Move /src to /subprojects

This will facilitate breaking up Nix into multiple packages for each
component with Meson.

subprojects/libexpr/eval-error.hh

@@ -0,0 +1,121 @@
+#pragma once
+
+#include "error.hh"
+#include "pos-idx.hh"
+
+namespace nix {
+
+struct Env;
+struct Expr;
+struct Value;
+
+class EvalState;
+template<class T>
+class EvalErrorBuilder;
+
+/**
+ * Base class for all errors that occur during evaluation.
+ *
+ * Most subclasses should inherit from `EvalError` instead of this class.
+ */
+class EvalBaseError : public Error
+{
+    template<class T>
+    friend class EvalErrorBuilder;
+public:
+    EvalState & state;
+
+    EvalBaseError(EvalState & state, ErrorInfo && errorInfo)
+        : Error(errorInfo)
+        , state(state)
+    {
+    }
+
+    template<typename... Args>
+    explicit EvalBaseError(EvalState & state, const std::string & formatString, const Args &... formatArgs)
+        : Error(formatString, formatArgs...)
+        , state(state)
+    {
+    }
+};
+
+/**
+ * `EvalError` is the base class for almost all errors that occur during evaluation.
+ *
+ * All instances of `EvalError` should show a degree of purity that allows them to be
+ * cached in pure mode. This means that they should not depend on the configuration or the overall environment.
+ */
+MakeError(EvalError, EvalBaseError);
+MakeError(ParseError, Error);
+MakeError(AssertionError, EvalError);
+MakeError(ThrownError, AssertionError);
+MakeError(Abort, EvalError);
+MakeError(TypeError, EvalError);
+MakeError(UndefinedVarError, EvalError);
+MakeError(MissingArgumentError, EvalError);
+MakeError(InfiniteRecursionError, EvalError);
+
+struct InvalidPathError : public EvalError
+{
+public:
+    Path path;
+    InvalidPathError(EvalState & state, const Path & path)
+        : EvalError(state, "path '%s' is not valid", path)
+    {
+    }
+};
+
+/**
+ * `EvalErrorBuilder`s may only be constructed by `EvalState`. The `debugThrow`
+ * method must be the final method in any such `EvalErrorBuilder` usage, and it
+ * handles deleting the object.
+ */
+template<class T>
+class EvalErrorBuilder final
+{
+    friend class EvalState;
+
+    template<typename... Args>
+    explicit EvalErrorBuilder(EvalState & state, const Args &... args)
+        : error(T(state, args...))
+    {
+    }
+
+public:
+    T error;
+
+    [[nodiscard, gnu::noinline]] EvalErrorBuilder<T> & withExitStatus(unsigned int exitStatus);
+
+    [[nodiscard, gnu::noinline]] EvalErrorBuilder<T> & atPos(PosIdx pos);
+
+    [[nodiscard, gnu::noinline]] EvalErrorBuilder<T> & atPos(Value & value, PosIdx fallback = noPos);
+
+    [[nodiscard, gnu::noinline]] EvalErrorBuilder<T> & withTrace(PosIdx pos, const std::string_view text);
+
+    [[nodiscard, gnu::noinline]] EvalErrorBuilder<T> & withFrameTrace(PosIdx pos, const std::string_view text);
+
+    [[nodiscard, gnu::noinline]] EvalErrorBuilder<T> & withSuggestions(Suggestions & s);
+
+    [[nodiscard, gnu::noinline]] EvalErrorBuilder<T> & withFrame(const Env & e, const Expr & ex);
+
+    [[nodiscard, gnu::noinline]] EvalErrorBuilder<T> & addTrace(PosIdx pos, HintFmt hint);
+
+    [[nodiscard, gnu::noinline]] EvalErrorBuilder<T> & setIsFromExpr();
+
+    template<typename... Args>
+    [[nodiscard, gnu::noinline]] EvalErrorBuilder<T> &
+    addTrace(PosIdx pos, std::string_view formatString, const Args &... formatArgs);
+
+    /**
+     * Delete the `EvalErrorBuilder` and throw the underlying exception.
+     */
+    [[gnu::noinline, gnu::noreturn]] void debugThrow();
+
+    /**
+     * A programming error or fatal condition occurred. Abort the process for core dump and debugging.
+     * This does not print a proper backtrace, because unwinding the stack is destructive.
+     */
+    [[gnu::noinline, gnu::noreturn]] void panic();
+};
+
+}