New features

Project version can be specified with a file

Meson can be instructed to load a project's version string from an external file like this:

project('foo', 'c' version: files('VERSION'))

The version file must contain exactly one line of text which will be used as the project's version. If the line ends in a newline character, it is removed.

Support for reading files at configuration time with the fs module

Reading text files during configuration is now supported. This can be done at any time after project has been called

project('myproject', 'c')
license_text = run_command(
    find_program('python3'), '-c', 'print(open("COPYING").read())'
).stdout().strip()
about_header = configuration_data()
about_header.add('COPYRIGHT', license_text)
about_header.add('ABOUT_STRING', meson.project_name())
...

There are several problems with the above approach:

  1. It's ugly and confusing
  2. If COPYING changes after configuration, Meson won't correctly rebuild when configuration data is based on the data in COPYING
  3. It has extra overhead

fs.read replaces the above idiom thus:

project('myproject', 'c')
fs = import('fs')
license_text = fs.read('COPYING').strip()
about_header = configuration_data()
about_header.add('COPYRIGHT', license_text)
about_header.add('ABOUT_STRING', meson.project_name())
...

They are not equivalent, though. Files read with fs.read create a configuration dependency on the file, and so if the COPYING file is modified, Meson will automatically reconfigure, guaranteeing the build is consistent. It can be used for any properly encoded text files. It supports specification of non utf-8 encodings too, so if you're stuck with text files in a different encoding, it can be passed as an argument. See the meson object documentation for details.

meson install --dry-run

New option to meson install command that does not actually install files, but only prints messages.

Experimental support for C++ modules in Visual Studio

Modules are a new C++ 20 feature for organising source code aiming to increase compilation speed and reliability. This support is experimental and may change in future releases. It only works with the latest preview release of Visual Studio.

Qt6 module

A module for Qt6 is now available with the same functionality as the Qt5 module.

Currently finding Qt6 is only available via qmake as pkg-config files aren't generated (see QTBUG-86080) and CMake support is not available for this module yet.

Unstable Rust module

A new unstable module has been added to make using Rust with Meson easier. Currently, it adds a single function to ease defining Rust tests, as well as a wrapper around bindgen, making it easier to use.

Meson test() now accepts protocol : 'rust'

This allows native Rust tests to be run and parsed by Meson; simply set the protocol to rust and Meson takes care of the rest.

MSVC/Clang-Cl Argument Changes/Cleanup

  • "Disable Debug" (/Od) is no longer manually specified for optimization levels {0,g} (it is already the default for MSVC).
  • "Run Time Checking" (/RTC1) removed from debug buildtype by default
  • Clang-CL debug buildtype arguments now match MSVC arguments
  • There is now no difference between buildtype flags and debug + optimization flags

The /Od flag has been removed, as it is already the default in the MSVC compilers, and conflicts with other user options.

/RTC1 conflicts with other RTC argument types as there are many different options, and has been removed by default. Run Time Checking can be enabled by manually adding /RTC1 or other RTC flags of your choice.

The debug buildtype for clang-cl added additional arguments compared to MSVC, which had more to do with optimization than debug. The arguments removed are /Ob0, /Od, /RTC1. (/Zi was also removed, but it is already added by default when debug is enabled.)

If these are important issues for you and would like builtin toggle options, please file an issue in the Meson bug tracker.

Buildtype remains even if dependent options are changed

Setting the buildtype option to a value sets the debug and optimization options to predefined values. Traditionally setting the options to other values would then change the buildtype to custom. This is confusing and means that you can't use, for example, debug level g in debug buildtype even though it would make sense under many circumstances.

Starting with this release, the buildtype is only changed when the user explicitly sets it; setting the build type modifies the debug and optimization options as before.

Passing internal dependencies to the compiler object

Methods on the compiler object (such as compiles, links, has_header) can be passed dependencies returned by declare_dependency, as long as they only specify compiler/linker arguments or other dependencies that satisfy the same requirements.

unstable_external_project improvements

  • Default arguments are added to add_project() in case some tags are not found in configure_options: '--prefix=@PREFIX@', '--libdir=@PREFIX@/@LIBDIR@', and '--includedir=@PREFIX@/@INCLUDEDIR@'. It was previously considered a fatal error to not specify them.

  • When the verbose keyword argument is not specified, or is false, command outputs are written on file in <builddir>/meson-logs/.

  • The LD environment variable is not passed any more when running the configure script. It caused issues because Meson sets LD to the CC linker wrapper but autotools expects it to be a real linker (e.g. /usr/bin/ld).

gnome.post_install()

Post-install update of various system wide caches. Each script will be executed only once even if gnome.post_install() is called multiple times from multiple subprojects. If DESTDIR is specified during installation all scripts will be skipped.

Currently supports glib-compile-schemas, gio-querymodules, and gtk-update-icon-cache.

"Edit and continue" (/ZI) is no longer used by default for Visual Studio

Meson was adding the /ZI compiler argument as an argument for Visual Studio in debug mode. This enables the edit-and-continue debugging in Visual Studio IDE's.

Unfortunately, it is also extremely expensive and breaks certain use cases such as link time code generation. Edit and continue can be enabled by manually by adding /ZI to compiler arguments.

The /ZI argument has now been replaced by the /Zi argument for debug builds.

If this is an important issue for you and would like a builtin toggle option, please file an issue in the Meson bug tracker.

Minimum required Python version updated to 3.6

Meson now requires at least Python version 3.6 to run as Python 3.5 reaches EOL on September 2020. In practice this should only affect people developing on Ubuntu Xenial, which will similarly reach EOL in April 2021.

Packaging a subproject

The meson dist command can now create a distribution tarball for a subproject in the same git repository as the main project. This can be useful if parts of the project (e.g. libraries) can be built and distributed separately. In that case they can be moved into subprojects/mysub and running meson dist in that directory will now create a tarball containing only the source code from that subdir and not the rest of the main project or other subprojects.

For example:

git clone https://github.com/myproject
cd myproject/subprojects/mysubproject
meson builddir
meson dist -C builddir

custom_target() and run_target() now accepts an env keyword argument

Environment variables can now be passed to the custom_target() command.

env = environment()
env.append('PATH', '/foo')
custom_target(..., env: env)
custom_target(..., env: {'MY_ENV': 'value'})
custom_target(..., env: ['MY_ENV=value'])

summary() accepts external programs or dependencies

External program objects and dependency objects can be passed to summary() as the value to be printed.

CMake find_package version support

It is now possible to specify a requested package version for the CMake dependency backend via the new cmake_package_version kwarg in the dependency function.

meson test only rebuilds test dependencies

Until now, meson test rebuilt the whole project independent of the requested tests and their dependencies. With this release, meson test will only rebuild what is needed for the tests or suites that will be run. This feature can be used, for example, to speed up bisecting regressions using commands like the following:

git bisect start <broken commit> <working commit>
git bisect run meson test <failing test name>

This would find the broken commit automatically while at each step rebuilding only those pieces of code needed to run the test.

However, this change could cause failures when upgrading to 0.57, if the dependencies are not specified correctly in meson.build.

The add_*_script methods now accept a File as the first argument

Meson now accepts file objects, including those produced by configure_file, as the first parameter of the various add_*_script methods

install_script = configure_file(
  configuration : conf,
  input : 'myscript.py.in',
  output : 'myscript.py',
)

meson.add_install_script(install_script, other, params)

Unity build with Vala disabled

The approach that meson has used for Vala unity builds is incorrect, we combine the generated C files like we would any other C file. This is very fragile however, as the Vala compiler generates helper functions and macros which work fine when each file is a separate translation unit, but fail when they are combined.

New logging format for meson test

The console output format for meson test has changed in several ways. The major changes are:

  • if stdout is a tty, meson includes a progress report.

  • if --print-errorlogs is specified, the logs are printed as tests run rather than afterwards. All the error logs are printed rather than only the first ten.

  • if --verbose is specified and --num-processes specifies more than one concurrent test, test output is buffered and printed after the test finishes.

  • the console logs include a reproducer command. If --verbose is specified, the command is printed for all tests at the time they start; otherwise, it is printed for failing tests at the time the test finishes.

  • for TAP and Rust tests, Meson is able to report individual subtests. If --verbose is specified, all tests are reported. If --print-errorlogs is specified, only failures are.

In addition, if --verbose was specified, Meson used not to generate logs. This limitation has now been removed.

These changes make the default ninja test output more readable, while --verbose output provides detailed, human-readable logs that are well suited to CI environments.

Specify DESTDIR on command line

meson install command now has a --destdir argument that overrides DESTDIR from environment.

Skip install scripts if DESTDIR is set

meson.add_install_script() now has skip_if_destdir keyword argument. If set to true the script won't be run if DESTDIR is set during installation. This is useful in the case the script updates system wide caches, or performs other tasks that are only needed when copying files into final destination.

Add support for prelinked static libraries

The static library gains a new prelink keyword argument that can be used to prelink object files in that target. This is currently only supported for the GNU toolchain, patches to add it to other compilers are most welcome.

Rust now has an std option

Rust calls these editions, however, Meson generally refers to such language versions as "standards", or std for short. Therefore, Meson's Rust support uses std for consistency with other languages.

Ctrl-C behavior in meson test

Starting from this version, sending a SIGINT signal (or pressing Ctrl-C) to meson test will interrupt the longest running test. Pressing Ctrl-C three times within a second will exit meson test.

Support added for LLVM's thinLTO

A new b_lto_mode option has been added, which may be set to default or thin. Thin only works for clang, and only with gnu gold, lld variants, or ld64.

test() timeout and timeout_multiplier value <= 0

test(..., timeout: 0), or negative value, used to abort the test immediately but now instead allow infinite duration. Note that omitting the timeout keyword argument still defaults to 30s timeout.

Likewise, add_test_setup(..., timeout_multiplier: 0), or meson test --timeout-multiplier 0, or negative value, disable tests timeout.

Knob to control LTO thread

Both the gnu linker and lld support using threads for speeding up LTO, meson now provides a knob for this: -Db_lto_threads. Currently this is only supported for clang and gcc. Any positive integer is supported, 0 means auto. If the compiler or linker implements it's on auto we use that, otherwise the number of threads on the machine is used.

summary() now uses left alignment for both keys and values

Previously it aligned keys toward the center, but this was deemed harder to read than having everything left aligned.

// is now allowed as a function id for meson rewrite.

msys bash may expand / to a path, breaking meson rewrite kwargs set project / .... Passing // will be converted to / by msys bash but in order to keep usage shell-agnostic, this release also allows // as the id. This way, meson rewrite kwargs set project // ... will work in both msys bash and other shells.

Get keys of configuration data object

All keys of the configuration_data object can be obtained with the keys() method as an alphabetically sorted array.

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