Version 0.11.0

 * Initial work on header unit importation and include translation support.

   In particular, for GCC, the (experimental) module mapper approach is now
   used to handle header unit importation, include translation, and headers
   dependency extraction, all with support for auto-generated headers.

 * Generalized target/prerequisite variable blocks.

   Target/prerequisite-specific variable blocks can now be present even if
   there are prerequisites. For example, now instead of:

   exe{foo}: cxx{foo}
   exe{foo}: cc.loptions += -rdynamic

   Or:

   exe{foo}: cxx{foo}
   exe{foo}:
   {
     cc.loptions += -rdynamic
     cc.libs += -ldl
   }

   We can write:

   exe{foo}: cxx{foo}
   {
     cc.loptions += -rdynamic
     cc.libs += -ldl
   }

   This also works with dependency chains in which case the block applies
   to the set of prerequisites (note: not targets) before the last ':'. For
   example:

   ./: exe{foo}: libue{foo}: cxx{foo}
   {
     bin.whole = false  # Applies to the libue{foo} prerequisite.
   }

 * Support for ad hoc target groups.

   In certain cases we may need to instruct the underlying tool (compiler,
   linker, etc) to produce additional outputs. For example, we may want to
   request the compiler to produce an assembler listing or the linker to
   produce a map file. While we could already pass the required options, the
   resulting files will not be part of the build state. Specifically, they
   will not be cleaned up and we cannot use them as prerequisites of other
   targets.

   Ad hoc target groups allow us to specify that updating a target produces
   additional outputs, called ad hoc group members. For example:

   <exe{hello} file{hello.map}>: cxx{hello}
   {
     cc.loptions += "-Wl,-Map=$out_base/hello.map"
   }

   <obje{hello} file{hello.lst}>:
   {
     cc.coptions += "-Wa,-amhls=$out_base/hello.lst"
   }

   Note also that all things ad hoc (prerequisites, targets, rules) are still
   under active development so further improvements (such as not having to
   repeat names twice) are likely.

 * New config.{c,cxx}.std configuration variables that, if present, override
   {c,cxx}.std specified at the project level.

   In particular, this allows forcing a specific standard for all the projects
   in a build configuration, for example:

   $ b create: exp-conf/,cc config.cxx=g++ config.cxx.std=experimental

 * New --dry-run|-n option instructs build rules to print commands without
   actually executing them.

   Note that commands that are required to create an accurate build state will
   still be executed and the extracted auxiliary dependency information saved.
   In other words, this is not the "don't touch the filesystem" mode but
   rather "do minimum amount of work to show what needs to be done". In
   particular, this mode is useful to quickly generate the compilation
   database, for example:

   $ b -vn clean update |& compiledb

 * Ability to disable automatic rpath, support for custom rpath-link.

   Specifically, the new config.bin.rpath.auto variable can be used to disable
   automatic addition of prerequisite library rpaths, for example:

   $ b config.bin.rpath.auto=false

   Note that in this case rpath-link is still added where normally required
   and for target platforms that support it (Linux and *BSD).

   The new config.bin.rpath_link and config.bin.rpath_link.auto have the same
   semantics as config.bin.rpath* but for rpath-link.

 * Enable MSVC strict mode (/permissive-) for 'experimental' standard starting
   from version 15.5.

Version 0.10.0

 * Support for an alternative build file/directory naming scheme.

   Now the build/*.build, buildfile, and .buildignore filesystem entries in
   a project can alternatively (but consistently) be called build2/*.build2,
   build2file, and .build2ignore. See a note at the beginning of the Project
   Structure section in the manual for details (motivation, restrictions,
   etc).

 * Support for multiple variable overrides.

   Now we can do:

   $ b config.cxx.coptions=-O3 config.cxx.coptions=-O0

   Or even:

   $ b config.cxx.coptions=-O3 config.cxx.coptions+=-g

 * Support for MSVC 16 (2019).

 * Support for automatic switching to option files (AKA response files) on
   Windows if the linker command line is too long.

   This covers both MSVC link.exe/lib.exe and MinGW gcc.exe/ar.exe.

Version 0.9.0

 * New "Diagnostics and Debugging" section in the manual on debugging build
   issues.

 * Support for dependency chains.

   Now instead of:

   ./: exe{foo}
   exe{foo}: cxx{*}

   We can write:

   ./: exe{foo}: cxx{*}

   Or even:

   ./: exe{foo}: libue{foo}: cxx{*}

   This can be combined with prerequisite-specific variables (which naturally
   only apply to the last set of prerequisites in the chain):

   ./: exe{foo}: libue{foo}: bin.whole = false

 * Support for target and prerequisite specific variable blocks.

   For example, now instead of:

   lib{foo}: cxx.loptions += -static
   lib{foo}: cxx.libs += -lpthread

   We can write:

   lib{foo}:
   {
     cxx.loptions += -static
     cxx.libs += -lpthread
   }

   The same works for prerequisites as well as target type/patterns. For
   example:

   exe{*.test}:
   {
     test = true
     install = false
   }

 * Fallback to loading outer buildfile if there isn't one in the target's
   directory (src_base).

   This covers the case where the target is defined in the outer buildfile
   which is common with non-intrusive project conversions where everything is
   built from a single root buildfile.

 * Command line variable override scope syntax is now consistent with
   buildfile syntax.

   Before:

   $ b dir/:foo=bar ...

   After:

   $ b dir/foo=bar

   Alternatively (the buildfile syntax):

   $ b 'dir/ foo=bar'

   Note that the (rarely used) scope visibility modifier now leads to a double
   slash:

   $ b dir//foo=bar

 * Support for relative to base scope command line variable overrides.

   Currently, if we do:

   $ b dir/ ./foo=bar

   The scope the foo=bar is set on is relative to CWD, not dir/. While this
   may seem wrong at first, this is the least surprising behavior when we take
   into account that there can be multiple dir/'s.

   Sometimes, however, we do want the override directory to be treated
   relative to (every) target's base scope that we are building. To support
   this we are extending the '.' and '..' special directory names (which are
   still resolved relative to CWD) with '...', which means "relative to the
   base scope of every target in the buildspec". For example:

   $ b dir/ .../foo=bar

   Is equivalent to:

   $ b dir/ dir/foo=bar

   And:

   $ b liba/ libb/ .../tests/foo=bar

   Is equivalent to:

   $ b liba/ libb/ liba/tests/foo=bar libb/tests/foo=bar

 * New config.{c,cxx}.{id,version,target} configuration variables.

   These variables allow overriding guessed compiler id/version/target, for
   example, in case of mis-guesses or when working with compilers that don't
   report their base (e.g., GCC, Clang) with -v/--version (common in the
   embedded space).

 * New --[no-]mtime-check options to control backwards modification time
   checks at runtime.

   By default the checks are enabled only for the staged toolchain.

 * New --dump <phase> option, remove state dumping from verbosity level 6.

 * The info meta-operation now prints the list of operations and meta-
   operations supported by the project.

 * New sleep Testscript builtin.

Version 0.8.0

 * BREAKING: rename the .test extension (Testscript file) to .testscript and
   the test{} target type to testscript{}.

 * Introduction chapter in the build system manual.

   The introduction covers every aspect of the build infrastructure, including
   the underlying concepts, for the canonical executable and library projects
   as produced by bdep-new(1).

 * New 'in' build system module.

   Given test.in containing something along these lines:

   foo = $foo$

   Now we can do:

   using in

   file{test}: in{test.in}
   file{test}: foo = FOO

   The alternative variable substitution symbol can be specified with the
   in.symbol variable and lax (instead of the default strict) mode with
   in.substitution. For example:

   file{test}: in.symbol = '@'
   file{test}: in.substitution = lax

 * New 'bash' build system module that provides modularization support for bash
   scripts. See the build system manual for all the details.

 * Support for 'binless' (binary-less aka header-only) libraries.

   A header-only library (or, in the future, a module interface-only library)
   is not a different kind of library compared to static/shared libraries but
   is rather a binary-less, or binless for short, static or shared library.
   Whether a library is binless is determined dynamically and automatically
   based on the absence of source file prerequisites. See the build system
   manual for details.

 * Use thin archives for utility libraries if available.

   Thin archives are supported by GNU ar since binutils 2.19.1 and LLVM ar
   since LLVM 3.8.0.

 * Support for archive checksum generation during distribution:

   Now we can do:

   $ b dist: ... \
   config.dist.archives='tar.gz zip' \
   config.dist.checksums='sha1 sha256'

   And end up with .tar.gz.sha1, .tar.gz.sha256, .zip.sha1, and .zip.sha256
   checksum files in addition to archives.

 * Support for excluded and ad hoc prerequisites:

   The inclusion/exclusion is controlled via the 'include' prerequisite-
   specific variable. Valid values are:

   false  - exclude
   true   - include
   adhoc  - include but treat as an ad hoc input

   For example:

   lib{foo}: cxx{win32-utility}: include = ($cxx.targe.class == 'windows')
   exe{bar}: libs{plugin}: include = adhoc

 * C++ Modules support:

   - handle the leading 'module;' marker (p0713)
   - switch to new GCC module interface (-fmodule-mapper)
   - force reprocessing for module interface units if compiling with MSVC

 * Testscript:

   - new mv builtin
   - new --after <ref-file> option in touch builtin

 * New $process.run() and $process.run_regex() functions:

   $process.run(<prog>[ <args>...])

   Return trimmed stdout.

   $process.run_regex(<prog>[ <args>...], <pat> [, <fmt>])

   Return stdout lines matched and optionally processed with regex.

   Each line of stdout (including the customary trailing blank) is matched (as
   a whole) against <pat> and, if successful, returned, optionally processed
   with <fmt>, as an element of a list.

 * Support for name patterns without wildcard characters.

   In particular, this allows the "if-exists" specification of prerequisites,
   for example:

   for t: $tests
     exe{$t}: cxx{$t} test{+$t}

 * Functions for decomposing name as target/prerequisite name:

   $name.name()
   $name.extension()
   $name.directory()
   $name.target_type()
   $name.project()

 * Add support for default extension specification, trailing dot escaping.

   For example:

   cxx{*}: extension = cxx

   cxx{foo}         # foo.cxx
   cxx{foo.test}    # foo.test      (probably what we want...)
   cxx{foo.test...} # foo.test.cxx  (... is this)
   cxx{foo..}       # foo.
   cxx{foo....}     # foo..
   cxx{foo.....}    # error (must come in escape pairs)

 * Use (native) C and C++ compilers we were built with as defaults for
   config.c and config.cxx, respectively.

 * Implement missing pieces in utility libraries support. In particular, we
   can now build static libraries out of utility libraries.

 * Built-in support for Windows module definition files (.def/def{}).

 * Project names are now sanitized when forming the config.import.<proj>
   variables. Specifically, '-', '+', and '.' are replaced with '_' to form a
   "canonical" variable name.

Version 0.7.0

 * Initial support for Clang targeting MSVC runtime (native Clang interface,
   not the clang-cl wrapper).

 * C++ Modules TS introduction, build system support, and design guidelines
   documentation.

 * New {c,cxx}.guess modules.

   These can be loaded before {c,cxx} to guess the compiler. Based on this
   information we can then choose the standard, experimental features, etc.
   For example:

   using cxx.guess

   if ($cxx.id == 'clang')
     cxx.features.modules = false

   cxx.std = experimental

   using cxx

 * New {c,cxx}.class variables.

   Compiler class describes a set of compilers that follow more or less
   the same command line interface. Compilers that don't belong to any of
   the existing classes are in classes of their own (say, Sun CC would be
   on its own if we were to support it).

   Currently defined compiler classes:

   gcc   gcc, clang, clang-apple, icc (on non-Windows)
   msvc  msvc, clang-cl, icc (Windows)

 * Support for C/C++ runtime/stdlib detection ({c,cxx}.{runtime,stdlib}
   variables; see cc/guess.hxx for possible values).


 * New __build2_preprocess macro.

   If cc.reprocess is true, the __build2_preprocess is defined during
   dependency extraction. This can be used to work around separate
   preprocessing bugs in the compiler.

 * Support for for-loop. The semantics is similar to the C++11 range-based
   for:

   list = 1 2 3
   for i: $list
     print $i

   Note that there is no scoping of any kind for the loop variable ('i' in the
   above example). In the future the plan is to also support more general
   while-loop as well as break and continue.

 * New info meta operation.

   This meta operation can be used to print basic information (name, version,
   source/output roots, etc) for one or more projects.

 * New update-for-{test,install} operation aliases.

 * Support for forwarded configurations with target backlinking. See the
   configure meta-operation discussion in b(1) for details.

 * Improvements to the in module (in.symbol, in.substitution={strict|lax}).

 * New $directory(), $base(), $leaf() and $extension() path functions.

 * New $regex.split(), $regex.merge() and $regex.apply() functions.

 * Support for (parallel) bootstrapping using GNU make makefile.

 * Support for chroot'ed install (aka DESTDIR):

   b config.install.root=/usr config.install.chroot=/tmp/install

 * Support for prerequisite-specific variables, used for the bin.whole
   variable ("link whole archive").

 * Regularize directory target/scope-specific variable assignment syntax:

   $out_root/: foo = bar # target
   $out_root/  foo = bar # scope
   $out_root/
   {
     foo = bar           # scope
   }

 * Support for structured result output (--structured-result).

 * Support for build hooks.

   The following buildfiles are loaded (if present) at appropriate times from
   the out_root subdirectories of a project:

   build/bootstrap/pre-*.build   # before loading bootstrap.build
   build/bootstrap/post-*.build  # after  loading bootstrap.build
   build/root/pre-*.build        # before loading root.build
   build/root/post-*.build       # after  loading root.build

 * New run directive.

   Now it is possible to:

   run echo 'foo = bar'
   print $foo

 * New dump directive.

   It can be used to print (to stderr) a human-readable representation of the
   current scope or a list of targets. For example:

   dump                           # Dump current scope.
   dump lib{foo} details/exe{bar} # Dump two targets.

   This is primarily useful for debugging as well as to write build system
   tests.

Version 0.6.0

  * C++ Modules TS support for GCC, Clang, and VC.

    The new 'experimental' value of the cxx.std variable enables modules
    support if provided by the C++ compiler. The cxx.features.modules boolean
    variable can be used to control/query C++ modules enablement.

    See the "C++ Module Support" section in the build system manual for all
    the details.

  * Precise change detection for C and C++ sources.

    The build system now calculates a checksum of the preprocessed token
    stream and avoids recompilation if the changes are ignorable (whitespaces,
    comments, unused macros, etc). To minimize confusion ("I've changed my
    code but nothing got updated"), the build system prints a 'skip' line for
    ignored changes.

  * Initial support for utility libraries.

    A utility library is an archive that "mimics" the object file type
    (executable, static library, or shared library) of its "primary" target.
    Unless explicitly overridden, utility libraries are linked in the "whole
    archive" mode. For example:

    exe{prog}: cxx{prog} libu{prog}
    libu{prog}: cxx{* -prog}

    # Unit tests.
    #
    tests/
    {
      libu{*}: bin.whole = false # Don't link whole.

      exe{test1}: cxx{test1} ../libu{prog}
      exe{test2}: cxx{test2} ../libu{prog}
    }

    This change adds the new target group libu{} and its libue{}, libua{}, and
    libus{} members. Note that the bin.whole variable can also be used on
    normal static libraries.

  * Progress display.

    The build system will now display build progress for low verbosity levels
    and if printing to a terminal. It can also be explicitly requested with
    the -p|--progress option and suppressed with --no-progress.

    Note that it is safe to enable progress even when redirecting to a file,
    for example:

    b -p 2>&1 | tee build.log

  * Support for generating pkg-config's .pc files on install.

    These files are now generated by default and automatically for libraries
    being installed provided the version, project.summary, and project.url
    variables are defined. The version module has been improved to extract the
    summary and url in addition to the version from the manifest.

  * Support for the '20' cxx.std value (C++20/c++2a).

  * The fail, warn, info, and text directives in addition to print. For
    example:

    if ($cxx.id.type == 'msvc')
      fail 'msvc is not supported'

  * New build system functions:

    - $getenv()                         -- query environment variable value
    - $filesystem.path_{search,match}() -- wildcard pattern search/match
    - $regex.{match,search,replace}()   -- regex match/search/replace

  * New Testscript builtins:

    - ln
    - exit (pseudo-builtin)

  * Separate C and C++ (partial) preprocessing and compilation for Clang, GCC,
    and VC.

    This is part of the infrastructure that is relied upon by the C++ modules
    support, precise change detection support, and, in the future, by
    distributed compilation.

    There is also the ability to limit the amount of preprocessing done on a
    source file by setting the {c,cxx}.preprocessed variables. Valid values
    are 'none' (not preprocessed), 'includes' (no #include directives in the
    source), 'modules' (as above plus no module declarations depend on the
    preprocessor, for example, #ifdef, etc.), and 'all' (the source is fully
    preprocessed). Note that for 'all' the source may still contain comments
    and line continuations.

    While normally unnecessary, the use of the (partially) preprocessed output
    in compilation can be disabled. This can be done from a buildfile for a
    scope (including project root scope) and per target via the cc.reprocess
    variable:

    cc.reprocess = true
    obj{hello}: cc.reprocess = false

    As well as externally via the config.cc.reprocess variable:

    b config.cc.reprocess=true

Version 0.5.0

  * Parallel build system execution, including header dependency extraction
    and compilation.

  * Support for Testscript, a shell-like language for portable and parallel
    execution of tests. See the Testscript manual for details.

  * Support for name generation with wildcard patterns. For example:

    exe{hello}: cxx{*}

    Or:

    ./: {*/ -build/}

    See the build system manual for details.

  * New module, version, automates project version management. See the build
    system manual for details.

  * Support for VC15, C++ standard selection in VC14U3 and up.

  * New meta-operation, create, allows the creation and configuration of an
    amalgamation project. See b(1) for details.

  * Alternative, shell-friendly command line buildspec and variable assignment
    syntax. For example:

    b test: foo/ bar/
    b config.import.libhello = ../libhello/

    See b(1) for details.

  * Automatic loading of directory buildfiles, implied directory buildfiles.
    Now instead of explicitly writing:

    d = foo/ bar/
    ./: $d doc{README}
    include $d

    We can just write:

    ./: foo/ bar/ doc{README}

    And if our buildfile simply builds all the subdirectories:

    ./: */

    Then it can be omitted altogether.

  * Support of the PATH-based search as a fallback import mechanism for exe{}
    targets.

  * Support for the 'latest' value in the cxx.std variable which can be used
    to request the latest C++ standard available in the compiler.

  * Ternary and logical operators support in eval contexts.

  * Initial support for build system functions. See build2/function*.?xx for
    early details.

  * Assert directive. The grammar is as follows:

    assert <expression> [<description>]
    assert! <expression> [<description>]

    The expression must evaluate to 'true' or 'false', just like in if-else.

Version 0.4.0

  * Support for Windows.

    The toolchain can now be built and used on Windows with either MSVC or
    MinGW GCC.

    With VC, the toolchain can be built with version 14 Update 2 or later and
    used with any version from 7.1. /MD and, for C++, /EHsc are default but
    are overridden if an explicit value is specified in the coptions variable.

  * Support for C compilation.

    There is now the 'c' module in addition to 'cxx' as well as 'cc', which
    stands for C-common. Mixed source (C and C++) building is also supported.

  * Integration with pkg-config.

    Note that build2 doesn't use pkg-config to actually locate the libraries
    (because this functionality of pkg-config is broken when it comes to
    cross-compilation). Rather, it searches for the library (in the
    directories extracted from the compiler) itself and then looks for the
    corresponding .pc file (normally in the pkgconfig/ subdirectory of where
    it found the library). It then calls pkg-config to extract any additional
    options that might be needed to use the library from this specific .pc
    file.

  * Initial support for library versioning.

    Currently, only platform-independent versions are supported. They get
    appended to the library name/soname. For example:

    lib{foo}: bin.lib.version = @-1.2

    This will produce libfoo-1.2.so, libfoo-1.2.dll, etc.

    In the future the plan is to support platform-specific versions, for
    example:

    lib{foo}: bin.lib.version = linux@1.2.3 freebsd@1.2 windows@1.2

  * Library dependency export support.

    In build2 a library dependency on another library is either an "interface"
    or "implementation". If it is an interface, then everyone who links this
    library should also be linking the interface dependency. A good example of
    an interface dependency is a library API that is called in an inline
    function.

    Interface dependencies of a library should be explicitly listed in the
    *.export.libs variable (where we can now list target names). The typical
    usage will be along these lines:

    import int_libs  = libformat%lib{format}
    import int_libs += ...

    import imp_libs  = libprint%lib{print}
    import imp_libs += ...

    lib{hello}: ... $imp_libs $int_libs

    lib{hello}: cxx.export.libs = $int_libs

    There is support for symbol exporting on Windows and build2 now also does
    all the right things when linking static vs shared libraries with regards
    to which library dependencies to link, which -rpath/-rpath-link options to
    pass, etc.

  * Support for the uninstall operation in addition to install.

  * Support for preserving subdirectories when installing.

    This is useful, for example, when installing headers:

    install.include = $install.include/foo/
    install.include.subdirs = true

    The base for calculating the subdirectories is the scope where the subdirs
    value is set.

  * Support for installing as a different file name.

    Now the install variable is a path, not dir_path. If it is a directory
    (ends with a trailing slash), then the target is installed into this
    directory with the same name. Otherwise, the entire path is used as the
    installation destination.

  * Support for config.bin.{,lib,exe}.{prefix,suffix}.

    This replaces the bin.libprefix functionality.

  * Support for global config.install.{cmd,options,sudo,mode,dir_mode}.

    This way we can do:

    b install \
      config.install.data_root=/opt/data \
      config.install.exec_root=/opt/exec \
      config.install.sudo=sudo

  * The new -V option is an alias for --verbose 3 (show all commands).

  * Support for specifying directories in config.dist.archives.

    For example, this command will create /tmp/foo-X.Y.Z.tar.xz:

    b foo/ config.dist.archives=/tmp/tar.xz

  * The cxx (and c) module is now project root-only.

    This means these modules can only be loaded in the project root scope
    (normally root.build). Also, the c.std and cxx.std values must now be set
    before loading the module to take effect.

  * The test, dist, install, and extension variables now have target
    visibility to prevent accidental "reuse" for other purposes.

  * An empty config.import.* value is now treated as an instruction to skip
    subproject search. Also, explicit config.import.* values now take
    precedence over the subproject search.

  * Search for subprojects is no longer recursive. In the future the plan is
    to allow specifying wildcard paths (* and **) in the subprojects variable.

  * Support out-qualified target syntax for setting target-specific variables
    on targets from src_base. For example:

    doc{INSTALL}@./: install = false

  * Only "effective escaping" (['"\$(]) is now performed for values on the
    command line. This makes for a more usable interface on Windows provided
    we use "sane" paths (no spaces, no (), etc).

  * The default variable override scope has been changed from "projects and
    subprojects" to "amalgamation".

    The "projects and subprojects" semantics resulted in counter-intuitive
    behavior. For example, in a project with tests/ as a subproject if one
    builds one of the tests directly with a non-global override (say C++
    compiler), then the main project would be built without the overrides. In
    this light, overriding in the whole amalgamation seems like the right
    thing to do. The old behavior can still be obtained with explicit scope
    qualification, for example:

    b ./:foo=bar

  * The config.build format has been made more readable. Specifically, the
    order is now from the higher-level modules (e.g., c, cxx) to the
    lower-level (e.g., binutils) with imports coming first. The file now also
    includes an explicit version for incompatibility detected/migration in
    the future.

  * Support for <, >, <=, >= in the eval context.

    Now we can write:

    if ($build.version >= 40000)

  * Support for single line if-blocks.

    Now we can write:

    if true
      print true
    else
      print false

    Instead of having to do:

    if true
    {
      print true
    }
    else
    {
      print false
    }

  * Support for prepend/append in target type/pattern-specific variables.

    Semantically, these are similar to variable overrides and are essentially
    treated as "templates" that are applied on lookup to the "stem" value that
    is specific to the target type/name. For example:

    x = [string] a
    file{f*}: x =+ b

    sub/:
    {
      file{*}: x += c

      print $(file{foo}:x)  # abc
      print $(file{bar}:x)  # ac
    }

  * The obj*{} target type to exe/lib mapping has been redesigned.

    Specifically:

    - objso{} and libso{} target types have been renamed to objs{} and libs{}

    - obje{} has been added (so now we have obje{}, obja{}, and objs{})

    - obje{} is now used for building exe{}

    - object file extensions now use "hierarchical extensions" that reflect
      the extension of the corresponding exe/lib target (instead of the -so
      suffix we used), specifically:

      obje{}: foo.o, (UNIX), foo.exe.o (MinGW), foo.exe.obj (MSVC)
      obja{}: foo.a.o (UNIX, MinGW), foo.lib.obj (MSVC)
      objs{}: foo.so.o (UNIX), foo.dylib.o (Darwin), foo.dll.o (MinGW),
              foo.dll.obj (MSVC)

    We now also have libi{} which is the Windows DLL import library. When
    used, it is the first ad hoc group member of libs{}.

Version 0.3.0

  * Support for High Fidelity Builds (HFB).

    The C++ compile and link rules now detect when the compiler, options, or
    input file set have changed and trigger the update of the target. Some
    examples of the events that would now trigger an automatic update are:

    * compiler change (e.g., g++ to clang++), upgrade, or reconfiguration
    * change of compile/link options (e.g., -O2 to -O3)
    * replacement of a source file (e.g., foo.cpp with foo.cxx)
    * removal of a file from a library/executable

  * New command line variable override semantics. A command line variable can
    be an override (=), prefix (=+), or suffix (+=), for example:

    b config.cxx=clang++ config.cxx.coptions+=-g config.cxx.poptions=+-I/tmp

    Prefixes/suffixes are applied at the outsets of values set in buildfiles,
    provided these values were set (in those buildfiles) using =+/+= and not
    an expansion, for example:

    b x=+P x+=S

    x = y
    print $x # P y S

    x =+ p
    x += s
    print $x # P p y s S

    But:

    x = A $x B
    print $x # A P p y s S B

    By default an override applies to all the projects mentioned in the
    buildspec as well as to their subprojects. We can restrict an override to
    not apply to subprojects by prefixing it with '%', for example:

    b %config.cxx=clang++ configure

    An override can also be made global (i.e., it applies to all projects,
    including the imported ones) by prefixing it with '!'. As an example,
    compare these two command lines:

    b config.cxx.coptions+=-g
    b '!config.cxx.coptions+=-g'

    In the first case only the current project and its subprojects will be
    recompiled with the debug information. In the second case, everything that
    the current project requires (e.g., imported libraries) will be rebuilt
    with the debug information.

    Finally, we can also specify the scope from which an override should
    apply. For example, we may only want to rebuild tests with the debug
    information:

    b tests/:config.cxx.coptions+=-g

  * Attribute support. Attributes are key or key=value pairs enclosed in []
    and separated with spaces. They come before the entity they apply to.
    Currently we recognize attributes for variables and values. For variables
    we recognize the following keys as types:

    bool
    uint64
    string
    path
    dir_path
    abs_dir_path
    name
    strings
    paths
    dir_paths
    names

    For example:

    [uint64] x = 01
    print $x # 1
    x += 1
    print $x # 2

    Note that variable types are global, which means you could type a variable
    that is used by another project for something completely different. As a
    result, typing of values (see below) is recommended over variables. If you
    do type a variable, make sure it has a namespace (typing of unqualified
    variables may become illegal).

    For values we recognize the same set of types plus 'null'. The value type
    is preserved in prepend/append (=+/+=) but not in assignment. For example:

    x = [uint64] 01
    print $x # 1
    x += 1
    print $x # 2

    x = [string] 01
    print $x # 01
    x += 1
    print $x # 011

    x = [null]
    print $x # [null]

    Value attributes can also be used in the evaluation contexts, for example:

    if ($x == [null])

    if ([uint64] $x == [uint64] 0)

  * Support for scope/target-qualified variable expansion. For example:

    print $(dir/:x)
    print $(file{target}:x)
    print $(dir/file{target}:x)

  * Command line options, variables, and buildspec can now be specified in any
    order. This is especially useful if you want to re-run the previous
    command with -v or add a forgotten config variable:

    b test -v
    b configure config.cxx=clang++

  * Support for the Intel C++ compiler on Linux.

  * Implement C++ compiler detection. Currently recognized compilers and their
    ids (in the <type>[-<variant>] form):

      gcc            GCC
      clang          Vanilla Clang
      clang-apple    Apple Clang (and the g++ "alias")
      icc            Intel icpc
      msvc           Microsoft cl.exe

    The compiler id, version, and other information is available via the
    following build system variables:

    cxx.id
    cxx.id.{type,variant}
    cxx.version
    cxx.version.{major,minor,patch,build}
    cxx.signature
    cxx.checksum
    cxx.target
    cxx.target.{cpu,vendor,system,version,class}

  * Implement ar/ranlib detection. The following information is available
    via the build system variables:

    bin.ar.signature
    bin.ar.checksum
    bin.ranlib.signature
    bin.ranlib.checksum

  * On update for install the C++ link rule no longer uses the -rpath
    mechanism for finding prerequisite libraries.

  * Set build.host, build.host.{cpu,vendor,system,version,class} build system
    variables to the host triplet. By default it is set to the compiler target
    build2 was built with but a more precise value can be obtained with the
    --config-guess option.

  * Set build.version, build.version.{major,minor,patch,release,string} build
    system variables to the build2 version.

  * Extracted header dependencies (-M*) are now cached in the auxiliary
    dependency (.d) files rather than being re-extracted on every run. This
    speeds up the up-to-date check significantly.

  * Revert back to only cleaning prerequisites if they are in the same project.

    Cleaning everything as long as it is in the same strong amalgamation had
    some undesirable side effects. For example, in bpkg, upgrading a package
    (which requires clean/reconfigure) led to all its prerequisites being
    cleaned as well and then rebuilt. That was surprising, to say the least.

  * Allow escaping in double-quoted strings.

  * Implement --buildfile option that can be used to specify the alternative
    file to read build information from. If '-' is specified, read from STDIN.

  * New scoping semantics. The src tree paths are no longer entered into the
    scope map. Instead, targets from the src tree now include their out tree
    directories (which are, in essence, their "configuration", with regards to
    variable lookup). The only user-visible result of this change is the extra
    '@<out-dir>/' suffix that is added when a target is printed, for example,
    as part of the compilation command lines.

Version 0.2.0

  * First public release.
