custom_target: Substitute @OUTPUT@ and @INPUT properly

They weren't being substituted if they were a part of a command
argument, ala --output=@OUTPUT@, etc.

Closes https://github.com/mesonbuild/meson/issues/824

mesonbuild/backend/backends.py

@@ -572,12 +572,29 @@ class Backend():
                 i = i.replace('@INPUT%d@' % j, src)
             for (j, res) in enumerate(ofilenames):
                 i = i.replace('@OUTPUT%d@' % j, res)
+            if '@INPUT@' in i:
+                msg = 'Custom target {} has @INPUT@ in the command, but'.format(target.name)
+                if len(srcs) == 0:
+                    raise MesonException(msg + ' no input files')
                 if i == '@INPUT@':
                     cmd += srcs
-            elif i == '@OUTPUT@':
+                    continue
-                cmd += ofilenames
                 else:
-                if '@OUTDIR@' in i:
+                    if len(srcs) > 1:
+                        raise MesonException(msg + ' more than one input file')
+                    i = i.replace('@INPUT@', srcs[0])
+            elif '@OUTPUT@' in i:
+                msg = 'Custom target {} has @OUTPUT@ in the command, but'.format(target.name)
+                if len(ofilenames) == 0:
+                    raise MesonException(msg + ' no output files')
+                if i == '@OUTPUT@':
+                    cmd += ofilenames
+                    continue
+                else:
+                    if len(ofilenames) > 1:
+                        raise MesonException(msg + ' more than one output file')
+                    i = i.replace('@OUTPUT@', ofilenames[0])
+            elif '@OUTDIR@' in i:
                 i = i.replace('@OUTDIR@', outdir)
             elif '@DEPFILE@' in i:
                 if target.depfile is None:

test cases/common/56 custom target/meson.build

@@ -9,7 +9,7 @@ comp = '@0@/@1@'.format(meson.current_source_dir(), 'my_compiler.py')
 mytarget = custom_target('bindat',
 output : 'data.dat',
 input : 'data_source.txt',
-command : [python, comp, '@INPUT@', '@OUTPUT@'],
+command : [python, comp, '--input=@INPUT@', '--output=@OUTPUT@'],
 install : true,
 install_dir : 'subdir'
 )

test cases/common/56 custom target/my_compiler.py

@@ -3,13 +3,14 @@
 import sys
 
 if __name__ == '__main__':
-    if len(sys.argv) != 3:
+    if len(sys.argv) != 3 or not sys.argv[1].startswith('--input') or \
-        print(sys.argv[0], 'input_file output_file')
+       not sys.argv[2].startswith('--output'):
+        print(sys.argv[0], '--input=input_file --output=output_file')
         sys.exit(1)
-    with open(sys.argv[1]) as f:
+    with open(sys.argv[1].split('=')[1]) as f:
         ifile = f.read()
     if ifile != 'This is a text only input file.\n':
         print('Malformed input')
         sys.exit(1)
-    with open(sys.argv[2], 'w') as ofile:
+    with open(sys.argv[2].split('=')[1], 'w') as ofile:
         ofile.write('This is a binary output file.\n')