Move /src to /subprojects

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

subprojects/libcmd/markdown.cc

@@ -0,0 +1,73 @@
+#include "markdown.hh"
+#include "environment-variables.hh"
+#include "error.hh"
+#include "finally.hh"
+#include "terminal.hh"
+
+#if HAVE_LOWDOWN
+#  include <sys/queue.h>
+#  include <lowdown.h>
+#endif
+
+namespace nix {
+
+#if HAVE_LOWDOWN
+static std::string doRenderMarkdownToTerminal(std::string_view markdown)
+{
+    int windowWidth = getWindowSize().second;
+
+    struct lowdown_opts opts
+    {
+        .type = LOWDOWN_TERM,
+        .maxdepth = 20,
+        .cols = (size_t) std::max(windowWidth - 5, 60),
+        .hmargin = 0,
+        .vmargin = 0,
+        .feat = LOWDOWN_COMMONMARK | LOWDOWN_FENCED | LOWDOWN_DEFLIST | LOWDOWN_TABLES,
+        .oflags = LOWDOWN_TERM_NOLINK,
+    };
+
+    auto doc = lowdown_doc_new(&opts);
+    if (!doc)
+        throw Error("cannot allocate Markdown document");
+    Finally freeDoc([&]() { lowdown_doc_free(doc); });
+
+    size_t maxn = 0;
+    auto node = lowdown_doc_parse(doc, &maxn, markdown.data(), markdown.size(), nullptr);
+    if (!node)
+        throw Error("cannot parse Markdown document");
+    Finally freeNode([&]() { lowdown_node_free(node); });
+
+    auto renderer = lowdown_term_new(&opts);
+    if (!renderer)
+        throw Error("cannot allocate Markdown renderer");
+    Finally freeRenderer([&]() { lowdown_term_free(renderer); });
+
+    auto buf = lowdown_buf_new(16384);
+    if (!buf)
+        throw Error("cannot allocate Markdown output buffer");
+    Finally freeBuffer([&]() { lowdown_buf_free(buf); });
+
+    int rndr_res = lowdown_term_rndr(buf, renderer, node);
+    if (!rndr_res)
+        throw Error("allocation error while rendering Markdown");
+
+    return filterANSIEscapes(std::string(buf->data, buf->size), !isTTY());
+}
+
+std::string renderMarkdownToTerminal(std::string_view markdown)
+{
+    if (auto e = getEnv("_NIX_TEST_RAW_MARKDOWN"); e && *e == "1")
+        return std::string(markdown);
+    else
+        return doRenderMarkdownToTerminal(markdown);
+}
+
+#else
+std::string renderMarkdownToTerminal(std::string_view markdown)
+{
+    return std::string(markdown);
+}
+#endif
+
+} // namespace nix