Move unit tests to the location Meson expects them to be

Everything that is a separate subproject should live in the subprojects
directory.

Progress on #2503

This reverts commit 451f8a8c19.

src/libutil-tests/position.cc

@@ -0,0 +1,122 @@
+#include <gtest/gtest.h>
+
+#include "position.hh"
+
+namespace nix {
+
+inline Pos::Origin makeStdin(std::string s)
+{
+    return Pos::Stdin{make_ref<std::string>(s)};
+}
+
+TEST(Position, getSnippetUpTo_0)
+{
+    Pos::Origin o = makeStdin("");
+    Pos p(1, 1, o);
+    ASSERT_EQ(p.getSnippetUpTo(p), "");
+}
+TEST(Position, getSnippetUpTo_1)
+{
+    Pos::Origin o = makeStdin("x");
+    {
+        // NOTE: line and column are actually 1-based indexes
+        Pos start(0, 0, o);
+        Pos end(99, 99, o);
+        ASSERT_EQ(start.getSnippetUpTo(start), "");
+        ASSERT_EQ(start.getSnippetUpTo(end), "x");
+        ASSERT_EQ(end.getSnippetUpTo(end), "");
+        ASSERT_EQ(end.getSnippetUpTo(start), std::nullopt);
+    }
+    {
+        // NOTE: line and column are actually 1-based indexes
+        Pos start(0, 99, o);
+        Pos end(99, 0, o);
+        ASSERT_EQ(start.getSnippetUpTo(start), "");
+
+        // "x" might be preferable, but we only care about not crashing for invalid inputs
+        ASSERT_EQ(start.getSnippetUpTo(end), "");
+
+        ASSERT_EQ(end.getSnippetUpTo(end), "");
+        ASSERT_EQ(end.getSnippetUpTo(start), std::nullopt);
+    }
+    {
+        Pos start(1, 1, o);
+        Pos end(1, 99, o);
+        ASSERT_EQ(start.getSnippetUpTo(start), "");
+        ASSERT_EQ(start.getSnippetUpTo(end), "x");
+        ASSERT_EQ(end.getSnippetUpTo(end), "");
+        ASSERT_EQ(end.getSnippetUpTo(start), "");
+    }
+    {
+        Pos start(1, 1, o);
+        Pos end(99, 99, o);
+        ASSERT_EQ(start.getSnippetUpTo(start), "");
+        ASSERT_EQ(start.getSnippetUpTo(end), "x");
+        ASSERT_EQ(end.getSnippetUpTo(end), "");
+        ASSERT_EQ(end.getSnippetUpTo(start), std::nullopt);
+    }
+}
+TEST(Position, getSnippetUpTo_2)
+{
+    Pos::Origin o = makeStdin("asdf\njkl\nqwer");
+    {
+        Pos start(1, 1, o);
+        Pos end(1, 2, o);
+        ASSERT_EQ(start.getSnippetUpTo(start), "");
+        ASSERT_EQ(start.getSnippetUpTo(end), "a");
+        ASSERT_EQ(end.getSnippetUpTo(end), "");
+
+        // nullopt? I feel like changing the column handling would just make it more fragile
+        ASSERT_EQ(end.getSnippetUpTo(start), "");
+    }
+    {
+        Pos start(1, 2, o);
+        Pos end(1, 3, o);
+        ASSERT_EQ(start.getSnippetUpTo(end), "s");
+    }
+    {
+        Pos start(1, 2, o);
+        Pos end(2, 2, o);
+        ASSERT_EQ(start.getSnippetUpTo(end), "sdf\nj");
+    }
+    {
+        Pos start(1, 2, o);
+        Pos end(3, 2, o);
+        ASSERT_EQ(start.getSnippetUpTo(end), "sdf\njkl\nq");
+    }
+    {
+        Pos start(1, 2, o);
+        Pos end(2, 99, o);
+        ASSERT_EQ(start.getSnippetUpTo(end), "sdf\njkl");
+    }
+    {
+        Pos start(1, 4, o);
+        Pos end(2, 99, o);
+        ASSERT_EQ(start.getSnippetUpTo(end), "f\njkl");
+    }
+    {
+        Pos start(1, 5, o);
+        Pos end(2, 99, o);
+        ASSERT_EQ(start.getSnippetUpTo(end), "\njkl");
+    }
+    {
+        Pos start(1, 6, o); // invalid: starting column past last "line character", ie at the newline
+        Pos end(2, 99, o);
+        ASSERT_EQ(start.getSnippetUpTo(end), "\njkl"); // jkl might be acceptable for this invalid start position
+    }
+    {
+        Pos start(1, 1, o);
+        Pos end(2, 0, o); // invalid
+        ASSERT_EQ(start.getSnippetUpTo(end), "asdf\n");
+    }
+}
+
+TEST(Position, example_1)
+{
+    Pos::Origin o = makeStdin("  unambiguous = \n    /** Very close */\n    x: x;\n# ok\n");
+    Pos start(2, 5, o);
+    Pos end(2, 22, o);
+    ASSERT_EQ(start.getSnippetUpTo(end), "/** Very close */");
+}
+
+} // namespace nix