[red-knot] support fstring expressions

[Slyces]

Summary

Implement inference for f-string, contributes to #12701.

First Implementation

When looking at the way mypy handles things, I noticed the following:

• No variables (e.g. f"hello") ⇒ LiteralString
• Any variable (e.g. f"number {1}") ⇒ str

My first commit (1ba5d0f) implements exactly this logic, except that we deal with string literals just like infer_string_literal_expression (if below MAX_STRING_LITERAL_SIZE, show Literal["exact string"])

Second Implementation

My second commit (90326ce) pushes things a bit further to handle cases where the expression within the f-string are all literal values (string representation known at static time).

Here's an example of when this could happen in code:

BASE_URL = "https://httpbin.org"
VERSION = "v1"
endpoint = f"{BASE_URL}/{VERSION}/post"  # Literal["https://httpbin.org/v1/post"]

As this can be sightly more costly (additional allocations), I don't know if we want this feature.

Test Plan

• Added a test fstring_expression covering all cases I can think of

[MichaReiser]

Oh wow nice and thanks for explaining how mypy handles this.

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

[carljm]

This is great, thank you! I think there are still a couple things to address before landing.

[Slyces]

I implemented the methods Type::repr to use in f-strings inference.

While doing so, I figured out a couple things:

• String replacement in f-strings is actually using str (see here)
• The str method uses repr as a fallback
• There is a rabbit-hole of the format builtin (see here) to deal with things like f"total ${number:01}" that are out of scope with this PR

[AlexWaygood]

String replacement in f-strings is actually using str (see here)

Ah! Very good point. I forgot about this -- str() is used unless !r is given as the format specifier, in which case repr is used :-)

[JelleZijlstra]

String replacement in f-strings is actually using str

It actually uses __format__ (unless you use the !s modifier). However, for most objects, this is the same.

[carljm]

This is very close, thanks for sticking with it! Just a couple small comments yet.