Move /src to /subprojects

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

subprojects/libutil/source-path.hh

@@ -0,0 +1,135 @@
+#pragma once
+/**
+ * @file
+ *
+ * @brief SourcePath
+ */
+
+#include "ref.hh"
+#include "canon-path.hh"
+#include "source-accessor.hh"
+#include "std-hash.hh"
+
+namespace nix {
+
+/**
+ * An abstraction for accessing source files during
+ * evaluation. Currently, it's just a wrapper around `CanonPath` that
+ * accesses files in the regular filesystem, but in the future it will
+ * support fetching files in other ways.
+ */
+struct SourcePath
+{
+    ref<SourceAccessor> accessor;
+    CanonPath path;
+
+    SourcePath(ref<SourceAccessor> accessor, CanonPath path = CanonPath::root)
+        : accessor(std::move(accessor))
+        , path(std::move(path))
+    { }
+
+    std::string_view baseName() const;
+
+    /**
+     * Construct the parent of this `SourcePath`. Aborts if `this`
+     * denotes the root.
+     */
+    SourcePath parent() const;
+
+    /**
+     * If this `SourcePath` denotes a regular file (not a symlink),
+     * return its contents; otherwise throw an error.
+     */
+    std::string readFile() const;
+
+    void readFile(
+        Sink & sink,
+        std::function<void(uint64_t)> sizeCallback = [](uint64_t size){}) const
+    { return accessor->readFile(path, sink, sizeCallback); }
+
+    /**
+     * Return whether this `SourcePath` denotes a file (of any type)
+     * that exists
+    */
+    bool pathExists() const;
+
+    /**
+     * Return stats about this `SourcePath`, or throw an exception if
+     * it doesn't exist.
+     */
+    SourceAccessor::Stat lstat() const;
+
+    /**
+     * Return stats about this `SourcePath`, or std::nullopt if it
+     * doesn't exist.
+     */
+    std::optional<SourceAccessor::Stat> maybeLstat() const;
+
+    /**
+     * If this `SourcePath` denotes a directory (not a symlink),
+     * return its directory entries; otherwise throw an error.
+     */
+    SourceAccessor::DirEntries readDirectory() const;
+
+    /**
+     * If this `SourcePath` denotes a symlink, return its target;
+     * otherwise throw an error.
+     */
+    std::string readLink() const;
+
+    /**
+     * Dump this `SourcePath` to `sink` as a NAR archive.
+     */
+    void dumpPath(
+        Sink & sink,
+        PathFilter & filter = defaultPathFilter) const;
+
+    /**
+     * Return the location of this path in the "real" filesystem, if
+     * it has a physical location.
+     */
+    std::optional<std::filesystem::path> getPhysicalPath() const;
+
+    std::string to_string() const;
+
+    /**
+     * Append a `CanonPath` to this path.
+     */
+    SourcePath operator / (const CanonPath & x) const;
+
+    /**
+     * Append a single component `c` to this path. `c` must not
+     * contain a slash. A slash is implicitly added between this path
+     * and `c`.
+     */
+    SourcePath operator / (std::string_view c) const;
+
+    bool operator==(const SourcePath & x) const noexcept;
+    std::strong_ordering operator<=>(const SourcePath & x) const noexcept;
+
+    /**
+     * Convenience wrapper around `SourceAccessor::resolveSymlinks()`.
+     */
+    SourcePath resolveSymlinks(
+        SymlinkResolution mode = SymlinkResolution::Full) const
+    {
+        return {accessor, accessor->resolveSymlinks(path, mode)};
+    }
+
+    friend class std::hash<nix::SourcePath>;
+};
+
+std::ostream & operator << (std::ostream & str, const SourcePath & path);
+
+}
+
+template<>
+struct std::hash<nix::SourcePath>
+{
+    std::size_t operator()(const nix::SourcePath & s) const noexcept
+    {
+        std::size_t hash = 0;
+        hash_combine(hash, s.accessor->number, s.path);
+        return hash;
+    }
+};