Please add to provides "xdg-desktop-portal-impl"

@RuairidhWilliams: Ah, good catch. I've implemented the change.

The build fails for me because I have CARGO_TARGET_DIR set to a different dir in my environment. I think this is because export CARGO_TARGET_DIR=target is set in the build() but also needs to be set in the package(). make install uses $(CARGO_TARGET_DIR) to find the executable to install so should be set to the same value in both steps.

My suggestion:

 package() {
   cd "${pkgname%-git}"
+  export CARGO_TARGET_DIR=target
   make prefix='/usr' libexecdir='/usr/lib' DESTDIR="$pkgdir" install
 }

The build fails for me on Artix Linux with the following error message:

error: failed to run custom build command for `cosmic-files v0.1.0 (https://github.com/pop-os/cosmic-files#66cef478)`

Caused by:
  process didn't exit successfully: `/home/user/.cache/yay/xdg-desktop-portal-cosmic-git/src/xdg-desktop-portal-cosmic/target/release/build/cosmic-files-729d7f483f73afdc/build-script-build` (exit status: 1)
  --- stdout
  cargo:rerun-if-changed=i18n
  cargo:rerun-if-env-changed=VERGEN_GIT_COMMIT_DATE
  cargo:rerun-if-env-changed=VERGEN_GIT_SHA

  --- stderr
  Error: not within a suitable 'git' worktree!