pkgconfig module: allow custom variables to reference builtin directories

Automatically generate additional variables and write them into the
generated pkg-config file.

This means projects no longer need to manually define the ones they
use, which is annoying for dataonly usages (it used to forbid setting
the base library-relevant "reserved" ones, and now allows it only for
dataonly. But it's bloat to manualy list them anyway).

It also fixes a regression in commit
248e6cf473 which caused libdir to not be
set, and to be unsettable, if the pkg-config file has no libraries but
uses the ${libdir} expansion in a custom variable. This could be
considered likely a case for dataonly, but it's not guaranteed.

docs/markdown/snippets/pkgconfig_directory_variables.md

@@ -0,0 +1,31 @@
+## pkgconfig.generate will now include variables for builtin directories when referenced
+
+When using the `variables:` family of kwargs to `pkgconfig.generate` to refer
+to installed paths, traditionally only `prefix`, `includedir`, and `libdir`
+were available by default, and generating a correct (relocatable) pkg-config
+file required manually constructing variables for e.g. `datadir`.
+
+Meson now checks each variable to see if it begins with a reference to a
+standard directory, and if so, adds it to the list of directories for which a
+builtin variable is created.
+
+For example, before it was necessary to do this:
+```meson
+pkgconfig.generate(
+    name: 'bash-completion',
+    description: 'programmable completion for the bash shell',
+    dataonly: true,
+    variables: {
+        'prefix': get_option('prefix'),
+        'datadir': join_paths('${prefix}', get_option('datadir')),
+        'sysconfdir': join_paths('${prefix}', get_option('sysconfdir')),
+
+        'compatdir': '${sysconfdir}/bash_completion.d',
+        'completionsdir': '${datadir}/bash-completion/completions',
+        'helpersdir': '${datadir}/bash-completion/helpers',
+    },
+    install_dir: join_paths(get_option('datadir'), 'pkgconfig'),
+)
+```
+
+Now the first three variables are not needed.

mesonbuild/modules/pkgconfig.py

@@ -22,8 +22,9 @@ from .. import build
 from .. import dependencies
 from .. import mesonlib
 from .. import mlog
+from ..coredata import BUILTIN_DIR_OPTIONS
 from ..dependencies import ThreadDependency
-from ..interpreterbase import permittedKwargs, FeatureNew, FeatureNewKwargs
+from ..interpreterbase import permittedKwargs, FeatureNew, FeatureDeprecated, FeatureNewKwargs
 
 if T.TYPE_CHECKING:
     from . import ModuleState
@@ -329,6 +330,41 @@ class PkgConfigModule(ExtensionModule):
                                  url, version, pcfile, conflicts, variables,
                                  unescaped_variables, uninstalled=False, dataonly=False):
         coredata = state.environment.get_coredata()
+        referenced_vars = set()
+        optnames = [x.name for x in BUILTIN_DIR_OPTIONS.keys()]
+
+        if not dataonly:
+            # includedir is always implied, although libdir may not be
+            # needed for header-only libraries
+            referenced_vars |= {'prefix', 'includedir'}
+            if deps.pub_libs or deps.priv_libs:
+                referenced_vars |= {'libdir'}
+        # also automatically infer variables referenced in other variables
+        implicit_vars_warning = False
+        redundant_vars_warning = False
+        varnames = set()
+        varstrings = set()
+        for k, v in variables + unescaped_variables:
+            varnames |= {k}
+            varstrings |= {v}
+        for optname in optnames:
+            optvar = f'${{{optname}}}'
+            if any(x.startswith(optvar) for x in varstrings):
+                if optname in varnames:
+                    redundant_vars_warning = True
+                else:
+                    # these 3 vars were always "implicit"
+                    if dataonly or optname not in {'prefix', 'includedir', 'libdir'}:
+                        implicit_vars_warning = True
+                    referenced_vars |= {'prefix', optname}
+        if redundant_vars_warning:
+            FeatureDeprecated.single_use('pkgconfig.generate variable for builtin directories', '0.62.0',
+                                         state.subproject, 'They will be automatically included when referenced',
+                                         state.current_node)
+        if implicit_vars_warning:
+            FeatureNew.single_use('pkgconfig.generate implicit variable for builtin directories', '0.62.0',
+                                  state.subproject, location=state.current_node)
+
         if uninstalled:
             outdir = os.path.join(state.environment.build_dir, 'meson-uninstalled')
             if not os.path.exists(outdir):
@@ -338,18 +374,17 @@ class PkgConfigModule(ExtensionModule):
         else:
             outdir = state.environment.scratch_dir
             prefix = PurePath(coredata.get_option(mesonlib.OptionKey('prefix')))
-        # These always return paths relative to prefix
-        libdir = PurePath(coredata.get_option(mesonlib.OptionKey('libdir')))
-        incdir = PurePath(coredata.get_option(mesonlib.OptionKey('includedir')))
         fname = os.path.join(outdir, pcfile)
         with open(fname, 'w', encoding='utf-8') as ofile:
-            if not dataonly:
-                ofile.write('prefix={}\n'.format(self._escape(prefix)))
-                if uninstalled:
-                    ofile.write('srcdir={}\n'.format(self._escape(srcdir)))
-                if deps.pub_libs or deps.priv_libs:
-                    ofile.write('libdir={}\n'.format(self._escape('${prefix}' / libdir)))
-                ofile.write('includedir={}\n'.format(self._escape('${prefix}' / incdir)))
+            for optname in optnames:
+                if optname in referenced_vars - varnames:
+                    if optname == 'prefix':
+                        ofile.write('prefix={}\n'.format(self._escape(prefix)))
+                    else:
+                        dirpath = PurePath(coredata.get_option(mesonlib.OptionKey(optname)))
+                        ofile.write('{}={}\n'.format(optname, self._escape('${prefix}' / dirpath)))
+            if uninstalled and not dataonly:
+                ofile.write('srcdir={}\n'.format(self._escape(srcdir)))
             if variables or unescaped_variables:
                 ofile.write('\n')
             for k, v in variables:

test cases/common/44 pkgconfig-gen/meson.build

@@ -1,4 +1,4 @@
-project('pkgconfig-gen', 'c')
+project('pkgconfig-gen', 'c', meson_version: '>=0.60.0')
 
 # Some CI runners does not have zlib, just skip them as we need some common
 # external dependency.
@@ -149,3 +149,17 @@ ct = custom_target('stat3',
 )
 simple6 = library('simple6', link_with: ct)
 pkgg.generate(simple6)
+
+# implicit variables
+pkgg.generate(
+  name : 'libvartest',
+  description : 'Check that implicit vars are created',
+  version : libver,
+  variables: ['datadir=${prefix}/data', 'foo=${datadir}/foo', 'bar=${bindir}/bar']
+)
+pkgg.generate(
+  name : 'libvartest2',
+  description : 'Check that libdir is not an implicit var',
+  version : libver,
+  variables: ['bar=${libdir}/bar']
+)

test cases/common/44 pkgconfig-gen/test.json

@@ -7,11 +7,26 @@
     {"type": "file", "file": "usr/lib/pkgconfig/libfoo.pc"},
     {"type": "file", "file": "usr/lib/pkgconfig/libhello.pc"},
     {"type": "file", "file": "usr/lib/pkgconfig/libhello_nolib.pc"},
+    {"type": "file", "file": "usr/lib/pkgconfig/libvartest.pc"},
+    {"type": "file", "file": "usr/lib/pkgconfig/libvartest2.pc"},
     {"type": "file", "file": "usr/lib/pkgconfig/simple2.pc"},
     {"type": "file", "file": "usr/lib/pkgconfig/simple3.pc"},
     {"type": "file", "file": "usr/lib/pkgconfig/simple5.pc"},
     {"type": "file", "file": "usr/lib/pkgconfig/simple6.pc"},
     {"type": "file", "file": "usr/lib/pkgconfig/ct.pc"},
     {"type": "file", "file": "usr/lib/pkgconfig/ct0.pc"}
+  ],
+  "stdout": [
+    {
+      "line": "test cases/common/44 pkgconfig-gen/meson.build:158: WARNING: Project targeting '>=0.60.0' but tried to use feature introduced in '0.62.0': pkgconfig.generate implicit variable for builtin directories."
+    },
+    {
+      "line": "test cases/common/44 pkgconfig-gen/meson.build:164: WARNING: Project targeting '>=0.60.0' but tried to use feature introduced in '0.62.0': pkgconfig.generate implicit variable for builtin directories.",
+      "count": 0
+    },
+    {
+      "comment": "This will either match in the future-deprecated notice summary, or match the warning summary",
+      "line": " * 0.62.0: {'pkgconfig.generate variable for builtin directories'}"
+    }
   ]
 }