Multiline f-strings aren't formatted correctly

[Alphasite]

Describe the bug

When you have multiple f-strings (I suspect without a + between them) then when it reformats them it
will just remove the new lines without merging the strings into a single compound string, e.g. f"{1}" f" {2}"
rather than f"{1} {2}"

To Reproduce

c.run(
    f"docker login {self.hostname} "
    f"--username {self.username} "
    f"--password {self.password}",
)

For example, take this code:

this = "code"

And run it with these arguments:

$ poetry run black src

The resulting error is:

         c.run(
-            f"docker login {self.hostname} "
-            f"--username {self.username} "
-            f"--password {self.password}",
+            f"docker login {self.hostname} " f"--username {self.username} " f"--password {self.password}",
         )

Expected behavior

         c.run(
-            f"docker login {self.hostname} "
-            f"--username {self.username} "
-            f"--password {self.password}",
+            f"docker login {self.hostname} --username {self.username} --password {self.password}",
         )

Environment

• Black's version: black, 21.12b0 (compiled: no)
• OS and Python version: Mac/Python 3.9

Additional context

[felix-hilden]

I believe this is fixed by using black --experimental-string-processing, which will soon become standard functionality, tracked in #2188.

$ black -c "f\"a\" f\"b\""
f"a" f"b"

$ black -c "f\"a\" f\"b\"" --experimental-string-processing
f"ab"

[Alphasite]

Oh thank you!