Official announcement: Elixir v1.15 released - The Elixir programming language

This release requires Erlang/OTP 24 and later.

Elixir v1.15 is a smaller release with focused improvements
on compilation and boot times. This release also completes
our integration process with Erlang/OTP logger, bringing new
features such as log rotation and compaction out of the box.

You will also find additional convenience functions in Code,
Map, Keyword, all Calendar modules, and others.

Compile and boot-time improvements

The last several releases brought improvements to compilation
time and this version is no different. In particular, Elixir
now caches and prunes load paths before compilation, ensuring your
project (and dependencies!) compile faster and in an environment
closer to production.

In a nutshell the Erlang VM loads modules from code paths. Each
application that ships with Erlang and Elixir plus each dependency
become an entry in your code path. The larger the code path, the
more work Erlang has to do in order to find a module.

In previous versions, Mix would only add entries to the load paths.
Therefore, if you compiled 20 dependencies and you went to compile
the 21st, the code path would have 21 entries (plus all Erlang and
Elixir apps). This allowed modules from unrelated dependencies to
be seen and made compilation slower the more dependencies you had.
With this release, we will now prune the code paths to only the ones
listed as dependencies, bringing the behaviour closer to mix release.

Furthermore, Erlang/OTP 26 allows us to start applications
concurrently and cache the code path lookups, decreasing the cost of
booting applications. The combination of Elixir v1.15 and Erlang/OTP 26
should reduce the boot time of applications, such as when starting
iex -S mix or running a single test with mix test, from 5% to 30%.

The compiler is also smarter in several ways: @behaviour declarations
no longer add compile-time dependencies and aliases in patterns and
guards add no dependency whatsoever, as no dispatching happens. Furthermore,
Mix now tracks the digests of @external_resource files, reducing the
amount of recompilation when swapping branches. Finally, dependencies
are automatically recompiled when their compile-time configuration changes.

Potential incompatibilities

Due to the code path pruning, if you have an application or dependency
that does not specify its dependencies on Erlang and Elixir application,
it may no longer compile successfully in Elixir v1.15. You can temporarily
disable code path pruning by setting prune_code_paths: false in your
mix.exs, although doing so may lead to runtime bugs that are only
manifested inside a mix release.

Compiler warnings and errors

The Elixir compiler can now emit many errors for a single file, making
sure more feedback is reported to developers before compilation is aborted.

In Elixir v1.14, an undefined function would be reported as:

** (CompileError) undefined function foo/0 (there is no such import)
    my_file.exs:1

In Elixir v1.15, the new reports will look like:

error: undefined function foo/0 (there is no such import)
  my_file.exs:1

** (CompileError) my_file.exs: cannot compile file (errors have been logged)

A new function, called Code.with_diagnostics/2, has been added so this
information can be leveraged by editors, allowing them to point to several
errors at once.

Potential incompatibilities

As part of this effort, the behaviour where undefined variables were
transformed into nullary function calls, often leading to confusing error
reports, has been disabled during project compilation. You can invoke
Code.compiler_options(on_undefined_variable: :warn)
at the top of your mix.exs to bring the old behaviour back.

Integration with Erlang/OTP logger

This release provides additional features such as global logger
metadata and file logging (with rotation and compaction) out-of-the-box!

This release also soft-deprecates Elixir’s Logger Backends in
favor of Erlang’s Logger handlers. Elixir will automatically
convert your :console backend configuration into the new
configuration. Previously, you would set:

config :logger, :console,
  level: :error,
  format: "$time $message $metadata"

Which is now translated to the equivalent:

config :logger, :default_handler,
  level: :error

config :logger, :default_formatter,
  format: "$time $message $metadata"

If you use Logger.Backends.Console with a custom device or other
backends, they are still fully supported and functional. If you
implement your own backends, you want to consider migrating to
:logger_backends
in the long term.

See the new Logger documentation for more information on the
new features and on compatibility.