxErrors
+{
+    template<typename Rule>
+    static constexpr auto message = error_message<Rule>;
+
+    template<typename Rule>
+    static constexpr bool raise_on_failure = false;
+};
+
+template<typename Rule>
+struct Control : p::must_if<SyntaxErrors>::control<Rule>
+{
+    template<typename ParseInput, typename... States>
+    [[noreturn]] static void raise(const ParseInput & in, States &&... st)
+    {
+        if (in.empty()) {
+            std::string expected;
+            if constexpr (constexpr auto msg = error_message<Rule>)
+                expected = fmt(", %s", msg);
+            throw p::parse_error("unexpected end of file" + expected, in);
+        }
+        p::must_if<SyntaxErrors>::control<Rule>::raise(in, st...);
+    }
+};
+
+struct ExprState
+    : grammar::
+          operator_semantics<ExprState, PosIdx, AttrPath, std::pair<PosIdx, std::unique_ptr<Expr>>>
+{
+    std::unique_ptr<Expr> popExprOnly() {
+        return std::move(popExpr().second);
+    }
+
+    template<typename Op, typename... Args>
+    std::unique_ptr<Expr> applyUnary(Args &&... args) {
+        return std::make_unique<Op>(popExprOnly(), std::forward<Args>(args)...);
+    }
+
+    template<typename Op>
+    std::unique_ptr<Expr> applyBinary(PosIdx pos) {
+        auto right = popExprOnly(), left = popExprOnly();
+        return std::make_unique<Op>(pos, std::move(left), std::move(right));
+    }
+
+    std::unique_ptr<Expr> call(PosIdx pos, Symbol fn, bool flip = false)
+    {
+        std::vector<std::unique_ptr<Expr>> args(2);
+        args[flip ? 0 : 1] = popExprOnly();
+        args[flip ? 1 : 0] = popExprOnly();
+        return std::make_unique<ExprCall>(pos, std::make_unique<ExprVar>(fn), std::move(args));
+    }
+
+    std::unique_ptr<Expr> order(PosIdx pos, bool less, State & state)
+    {
+        return call(pos, state.s.lessThan, !less);
+    }
+
+    std::unique_ptr<Expr> concatStrings(PosIdx pos)
+    {
+        std::vector<std::pair<PosIdx, std::unique_ptr<Expr>>> args(2);
+        args[1] = popExpr();
+        args[0] = popExpr();
+        return std::make_unique<ExprConcatStrings>(pos, false, std::move(args));
+    }
+
+    std::unique_ptr<Expr> negate(PosIdx pos, State & state)
+    {
+        std::vector<std::unique_ptr<Expr>> args(2);
+        args[0] = std::make_unique<ExprInt>(0);
+        args[1] = popExprOnly();
+        return std::make_unique<ExprCall>(pos, std::make_unique<ExprVar>(state.s.sub), std::move(args));
+    }
+
+    std::pair<PosIdx, std::unique_ptr<Expr>> applyOp(PosIdx pos, auto & op, State & state) {
+        using Op = grammar::op;
+
+        auto not_ = [] (auto e) {
+            return std::make_unique<ExprOpNot>(std::move(e));
+        };
+
+        return {
+            pos,
+            (overloaded {
+                [&] (Op::implies)     { return applyBinary<ExprOpImpl>(pos); },
+                [&] (Op::or_)         { return applyBinary<ExprOpOr>(pos); },
+                [&] (Op::and_)        { return applyBinary<ExprOpAnd>(pos); },
+                [&] (Op::equals)      { return applyBinary<ExprOpEq>(pos); },
+                [&] (Op::not_equals)  { return applyBinary<ExprOpNEq>(pos); },
+                [&] (Op::less)        { return order(pos, true, state); },
+                [&] (Op::greater_eq)  { return not_(order(pos, true, state)); },
+                [&] (Op::greater)     { return order(pos, false, state); },
+                [&] (Op::less_eq)     { return not_(order(pos, false, state)); },
+                [&] (Op::update)      { return applyBinary<ExprOpUpdate>(pos); },
+                [&] (Op::not_)        { return applyUnary<ExprOpNot>(); },
+                [&] (Op::plus)        { return concatStrings(pos); },
+                [&] (Op::minus)       { return call(pos, state.s.sub); },
+                [&] (Op::mul)         { return call(pos, state.s.mul); },
+                [&] (Op::div)         { return call(pos, state.s.div); },
+                [&] (Op::concat)      { return applyBinary<ExprOpConcatLists>(pos); },
+                [&] (has_attr & a)    { return applyUnary<ExprOpHasAttr>(std::move(a.path)); },
+                [&] (Op::unary_minus) { return negate(pos, state); },
+            })(op)
+        };
+    }
+
+    // always_inline is needed, otherwise pushOp slows down considerably
+    [[noreturn, gnu::always_inline]]
+    static void badOperator(PosIdx pos, State & state)
+    {
+        throw ParseError({
+            .msg = HintFmt("syntax error, unexpected operator"),
+            .pos = state.positions[pos]
+        });
+    }
+
+    template<typename Expr, typename... Args>
+    Expr & pushExpr(PosIdx pos, Args && ... args)
+    {
+        auto p = std::make_unique<Expr>(std::forward<Args>(args)...);
+        auto & result = *p;
+        exprs.emplace_back(pos, std::move(p));
+        return result;
+    }
+};
+
+struct SubexprState {
+private:
+    ExprState * up;
+
+public:
+    explicit SubexprState(ExprState & up, auto &...) : up(&up) {}
+    operator ExprState &() { return *up; }
+    ExprState * operator->() { return up; }
+};
+
+
+
+template<typename Rule>
+struct BuildAST : grammar::nothing<Rule> {};
+
+struct LambdaState : SubexprState {
+    using SubexprState::SubexprState;
+
+    Symbol arg;
+    std::unique_ptr<Formals> formals;
+};
+
+struct FormalsState : SubexprState {
+    using SubexprState::SubexprState;
+
+    Formals formals{};
+    Formal formal{};
+};
+
+template<> struct BuildAST<grammar::formal::name> {
+    static void apply(const auto & in, FormalsState & s, State & ps) {
+        s.formal = {
+            .pos = ps.at(in),
+            .name = ps.symbols.create(in.string_view()),
+        };
+    }
+};
+
+template<> struct BuildAST<grammar::formal> {
+    static void apply0(FormalsState & s, State &) {
+        s.formals.formals.emplace_back(std::move(s.formal));
+    }
+};
+
+template<> struct BuildAST<grammar::formal::default_value> {
+    static void apply0(FormalsState & s, State & ps) {
+        s.formal.def = s->popExprOnly();
+    }
+};
+
+template<> struct BuildAST<grammar::formals::ellipsis> {
+    static void apply0(FormalsState & s, State &) {
+        s.formals.ellipsis = true;
+    }
+};
+
+template<> struct BuildAST<grammar::formals> : change_head<FormalsState> {
+    static void success0(FormalsState & f, LambdaState & s, State &) {
+        s.formals = std::make_unique<Formals>(std::move(f.formals));
+    }
+};
+
+struct AttrState : SubexprState {
+    using SubexprState::SubexprState;
+
+    std::vector<AttrName> attrs;
+
+    void pushAttr(auto && attr, PosIdx) { attrs.emplace_back(std::move(attr)); }
+};
+
+template<> struct BuildAST<grammar::attr::simple> {
+    static void apply(const auto & in, auto & s, State & ps) {
+        s.pushAttr(ps.symbols.create(in.string_view()), ps.at(in));
+    }
+};
+
+template<> struct BuildAST<grammar::attr::string> {
+    static void apply(const auto & in, auto & s, State & ps) {
+        auto e = s->popExprOnly();
+        if (auto str = dynamic_cast<ExprString *>(e.get()))
+            s.pushAttr(ps.symbols.create(str->s), ps.at(in));
+        else
+            s.pushAttr(std::move(e), ps.at(in));
+    }
+};
+
+template<> struct BuildAST<grammar::attr::expr> : BuildAST<grammar::attr::string> {};
+
+struct BindingsState : SubexprState {
+    using SubexprState::SubexprState;
+
+    ExprAttrs attrs;
+    AttrPath path;
+    std::unique_ptr<Expr> value;
+};
+
+struct InheritState : SubexprState {
+    using SubexprState::SubexprState;
+
+    std::vector<std::pair<AttrName, PosIdx>> attrs;
+    std::unique_ptr<Expr> from;
+    PosIdx fromPos;
+
+    void pushAttr(auto && attr, PosIdx pos) { attrs.emplace_back(std::move(attr), pos); }
+};
+
+template<> struct BuildAST<grammar::inherit::from> {
+    static void apply(const auto & in, InheritState & s, State & ps) {
+        s.from = s->popExprOnly();
+        s.fromPos = ps.at(in);
+    }
+};
+
+template<> struct BuildAST<grammar::inherit> : change_head<InheritState> {
+    static void success0(InheritState & s, BindingsState & b, State & ps) {
+        auto & attrs = b.attrs.attrs;
+        // TODO this should not reuse generic attrpath rules.
+        for (auto & [i, iPos] : s.attrs) {
+            if (i.symbol)
+                continue;
+            if (auto str = dynamic_cast<ExprString *>(i.expr.get()))
+                i = AttrName(ps.symbols.create(str->s));
+            else {
+                throw ParseError({
+                    .msg = HintFmt("dynamic attributes not allowed in inherit"),
+                    .pos = ps.positions[iPos]
+                });
+            }
+        }
+        if (auto fromE = std::move(s.from)) {
+            if (!b.attrs.inheritFromExprs)
+                b.attrs.inheritFromExprs = std::make_unique<std::vector<std::unique_ptr<Expr>>>();
+            b.attrs.inheritFromExprs->push_back(std::move(fromE));
+            for (auto & [i, iPos] : s.attrs) {
+                if (attrs.find(i.symbol) != attrs.end())
+                    ps.dupAttr(i.symbol, iPos, attrs[i.symbol].pos);
+                auto from = std::make_unique<ExprInheritFrom>(s.fromPos, b.attrs.inheritFromExprs->size() - 1);
+                attrs.emplace(
+                    i.symbol,
+                    ExprAttrs::AttrDef(
+                        std::make_unique<ExprSelect>(iPos, std::move(from), i.symbol),
+                        iPos,
+                        ExprAttrs::AttrDef::Kind::InheritedFrom));
+            }
+        } else {
+            for (auto & [i, iPos] : s.attrs) {
+                if (attrs.find(i.symbol) != attrs.end())
+                    ps.dupAttr(i.symbol, iPos, attrs[i.symbol].pos);
+                attrs.emplace(
+                    i.symbol,
+                    ExprAttrs::AttrDef(
+                        std::make_unique<ExprVar>(iPos, i.symbol),
+                        iPos,
+                        ExprAttrs::AttrDef::Kind::Inherited));
+            }
+        }
+    }
+};
+
+template<> struct BuildAST<grammar::binding::path> : change_head<AttrState> {
+    static void success0(AttrState & a, BindingsState & s, State & ps) {
+        s.path = std::move(a.attrs);
+    }
+};
+
+template<> struct BuildAST<grammar::binding::value> {
+    static void apply0(BindingsState & s, State & ps) {
+        s.value = s->popExprOnly();
+    }
+};
+
+template<> struct BuildAST<grammar::binding> {
+    static void apply(const auto & in, BindingsState & s, State & ps) {
+        ps.addAttr(&s.attrs, std::move(s.path), std::move(s.value), ps.at(in));
+    }
+};
+
+template<> struct BuildAST<grammar::expr::id> {
+    static void apply(const auto & in, ExprState & s, State & ps) {
+        if (in.string_view() == "__curPos")
+            s.pushExpr<ExprPos>(ps.at(in), ps.at(in));
+        else
+            s.pushExpr<ExprVar>(ps.at(in), ps.at(in), ps.symbols.create(in.string_view()));
+    }
+};
+
+template<> struct BuildAST<grammar::expr::int_> {
+    static void apply(const auto & in, ExprState & s, State & ps) {
+        int64_t v;
+        if (std::from_chars(in.begin(), in.end(), v).ec != std::errc{}) {
+            throw ParseError({
+                .msg = HintFmt("invalid integer '%1%'", in.string_view()),
+                .pos = ps.positions[ps.at(in)],
+            });
+        }
+        s.pushExpr<ExprInt>(noPos, v);
+    }
+};
+
+template<> struct BuildAST<grammar::expr::float_> {
+    static void apply(const auto & in, ExprState & s, State & ps) {
+        // copy the input into a temporary string so we can call stod.
+        // can't use from_chars because libc++ (thus darwin) does not have it,
+        // and floats are not performance-sensitive anyway. if they were you'd
+        // be in much bigger trouble than this.
+        //
+        // we also get to do a locale-save dance because stod is locale-aware and
+        // something (a plugin?) may have called setlocale or uselocale.
+        static struct locale_hack {
+            locale_t posix;
+            locale_hack(): posix(newlocale(LC_ALL_MASK, "POSIX", 0))
+            {
+                if (posix == 0)
+                    throw SysError("could not get POSIX locale");
+            }
+        } locale;
+
+        auto tmp = in.string();
+        double v = [&] {
+            auto oldLocale = uselocale(locale.posix);
+            Finally resetLocale([=] { uselocale(oldLocale); });
+            try {
+                return std::stod(tmp);
+            } catch (...) {
+                throw ParseError({
+                    .msg = HintFmt("invalid float '%1%'", in.string_view()),
+                    .pos = ps.positions[ps.at(in)],
+                });
+            }
+        }();
+        s.pushExpr<ExprFloat>(noPos, v);
+    }
+};
+
+struct StringState : SubexprState {
+    using SubexprState::SubexprState;
+
+    std::string currentLiteral;
+    PosIdx currentPos;
+    std::vector<std::pair<nix::PosIdx, std::unique_ptr<Expr>>> parts;
+
+    void append(PosIdx pos, std::string_view s)
+    {
+        if (currentLiteral.empty())
+            currentPos = pos;
+        currentLiteral += s;
+    }
+
+    // FIXME this truncates strings on NUL for compat with the old parser. ideally
+    // we should use the decomposition the g gives us instead of iterating over
+    // the entire string again.
+    static void unescapeStr(std::string & str)
+    {
+        char * s = str.data();
+        char * t = s;
+        char c;
+        while ((c = *s++)) {
+            if (c == '\\') {
+                c = *s++;
+                if (c == 'n') *t = '\n';
+                else if (c == 'r') *t = '\r';
+                else if (c == 't') *t = '\t';
+                else *t = c;
+            }
+            else if (c == '\r') {
+                /* Normalise CR and CR/LF into LF. */
+                *t = '\n';
+                if (*s == '\n') s++; /* cr/lf */
+            }
+            else *t = c;
+            t++;
+        }
+        str.resize(t - str.data());
+    }
+
+    void endLiteral()
+    {
+        if (!currentLiteral.empty()) {
+            unescapeStr(currentLiteral);
+            parts.emplace_back(currentPos, std::make_unique<ExprString>(std::move(currentLiteral)));
+        }
+    }
+
+    std::unique_ptr<Expr> finish()
+    {
+        if (parts.empty()) {
+            unescapeStr(currentLiteral);
+            return std::make_unique<ExprString>(std::move(currentLiteral));
+        } else {
+            endLiteral();
+            auto pos = parts[0].first;
+            return std::make_unique<ExprConcatStrings>(pos, true, std::move(parts));
+        }
+    }
+};
+
+template<typename... Content> struct BuildAST<grammar::string::literal<Content...>> {
+    static void apply(const auto & in, StringState & s, State & ps) {
+        s.append(ps.at(in), in.string_view());
+    }
+};
+
+template<> struct BuildAST<grammar::string::cr_lf> {
+    static void apply(const auto & in, StringState & s, State & ps) {
+        s.append(ps.at(in), in.string_view()); // FIXME compat with old parser
+    }
+};
+
+template<> struct BuildAST<grammar::string::interpolation> {
+    static void apply(const auto & in, StringState & s, State & ps) {
+        s.endLiteral();
+        s.parts.emplace_back(ps.at(in), s->popExprOnly());
+    }
+};
+
+template<> struct BuildAST<grammar::string::escape> {
+    static void apply(const auto & in, StringState & s, State & ps) {
+        s.append(ps.at(in), "\\"); // FIXME compat with old parser
+        s.append(ps.at(in), in.string_view());
+    }
+};
+
+template<> struct BuildAST<grammar::string> : change_head<StringState> {
+    static void success0(StringState & s, ExprState & e, State &) {
+        e.exprs.emplace_back(noPos, s.finish());
+    }
+};
+
+struct IndStringState : SubexprState {
+    using SubexprState::SubexprState;
+
+    std::vector<std::pair<PosIdx, std::variant<std::unique_ptr<Expr>, StringToken>>> parts;
+};
+
+template<bool Indented, typename... Content>
+struct BuildAST<grammar::ind_string::literal<Indented, Content...>> {
+    static void apply(const auto & in, IndStringState & s, State & ps) {
+        s.parts.emplace_back(ps.at(in), StringToken{in.string_view(), Indented});
+    }
+};
+
+template<> struct BuildAST<grammar::ind_string::interpolation> {
+    static void apply(const auto & in, IndStringState & s, State & ps) {
+        s.parts.emplace_back(ps.at(in), s->popExprOnly());
+    }
+};
+
+template<> struct BuildAST<grammar::ind_string::escape> {
+    static void apply(const auto & in, IndStringState & s, State & ps) {
+        switch (*in.begin()) {
+        case 'n': s.parts.emplace_back(ps.at(in), StringToken{"\n"}); break;
+        case 'r': s.parts.emplace_back(ps.at(in), StringToken{"\r"}); break;
+        case 't': s.parts.emplace_back(ps.at(in), StringToken{"\t"}); break;
+        default:  s.parts.emplace_back(ps.at(in), StringToken{in.string_view()}); break;
+        }
+    }
+};
+
+template<> struct BuildAST<grammar::ind_string> : change_head<IndStringState> {
+    static void success(const auto & in, IndStringState & s, ExprState & e, State & ps) {
+        e.exprs.emplace_back(noPos, ps.stripIndentation(ps.at(in), std::move(s.parts)));
+    }
+};
+
+template<typename... Content> struct BuildAST<grammar::path::literal<Content...>> {
+    static void apply(const auto & in, StringState & s, State & ps) {
+        s.append(ps.at(in), in.string_view());
+        s.endLiteral();
+    }
+};
+
+template<> struct BuildAST<grammar::path::interpolation> : BuildAST<grammar::string::interpolation> {};
+
+template<> struct BuildAST<grammar::path::anchor> {
+    static void apply(const auto & in, StringState & s, State & ps) {
+        Path path(absPath(in.string(), ps.basePath.path.abs()));
+        /* add back in the trailing '/' to the first segment */
+        if (in.string_view().ends_with('/') && in.size() > 1)
+            path += "/";
+        s.parts.emplace_back(ps.at(in), new ExprPath(std::move(path)));
+    }
+};
+
+template<> struct BuildAST<grammar::path::home_anchor> {
+    static void apply(const auto & in, StringState & s, State & ps) {
+        if (evalSettings.pureEval)
+            throw Error("the path '%s' can not be resolved in pure mode", in.string_view());
+        Path path(getHome() + in.string_view().substr(1));
+        s.parts.emplace_back(ps.at(in), new ExprPath(std::move(path)));
+    }
+};
+
+template<> struct BuildAST<grammar::path::searched_path> {
+    static void apply(const auto & in, StringState & s, State & ps) {
+        std::vector<std::unique_ptr<Expr>> args{2};
+        args[0] = std::make_unique<ExprVar>(ps.s.nixPath);
+        args[1] = std::make_unique<ExprString>(in.string());
+        s.parts.emplace_back(
+            ps.at(in),
+            std::make_unique<ExprCall>(
+                ps.at(in),
+                std::make_unique<ExprVar>(ps.s.findFile),
+                std::move(args)));
+    }
+};
+
+template<> struct BuildAST<grammar::path> : change_head<StringState> {
+    template<typename E>
+    static void check_slash(PosIdx end, StringState & s, State & ps) {
+        auto e = dynamic_cast<E *>(s.parts.back().second.get());
+        if (!e || !e->s.ends_with('/'))
+            return;
+        if (s.parts.size() > 1 || e->s != "/")
+            throw ParseError({
+                .msg = HintFmt("path has a trailing slash"),
+                .pos = ps.positions[end],
+            });
+    }
+
+    static void success(const auto & in, StringState & s, ExprState & e, State & ps) {
+        s.endLiteral();
+        check_slash<ExprPath>(ps.atEnd(in), s, ps);
+        check_slash<ExprString>(ps.atEnd(in), s, ps);
+        if (s.parts.size() == 1) {
+            e.exprs.emplace_back(noPos, std::move(s.parts.back().second));
+        } else {
+            e.pushExpr<ExprConcatStrings>(ps.at(in), ps.at(in), false, std::move(s.parts));
+        }
+    }
+};
+
+// strings and paths sare handled fully by the grammar-level rule for now
+template<> struct BuildAST<grammar::expr::string> : p::maybe_nothing {};
+template<> struct BuildAST<grammar::expr::ind_string> : p::maybe_nothing {};
+template<> struct BuildAST<grammar::expr::path> : p::maybe_nothing {};
+
+template<> struct BuildAST<grammar::expr::uri> {
+    static void apply(const auto & in, ExprState & s, State & ps) {
+       static bool noURLLiterals = experimentalFeatureSettings.isEnabled(Xp::NoUrlLiterals);
+       if (noURLLiterals)
+           throw ParseError({
+               .msg = HintFmt("URL literals are disabled"),
+               .pos = ps.positions[ps.at(in)]
+           });
+       s.pushExpr<ExprString>(ps.at(in), in.string());
+    }
+};
+
+template<> struct BuildAST<grammar::expr::ancient_let> : change_head<BindingsState> {
+    static void success(const auto & in, BindingsState & b, ExprState & s, State & ps) {
+        b.attrs.pos = ps.at(in);
+        b.attrs.recursive = true;
+        s.pushExpr<ExprSelect>(b.attrs.pos, b.attrs.pos, std::make_unique<ExprAttrs>(std::move(b.attrs)), ps.s.body);
+    }
+};
+
+template<> struct BuildAST<grammar::expr::rec_set> : change_head<BindingsState> {
+    static void success(const auto & in, BindingsState & b, ExprState & s, State & ps) {
+        b.attrs.pos = ps.at(in);
+        b.attrs.recursive = true;
+        s.pushExpr<ExprAttrs>(b.attrs.pos, std::move(b.attrs));
+    }
+};
+
+template<> struct BuildAST<grammar::expr::set> : change_head<BindingsState> {
+    static void success(const auto & in, BindingsState & b, ExprState & s, State & ps) {
+        b.attrs.pos = ps.at(in);
+        s.pushExpr<ExprAttrs>(b.attrs.pos, std::move(b.attrs));
+    }
+};
+
+using ListState = std::vector<std::unique_ptr<Expr>>;
+
+template<> struct BuildAST<grammar::expr::list> : change_head<ListState> {
+    static void success(const auto & in, ListState & ls, ExprState & s, State & ps) {
+        auto e = std::make_unique<ExprList>();
+        e->elems = std::move(ls);
+        s.exprs.emplace_back(ps.at(in), std::move(e));
+    }
+};
+
+template<> struct BuildAST<grammar::expr::list::entry> : change_head<ExprState> {
+    static void success0(ExprState & e, ListState & s, State & ps) {
+        s.emplace_back(e.finish(ps).second);
+    }
+};
+
+struct SelectState : SubexprState {
+    using SubexprState::SubexprState;
+
+    PosIdx pos;
+    ExprSelect * e = nullptr;
+};
+
+template<> struct BuildAST<grammar::expr::select::head> {
+    static void apply(const auto & in, SelectState & s, State & ps) {
+        s.pos = ps.at(in);
+    }
+};
+
+template<> struct BuildAST<grammar::expr::select::attr> : change_head<AttrState> {
+    static void success0(AttrState & a, SelectState & s, State &) {
+        s.e = &s->pushExpr<ExprSelect>(s.pos, s.pos, s->popExprOnly(), std::move(a.attrs), nullptr);
+    }
+};
+
+template<> struct BuildAST<grammar::expr::select::attr_or> {
+    static void apply0(SelectState & s, State &) {
+        s.e->def = s->popExprOnly();
+    }
+};
+
+template<> struct BuildAST<grammar::expr::select::as_app_or> {
+    static void apply(const auto & in, SelectState & s, State & ps) {
+        std::vector<std::unique_ptr<Expr>> args(1);
+        args[0] = std::make_unique<ExprVar>(ps.at(in), ps.s.or_);
+        s->pushExpr<ExprCall>(s.pos, s.pos, s->popExprOnly(), std::move(args));
+    }
+};
+
+template<> struct BuildAST<grammar::expr::select> : change_head<SelectState> {
+    static void success0(const auto &...) {}
+};
+
+struct AppState : SubexprState {
+    using SubexprState::SubexprState;
+
+    PosIdx pos;
+    ExprCall * e = nullptr;
+};
+
+template<> struct BuildAST<grammar::expr::app::select_or_fn> {
+    static void apply(const auto & in, AppState & s, State & ps) {
+        s.pos = ps.at(in);
+    }
+};
+
+template<> struct BuildAST<grammar::expr::app::first_arg> {
+    static void apply(auto & in, AppState & s, State & ps) {
+        auto arg = s->popExprOnly(), fn = s->popExprOnly();
+        if ((s.e = dynamic_cast<ExprCall *>(fn.get()))) {
+            // TODO remove.
+            // AST compat with old parser, semantics are the same.
+            // this can happen on occasions such as `<p> <p>` or `a or b or`,
+            // neither of which are super worth optimizing.
+            s.e->args.push_back(std::move(arg));
+            s->exprs.emplace_back(noPos, std::move(fn));
+        } else {
+            std::vector<std::unique_ptr<Expr>> args{1};
+            args[0] = std::move(arg);
+            s.e = &s->pushExpr<ExprCall>(s.pos, s.pos, std::move(fn), std::move(args));
+        }
+    }
+};
+
+template<> struct BuildAST<grammar::expr::app::another_arg> {
+    static void apply0(AppState & s, State & ps) {
+        s.e->args.push_back(s->popExprOnly());
+    }
+};
+
+template<> struct BuildAST<grammar::expr::app> : change_head<AppState> {
+    static void success0(const auto &...) {}
+};
+
+template<typename Op> struct BuildAST<grammar::expr::operator_<Op>> {
+    static void apply(const auto & in, ExprState & s, State & ps) {
+        s.pushOp(ps.at(in), Op{}, ps);
+    }
+};
+template<> struct BuildAST<grammar::expr::operator_<grammar::op::has_attr>> : change_head<AttrState> {
+    static void success(const auto & in, AttrState & a, ExprState & s, State & ps) {
+        s.pushOp(ps.at(in), ExprState::has_attr{{}, std::move(a.attrs)}, ps);
+    }
+};
+
+template<> struct BuildAST<grammar::expr::lambda::arg> {
+    static void apply(const auto & in, LambdaState & s, State & ps) {
+        s.arg = ps.symbols.create(in.string_view());
+    }
+};
+
+template<> struct BuildAST<grammar::expr::lambda> : change_head<LambdaState> {
+    static void success(const auto & in, LambdaState & l, ExprState & s, State & ps) {
+        if (l.formals)
+            l.formals = ps.validateFormals(std::move(l.formals), ps.at(in), l.arg);
+        s.pushExpr<ExprLambda>(ps.at(in), ps.at(in), l.arg, std::move(l.formals), l->popExprOnly());
+    }
+};
+
+template<> struct BuildAST<grammar::expr::assert_> {
+    static void apply(const auto & in, ExprState & s, State & ps) {
+        auto body = s.popExprOnly(), cond = s.popExprOnly();
+        s.pushExpr<ExprAssert>(ps.at(in), ps.at(in), std::move(cond), std::move(body));
+    }
+};
+
+template<> struct BuildAST<grammar::expr::with> {
+    static void apply(const auto & in, ExprState & s, State & ps) {
+        auto body = s.popExprOnly(), scope = s.popExprOnly();
+        s.pushExpr<ExprWith>(ps.at(in), ps.at(in), std::move(scope), std::move(body));
+    }
+};
+
+template<> struct BuildAST<grammar::expr::let> : change_head<BindingsState> {
+    static void success(const auto & in, BindingsState & b, ExprState & s, State & ps) {
+        if (!b.attrs.dynamicAttrs.empty())
+            throw ParseError({
+                .msg = HintFmt("dynamic attributes not allowed in let"),
+                .pos = ps.positions[ps.at(in)]
+            });
+
+        s.pushExpr<ExprLet>(ps.at(in), std::make_unique<ExprAttrs>(std::move(b.attrs)), b->popExprOnly());
+    }
+};
+
+template<> struct BuildAST<grammar::expr::if_> {
+    static void apply(const auto & in, ExprState & s, State & ps) {
+        auto else_ = s.popExprOnly(), then = s.popExprOnly(), cond = s.popExprOnly();
+        s.pushExpr<ExprIf>(ps.at(in), ps.at(in), std::move(cond), std::move(then), std::move(else_));
+    }
+};
+
+template<> struct BuildAST<grammar::expr> : change_head<ExprState> {
+    static void success0(ExprState & inner, ExprState & outer, State & ps) {
+        outer.exprs.push_back(inner.finish(ps));
+    }
+};
+
+}
+}
+
+namespace nix {
+
+Expr * EvalState::parse(
+    char * text,
+    size_t length,
+    Pos::Origin origin,
+    const SourcePath & basePath,
+    std::shared_ptr<StaticEnv> & staticEnv)
+{
+    parser::State s = {
+        symbols,
+        positions,
+        basePath,
+        positions.addOrigin(origin, length),
+        exprSymbols,
+    };
+    parser::ExprState x;
+
+    assert(length >= 2);
+    assert(text[length - 1] == 0);
+    assert(text[length - 2] == 0);
+    length -= 2;
+
+    p::string_input<p::tracking_mode::lazy> inp{std::string_view{text, length}, "input"};
+    try {
+        p::parse<parser::grammar::root, parser::BuildAST, parser::Control>(inp, x, s);
+    } catch (p::parse_error & e) {
+        auto pos = e.positions().back();
+        throw ParseError({
+            .msg = HintFmt("syntax error, %s", e.message()),
+            .pos = positions[s.positions.add(s.origin, pos.byte)]
+        });
+    }
+
+    auto [_pos, result] = x.finish(s);
+    result->bindVars(*this, staticEnv);
+    return result.release();
+}
+
+}