diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 91fb3ede5..96a0eb7f5 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -940,12 +940,11 @@ void EvalState::mkThunk_(Value & v, Expr * expr)
 
 void EvalState::mkPos(Value & v, PosIdx p)
 {
-    auto pos = positions[p];
-    if (auto path = std::get_if<SourcePath>(&pos.origin)) {
+    auto origin = positions.originOf(p);
+    if (auto path = std::get_if<SourcePath>(&origin)) {
         auto attrs = buildBindings(3);
         attrs.alloc(sFile).mkString(path->path.abs());
-        attrs.alloc(sLine).mkInt(pos.line);
-        attrs.alloc(sColumn).mkInt(pos.column);
+        makePositionThunks(*this, p, attrs.alloc(sLine), attrs.alloc(sColumn));
         v.mkAttrs(attrs);
     } else
         v.mkNull();
diff --git a/src/libexpr/flake/flake.cc b/src/libexpr/flake/flake.cc
index 305d84a68..72e98f6bb 100644
--- a/src/libexpr/flake/flake.cc
+++ b/src/libexpr/flake/flake.cc
@@ -227,11 +227,10 @@ static Flake getFlake(
         .sourceInfo = std::make_shared<fetchers::Tree>(std::move(sourceInfo))
     };
 
+    // NOTE evalFile forces vInfo to be an attrset because mustBeTrivial is true.
     Value vInfo;
     state.evalFile(CanonPath(flakeFile), vInfo, true); // FIXME: symlink attack
 
-    expectType(state, nAttrs, vInfo, state.positions.add({CanonPath(flakeFile)}, 1, 1));
-
     if (auto description = vInfo.attrs->get(state.sDescription)) {
         expectType(state, nString, *description->value, description->pos);
         flake.description = description->value->string.s;
diff --git a/src/libexpr/lexer.l b/src/libexpr/lexer.l
index adb9d9757..1780f5cb9 100644
--- a/src/libexpr/lexer.l
+++ b/src/libexpr/lexer.l
@@ -39,33 +39,16 @@ namespace nix {
 
 static void initLoc(YYLTYPE * loc)
 {
-    loc->first_line = loc->last_line = 1;
-    loc->first_column = loc->last_column = 1;
+    loc->first_line = loc->last_line = 0;
+    loc->first_column = loc->last_column = 0;
 }
 
 static void adjustLoc(YYLTYPE * loc, const char * s, size_t len)
 {
     loc->stash();
 
-    loc->first_line = loc->last_line;
     loc->first_column = loc->last_column;
-
-    for (size_t i = 0; i < len; i++) {
-       switch (*s++) {
-       case '\r':
-           if (*s == '\n') { /* cr/lf */
-               i++;
-               s++;
-           }
-           /* fall through */
-       case '\n':
-           ++loc->last_line;
-           loc->last_column = 1;
-           break;
-       default:
-           ++loc->last_column;
-       }
-    }
+    loc->last_column += len;
 }
 
 
diff --git a/src/libexpr/nixexpr.cc b/src/libexpr/nixexpr.cc
index c591916e3..a9989b59e 100644
--- a/src/libexpr/nixexpr.cc
+++ b/src/libexpr/nixexpr.cc
@@ -581,6 +581,39 @@ std::string ExprLambda::showNamePos(const EvalState & state) const
 
 
 
+/* Position table. */
+
+Pos PosTable::operator[](PosIdx p) const
+{
+    auto origin = resolve(p);
+    if (!origin)
+        return {};
+
+    const auto offset = origin->offsetOf(p);
+
+    Pos result{0, 0, origin->origin};
+    auto lines = this->lines.lock();
+    auto linesForInput = (*lines)[origin->offset];
+
+    if (linesForInput.empty()) {
+        auto source = result.getSource().value_or("");
+        const char * begin = source.data();
+        for (Pos::LinesIterator it(source), end; it != end; it++)
+            linesForInput.push_back(it->data() - begin);
+        if (linesForInput.empty())
+            linesForInput.push_back(0);
+    }
+    // as above: the first line starts at byte 0 and is always present
+    auto lineStartOffset = std::prev(
+        std::upper_bound(linesForInput.begin(), linesForInput.end(), offset));
+
+    result.line = 1 + (lineStartOffset - linesForInput.begin());
+    result.column = 1 + (offset - *lineStartOffset);
+    return result;
+}
+
+
+
 /* Symbol table. */
 
 size_t SymbolTable::totalSize() const
diff --git a/src/libexpr/nixexpr.hh b/src/libexpr/nixexpr.hh
index f0265a896..e5755bb99 100644
--- a/src/libexpr/nixexpr.hh
+++ b/src/libexpr/nixexpr.hh
@@ -7,7 +7,6 @@
 #include "value.hh"
 #include "symbol-table.hh"
 #include "error.hh"
-#include "chunked-vector.hh"
 #include "position.hh"
 #include "eval-error.hh"
 #include "pos-idx.hh"
diff --git a/src/libexpr/parser-state.hh b/src/libexpr/parser-state.hh
index a83d8c8b2..247a6bbff 100644
--- a/src/libexpr/parser-state.hh
+++ b/src/libexpr/parser-state.hh
@@ -24,20 +24,15 @@ struct ParserLocation
     int last_line, last_column;
 
     // backup to recover from yyless(0)
-    int stashed_first_line, stashed_first_column;
-    int stashed_last_line, stashed_last_column;
+    int stashed_first_column, stashed_last_column;
 
     void stash() {
-        stashed_first_line = first_line;
         stashed_first_column = first_column;
-        stashed_last_line = last_line;
         stashed_last_column = last_column;
     }
 
     void unstash() {
-        first_line = stashed_first_line;
         first_column = stashed_first_column;
-        last_line = stashed_last_line;
         last_column = stashed_last_column;
     }
 };
@@ -275,7 +270,7 @@ inline Expr * ParserState::stripIndentation(const PosIdx pos,
 
 inline PosIdx ParserState::at(const ParserLocation & loc)
 {
-    return positions.add(origin, loc.first_line, loc.first_column);
+    return positions.add(origin, loc.first_column);
 }
 
 }
diff --git a/src/libexpr/parser.y b/src/libexpr/parser.y
index 727ee94b4..5424d8741 100644
--- a/src/libexpr/parser.y
+++ b/src/libexpr/parser.y
@@ -435,7 +435,7 @@ Expr * parseExprFromBuf(
         .symbols = symbols,
         .positions = positions,
         .basePath = basePath,
-        .origin = {origin},
+        .origin = positions.addOrigin(origin, length),
         .s = astSymbols,
     };
 
diff --git a/src/libexpr/pos-idx.hh b/src/libexpr/pos-idx.hh
index 9949f1dc5..e94fd85c6 100644
--- a/src/libexpr/pos-idx.hh
+++ b/src/libexpr/pos-idx.hh
@@ -6,6 +6,7 @@ namespace nix {
 
 class PosIdx
 {
+    friend struct LazyPosAcessors;
     friend class PosTable;
 
 private:
diff --git a/src/libexpr/pos-table.hh b/src/libexpr/pos-table.hh
index 1decf3c85..8a0a3ba86 100644
--- a/src/libexpr/pos-table.hh
+++ b/src/libexpr/pos-table.hh
@@ -7,6 +7,7 @@
 #include "chunked-vector.hh"
 #include "pos-idx.hh"
 #include "position.hh"
+#include "sync.hh"
 
 namespace nix {
 
@@ -17,66 +18,69 @@ public:
     {
         friend PosTable;
     private:
-        // must always be invalid by default, add() replaces this with the actual value.
-        // subsequent add() calls use this index as a token to quickly check whether the
-        // current origins.back() can be reused or not.
-        mutable uint32_t idx = std::numeric_limits<uint32_t>::max();
+        uint32_t offset;
 
-        // Used for searching in PosTable::[].
-        explicit Origin(uint32_t idx)
-            : idx(idx)
-            , origin{std::monostate()}
-        {
-        }
+        Origin(Pos::Origin origin, uint32_t offset, size_t size):
+            offset(offset), origin(origin), size(size)
+        {}
 
     public:
         const Pos::Origin origin;
+        const size_t size;
 
-        Origin(Pos::Origin origin)
-            : origin(origin)
+        uint32_t offsetOf(PosIdx p) const
         {
+            return p.id - 1 - offset;
         }
     };
 
-    struct Offset
-    {
-        uint32_t line, column;
-    };
-
 private:
-    std::vector<Origin> origins;
-    ChunkedVector<Offset, 8192> offsets;
+    using Lines = std::vector<uint32_t>;
 
-public:
-    PosTable()
-        : offsets(1024)
-    {
-        origins.reserve(1024);
-    }
+    std::map<uint32_t, Origin> origins;
+    mutable Sync<std::map<uint32_t, Lines>> lines;
 
-    PosIdx add(const Origin & origin, uint32_t line, uint32_t column)
+    const Origin * resolve(PosIdx p) const
     {
-        const auto idx = offsets.add({line, column}).second;
-        if (origins.empty() || origins.back().idx != origin.idx) {
-            origin.idx = idx;
-            origins.push_back(origin);
-        }
-        return PosIdx(idx + 1);
-    }
+        if (p.id == 0)
+            return nullptr;
 
-    Pos operator[](PosIdx p) const
-    {
-        if (p.id == 0 || p.id > offsets.size())
-            return {};
         const auto idx = p.id - 1;
         /* we want the last key <= idx, so we'll take prev(first key > idx).
-           this is guaranteed to never rewind origin.begin because the first
-           key is always 0. */
-        const auto pastOrigin = std::upper_bound(
-            origins.begin(), origins.end(), Origin(idx), [](const auto & a, const auto & b) { return a.idx < b.idx; });
-        const auto origin = *std::prev(pastOrigin);
-        const auto offset = offsets[idx];
-        return {offset.line, offset.column, origin.origin};
+            this is guaranteed to never rewind origin.begin because the first
+            key is always 0. */
+        const auto pastOrigin = origins.upper_bound(idx);
+        return &std::prev(pastOrigin)->second;
+    }
+
+public:
+    Origin addOrigin(Pos::Origin origin, size_t size)
+    {
+        uint32_t offset = 0;
+        if (auto it = origins.rbegin(); it != origins.rend())
+            offset = it->first + it->second.size;
+        // +1 because all PosIdx are offset by 1 to begin with, and
+        // another +1 to ensure that all origins can point to EOF, eg
+        // on (invalid) empty inputs.
+        if (2 + offset + size < offset)
+            return Origin{origin, offset, 0};
+        return origins.emplace(offset, Origin{origin, offset, size}).first->second;
+    }
+
+    PosIdx add(const Origin & origin, size_t offset)
+    {
+        if (offset > origin.size)
+            return PosIdx();
+        return PosIdx(1 + origin.offset + offset);
+    }
+
+    Pos operator[](PosIdx p) const;
+
+    Pos::Origin originOf(PosIdx p) const
+    {
+        if (auto o = resolve(p))
+            return o->origin;
+        return std::monostate{};
     }
 };
 
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 83eccb818..e0245c2d4 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -2492,6 +2492,54 @@ static RegisterPrimOp primop_unsafeGetAttrPos(PrimOp {
     .fun = prim_unsafeGetAttrPos,
 });
 
+// access to exact position information (ie, line and colum numbers) is deferred
+// due to the cost associated with calculating that information and how rarely
+// it is used in practice. this is achieved by creating thunks to otherwise
+// inaccessible primops that are not exposed as __op or under builtins to turn
+// the internal PosIdx back into a line and column number, respectively. exposing
+// these primops in any way would at best be not useful and at worst create wildly
+// indeterministic eval results depending on parse order of files.
+//
+// in a simpler world this would instead be implemented as another kind of thunk,
+// but each type of thunk has an associated runtime cost in the current evaluator.
+// as with black holes this cost is too high to justify another thunk type to check
+// for in the very hot path that is forceValue.
+static struct LazyPosAcessors {
+    PrimOp primop_lineOfPos{
+        .arity = 1,
+        .fun = [] (EvalState & state, PosIdx pos, Value * * args, Value & v) {
+            v.mkInt(state.positions[PosIdx(args[0]->integer)].line);
+        }
+    };
+    PrimOp primop_columnOfPos{
+        .arity = 1,
+        .fun = [] (EvalState & state, PosIdx pos, Value * * args, Value & v) {
+            v.mkInt(state.positions[PosIdx(args[0]->integer)].column);
+        }
+    };
+
+    Value lineOfPos, columnOfPos;
+
+    LazyPosAcessors()
+    {
+        lineOfPos.mkPrimOp(&primop_lineOfPos);
+        columnOfPos.mkPrimOp(&primop_columnOfPos);
+    }
+
+    void operator()(EvalState & state, const PosIdx pos, Value & line, Value & column)
+    {
+        Value * posV = state.allocValue();
+        posV->mkInt(pos.id);
+        line.mkApp(&lineOfPos, posV);
+        column.mkApp(&columnOfPos, posV);
+    }
+} makeLazyPosAccessors;
+
+void makePositionThunks(EvalState & state, const PosIdx pos, Value & line, Value & column)
+{
+    makeLazyPosAccessors(state, pos, line, column);
+}
+
 /* Dynamic version of the `?' operator. */
 static void prim_hasAttr(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
diff --git a/src/libexpr/primops.hh b/src/libexpr/primops.hh
index 45486608f..9f76975db 100644
--- a/src/libexpr/primops.hh
+++ b/src/libexpr/primops.hh
@@ -51,4 +51,6 @@ void prim_importNative(EvalState & state, const PosIdx pos, Value * * args, Valu
  */
 void prim_exec(EvalState & state, const PosIdx pos, Value * * args, Value & v);
 
+void makePositionThunks(EvalState & state, const PosIdx pos, Value & line, Value & column);
+
 }
diff --git a/tests/functional/lang/eval-okay-unsafeGetAttrPos.exp b/tests/functional/lang/eval-okay-unsafeGetAttrPos.exp
new file mode 100644
index 000000000..c48e03014
--- /dev/null
+++ b/tests/functional/lang/eval-okay-unsafeGetAttrPos.exp
@@ -0,0 +1 @@
+{ column = 4; file = "/pwd/lang/eval-okay-unsafeGetAttrPos.imported-nix"; line = 6; }
diff --git a/tests/functional/lang/eval-okay-unsafeGetAttrPos.imported-nix b/tests/functional/lang/eval-okay-unsafeGetAttrPos.imported-nix
new file mode 100644
index 000000000..bdc364e76
--- /dev/null
+++ b/tests/functional/lang/eval-okay-unsafeGetAttrPos.imported-nix
@@ -0,0 +1,4 @@
+( 
+ 
 
+ 
+
 { y = "x"; })
diff --git a/tests/functional/lang/eval-okay-unsafeGetAttrPos.nix b/tests/functional/lang/eval-okay-unsafeGetAttrPos.nix
new file mode 100644
index 000000000..6d2cb86a6
--- /dev/null
+++ b/tests/functional/lang/eval-okay-unsafeGetAttrPos.nix
@@ -0,0 +1,4 @@
+let
+  pos = builtins.unsafeGetAttrPos "y" (import ./eval-okay-unsafeGetAttrPos.imported-nix);
+in
+pos
diff --git a/tests/unit/libexpr/value/print.cc b/tests/unit/libexpr/value/print.cc
index aabf156c2..d2d699a64 100644
--- a/tests/unit/libexpr/value/print.cc
+++ b/tests/unit/libexpr/value/print.cc
@@ -110,8 +110,8 @@ TEST_F(ValuePrintingTests, vLambda)
         .up = nullptr,
         .values = { }
     };
-    PosTable::Origin origin((std::monostate()));
-    auto posIdx = state.positions.add(origin, 1, 1);
+    PosTable::Origin origin = state.positions.addOrigin(std::monostate(), 1);
+    auto posIdx = state.positions.add(origin, 0);
     auto body = ExprInt(0);
     auto formals = Formals {};
 
@@ -558,8 +558,8 @@ TEST_F(ValuePrintingTests, ansiColorsLambda)
         .up = nullptr,
         .values = { }
     };
-    PosTable::Origin origin((std::monostate()));
-    auto posIdx = state.positions.add(origin, 1, 1);
+    PosTable::Origin origin = state.positions.addOrigin(std::monostate(), 1);
+    auto posIdx = state.positions.add(origin, 0);
     auto body = ExprInt(0);
     auto formals = Formals {};