build: add an incremental GUI type-check via the nix devShell

Verifying a GUI change meant `nix build .#syn-gui`, which starts from a clean
sandbox every time: crane caches the ~600 dependencies, but the workspace
crates rebuild from scratch in release, then a 107 MB binary is copied back
from the remote builder. Fifteen minutes to learn whether a one-line change
type-checks.

The devShell already had the Rust toolchain but none of the GUI's native
libraries — those were declared only inside the syn-gui derivation — so a
plain `cargo check -p gui-app` there died on fontconfig or alsa, which is
what forced the hermetic build in the first place.

Factors those into a shared `guiBuildInputs` used by both syn-gui and the
devShell, adds pkg-config and bindgenHook (for mupdf-sys's build.rs), and
sets LD_LIBRARY_PATH so a binary built in the shell can also run — gpui
dlopen()s wayland/X11/Vulkan/ALSA rather than linking them, which the
packaged build handles with an rpath instead.

`just check-gui-nix` then runs a normal incremental cargo check against
rust/target: measured 4m30s cold, 14s warm, against ~15 minutes for the
hermetic build. `build-gui-nix` stays for final verification and for
producing a binary you can actually run.

Worth noting the gnarly mupdf postPatch workaround in syn-gui does not apply
here: it exists only because crane vendors dependencies into the read-only
Nix store, and a devShell build uses the normal writable cargo registry.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014qebJeotZq6FvrDeTDBzeT

flake.nix

@@ -84,6 +84,22 @@
           ];
         };
 
+        # Native libraries the gpui desktop app needs on top of `commonArgs`.
+        # Shared with `devShells.default` so a plain `cargo check -p gui-app`
+        # works there too — without these a local build dies on fontconfig or
+        # alsa, which is exactly the thing that forces a full hermetic rebuild
+        # just to type-check a GUI change.
+        guiBuildInputs = pkgs.lib.optionals pkgs.stdenv.isLinux [
+          pkgs.libX11
+          pkgs.libxcursor
+          pkgs.libxrandr
+          pkgs.libxi
+          pkgs.libGL
+          pkgs.alsa-lib
+          pkgs.vulkan-loader
+          pkgs.libxcb
+        ];
+
         # Build workspace deps once (shared across all binaries)
         cargoArtifacts = craneLib.buildDepsOnly commonArgs;
 
@@ -137,16 +153,7 @@
             pkgs.rustPlatform.bindgenHook
           ];
 
-          buildInputs = commonArgs.buildInputs ++ pkgs.lib.optionals pkgs.stdenv.isLinux [
-            pkgs.libX11
-            pkgs.libxcursor
-            pkgs.libxrandr
-            pkgs.libxi
-            pkgs.libGL
-            pkgs.alsa-lib
-            pkgs.vulkan-loader
-            pkgs.libxcb
-          ];
+          buildInputs = commonArgs.buildInputs ++ guiBuildInputs;
 
           # gpui needs to find wayland/X11/Vulkan/ALSA libs at runtime
           postFixup = pkgs.lib.optionalString pkgs.stdenv.isLinux ''
@@ -186,6 +193,19 @@
         };
 
         devShells.default = pkgs.mkShell {
+          # `bindgenHook` sets LIBCLANG_PATH etc. for mupdf-sys's build.rs. It
+          # belongs in nativeBuildInputs so its setup hook actually runs.
+          nativeBuildInputs = [
+            pkgs.pkg-config
+            pkgs.rustPlatform.bindgenHook
+          ];
+
+          # commonArgs.buildInputs + guiBuildInputs mirror what the `syn-gui`
+          # derivation links against, so `cargo build`/`cargo check` inside this
+          # shell finds the same native libraries the hermetic build does. The
+          # payoff is incremental compilation: the hermetic build starts from a
+          # clean sandbox every time, so type-checking a one-line GUI change
+          # costs a full release rebuild of the workspace.
           buildInputs = [
             buildApkCi
             createReleaseCi
@@ -194,7 +214,13 @@
             androidSdk
             pkgs.jdk17
             pkgs.just
-          ];
+          ] ++ commonArgs.buildInputs ++ guiBuildInputs;
+
+          # gpui dlopen()s wayland/X11/Vulkan/ALSA at runtime rather than
+          # linking them, so a binary built in this shell needs them on the
+          # library path to actually run (the packaged build patchelfs an rpath
+          # in instead).
+          LD_LIBRARY_PATH = pkgs.lib.makeLibraryPath (commonArgs.buildInputs ++ guiBuildInputs);
 
           ANDROID_HOME = "${androidSdk}/libexec/android-sdk";
           ANDROID_SDK_ROOT = "${androidSdk}/libexec/android-sdk";

justfile

@@ -199,6 +199,21 @@ build-gui-nix:
     nix build .#syn-gui --max-jobs auto --cores 0
 
 
+# Type-check the gpui desktop GUI inside the nix devShell.
+#
+# Prefer this over `build-gui-nix` while iterating: the hermetic build starts
+# from a clean sandbox every time, so it rebuilds the whole workspace in
+# release just to check a one-line change. This runs a normal incremental
+# `cargo check` against `rust/target`, with the same native libraries the
+# packaged build links against — seconds instead of minutes, after the first
+# run warms the cache. Use `build-gui-nix` for final verification, or when you
+# need a binary you can actually run.
+[doc: "Type-check the gpui GUI incrementally in the nix devShell"]
+[group: "build"]
+check-gui-nix *args:
+    nix develop -c cargo check --manifest-path rust/gui-app/Cargo.toml {{args}}
+
+
 # Run all formatters
 [group: "format"]
 fmt-all: fmt-kotlin fmt-rust