ninjabackend: optimize length_estimate

Optimize the regular expression so that the variable expansion part
always ends up in group 1, and the trailer after the variable is discarded
in the same match.  Do not use re.sub to remove braces, and do not bother
building the expanded command, just adjust the estimated length on the fly.

functools.reduce is extremely slow, so I am keeping ' '.join(chunk).

On a QEMU build the time spend in the function goes from 1.072s to 0.757s.

mesonbuild/backend/ninjabackend.py

@@ -265,18 +265,20 @@ class NinjaRule:
 
         # expand variables in command
         command = ' '.join([self._quoter(x) for x in self.command + self.args])
-        expanded_command = ''
-        for m in re.finditer(r'(\${\w*})|(\$\w*)|([^$]*)', command):
-            chunk = m.group()
-            if chunk.startswith('$'):
-                chunk = chunk[1:]
-                chunk = re.sub(r'{(.*)}', r'\1', chunk)
-                chunk = ninja_vars.get(chunk, [])  # undefined ninja variables are empty
-                chunk = ' '.join(chunk)
-            expanded_command += chunk
+        estimate = len(command)
+        for m in re.finditer(r'(\${\w*}|\$\w*)?[^$]*', command):
+            if m.start(1) != -1:
+                estimate -= m.end(1) - m.start(1) + 1
+                chunk = m.group(1)
+                if chunk[1] == '{':
+                    chunk = chunk[2:-1]
+                else:
+                    chunk = chunk[1:]
+                chunk = ninja_vars.get(chunk, []) # undefined ninja variables are empty
+                estimate += len(' '.join(chunk))
 
         # determine command length
-        return len(expanded_command)
+        return estimate
 
 class NinjaBuildElement:
     def __init__(self, all_outputs, outfilenames, rulename, infilenames, implicit_outs=None):