Merge pull request #9634 from 9999years/combine-abstract-pos-and-pos

Combine `AbstractPos`, `PosAdapter`, and `Pos`

(cherry picked from commit 113499d16fc87d53b73fb62fe6242154909756ed)

===

this is a bit cursed because originally it was based on InputAccessor
code that we don't have and moved/patched features we likewise don't
have (fetchToStore caching, all the individual accessors,
ContentAddressMethod). the commit is adjusted accordingly to
match (remove caching, ignore accessors, use FileIngestionMethod).

note that `state.rootPath . CanonPath == abs` and
computeStorePathForPath works relative to cwd, so the slight rewrite in
the moved fetchToStore is legal.

Change-Id: I05fd340c273f0bcc8ffabfebdc4a88b98083bce5

24 files changed, 311 insertions, 241 deletions

diff --git a/src/libcmd/editor-for.cc b/src/libcmd/editor-for.cc
index a17c6f12a..e5f762104 100644
--- a/src/libcmd/editor-for.cc
+++ b/src/libcmd/editor-for.cc
@@ -1,5 +1,6 @@
 #include "util.hh"
 #include "editor-for.hh"
+#include "source-path.hh"
 
 namespace nix {
 
diff --git a/src/libcmd/editor-for.hh b/src/libcmd/editor-for.hh
index fbf4307c9..8acd7011e 100644
--- a/src/libcmd/editor-for.hh
+++ b/src/libcmd/editor-for.hh
@@ -2,7 +2,7 @@
 ///@file
 
 #include "types.hh"
-#include "input-accessor.hh"
+#include "source-path.hh"
 
 namespace nix {
 
diff --git a/src/libcmd/installable-value.cc b/src/libcmd/installable-value.cc
index 08ad35105..c8a3e1b21 100644
--- a/src/libcmd/installable-value.cc
+++ b/src/libcmd/installable-value.cc
@@ -1,5 +1,6 @@
 #include "installable-value.hh"
 #include "eval-cache.hh"
+#include "fetch-to-store.hh"
 
 namespace nix {
 
@@ -44,7 +45,7 @@ ref<InstallableValue> InstallableValue::require(ref<Installable> installable)
 std::optional<DerivedPathWithInfo> InstallableValue::trySinglePathToDerivedPaths(Value & v, const PosIdx pos, std::string_view errorCtx)
 {
     if (v.type() == nPath) {
-        auto storePath = v.path().fetchToStore(state->store);
+        auto storePath = fetchToStore(*state->store, v.path());
         return {{
             .path = DerivedPath::Opaque {
                 .path = std::move(storePath),
diff --git a/src/libcmd/repl.cc b/src/libcmd/repl.cc
index 6181e3eb2..83d719bfe 100644
--- a/src/libcmd/repl.cc
+++ b/src/libcmd/repl.cc
@@ -218,7 +218,7 @@ static std::ostream & showDebugTrace(std::ostream & out, const PosTable & positi
     // prefer direct pos, but if noPos then try the expr.
     auto pos = dt.pos
         ? dt.pos
-        : static_cast<std::shared_ptr<AbstractPos>>(positions[dt.expr.getPos() ? dt.expr.getPos() : noPos]);
+        : positions[dt.expr.getPos() ? dt.expr.getPos() : noPos];
 
     if (pos) {
         out << *pos;
diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index debeaf75f..8e3077a67 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -14,6 +14,7 @@
 #include "profiles.hh"
 #include "print.hh"
 #include "gc-small-vector.hh"
+#include "fetch-to-store.hh"
 
 #include <algorithm>
 #include <chrono>
@@ -879,7 +880,7 @@ void EvalState::runDebugRepl(const Error * error, const Env & env, const Expr &
         ? std::make_unique<DebugTraceStacker>(
             *this,
             DebugTrace {
-                .pos = error->info().errPos ? error->info().errPos : static_cast<std::shared_ptr<AbstractPos>>(positions[expr.getPos()]),
+                .pos = error->info().errPos ? error->info().errPos : positions[expr.getPos()],
                 .expr = expr,
                 .env = env,
                 .hint = error->info().msg,
@@ -916,7 +917,7 @@ static std::unique_ptr<DebugTraceStacker> makeDebugTraceStacker(
     EvalState & state,
     Expr & expr,
     Env & env,
-    std::shared_ptr<AbstractPos> && pos,
+    std::shared_ptr<Pos> && pos,
     const char * s,
     const std::string & s2)
 {
@@ -1214,7 +1215,7 @@ void EvalState::cacheFile(
                 *this,
                 *e,
                 this->baseEnv,
-                e->getPos() ? static_cast<std::shared_ptr<AbstractPos>>(positions[e->getPos()]) : nullptr,
+                e->getPos() ? std::make_shared<Pos>(positions[e->getPos()]) : nullptr,
                 "while evaluating the file '%1%':", resolvedPath.to_string())
             : nullptr;
 
@@ -2388,7 +2389,7 @@ StorePath EvalState::copyPathToStore(NixStringContext & context, const SourcePat
     auto dstPath = i != srcToStore.end()
         ? i->second
         : [&]() {
-            auto dstPath = path.fetchToStore(store, path.baseName(), nullptr, repair);
+            auto dstPath = fetchToStore(*store, path, path.baseName(), FileIngestionMethod::Recursive, nullptr, repair);
             allowPath(dstPath);
             srcToStore.insert_or_assign(path, dstPath);
             printMsg(lvlChatty, "copied source '%1%' -> '%2%'", path, store->printStorePath(dstPath));
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index 9cf50ba6c..d070fdab8 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -141,7 +141,7 @@ struct RegexCache;
 std::shared_ptr<RegexCache> makeRegexCache();
 
 struct DebugTrace {
-    std::shared_ptr<AbstractPos> pos;
+    std::shared_ptr<Pos> pos;
     const Expr & expr;
     const Env & env;
     hintformat hint;
diff --git a/src/libexpr/nixexpr.cc b/src/libexpr/nixexpr.cc
index b34112be1..eb5d0d3d4 100644
--- a/src/libexpr/nixexpr.cc
+++ b/src/libexpr/nixexpr.cc
@@ -11,58 +11,6 @@ namespace nix {
 
 ExprBlackHole eBlackHole;
 
-struct PosAdapter : AbstractPos
-{
-    Pos::Origin origin;
-
-    PosAdapter(Pos::Origin origin)
-        : origin(std::move(origin))
-    {
-    }
-
-    std::optional<std::string> getSource() const override
-    {
-        return std::visit(overloaded {
-            [](const Pos::none_tag &) -> std::optional<std::string> {
-                return std::nullopt;
-            },
-            [](const Pos::Stdin & s) -> std::optional<std::string> {
-                // Get rid of the null terminators added by the parser.
-                return std::string(s.source->c_str());
-            },
-            [](const Pos::String & s) -> std::optional<std::string> {
-                // Get rid of the null terminators added by the parser.
-                return std::string(s.source->c_str());
-            },
-            [](const SourcePath & path) -> std::optional<std::string> {
-                try {
-                    return path.readFile();
-                } catch (Error &) {
-                    return std::nullopt;
-                }
-            }
-        }, origin);
-    }
-
-    void print(std::ostream & out) const override
-    {
-        std::visit(overloaded {
-            [&](const Pos::none_tag &) { out << "«none»"; },
-            [&](const Pos::Stdin &) { out << "«stdin»"; },
-            [&](const Pos::String & s) { out << "«string»"; },
-            [&](const SourcePath & path) { out << path; }
-        }, origin);
-    }
-};
-
-Pos::operator std::shared_ptr<AbstractPos>() const
-{
-    auto pos = std::make_shared<PosAdapter>(origin);
-    pos->line = line;
-    pos->column = column;
-    return pos;
-}
-
 // FIXME: remove, because *symbols* are abstract and do not have a single
 //        textual representation; see printIdentifier()
 std::ostream & operator <<(std::ostream & str, const SymbolStr & symbol)
@@ -268,17 +216,6 @@ void ExprPos::show(const SymbolTable & symbols, std::ostream & str) const
 }
 
 
-std::ostream & operator << (std::ostream & str, const Pos & pos)
-{
-    if (auto pos2 = (std::shared_ptr<AbstractPos>) pos) {
-        str << *pos2;
-    } else
-        str << "undefined position";
-
-    return str;
-}
-
-
 std::string showAttrPath(const SymbolTable & symbols, const AttrPath & attrPath)
 {
     std::ostringstream out;
diff --git a/src/libexpr/nixexpr.hh b/src/libexpr/nixexpr.hh
index b38f19b41..6f2a4c0f9 100644
--- a/src/libexpr/nixexpr.hh
+++ b/src/libexpr/nixexpr.hh
@@ -8,6 +8,7 @@
 #include "symbol-table.hh"
 #include "error.hh"
 #include "chunked-vector.hh"
+#include "position.hh"
 
 namespace nix {
 
@@ -29,27 +30,6 @@ public:
     using EvalError::EvalError;
 };
 
-/**
- * Position objects.
- */
-struct Pos
-{
-    uint32_t line;
-    uint32_t column;
-
-    struct none_tag { };
-    struct Stdin { ref<std::string> source; };
-    struct String { ref<std::string> source; };
-
-    typedef std::variant<none_tag, Stdin, String, SourcePath> Origin;
-
-    Origin origin;
-
-    explicit operator bool() const { return line > 0; }
-
-    operator std::shared_ptr<AbstractPos>() const;
-};
-
 class PosIdx {
     friend class PosTable;
 
@@ -82,7 +62,7 @@ public:
         mutable uint32_t idx = std::numeric_limits<uint32_t>::max();
 
         // Used for searching in PosTable::[].
-        explicit Origin(uint32_t idx): idx(idx), origin{Pos::none_tag()} {}
+        explicit Origin(uint32_t idx): idx(idx), origin{std::monostate()} {}
 
     public:
         const Pos::Origin origin;
@@ -133,8 +113,6 @@ public:
 
 inline PosIdx noPos = {};
 
-std::ostream & operator << (std::ostream & str, const Pos & pos);
-
 
 struct Env;
 struct Value;
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index d64f2d51e..1a961582f 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -14,6 +14,7 @@
 #include "value-to-json.hh"
 #include "value-to-xml.hh"
 #include "primops.hh"
+#include "fetch-to-store.hh"
 
 #include <boost/container/small_vector.hpp>
 #include <nlohmann/json.hpp>
@@ -2235,9 +2236,8 @@ static void addPath(
             });
 
         if (!expectedHash || !state.store->isValidPath(*expectedStorePath)) {
-            StorePath dstPath = settings.readOnlyMode
-                ? state.store->computeStorePathForPath(name, path, method, htSHA256, filter).first
-                : state.store->addToStore(name, path, method, htSHA256, filter, state.repair, refs);
+            auto dstPath = fetchToStore(
+                *state.store, state.rootPath(CanonPath(path)), name, method, &filter, state.repair);
             if (expectedHash && expectedStorePath != dstPath)
                 state.debugThrowLastTrace(Error("store path mismatch in (possibly filtered) path added from '%s'", path));
             state.allowAndSetStorePathString(dstPath, v);
diff --git a/src/libexpr/value.hh b/src/libexpr/value.hh
index 224ffac9c..f90082e61 100644
--- a/src/libexpr/value.hh
+++ b/src/libexpr/value.hh
@@ -7,6 +7,7 @@
 #include "symbol-table.hh"
 #include "value/context.hh"
 #include "input-accessor.hh"
+#include "source-path.hh"
 
 #if HAVE_BOEHMGC
 #include <gc/gc_allocator.h>
diff --git a/src/libfetchers/fetch-to-store.cc b/src/libfetchers/fetch-to-store.cc
new file mode 100644
index 000000000..f830a4963
--- /dev/null
+++ b/src/libfetchers/fetch-to-store.cc
@@ -0,0 +1,26 @@
+#include "fetch-to-store.hh"
+#include "fetchers.hh"
+#include "cache.hh"
+
+namespace nix {
+
+StorePath fetchToStore(
+    Store & store,
+    const SourcePath & path,
+    std::string_view name,
+    FileIngestionMethod method,
+    PathFilter * filter,
+    RepairFlag repair)
+{
+    Activity act(*logger, lvlChatty, actUnknown, fmt("copying '%s' to the store", path));
+
+    auto filter2 = filter ? *filter : defaultPathFilter;
+
+    return
+        settings.readOnlyMode
+        ? store.computeStorePathForPath(name, path.path.abs(), method, htSHA256, filter2).first
+        : store.addToStore(name, path.path.abs(), method, htSHA256, filter2, repair);
+}
+
+
+}
diff --git a/src/libfetchers/fetch-to-store.hh b/src/libfetchers/fetch-to-store.hh
new file mode 100644
index 000000000..eef269071
--- /dev/null
+++ b/src/libfetchers/fetch-to-store.hh
@@ -0,0 +1,22 @@
+#pragma once
+
+#include "source-path.hh"
+#include "store-api.hh"
+#include "util.hh"
+#include "repair-flag.hh"
+#include "content-address.hh"
+
+namespace nix {
+
+/**
+ * Copy the `path` to the Nix store.
+ */
+StorePath fetchToStore(
+    Store & store,
+    const SourcePath & path,
+    std::string_view name = "source",
+    FileIngestionMethod method = FileIngestionMethod::Recursive,
+    PathFilter * filter = nullptr,
+    RepairFlag repair = NoRepair);
+
+}
diff --git a/src/libfetchers/fetchers.cc b/src/libfetchers/fetchers.cc
index e683b9f80..530473023 100644
--- a/src/libfetchers/fetchers.cc
+++ b/src/libfetchers/fetchers.cc
@@ -1,5 +1,7 @@
 #include "fetchers.hh"
 #include "store-api.hh"
+#include "source-path.hh"
+#include "fetch-to-store.hh"
 
 #include <nlohmann/json.hpp>
 
diff --git a/src/libstore/store-api.hh b/src/libstore/store-api.hh
index 3e49c3d95..88c182b94 100644
--- a/src/libstore/store-api.hh
+++ b/src/libstore/store-api.hh
@@ -14,6 +14,7 @@
 #include "config.hh"
 #include "path-info.hh"
 #include "repair-flag.hh"
+#include "source-path.hh"
 
 #include <nlohmann/json_fwd.hpp>
 #include <atomic>
diff --git a/src/libutil/error.cc b/src/libutil/error.cc
index 36afb22ae..a9e3b168f 100644
--- a/src/libutil/error.cc
+++ b/src/libutil/error.cc
@@ -1,4 +1,5 @@
 #include "error.hh"
+#include "position.hh"
 
 #include <iostream>
 #include <optional>
@@ -9,7 +10,7 @@ namespace nix {
 
 const std::string nativeSystem = SYSTEM;
 
-void BaseError::addTrace(std::shared_ptr<AbstractPos> && e, hintformat hint, bool frame)
+void BaseError::addTrace(std::shared_ptr<Pos> && e, hintformat hint, bool frame)
 {
     err.traces.push_front(Trace { .pos = std::move(e), .hint = hint, .frame = frame });
 }
@@ -40,15 +41,6 @@ std::ostream & operator <<(std::ostream & os, const hintformat & hf)
     return os << hf.str();
 }
 
-std::ostream & operator <<(std::ostream & str, const AbstractPos & pos)
-{
-    pos.print(str);
-    str << ":" << pos.line;
-    if (pos.column > 0)
-        str << ":" << pos.column;
-    return str;
-}
-
 /**
  * An arbitrarily defined value comparison for the purpose of using traces in the key of a sorted container.
  */
@@ -75,49 +67,10 @@ inline bool operator> (const Trace& lhs, const Trace& rhs) { return rhs < lhs; }
 inline bool operator<=(const Trace& lhs, const Trace& rhs) { return !(lhs > rhs); }
 inline bool operator>=(const Trace& lhs, const Trace& rhs) { return !(lhs < rhs); }
 
-std::optional<LinesOfCode> AbstractPos::getCodeLines() const
-{
-    if (line == 0)
-        return std::nullopt;
-
-    if (auto source = getSource()) {
-
-        std::istringstream iss(*source);
-        // count the newlines.
-        int count = 0;
-        std::string curLine;
-        int pl = line - 1;
-
-        LinesOfCode loc;
-
-        do {
-            std::getline(iss, curLine);
-            ++count;
-            if (count < pl)
-                ;
-            else if (count == pl) {
-                loc.prevLineOfCode = curLine;
-            } else if (count == pl + 1) {
-                loc.errLineOfCode = curLine;
-            } else if (count == pl + 2) {
-                loc.nextLineOfCode = curLine;
-                break;
-            }
-
-            if (!iss.good())
-                break;
-        } while (true);
-
-        return loc;
-    }
-
-    return std::nullopt;
-}
-
 // print lines of code to the ostream, indicating the error column.
 void printCodeLines(std::ostream & out,
     const std::string & prefix,
-    const AbstractPos & errPos,
+    const Pos & errPos,
     const LinesOfCode & loc)
 {
     // previous line of code.
@@ -195,7 +148,7 @@ static bool printUnknownLocations = getEnv("_NIX_DEVELOPER_SHOW_UNKNOWN_LOCATION
  *
  * @return true if a position was printed.
  */
-static bool printPosMaybe(std::ostream & oss, std::string_view indent, const std::shared_ptr<AbstractPos> & pos) {
+static bool printPosMaybe(std::ostream & oss, std::string_view indent, const std::shared_ptr<Pos> & pos) {
     bool hasPos = pos && *pos;
     if (hasPos) {
         oss << indent << ANSI_BLUE << "at " ANSI_WARNING << *pos << ANSI_NORMAL << ":";
diff --git a/src/libutil/error.hh b/src/libutil/error.hh
index baffca128..234cbe1f6 100644
--- a/src/libutil/error.hh
+++ b/src/libutil/error.hh
@@ -63,45 +63,15 @@ struct LinesOfCode {
     std::optional<std::string> nextLineOfCode;
 };
 
-/**
- * An abstract type that represents a location in a source file.
- */
-struct AbstractPos
-{
-    uint32_t line = 0;
-    uint32_t column = 0;
-
-    /**
-     * An AbstractPos may be a "null object", representing an unknown position.
-     *
-     * Return true if this position is known.
-     */
-    inline operator bool() const { return line != 0; };
-
-    /**
-     * Return the contents of the source file.
-     */
-    virtual std::optional<std::string> getSource() const
-    { return std::nullopt; };
-
-    virtual void print(std::ostream & out) const = 0;
-
-    std::optional<LinesOfCode> getCodeLines() const;
-
-    virtual ~AbstractPos() = default;
-
-    inline auto operator<=>(const AbstractPos& rhs) const = default;
-};
-
-std::ostream & operator << (std::ostream & str, const AbstractPos & pos);
+struct Pos;
 
 void printCodeLines(std::ostream & out,
     const std::string & prefix,
-    const AbstractPos & errPos,
+    const Pos & errPos,
     const LinesOfCode & loc);
 
 struct Trace {
-    std::shared_ptr<AbstractPos> pos;
+    std::shared_ptr<Pos> pos;
     hintformat hint;
     bool frame;
 };
@@ -114,7 +84,7 @@ inline bool operator>=(const Trace& lhs, const Trace& rhs);
 struct ErrorInfo {
     Verbosity level;
     hintformat msg;
-    std::shared_ptr<AbstractPos> errPos;
+    std::shared_ptr<Pos> errPos;
     std::list<Trace> traces;
 
     Suggestions suggestions;
@@ -185,12 +155,12 @@ public:
     }
 
     template<typename... Args>
-    void addTrace(std::shared_ptr<AbstractPos> && e, std::string_view fs, const Args & ... args)
+    void addTrace(std::shared_ptr<Pos> && e, std::string_view fs, const Args & ... args)
     {
         addTrace(std::move(e), hintfmt(std::string(fs), args...));
     }
 
-    void addTrace(std::shared_ptr<AbstractPos> && e, hintformat hint, bool frame = false);
+    void addTrace(std::shared_ptr<Pos> && e, hintformat hint, bool frame = false);
 
     bool hasTrace() const { return !err.traces.empty(); }
 
diff --git a/src/libutil/input-accessor.hh b/src/libutil/input-accessor.hh
new file mode 100644
index 000000000..740175af0
--- /dev/null
+++ b/src/libutil/input-accessor.hh
@@ -0,0 +1,35 @@
+#pragma once
+///@file
+
+#include "types.hh"
+#include "archive.hh"
+
+namespace nix {
+
+struct InputAccessor
+{
+    enum Type {
+      tRegular, tSymlink, tDirectory,
+      /**
+        Any other node types that may be encountered on the file system, such as device nodes, sockets, named pipe, and possibly even more exotic things.
+
+        Responsible for `"unknown"` from `builtins.readFileType "/dev/null"`.
+
+        Unlike `DT_UNKNOWN`, this must not be used for deferring the lookup of types.
+      */
+      tMisc
+    };
+
+    struct Stat
+    {
+        Type type = tMisc;
+        //uint64_t fileSize = 0; // regular files only
+        bool isExecutable = false; // regular files only
+    };
+
+    typedef std::optional<Type> DirEntry;
+
+    typedef std::map<std::string, DirEntry> DirEntries;
+};
+
+}
diff --git a/src/libutil/logging.cc b/src/libutil/logging.cc
index 9d7a141b3..b4b180760 100644
--- a/src/libutil/logging.cc
+++ b/src/libutil/logging.cc
@@ -1,6 +1,8 @@
 #include "logging.hh"
 #include "util.hh"
 #include "config.hh"
+#include "source-path.hh"
+#include "position.hh"
 
 #include <atomic>
 #include <nlohmann/json.hpp>
@@ -133,13 +135,13 @@ Activity::Activity(Logger & logger, Verbosity lvl, ActivityType type,
     logger.startActivity(id, lvl, type, s, fields, parent);
 }
 
-void to_json(nlohmann::json & json, std::shared_ptr<AbstractPos> pos)
+void to_json(nlohmann::json & json, std::shared_ptr<Pos> pos)
 {
     if (pos) {
         json["line"] = pos->line;
         json["column"] = pos->column;
         std::ostringstream str;
-        pos->print(str);
+        pos->print(str, true);
         json["file"] = str.str();
     } else {
         json["line"] = nullptr;
diff --git a/src/libutil/position.cc b/src/libutil/position.cc
new file mode 100644
index 000000000..b39a5a1d4
--- /dev/null
+++ b/src/libutil/position.cc
@@ -0,0 +1,112 @@
+#include "position.hh"
+
+namespace nix {
+
+Pos::Pos(const Pos * other)
+{
+    if (!other) {
+        return;
+    }
+    line = other->line;
+    column = other->column;
+    origin = std::move(other->origin);
+}
+
+Pos::operator std::shared_ptr<Pos>() const
+{
+    return std::make_shared<Pos>(&*this);
+}
+
+bool Pos::operator<(const Pos &rhs) const
+{
+    return std::forward_as_tuple(line, column, origin)
+        < std::forward_as_tuple(rhs.line, rhs.column, rhs.origin);
+}
+
+std::optional<LinesOfCode> Pos::getCodeLines() const
+{
+    if (line == 0)
+        return std::nullopt;
+
+    if (auto source = getSource()) {
+
+        std::istringstream iss(*source);
+        // count the newlines.
+        int count = 0;
+        std::string curLine;
+        int pl = line - 1;
+
+        LinesOfCode loc;
+
+        do {
+            std::getline(iss, curLine);
+            ++count;
+            if (count < pl)
+                ;
+            else if (count == pl) {
+                loc.prevLineOfCode = curLine;
+            } else if (count == pl + 1) {
+                loc.errLineOfCode = curLine;
+            } else if (count == pl + 2) {
+                loc.nextLineOfCode = curLine;
+                break;
+            }
+
+            if (!iss.good())
+                break;
+        } while (true);
+
+        return loc;
+    }
+
+    return std::nullopt;
+}
+
+
+std::optional<std::string> Pos::getSource() const
+{
+    return std::visit(overloaded {
+        [](const std::monostate &) -> std::optional<std::string> {
+            return std::nullopt;
+        },
+        [](const Pos::Stdin & s) -> std::optional<std::string> {
+            // Get rid of the null terminators added by the parser.
+            return std::string(s.source->c_str());
+        },
+        [](const Pos::String & s) -> std::optional<std::string> {
+            // Get rid of the null terminators added by the parser.
+            return std::string(s.source->c_str());
+        },
+        [](const SourcePath & path) -> std::optional<std::string> {
+            try {
+                return path.readFile();
+            } catch (Error &) {
+                return std::nullopt;
+            }
+        }
+    }, origin);
+}
+
+void Pos::print(std::ostream & out, bool showOrigin) const
+{
+    if (showOrigin) {
+        std::visit(overloaded {
+            [&](const std::monostate &) { out << "«none»"; },
+            [&](const Pos::Stdin &) { out << "«stdin»"; },
+            [&](const Pos::String & s) { out << "«string»"; },
+            [&](const SourcePath & path) { out << path; }
+        }, origin);
+        out << ":";
+    }
+    out << line;
+    if (column > 0)
+        out << ":" << column;
+}
+
+std::ostream & operator<<(std::ostream & str, const Pos & pos)
+{
+    pos.print(str, true);
+    return str;
+}
+
+}
diff --git a/src/libutil/position.hh b/src/libutil/position.hh
new file mode 100644
index 000000000..a184997ed
--- /dev/null
+++ b/src/libutil/position.hh
@@ -0,0 +1,74 @@
+#pragma once
+/**
+ * @file
+ *
+ * @brief Pos and AbstractPos
+ */
+
+#include <cstdint>
+#include <string>
+
+#include "source-path.hh"
+
+namespace nix {
+
+/**
+ * A position and an origin for that position (like a source file).
+ */
+struct Pos
+{
+    uint32_t line = 0;
+    uint32_t column = 0;
+
+    struct Stdin {
+        ref<std::string> source;
+        bool operator==(const Stdin & rhs) const
+        { return *source == *rhs.source; }
+        bool operator!=(const Stdin & rhs) const
+        { return *source != *rhs.source; }
+        bool operator<(const Stdin & rhs) const
+        { return *source < *rhs.source; }
+    };
+    struct String {
+        ref<std::string> source;
+        bool operator==(const String & rhs) const
+        { return *source == *rhs.source; }
+        bool operator!=(const String & rhs) const
+        { return *source != *rhs.source; }
+        bool operator<(const String & rhs) const
+        { return *source < *rhs.source; }
+    };
+
+    typedef std::variant<std::monostate, Stdin, String, SourcePath> Origin;
+
+    Origin origin = std::monostate();
+
+    Pos() { }
+    Pos(uint32_t line, uint32_t column, Origin origin)
+        : line(line), column(column), origin(origin) { }
+    Pos(Pos & other) = default;
+    Pos(const Pos & other) = default;
+    Pos(Pos && other) = default;
+    Pos(const Pos * other);
+
+    explicit operator bool() const { return line > 0; }
+
+    operator std::shared_ptr<Pos>() const;
+
+    /**
+     * Return the contents of the source file.
+     */
+    std::optional<std::string> getSource() const;
+
+    void print(std::ostream & out, bool showOrigin) const;
+
+    std::optional<LinesOfCode> getCodeLines() const;
+
+    bool operator==(const Pos & rhs) const = default;
+    bool operator!=(const Pos & rhs) const = default;
+    bool operator<(const Pos & rhs) const;
+};
+
+std::ostream & operator<<(std::ostream & str, const Pos & pos);
+
+}
diff --git a/src/libutil/ref.hh b/src/libutil/ref.hh
index af5f8304c..5d0c3696d 100644
--- a/src/libutil/ref.hh
+++ b/src/libutil/ref.hh
@@ -1,6 +1,7 @@
 #pragma once
 ///@file
 
+#include <compare>
 #include <memory>
 #include <exception>
 #include <stdexcept>
diff --git a/src/libstore/repair-flag.hh b/src/libutil/repair-flag.hh
index f412d6a20..f412d6a20 100644
--- a/src/libstore/repair-flag.hh
+++ b/src/libutil/repair-flag.hh
diff --git a/src/libfetchers/input-accessor.cc b/src/libutil/source-path.cc
index f37a8058b..b6563ee90 100644
--- a/src/libfetchers/input-accessor.cc
+++ b/src/libutil/source-path.cc
@@ -1,5 +1,4 @@
-#include "input-accessor.hh"
-#include "store-api.hh"
+#include "source-path.hh"
 
 namespace nix {
 
@@ -57,18 +56,6 @@ InputAccessor::DirEntries SourcePath::readDirectory() const
     return res;
 }
 
-StorePath SourcePath::fetchToStore(
-    ref<Store> store,
-    std::string_view name,
-    PathFilter * filter,
-    RepairFlag repair) const
-{
-    return
-        settings.readOnlyMode
-        ? store->computeStorePathForPath(name, path.abs(), FileIngestionMethod::Recursive, htSHA256, filter ? *filter : defaultPathFilter).first
-        : store->addToStore(name, path.abs(), FileIngestionMethod::Recursive, htSHA256, filter ? *filter : defaultPathFilter, repair);
-}
-
 SourcePath SourcePath::resolveSymlinks() const
 {
     SourcePath res(CanonPath::root);
diff --git a/src/libfetchers/input-accessor.hh b/src/libutil/source-path.hh
index 5a2f17f62..54698de77 100644
--- a/src/libfetchers/input-accessor.hh
+++ b/src/libutil/source-path.hh
@@ -1,42 +1,17 @@
 #pragma once
+/**
+ * @file
+ *
+ * @brief SourcePath
+ */
 
 #include "ref.hh"
-#include "types.hh"
-#include "archive.hh"
 #include "canon-path.hh"
 #include "repair-flag.hh"
+#include "input-accessor.hh"
 
 namespace nix {
 
-class StorePath;
-class Store;
-
-struct InputAccessor
-{
-    enum Type {
-      tRegular, tSymlink, tDirectory,
-      /**
-        Any other node types that may be encountered on the file system, such as device nodes, sockets, named pipe, and possibly even more exotic things.
-
-        Responsible for `"unknown"` from `builtins.readFileType "/dev/null"`.
-
-        Unlike `DT_UNKNOWN`, this must not be used for deferring the lookup of types.
-      */
-      tMisc
-    };
-
-    struct Stat
-    {
-        Type type = tMisc;
-        //uint64_t fileSize = 0; // regular files only
-        bool isExecutable = false; // regular files only
-    };
-
-    typedef std::optional<Type> DirEntry;
-
-    typedef std::map<std::string, DirEntry> DirEntries;
-};
-
 /**
  * An abstraction for accessing source files during
  * evaluation. Currently, it's just a wrapper around `CanonPath` that
@@ -107,15 +82,6 @@ struct SourcePath
     { return nix::dumpPath(path.abs(), sink, filter); }
 
     /**
-     * Copy this `SourcePath` to the Nix store.
-     */
-    StorePath fetchToStore(
-        ref<Store> store,
-        std::string_view name = "source",
-        PathFilter * filter = nullptr,
-        RepairFlag repair = NoRepair) const;
-
-    /**
      * Return the location of this path in the "real" filesystem, if
      * it has a physical location.
      */