Implicit string concatenation produces different AST than CPython #4095

The AST produced by RustPython differs from CPython when parsing implicitly concatenated strings.

For example, given this source:

import ast

expr = """(
    'Hello, '
    'world!'
)"""

print(ast.dump(ast.parse(expr), include_attributes=True))

CPython produces a single Constant, with a value equal to the concatenated strings:

Expr(value=Constant(value='Hello, world!', kind=None, lineno=2, col_offset=4, end_lineno=3, end_col_offset=12), lineno=1, col_offset=0, end_lineno=4, end_col_offset=1)

Whereas RustPython produces a JoinedStr consisting of two Constant sub-expressions:

Expr { value: Located { location: Location { row: 2, column: 6 }, custom: (), node: JoinedStr { values: [Located { location: Location { row: 2, column: 6 }, custom: (), node: Constant { value: Str("Hello, "), kind: None } }, Located { location: Location { row: 3, column: 6 }, custom: (), node: Constant { value: Str("world!"), kind: None } }] } } }

There are some other differences too, e.g.:

• CPython seems to propagate the string kind of the first substring. So if the first substring is u'Hello, ', then the whole thing will be treated as Unicode. If the second substring is u'world!', CPython drops the u entirely. (I found this surprising.)
• If any of the substrings are f-strings, CPython returns a JoinedStr, but exhibits the same constant-collapsing as in the provided example.

As far as I can tell, the rules are (roughly):

• If every substring is a Constant, return a single Constant with the substrings concatened.
• If any substring is an f-string, return a JoinedStr, then flatten out the values into that top-level JoinedStr, collapsing any adjacent Constant expressions, so that you're left with Constant expressions interspersed with FormattedValue expressions.
• If the first substring has a "kind", apply that "kind" to any Constant expressions.

(I haven't been able to find this in any documentation, only from testing out CPython 3.8.12 itself.)

Happy to attempt a PR, unless this behavior is intentional.

(By the way: very impressive project!)