build: Add sandboxed mode

meson_options.txt

@@ -8,3 +8,12 @@ option(
   value: 'default',
   description: 'The build profile for Fretboard. One of "default" or "development".'
 )
+option(
+  'sandboxed',
+  type : 'boolean',
+  value : false,
+  description: 'Whether the build happens in a sandbox.' +
+               'When that is the case, cargo will not be able to download the dependencies during' +
+               'the build so they are assumed to be in meson.project_source_root()/cargo'
+)
+

src/meson.build

@@ -21,7 +21,13 @@ run_command(
 cargo_bin  = find_program('cargo')
 cargo_opt  = [ '--manifest-path', meson.project_source_root() / 'Cargo.toml' ]
 cargo_opt += [ '--target-dir', meson.project_build_root()  / 'src' ]
-cargo_env  = [ 'CARGO_HOME=' + meson.project_build_root()  / 'cargo-home' ]
+
+if get_option('sandboxed')
+  # This is the path used by flatpak-cargo-generator in flatpak-builder-tools
+  cargo_env = [ 'CARGO_HOME=' + meson.project_source_root() / 'cargo' ]
+else
+  cargo_env = [ 'CARGO_HOME=' + meson.project_build_root() / 'cargo-home' ]
+endif
 
 if get_option('profile') == 'default'
   cargo_opt += [ '--release' ]