[red-knot] Port comprehension tests to Markdown (#15688)

## Summary

Port comprehension tests from Rust to Markdown

I don' think the remaining tests in `infer.rs` should be ported to
Markdown, maybe except for the incremental-checking tests when (if ever)
we have support for that in the MD tests.


closes #13696

Author: sharkdp

crates/red_knot_python_semantic/resources/mdtest/comprehensions/basic.md

@@ -0,0 +1,149 @@
+# Comprehensions
+
+## Basic comprehensions
+
+```py
+class IntIterator:
+    def __next__(self) -> int:
+        return 42
+
+class IntIterable:
+    def __iter__(self) -> IntIterator:
+        return IntIterator()
+
+# revealed: int
+[reveal_type(x) for x in IntIterable()]
+
+class IteratorOfIterables:
+    def __next__(self) -> IntIterable:
+        return IntIterable()
+
+class IterableOfIterables:
+    def __iter__(self) -> IteratorOfIterables:
+        return IteratorOfIterables()
+
+# revealed: tuple[int, IntIterable]
+[reveal_type((x, y)) for y in IterableOfIterables() for x in y]
+
+# revealed: int
+{reveal_type(x): 0 for x in IntIterable()}
+
+# revealed: int
+{0: reveal_type(x) for x in IntIterable()}
+```
+
+## Nested comprehension
+
+```py
+class IntIterator:
+    def __next__(self) -> int:
+        return 42
+
+class IntIterable:
+    def __iter__(self) -> IntIterator:
+        return IntIterator()
+
+# revealed: tuple[int, int]
+[[reveal_type((x, y)) for x in IntIterable()] for y in IntIterable()]
+```
+
+## Comprehension referencing outer comprehension
+
+```py
+class IntIterator:
+    def __next__(self) -> int:
+        return 42
+
+class IntIterable:
+    def __iter__(self) -> IntIterator:
+        return IntIterator()
+
+class IteratorOfIterables:
+    def __next__(self) -> IntIterable:
+        return IntIterable()
+
+class IterableOfIterables:
+    def __iter__(self) -> IteratorOfIterables:
+        return IteratorOfIterables()
+
+# revealed: tuple[int, IntIterable]
+[[reveal_type((x, y)) for x in y] for y in IterableOfIterables()]
+```
+
+## Comprehension with unbound iterable
+
+Iterating over an unbound iterable yields `Unknown`:
+
+```py
+# error: [unresolved-reference] "Name `x` used when not defined"
+# revealed: Unknown
+[reveal_type(z) for z in x]
+
+class IntIterator:
+    def __next__(self) -> int:
+        return 42
+
+class IntIterable:
+    def __iter__(self) -> IntIterator:
+        return IntIterator()
+
+# error: [not-iterable] "Object of type `int` is not iterable"
+# revealed: tuple[int, Unknown]
+[reveal_type((x, z)) for x in IntIterable() for z in x]
+```
+
+## Starred expressions
+
+Starred expressions must be iterable
+
+```py
+class NotIterable: ...
+
+class Iterator:
+    def __next__(self) -> int:
+        return 42
+
+class Iterable:
+    def __iter__(self) -> Iterator: ...
+
+# This is fine:
+x = [*Iterable()]
+
+# error: [not-iterable] "Object of type `NotIterable` is not iterable"
+y = [*NotIterable()]
+```
+
+## Async comprehensions
+
+### Basic
+
+```py
+class AsyncIterator:
+    async def __anext__(self) -> int:
+        return 42
+
+class AsyncIterable:
+    def __aiter__(self) -> AsyncIterator:
+        return AsyncIterator()
+
+# revealed: @Todo(async iterables/iterators)
+[reveal_type(x) async for x in AsyncIterable()]
+```
+
+### Invalid async comprehension
+
+This tests that we understand that `async` comprehensions do *not* work according to the synchronous
+iteration protocol
+
+```py
+class Iterator:
+    def __next__(self) -> int:
+        return 42
+
+class Iterable:
+    def __iter__(self) -> Iterator:
+        return Iterator()
+
+# revealed: @Todo(async iterables/iterators)
+[reveal_type(x) async for x in Iterable()]
+```

crates/red_knot_python_semantic/resources/mdtest/comprehensions/invalid_syntax.md

@@ -0,0 +1,43 @@
+# Comprehensions with invalid syntax
+
+```py
+class IntIterator:
+    def __next__(self) -> int:
+        return 42
+
+class IntIterable:
+    def __iter__(self) -> IntIterator:
+        return IntIterator()
+
+# Missing 'in' keyword.
+
+# It's reasonably clear here what they *meant* to write,
+# so we'll still infer the correct type:
+
+# error: [invalid-syntax] "Expected 'in', found name"
+# revealed: int
+[reveal_type(a) for a IntIterable()]
+
+
+# Missing iteration variable
+
+# error: [invalid-syntax] "Expected an identifier, but found a keyword 'in' that cannot be used here"
+# error: [invalid-syntax] "Expected 'in', found name"
+# error: [unresolved-reference]
+# revealed: Unknown
+[reveal_type(b) for in IntIterable()]
+
+
+# Missing iterable
+
+# error: [invalid-syntax] "Expected an expression"
+# revealed: Unknown
+[reveal_type(c) for c in]
+
+
+# Missing 'in' keyword and missing iterable
+
+# error: [invalid-syntax] "Expected 'in', found ']'"
+# revealed: Unknown
+[reveal_type(d) for d]
+```