libexpr: Support structured error classes

While preparing PRs like #9753, I've had to change error messages in
dozens of code paths. It would be nice if instead of

    EvalError("expected 'boolean' but found '%1%'", showType(v))

we could write

    TypeError(v, "boolean")

or similar. Then, changing the error message could be a mechanical
refactor with the compiler pointing out places the constructor needs to
be changed, rather than the error-prone process of grepping through the
codebase. Structured errors would also help prevent the "same" error
from having multiple slightly different messages, and could be a first
step towards error codes / an error index.

This PR reworks the exception infrastructure in `libexpr` to
support exception types with different constructor signatures than
`BaseError`. Actually refactoring the exceptions to use structured data
will come in a future PR (this one is big enough already, as it has to
touch every exception in `libexpr`).

The core design is in `eval-error.hh`. Generally, errors like this:

    state.error("'%s' is not a string", getAttrPathStr())
      .debugThrow<TypeError>()

are transformed like this:

    state.error<TypeError>("'%s' is not a string", getAttrPathStr())
      .debugThrow()

The type annotation has moved from `ErrorBuilder::debugThrow` to
`EvalState::error`.

src/libutil/error.cc

@@ -335,7 +335,7 @@ std::ostream & showErrorInfo(std::ostream & out, const ErrorInfo & einfo, bool s
      *      try {
      *          e->eval(*this, env, v);
      *          if (v.type() != nAttrs)
-     *              throwTypeError("expected a set but found %1%", v);
+     *              error<TypeError>("expected a set but found %1%", v);
      *      } catch (Error & e) {
      *          e.addTrace(pos, errorCtx);
      *          throw;
@@ -349,7 +349,7 @@ std::ostream & showErrorInfo(std::ostream & out, const ErrorInfo & einfo, bool s
      *      e->eval(*this, env, v);
      *      try {
      *          if (v.type() != nAttrs)
-     *              throwTypeError("expected a set but found %1%", v);
+     *              error<TypeError>("expected a set but found %1%", v);
      *      } catch (Error & e) {
      *          e.addTrace(pos, errorCtx);
      *          throw;
@@ -411,7 +411,7 @@ std::ostream & showErrorInfo(std::ostream & out, const ErrorInfo & einfo, bool s
 
     oss << einfo.msg << "\n";
 
-    printPosMaybe(oss, "", einfo.errPos);
+    printPosMaybe(oss, "", einfo.pos);
 
     auto suggestions = einfo.suggestions.trim();
     if (!suggestions.suggestions.empty()) {

src/libutil/error.hh

@@ -84,9 +84,14 @@ inline bool operator>=(const Trace& lhs, const Trace& rhs);
 struct ErrorInfo {
     Verbosity level;
     hintformat msg;
-    std::shared_ptr<Pos> errPos;
+    std::shared_ptr<Pos> pos;
     std::list<Trace> traces;
 
+    /**
+     * Exit status.
+     */
+    unsigned int status = 1;
+
     Suggestions suggestions;
 
     static std::optional<std::string> programName;
@@ -103,18 +108,21 @@ class BaseError : public std::exception
 protected:
     mutable ErrorInfo err;
 
+    /**
+     * Cached formatted contents of `err.msg`.
+     */
     mutable std::optional<std::string> what_;
+    /**
+     * Format `err.msg` and set `what_` to the resulting value.
+     */
     const std::string & calcWhat() const;
 
 public:
-    unsigned int status = 1; // exit status
-
     BaseError(const BaseError &) = default;
 
     template<typename... Args>
     BaseError(unsigned int status, const Args & ... args)
-        : err { .level = lvlError, .msg = hintfmt(args...) }
-        , status(status)
+        : err { .level = lvlError, .msg = hintfmt(args...), .status = status }
     { }
 
     template<typename... Args>
@@ -149,6 +157,15 @@ public:
     const std::string & msg() const { return calcWhat(); }
     const ErrorInfo & info() const { calcWhat(); return err; }
 
+    void withExitStatus(unsigned int status)
+    {
+        err.status = status;
+    }
+
+    void atPos(std::shared_ptr<Pos> pos) {
+        err.pos = pos;
+    }
+
     void pushTrace(Trace trace)
     {
         err.traces.push_front(trace);

src/libutil/logging.cc

@@ -199,7 +199,7 @@ struct JSONLogger : Logger {
         json["level"] = ei.level;
         json["msg"] = oss.str();
         json["raw_msg"] = ei.msg.str();
-        to_json(json, ei.errPos);
+        to_json(json, ei.pos);
 
         if (loggerSettings.showTrace.get() && !ei.traces.empty()) {
             nlohmann::json traces = nlohmann::json::array();