Make the Store API more type-safe

Most functions now take a StorePath argument rather than a Path (which
is just an alias for std::string). The StorePath constructor ensures
that the path is syntactically correct (i.e. it looks like
<store-dir>/<base32-hash>-<name>). Similarly, functions like
buildPaths() now take a StorePathWithOutputs, rather than abusing Path
by adding a '!<outputs>' suffix.

Note that the StorePath type is implemented in Rust. This involves
some hackery to allow Rust values to be used directly in C++, via a
helper type whose destructor calls the Rust type's drop()
function. The main issue is the dynamic nature of C++ move semantics:
after we have moved a Rust value, we should not call the drop function
on the original value. So when we move a value, we set the original
value to bitwise zero, and the destructor only calls drop() if the
value is not bitwise zero. This should be sufficient for most types.

Also lots of minor cleanups to the C++ API to make it more modern
(e.g. using std::optional and std::string_view in some places).

src/libstore/path.hh

@@ -0,0 +1,81 @@
+#pragma once
+
+#include "rust-ffi.hh"
+
+namespace nix {
+
+/* See path.rs. */
+struct StorePath;
+
+extern "C" {
+    void ffi_StorePath_drop(void *);
+    bool ffi_StorePath_less_than(const StorePath & a, const StorePath & b);
+    bool ffi_StorePath_eq(const StorePath & a, const StorePath & b);
+    unsigned char * ffi_StorePath_hash_data(const StorePath & p);
+}
+
+struct StorePath : rust::Value<3 * sizeof(void *) + 24, ffi_StorePath_drop>
+{
+    static StorePath make(std::string_view path, std::string_view storeDir);
+
+    static StorePath make(unsigned char hash[20], std::string_view name);
+
+    static StorePath fromBaseName(std::string_view baseName);
+
+    rust::String to_string() const;
+
+    bool operator < (const StorePath & other) const
+    {
+        return ffi_StorePath_less_than(*this, other);
+    }
+
+    bool operator == (const StorePath & other) const
+    {
+        return ffi_StorePath_eq(*this, other);
+    }
+
+    bool operator != (const StorePath & other) const
+    {
+        return !(*this == other);
+    }
+
+    StorePath clone() const;
+
+    /* Check whether a file name ends with the extension for
+       derivations. */
+    bool isDerivation() const;
+
+    std::string_view name() const;
+
+    unsigned char * hashData() const
+    {
+        return ffi_StorePath_hash_data(*this);
+    }
+};
+
+typedef std::set<StorePath> StorePathSet;
+typedef std::vector<StorePath> StorePaths;
+
+StorePathSet cloneStorePathSet(const StorePathSet & paths);
+StorePathSet storePathsToSet(const StorePaths & paths);
+
+StorePathSet singleton(const StorePath & path);
+
+/* Size of the hash part of store paths, in base-32 characters. */
+const size_t storePathHashLen = 32; // i.e. 160 bits
+
+/* Extension of derivations in the Nix store. */
+const std::string drvExtension = ".drv";
+
+}
+
+namespace std {
+
+template<> struct hash<nix::StorePath> {
+    std::size_t operator()(const nix::StorePath & path) const noexcept
+    {
+        return * (std::size_t *) path.hashData();
+    }
+};
+
+}