dev-shell: Set mesonBuildType to debugoptimized

Previously, we got debug symbols implicitly because we were using
`separateDebugInfo = true`, which adds `-ggdb` to the compiler flags.

doc/manual/source/development/debugging.md

@@ -6,14 +6,7 @@ Additionally, see [Testing Nix](./testing.md) for further instructions on how to
 
 ## Building Nix with Debug Symbols
 
-In the development shell, set the `mesonBuildType` environment variable to `debug` before configuring the build:
-
-```console
-[nix-shell]$ export mesonBuildType=debugoptimized
-```
-
-Then, proceed to build Nix as described in [Building Nix](./building.md).
-This will build Nix with debug symbols, which are essential for effective debugging.
+In the development shell, `mesonBuildType` is set automatically to `debugoptimized`. This builds Nix with debug symbols, which are essential for effective debugging.
 
 It is also possible to build without optimization for faster build:
 

packaging/dev-shell.nix

@@ -258,10 +258,13 @@ pkgs.nixComponents2.nix-util.overrideAttrs (
 
     # We use this shell with the local checkout, not unpackPhase.
     src = null;
+
     # Workaround https://sourceware.org/pipermail/gdb-patches/2025-October/221398.html
     # Remove when gdb fix is rolled out everywhere.
     separateDebugInfo = false;
 
+    mesonBuildType = "debugoptimized";
+
     env = {
       # For `make format`, to work without installing pre-commit
       _NIX_PRE_COMMIT_HOOKS_CONFIG = "${(pkgs.formats.yaml { }).generate "pre-commit-config.yaml"