Replace deprecated xref: [exclude: ...] with elixirc_options: [no_warn_undefined: ...]

[gilbertwong96]

Elixir 1.20 deprecates the xref: [exclude: ...] project option in mix.exs in favor of elixirc_options: [no_warn_undefined: ...]. The new option suppresses the same undefined-module warning that the old xref exclude silenced for the optional Jason dependency.

Deprecation warning

warning: "xref: [exclude: ...]" in your mix.exs file is deprecated, instead use: "elixirc_options: [no_warn_undefined: ...]"
  (mix 1.20.1) lib/mix/tasks/compile.elixir.ex:243: Mix.Tasks.Compile.Elixir.xref_exclude_opts/2

Change

-      xref: [exclude: [Jason]]
+      elixirc_options: [no_warn_undefined: [Jason]]

Why [Jason] (bare module) rather than a tuple

The only references to Jason in the codebase are as default values for Application.get_env/3 (in lib/postgrex/types.ex, lib/postgrex/extensions/jsonb.ex, lib/postgrex/extensions/json.ex) — no Jason.foo/n function is called directly. The bare module form is the natural fit and matches the old xref: [exclude: [Jason]] semantics.

Verification

$ mix compile --force --warnings-as-errors
Compiling 70 files (.ex)
Generated postgrex app

No deprecation warning, no other warnings.

[josevalim]

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