Decode virtual paths in user-thrown errors

E.g. instead of

  error: Package ‘steam’ in /__virtual__/4/pkgs/games/steam/steam.nix:43 has an unfree license (‘unfreeRedistributable’), refusing to evaluate.

you now get

  error: Package ‘steam’ in «github:nixos/nixpkgs/b82ccafb54163ab9024e893e578d840577785fea»/pkgs/games/steam/steam.nix:43 has an unfree license (‘unfreeRedistributable’), refusing to evaluate.

src/libexpr/eval.hh

@@ -231,6 +231,11 @@ public:
     /* Decode a path encoded by `encodePath()`. */
     SourcePath decodePath(std::string_view s, PosIdx pos = noPos);
 
+    /* Decode all virtual paths in a string, i.e. all
+       /__virtual__/... substrings are replaced by the corresponding
+       input accessor. */
+    std::string decodePaths(std::string_view s);
+
     /* Allow access to a path. */
     void allowPath(const Path & path);
 

src/libexpr/paths.cc

@@ -63,4 +63,34 @@ SourcePath EvalState::decodePath(std::string_view s, PosIdx pos)
         return {rootFS, CanonPath(s)};
 }
 
+std::string EvalState::decodePaths(std::string_view s)
+{
+    std::string res;
+
+    size_t pos = 0;
+
+    while (true) {
+        auto m = s.find(marker, pos);
+        if (m == s.npos) {
+            res.append(s.substr(pos));
+            return res;
+        }
+
+        res.append(s.substr(pos, m - pos));
+
+        auto end = s.find_first_of(" \n\r\t'\"’:", m);
+        if (end == s.npos) end = s.size();
+
+        try {
+            auto path = decodePath(s.substr(m, end - m), noPos);
+            res.append(path.to_string());
+        } catch (...) {
+            throw;
+            res.append(s.substr(pos, end - m));
+        }
+
+        pos = end;
+    }
+}
+
 }

src/libexpr/primops.cc

@@ -770,7 +770,7 @@ static RegisterPrimOp primop_abort({
     .fun = [](EvalState & state, const PosIdx pos, Value * * args, Value & v)
     {
         PathSet context;
-        auto s = state.coerceToString(pos, *args[0], context).toOwned();
+        auto s = state.decodePaths(*state.coerceToString(pos, *args[0], context));
         state.debugThrowLastTrace(Abort("evaluation aborted with the following error message: '%1%'", s));
     }
 });
@@ -788,7 +788,7 @@ static RegisterPrimOp primop_throw({
     .fun = [](EvalState & state, const PosIdx pos, Value * * args, Value & v)
     {
       PathSet context;
-      auto s = state.coerceToString(pos, *args[0], context).toOwned();
+      auto s = state.decodePaths(*state.coerceToString(pos, *args[0], context));
       state.debugThrowLastTrace(ThrownError(s));
     }
 });
@@ -800,7 +800,7 @@ static void prim_addErrorContext(EvalState & state, const PosIdx pos, Value * *
         v = *args[1];
     } catch (Error & e) {
         PathSet context;
-        e.addTrace(nullptr, state.coerceToString(pos, *args[0], context).toOwned());
+        e.addTrace(nullptr, state.decodePaths(*state.coerceToString(pos, *args[0], context)));
         throw;
     }
 }