py: Implement partial PEP-498 (f-string) support.

This implements (most of) the PEP-498 spec for f-strings and is based on
https://github.com/micropython/micropython/pull/4998 by @klardotsh.

It is implemented in the lexer as a syntax translation to `str.format`:
  f"{a}" --> "{}".format(a)

It also supports:
  f"{a=}" --> "a={}".format(a)

This is done by extracting the arguments into a temporary vstr buffer,
then after the string has been tokenized, the lexer input queue is saved
and the contents of the temporary vstr buffer are injected into the lexer
instead.

There are four main limitations:
- raw f-strings (`fr` or `rf` prefixes) are not supported and will raise
  `SyntaxError: raw f-strings are not supported`.

- literal concatenation of f-strings with adjacent strings will fail
    "{}" f"{a}" --> "{}{}".format(a)    (str.format will incorrectly use
                                         the braces from the non-f-string)
    f"{a}" f"{a}" --> "{}".format(a) "{}".format(a) (cannot concatenate)

- PEP-498 requires the full parser to understand the interpolated
  argument, however because this entirely runs in the lexer it cannot
  resolve nested braces in expressions like
    f"{'}'}"

- The !r, !s, and !a conversions are not supported.

Includes tests and cpydiffs.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

1 files changed, 5 insertions, 0 deletions

diff --git a/py/mpconfig.h b/py/mpconfig.h
index a91c39b01..a5d639efe 100644
--- a/py/mpconfig.h
+++ b/py/mpconfig.h
@@ -875,6 +875,11 @@ typedef double mp_float_t;
 #define MICROPY_PY_ASYNC_AWAIT (1)
 #endif
 
+// Support for literal string interpolation, f-strings (see PEP 498, Python 3.6+)
+#ifndef MICROPY_PY_FSTRINGS
+#define MICROPY_PY_FSTRINGS (0)
+#endif
+
 // Support for assignment expressions with := (see PEP 572, Python 3.8+)
 #ifndef MICROPY_PY_ASSIGN_EXPR
 #define MICROPY_PY_ASSIGN_EXPR (1)