libcmd: add support for lowdown >= 3.0.0

lowdown 3.0.0 merged some flags into one to save on bits and did not add
any aliases for backward compatibility.

As with the changes for lowdown >= 1.4, we define a preprocessor flag to
gate the changes on and add a job to CI to ensure that lowdown < 3.0
keeps working (which is used by NixOS 25.11).

Unfortunately, we need to jump through some hoops to prevent nix and lix
from upstream Nixpkgs from being rebuilt due to a changed lowdown. Since
both implementation's packaging in Nixpkgs has their own package set /
fix point now, we can't simply inherit them from `prev` since they will
always be (re-)computed from the `final` fix point. As a consequence,
we need to expose our changed lowdown version at a non-default attribute
or break the builds of Nixpkgs derivations we test against.

Change-Id: I20a3e2fdaa05906f032ff66911c42867557fdd11
(cherry picked from commit af0390c27b)

flake.nix

@@ -219,6 +219,9 @@
             inherit versionSuffix officialRelease;
             stdenv = currentStdenv;
             busybox-sandbox-shell = final.busybox-sandbox-shell or final.default-busybox-sandbox-shell;
+            # See below
+            lowdown = final.lowdown_3_0;
+            lowdown-unsandboxed = final.lowdown_3_0.override { enableDarwinSandbox = false; };
           };
 
           lix-clang-tidy = final.callPackage ./subprojects/lix-clang-tidy { };
@@ -245,9 +248,21 @@
           # And same thing for our build-release-notes package.
           build-release-notes = final.nix.passthru.build-release-notes;
 
-          lowdown =
-            assert lib.versionAtLeast prev.lowdown.version "2.0.0";
-            prev.lowdown;
+          # As soon as Nixpkgs updates to >= 3.0.0, change to lowdown_2_0!
+          # We don't change the default version in order to not change the hash
+          # of Nix/Lix from upstream Nixpkgs.
+          lowdown_3_0 =
+            assert lib.versionOlder prev.lowdown.version "3.0.0";
+            prev.lowdown.overrideAttrs (
+              finalAttrs: _prevAttrs: {
+                version = "3.0.0";
+
+                src = final.fetchurl {
+                  url = "https://kristaps.bsd.lv/lowdown/snapshots/lowdown-${finalAttrs.version}.tar.gz";
+                  sha512 = "94e97234d598382c3c3dc27f9bfdb3a3a2fcf7dbb6a8df3c85ee09f27f792449034a41d49d9cfd3d8450d2de01b8562c20c3d120e65c81af4d7d6c9454119e93";
+                };
+              }
+            );
 
           capnproto = prev.capnproto.overrideAttrs (old: {
             patches =
@@ -275,6 +290,15 @@
         # Binary package for various platforms.
         build = forAllSystems (system: self.packages.${system}.nix);
 
+        # Ensure support for lowdown < 3.0 doesn't regress for NixOS 25.11
+        build-lowdown_2_0.aarch64-linux = lib.genAttrs [ "aarch64-linux" ] (
+          system:
+          self.packages.${system}.nix.override {
+            lowdown = nixpkgsFor.${system}.native.lowdown;
+            lowdown-unsandboxed = nixpkgsFor.${system}.native.lowdown-unsandboxed;
+          }
+        );
+
         # Building Lix twice in CI is expensive, but we can catch a lot of static
         # build regressions by at least making sure it evals and configures.
         configure-static = lib.genAttrs linux64BitSystems (

lix/libcmd/markdown.cc

@@ -74,7 +74,11 @@ std::string renderMarkdownToTerminal(std::string_view markdown, StandardOutputSt
         .vmargin = 0,
 #endif /* LOWDOWN_SEPARATE_TERM_OPTS */
         .feat = LOWDOWN_COMMONMARK | LOWDOWN_FENCED | LOWDOWN_DEFLIST | LOWDOWN_TABLES,
+#ifdef LOWDOWN_CONSOLIDATED_OFLAGS
+        .oflags = LOWDOWN_NOLINK,
+#else
         .oflags = LOWDOWN_TERM_NOLINK,
+#endif /* LOWDOWN_CONSOLIDATED_OFLAGS */
     };
     if (!shouldANSI(fileno)) {
         opts.oflags |= LOWDOWN_TERM_NOANSI;

meson.build

@@ -383,6 +383,11 @@ if lowdown.version().version_compare('>= 1.4.0')
   add_project_arguments('-DLOWDOWN_SEPARATE_TERM_OPTS', language: 'cpp')
 endif
 
+# TODO(sterni): drop the corresponding #ifdef after NixOS 25.11 is EOL which still distributes lowdown < 3.0.0
+if lowdown.version().version_compare('>= 3.0.0')
+  add_project_arguments('-DLOWDOWN_CONSOLIDATED_OFLAGS', language: 'cpp')
+endif
+
 # HACK(Qyriad): rapidcheck's pkg-config doesn't include the libs lol
 # Note: technically we 'check' for rapidcheck twice, for the internal-api-docs handling above,
 # but Meson will cache the result of the first one, and the required : arguments are different.