Revert "Merge pull request #13938 from NixOS/import-thunk"

This has multiple dangling pointer issues that lead to segfaults in e.g.:

nix eval --expr '(builtins.getFlake "github:nixos/nixpkgs/25.05")' --impure

This reverts commit ad175727e4, reversing
changes made to d314750174.

src/libutil/include/nix/util/canon-path.hh

@@ -8,8 +8,6 @@
 #include <set>
 #include <vector>
 
-#include <boost/container_hash/hash.hpp>
-
 namespace nix {
 
 /**
@@ -260,17 +258,11 @@ public:
      */
     std::string makeRelative(const CanonPath & path) const;
 
-    friend std::size_t hash_value(const CanonPath &);
+    friend struct std::hash<CanonPath>;
 };
 
 std::ostream & operator<<(std::ostream & stream, const CanonPath & path);
 
-inline std::size_t hash_value(const CanonPath & path)
-{
-    boost::hash<std::string_view> hasher;
-    return hasher(path.path);
-}
-
 } // namespace nix
 
 template<>
@@ -278,8 +270,8 @@ struct std::hash<nix::CanonPath>
 {
     using is_avalanching = std::true_type;
 
-    std::size_t operator()(const nix::CanonPath & path) const noexcept
+    std::size_t operator()(const nix::CanonPath & s) const noexcept
     {
-        return nix::hash_value(path);
+        return std::hash<std::string>{}(s.path);
     }
 };

src/libutil/include/nix/util/source-path.hh

@@ -119,23 +119,15 @@ struct SourcePath
 
 std::ostream & operator<<(std::ostream & str, const SourcePath & path);
 
-inline std::size_t hash_value(const SourcePath & path)
-{
-    std::size_t hash = 0;
-    boost::hash_combine(hash, path.accessor->number);
-    boost::hash_combine(hash, path.path);
-    return hash;
-}
-
 } // namespace nix
 
 template<>
 struct std::hash<nix::SourcePath>
 {
-    using is_avalanching = std::true_type;
-
     std::size_t operator()(const nix::SourcePath & s) const noexcept
     {
-        return nix::hash_value(s);
+        std::size_t hash = 0;
+        hash_combine(hash, s.accessor->number, s.path);
+        return hash;
     }
 };

src/libutil/include/nix/util/util.hh

@@ -220,17 +220,6 @@ typename T::mapped_type * get(T & map, const K & key)
 template<class T, typename K>
 typename T::mapped_type * get(T && map, const K & key) = delete;
 
-/**
- * Look up a value in a `boost::concurrent_flat_map`.
- */
-template<class T>
-std::optional<typename T::mapped_type> getConcurrent(const T & map, const typename T::key_type & key)
-{
-    std::optional<typename T::mapped_type> res;
-    map.cvisit(key, [&](auto & x) { res = x.second; });
-    return res;
-}
-
 /**
  * Get a value for the specified key from an associate container, or a default value if the key isn't present.
  */

src/libutil/posix-source-accessor.cc

@@ -95,7 +95,9 @@ std::optional<struct stat> PosixSourceAccessor::cachedLstat(const CanonPath & pa
     // former is not hashable on libc++.
     Path absPath = makeAbsPath(path).string();
 
-    if (auto res = getConcurrent(cache, absPath))
+    std::optional<Cache::mapped_type> res;
+    cache.cvisit(absPath, [&](auto & x) { res.emplace(x.second); });
+    if (res)
         return *res;
 
     auto st = nix::maybeLstat(absPath.c_str());