interpreter: Dependency variables can be empty string

There is no reason to forbid empty variables, PkgConfigCLI.variable()
even has code specifically for handling that case.

mesonbuild/interpreter/interpreter.py

@@ -702,6 +702,8 @@ class Interpreter(InterpreterBase, HoldableObject):
         srcdir = Path(self.environment.source_dir)
         # convert variables which refer to an -uninstalled.pc style datadir
         for k, v in variables.items():
+            if not v:
+                FeatureNew.single_use('empty variable value in declare_dependency', '1.4.0', self.subproject, location=node)
             try:
                 p = Path(v)
             except ValueError:

mesonbuild/interpreter/type_checking.py

@@ -144,8 +144,6 @@ def variables_validator(contents: T.Union[str, T.List[str], T.Dict[str, str]]) -
     for k, v in variables.items():
         if not k:
             return 'empty variable name'
-        if not v:
-            return 'empty variable value'
         if any(c.isspace() for c in k):
             return f'invalid whitespace in variable name {k!r}'
     return None

mesonbuild/modules/pkgconfig.py

@@ -680,6 +680,8 @@ class PkgConfigModule(NewExtensionModule):
             reserved = ['prefix', 'libdir', 'includedir']
             variables = []
             for name, value in vardict.items():
+                if not value:
+                    FeatureNew.single_use('empty variable value in pkg.generate', '1.4.0', state.subproject, location=state.current_node)
                 if not dataonly and name in reserved:
                     raise mesonlib.MesonException(f'Variable "{name}" is reserved')
                 variables.append((name, value))

test cases/failing/47 pkgconfig variables zero length value/meson.build

@@ -1,16 +0,0 @@
-project('variables-zero-length-value-test', 'c', version : '1.0')
-
-pkgg = import('pkgconfig')
-lib = shared_library('simple', 'simple.c')
-libver = '1.0'
-h = install_headers('simple.h')
-
-pkgg.generate(
-  libraries : [lib, '-lz'],
-  subdirs : '.',
-  version : libver,
-  name : 'libsimple',
-  filebase : 'simple',
-  description : 'A simple demo library.',
-  variables : [ 'key=' ]
-)

test cases/failing/47 pkgconfig variables zero length value/simple.c

@@ -1,5 +0,0 @@
-#include"simple.h"
-
-int simple_function() {
-    return 42;
-}

test cases/failing/47 pkgconfig variables zero length value/simple.h

@@ -1,6 +0,0 @@
-#ifndef SIMPLE_H_
-#define SIMPLE_H_
-
-int simple_function();
-
-#endif

test cases/failing/47 pkgconfig variables zero length value/test.json

@@ -1,7 +0,0 @@
-{
-  "stdout": [
-    {
-      "line": "test cases/failing/47 pkgconfig variables zero length value/meson.build:8:5: ERROR: pkgconfig.generate keyword argument \"variables\" empty variable value"
-    }
-  ]
-}