CHANGES:

- Second step of the deprecation of jbuilder: the `jbuilder` binary
  now emits a warning on every startup and both `jbuilder` and `dune`
  emit warnings when encountering `jbuild` files (ocaml/dune#1752, @diml)

- Change the layout of build artifacts inside _build. The new layout enables
  optimizations that depend on the presence of `.cmx` files of private modules
  (ocaml/dune#1676, @bobot)

- Fix merlin handling of private module visibility (ocaml/dune#1653 @bobot)

- unstable-fmt: use boxes to wrap some lists (ocaml/dune#1608, fix ocaml/dune#1153, @emillon,
  thanks to @rgrinberg)

- skip directories when looking up programs in the PATH (ocaml/dune#1628, fixes
  ocaml/dune#1616, @diml)

- Use `lsof` on macOS to implement `--stats` (ocaml/dune#1636, fixes ocaml/dune#1634, @xclerc)

- Generate `dune-package` files for every package. These files are installed and
  read instead of `META` files whenever they are available (ocaml/dune#1329, @rgrinberg)

- Fix preprocessing for libraries with `(include_subdirs ..)` (ocaml/dune#1624, fix ocaml/dune#1626,
  @nojb, @rgrinberg)

- Do not generate targets for archive that don't match the `modes` field.
  (ocaml/dune#1632, fix ocaml/dune#1617, @rgrinberg)

- When executing actions, open files lazily and close them as soon as
  possible in order to reduce the maximum number of file descriptors
  opened by Dune (ocaml/dune#1635, ocaml/dune#1643, fixes ocaml/dune#1633, @jonludlam, @rgrinberg,
  @diml)

- Reimplement the core of Dune using a new generic memoization system
  (ocaml/dune#1489, @rudihorn, @diml)

- Replace the broken cycle detection algorithm by a state of the art
  one from [this paper](https://doi.org/10.1145/2756553) (ocaml/dune#1489,
  @rudihorn)

- Get the correct environment node for multi project workspaces (ocaml/dune#1648,
  @rgrinberg)

- Add `dune compute` to call internal memoized functions (ocaml/dune#1528,
  @rudihorn, @diml)

- Add `--trace-file` option to trace dune internals (ocaml/dune#1639, fix ocaml/dune#1180, @emillon)

- Add `--no-print-directory` (borrowed from GNU make) to suppress
  `Entering directory` messages. (ocaml/dune#1668, @dra27)

- Remove `--stats` and track fd usage in `--trace-file` (ocaml/dune#1667, @emillon)

- Add virtual libraries feature and enable it by default (ocaml/dune#1430 fixes ocaml/dune#921,
  @rgrinberg)

- Fix handling of Control+C in watch mode (ocaml/dune#1678, fixes ocaml/dune#1671, @diml)

- Look for jsoo runtime in the same dir as the `js_of_ocaml` binary
  when the ocamlfind package is not available (ocaml/dune#1467, @nojb)

- Make the `seq` package available for OCaml >= 4.07 (ocaml/dune#1714, @rgrinberg)

- Add locations to error messages where a rule fails to generate targets and
  rules that require files outside the build/source directory. (ocaml/dune#1708, fixes
  ocaml/dune#848, @rgrinberg)

- Let `Configurator` handle `sizeof` (in addition to negative numbers).
  (ocaml/dune#1726, fixes ocaml/dune#1723, @Chris00)

- Fix an issue causing menhir generated parsers to fail to build in
  some cases. The fix is to systematically use `-short-paths` when
  calling `ocamlc -i` (ocaml/dune#1743, fix ocaml/dune#1504, @diml)

- Never raise when printing located errors. The code that would print the
  location excerpts was prone to raising. (ocaml/dune#1744, fix ocaml/dune#1736, @rgrinberg)

- Add a `dune upgrade` command for upgrading jbuilder projects to Dune
  (ocaml/dune#1749, @diml)

- When automatically creating a `dune-project` file, insert the
  detected name in it (ocaml/dune#1749, @diml)

- Add `(implicit_transitive_deps <bool>)` mode to dune projects. When this mode
  is turned off, transitive dependencies are not accessible. Only listed
  dependencies are directly accessible. (ocaml/dune#1734, ocaml/dune#430, @rgrinberg, @hnrgrgr)

- Add `toplevel` stanza. This stanza is used to define toplevels with libraries
  already preloaded. (ocaml/dune#1713, @rgrinberg)

- Generate `.merlin` files that account for normal preprocessors defined using a
  subset of the `action` language. (ocaml/dune#1768, @rgrinberg)

- Emit `(orig_src_dir <path>)` metadata in `dune-package` for dune packages
  built with `--store-orig-source-dir` command line flag (also controlled by
  `DUNE_STORE_ORIG_SOURCE_DIR` env variable). This is later used to generate
  `.merlin` with `S`-directives pointed to original source locations and thus
  allowing merlin to see those. (ocaml/dune#1750, @andreypopp)

- Improve the behavior of `dune promote` when the files to be promoted have been
  deleted. (ocaml/dune#1775, fixes ocaml/dune#1772, @diml)

- unstable-fmt: preserve comments (ocaml/dune#1766, @emillon)

- Pass flags correctly when using `staged_pps` (ocaml/dune#1779, fixes ocaml/dune#1774, @diml)

- Fix an issue with the use of `(mode promote)` in the menhir
  stanza. It was previously causing intermediate *mock* files to be
  promoted (ocaml/dune#1783, fixes ocaml/dune#1781, @diml)

- unstable-fmt: ignore files using OCaml syntax (ocaml/dune#1784, @emillon)

- Configurator: Add `which` function to replace the `which` command line utility
  in a cross platform way. (ocaml/dune#1773, fixes ocaml/dune#1705, @Chris00)

- Make configurator append paths to `$PKG_CONFIG_PATH` on macOS. Previously it
  was prepending paths and thus `$PKG_CONFIG_PATH` set by users could have been
  overridden by homebrew installed libraries (ocaml/dune#1785, @andreypopp)

- Disallow c/cxx sources that share an object file in the same stubs archive.
  This means that `foo.c` and `foo.cpp` can no longer exist in the same library.
  (ocaml/dune#1788, @rgrinberg)

- Forbid use of `%{targets}` (or `${@}` in jbuild files) inside
  preprocessing actions
  (ocaml/dune#1812, fixes ocaml/dune#1811, @diml)

- Add `DUNE_PROFILE` environment variable to easily set the profile. (ocaml/dune#1806,
  @rgrinberg)

- Deprecate the undocumented `(no_keep_locs)` field. It was only
  necessary until virtual libraries were supported (ocaml/dune#1822, fix ocaml/dune#1816,
  @diml)

- Rename `unstable-fmt` to `format-dune-file` and remove its `--inplace` option.
  (ocaml/dune#1821, @emillon).

- Autoformatting: `(using fmt 1.1)` will also format dune files (ocaml/dune#1821, @emillon).

- Autoformatting: record dependencies on `.ocamlformat-ignore` files (ocaml/dune#1824,
  fixes ocaml/dune#1793, @emillon)