Change parse_multiline to follow PEP 257.

The main change is that it avoids creating blank lines when processing
docstrings.

This also adds blank lines in various places to make the generated code
prettier.

lib/cretonne/meta/srcgen.py

@@ -168,12 +168,30 @@ def parse_multiline(s):
     # type: (str) -> List[str]
     """
     Given a multi-line string, split it into a sequence of lines after
-    stripping a common indentation. This is useful for strings defined with doc
-    strings:
+    stripping a common indentation, as described in the "trim" function
+    from PEP 257. This is useful for strings defined with doc strings:
         >>> parse_multiline('\\n    hello\\n    world\\n')
-        [None, 'hello', 'world']
+        ['hello', 'world']
     """
-    lines = s.splitlines()
-    indents = list(i for i in (_indent(l) for l in lines) if i)
-    indent = min(indents) if indents else 0
-    return list(l[indent:] if len(l) > indent else None for l in lines)
+    if not s:
+        return []
+    # Convert tabs to spaces (following the normal Python rules)
+    # and split into a list of lines:
+    lines = s.expandtabs().splitlines()
+    # Determine minimum indentation (first line doesn't count):
+    indent = sys.maxsize
+    for line in lines[1:]:
+        stripped = line.lstrip()
+        if stripped:
+            indent = min(indent, len(line) - len(stripped))
+    # Remove indentation (first line is special):
+    trimmed = [lines[0].strip()]
+    if indent < sys.maxsize:
+        for line in lines[1:]:
+            trimmed.append(line[indent:].rstrip())
+    # Strip off trailing and leading blank lines:
+    while trimmed and not trimmed[-1]:
+        trimmed.pop()
+    while trimmed and not trimmed[0]:
+        trimmed.pop(0)
+    return trimmed