[ty] Correctly handle calls to functions marked as returning Never / NoReturn (#18333)

## Summary

`ty` does not understand that calls to functions which have been
annotated as having a return type of `Never` / `NoReturn` are terminal.

This PR fixes that, by adding new reachability constraints when call
expressions are seen. If the call expression evaluates to `Never`, the
code following it will be considered to be unreachable. Note that, for
adding these constraints, we only consider call expressions at the
statement level, and that too only inside function scopes. This is
because otherwise, the number of such constraints becomes too high, and
evaluating them later on during type inference results in a major
performance degradation.

Fixes astral-sh/ty#180

## Test Plan

New mdtests.

## Ecosystem changes

This PR removes the following false-positives:
- "Function can implicitly return `None`, which is not assignable to
...".
- "Name `foo` used when possibly not defind" - because the branch in
which it is not defined has a `NoReturn` call, or when `foo` was
imported in a `try`, and the except had a `NoReturn` call.

---------

Co-authored-by: David Peter <mail@david-peter.de>

Author: abhijeetbodas2001

crates/ty_python_semantic/resources/mdtest/directives/assert_never.md

@@ -2,27 +2,53 @@
 
 ## Basic functionality
 
-<!-- snapshot-diagnostics -->
+`assert_never` makes sure that the type of the argument is `Never`.
 
-`assert_never` makes sure that the type of the argument is `Never`. If it is not, a
-`type-assertion-failure` diagnostic is emitted.
+### Correct usage
 
 ```py
 from typing_extensions import assert_never, Never, Any
 from ty_extensions import Unknown
 
-def _(never: Never, any_: Any, unknown: Unknown, flag: bool):
+def _(never: Never):
     assert_never(never)  # fine
+```
+
+### Diagnostics
+
+<!-- snapshot-diagnostics -->
 
+If it is not, a `type-assertion-failure` diagnostic is emitted.
+
+```py
+from typing_extensions import assert_never, Never, Any
+from ty_extensions import Unknown
+
+def _():
     assert_never(0)  # error: [type-assertion-failure]
+
+def _():
     assert_never("")  # error: [type-assertion-failure]
+
+def _():
     assert_never(None)  # error: [type-assertion-failure]
+
+def _():
     assert_never([])  # error: [type-assertion-failure]
+
+def _():
     assert_never({})  # error: [type-assertion-failure]
+
+def _():
     assert_never(())  # error: [type-assertion-failure]
+
+def _(flag: bool, never: Never):
     assert_never(1 if flag else never)  # error: [type-assertion-failure]
 
+def _(any_: Any):
     assert_never(any_)  # error: [type-assertion-failure]
+
+def _(unknown: Unknown):
     assert_never(unknown)  # error: [type-assertion-failure]
 ```
 

crates/ty_python_semantic/resources/mdtest/snapshots/assert_never.md_-_`assert_never`_-_Basic_functionality_(78e7b52096b8d36c).snap renamed to crates/ty_python_semantic/resources/mdtest/snapshots/assert_never.md_-_`assert_never`_-_Basic_functionality_-_Diagnostics_(be8f5d8b0718ee54).snap

@@ -3,7 +3,7 @@ source: crates/ty_test/src/lib.rs
 expression: snapshot
 ---
 ---
-mdtest name: assert_never.md - `assert_never` - Basic functionality
+mdtest name: assert_never.md - `assert_never` - Basic functionality - Diagnostics
 mdtest path: crates/ty_python_semantic/resources/mdtest/directives/assert_never.md
 ---
 
@@ -15,35 +15,47 @@ mdtest path: crates/ty_python_semantic/resources/mdtest/directives/assert_never.
  1 | from typing_extensions import assert_never, Never, Any
  2 | from ty_extensions import Unknown
  3 | 
- 4 | def _(never: Never, any_: Any, unknown: Unknown, flag: bool):
- 5 |     assert_never(never)  # fine
+ 4 | def _():
+ 5 |     assert_never(0)  # error: [type-assertion-failure]
  6 | 
- 7 |     assert_never(0)  # error: [type-assertion-failure]
+ 7 | def _():
  8 |     assert_never("")  # error: [type-assertion-failure]
- 9 |     assert_never(None)  # error: [type-assertion-failure]
-10 |     assert_never([])  # error: [type-assertion-failure]
-11 |     assert_never({})  # error: [type-assertion-failure]
-12 |     assert_never(())  # error: [type-assertion-failure]
-13 |     assert_never(1 if flag else never)  # error: [type-assertion-failure]
-14 | 
-15 |     assert_never(any_)  # error: [type-assertion-failure]
-16 |     assert_never(unknown)  # error: [type-assertion-failure]
+ 9 | 
+10 | def _():
+11 |     assert_never(None)  # error: [type-assertion-failure]
+12 | 
+13 | def _():
+14 |     assert_never([])  # error: [type-assertion-failure]
+15 | 
+16 | def _():
+17 |     assert_never({})  # error: [type-assertion-failure]
+18 | 
+19 | def _():
+20 |     assert_never(())  # error: [type-assertion-failure]
+21 | 
+22 | def _(flag: bool, never: Never):
+23 |     assert_never(1 if flag else never)  # error: [type-assertion-failure]
+24 | 
+25 | def _(any_: Any):
+26 |     assert_never(any_)  # error: [type-assertion-failure]
+27 | 
+28 | def _(unknown: Unknown):
+29 |     assert_never(unknown)  # error: [type-assertion-failure]
 ```
 
 # Diagnostics
 
 ```
 error[type-assertion-failure]: Argument does not have asserted type `Never`
- --> src/mdtest_snippet.py:7:5
+ --> src/mdtest_snippet.py:5:5
   |
-5 |     assert_never(never)  # fine
-6 |
-7 |     assert_never(0)  # error: [type-assertion-failure]
+4 | def _():
+5 |     assert_never(0)  # error: [type-assertion-failure]
   |     ^^^^^^^^^^^^^-^
   |                  |
   |                  Inferred type of argument is `Literal[0]`
-8 |     assert_never("")  # error: [type-assertion-failure]
-9 |     assert_never(None)  # error: [type-assertion-failure]
+6 |
+7 | def _():
   |
 info: `Never` and `Literal[0]` are not equivalent types
 info: rule `type-assertion-failure` is enabled by default
@@ -54,13 +66,13 @@ info: rule `type-assertion-failure` is enabled by default
 error[type-assertion-failure]: Argument does not have asserted type `Never`
   --> src/mdtest_snippet.py:8:5
    |
- 7 |     assert_never(0)  # error: [type-assertion-failure]
+ 7 | def _():
  8 |     assert_never("")  # error: [type-assertion-failure]
    |     ^^^^^^^^^^^^^--^
    |                  |
    |                  Inferred type of argument is `Literal[""]`
- 9 |     assert_never(None)  # error: [type-assertion-failure]
-10 |     assert_never([])  # error: [type-assertion-failure]
+ 9 |
+10 | def _():
    |
 info: `Never` and `Literal[""]` are not equivalent types
 info: rule `type-assertion-failure` is enabled by default
@@ -69,16 +81,15 @@ info: rule `type-assertion-failure` is enabled by default
 
 ```
 error[type-assertion-failure]: Argument does not have asserted type `Never`
-  --> src/mdtest_snippet.py:9:5
+  --> src/mdtest_snippet.py:11:5
    |
- 7 |     assert_never(0)  # error: [type-assertion-failure]
- 8 |     assert_never("")  # error: [type-assertion-failure]
- 9 |     assert_never(None)  # error: [type-assertion-failure]
+10 | def _():
+11 |     assert_never(None)  # error: [type-assertion-failure]
    |     ^^^^^^^^^^^^^----^
    |                  |
    |                  Inferred type of argument is `None`
-10 |     assert_never([])  # error: [type-assertion-failure]
-11 |     assert_never({})  # error: [type-assertion-failure]
+12 |
+13 | def _():
    |
 info: `Never` and `None` are not equivalent types
 info: rule `type-assertion-failure` is enabled by default
@@ -87,16 +98,15 @@ info: rule `type-assertion-failure` is enabled by default
 
 ```
 error[type-assertion-failure]: Argument does not have asserted type `Never`
-  --> src/mdtest_snippet.py:10:5
+  --> src/mdtest_snippet.py:14:5
    |
- 8 |     assert_never("")  # error: [type-assertion-failure]
- 9 |     assert_never(None)  # error: [type-assertion-failure]
-10 |     assert_never([])  # error: [type-assertion-failure]
+13 | def _():
+14 |     assert_never([])  # error: [type-assertion-failure]
    |     ^^^^^^^^^^^^^--^
    |                  |
    |                  Inferred type of argument is `list[Unknown]`
-11 |     assert_never({})  # error: [type-assertion-failure]
-12 |     assert_never(())  # error: [type-assertion-failure]
+15 |
+16 | def _():
    |
 info: `Never` and `list[Unknown]` are not equivalent types
 info: rule `type-assertion-failure` is enabled by default
@@ -105,16 +115,15 @@ info: rule `type-assertion-failure` is enabled by default
 
 ```
 error[type-assertion-failure]: Argument does not have asserted type `Never`
-  --> src/mdtest_snippet.py:11:5
+  --> src/mdtest_snippet.py:17:5
    |
- 9 |     assert_never(None)  # error: [type-assertion-failure]
-10 |     assert_never([])  # error: [type-assertion-failure]
-11 |     assert_never({})  # error: [type-assertion-failure]
+16 | def _():
+17 |     assert_never({})  # error: [type-assertion-failure]
    |     ^^^^^^^^^^^^^--^
    |                  |
    |                  Inferred type of argument is `dict[Unknown, Unknown]`
-12 |     assert_never(())  # error: [type-assertion-failure]
-13 |     assert_never(1 if flag else never)  # error: [type-assertion-failure]
+18 |
+19 | def _():
    |
 info: `Never` and `dict[Unknown, Unknown]` are not equivalent types
 info: rule `type-assertion-failure` is enabled by default
@@ -123,15 +132,15 @@ info: rule `type-assertion-failure` is enabled by default
 
 ```
 error[type-assertion-failure]: Argument does not have asserted type `Never`
-  --> src/mdtest_snippet.py:12:5
+  --> src/mdtest_snippet.py:20:5
    |
-10 |     assert_never([])  # error: [type-assertion-failure]
-11 |     assert_never({})  # error: [type-assertion-failure]
-12 |     assert_never(())  # error: [type-assertion-failure]
+19 | def _():
+20 |     assert_never(())  # error: [type-assertion-failure]
    |     ^^^^^^^^^^^^^--^
    |                  |
    |                  Inferred type of argument is `tuple[()]`
-13 |     assert_never(1 if flag else never)  # error: [type-assertion-failure]
+21 |
+22 | def _(flag: bool, never: Never):
    |
 info: `Never` and `tuple[()]` are not equivalent types
 info: rule `type-assertion-failure` is enabled by default
@@ -140,16 +149,15 @@ info: rule `type-assertion-failure` is enabled by default
 
 ```
 error[type-assertion-failure]: Argument does not have asserted type `Never`
-  --> src/mdtest_snippet.py:13:5
+  --> src/mdtest_snippet.py:23:5
    |
-11 |     assert_never({})  # error: [type-assertion-failure]
-12 |     assert_never(())  # error: [type-assertion-failure]
-13 |     assert_never(1 if flag else never)  # error: [type-assertion-failure]
+22 | def _(flag: bool, never: Never):
+23 |     assert_never(1 if flag else never)  # error: [type-assertion-failure]
    |     ^^^^^^^^^^^^^--------------------^
    |                  |
    |                  Inferred type of argument is `Literal[1]`
-14 |
-15 |     assert_never(any_)  # error: [type-assertion-failure]
+24 |
+25 | def _(any_: Any):
    |
 info: `Never` and `Literal[1]` are not equivalent types
 info: rule `type-assertion-failure` is enabled by default
@@ -158,15 +166,15 @@ info: rule `type-assertion-failure` is enabled by default
 
 ```
 error[type-assertion-failure]: Argument does not have asserted type `Never`
-  --> src/mdtest_snippet.py:15:5
+  --> src/mdtest_snippet.py:26:5
    |
-13 |     assert_never(1 if flag else never)  # error: [type-assertion-failure]
-14 |
-15 |     assert_never(any_)  # error: [type-assertion-failure]
+25 | def _(any_: Any):
+26 |     assert_never(any_)  # error: [type-assertion-failure]
    |     ^^^^^^^^^^^^^----^
    |                  |
    |                  Inferred type of argument is `Any`
-16 |     assert_never(unknown)  # error: [type-assertion-failure]
+27 |
+28 | def _(unknown: Unknown):
    |
 info: `Never` and `Any` are not equivalent types
 info: rule `type-assertion-failure` is enabled by default
@@ -175,10 +183,10 @@ info: rule `type-assertion-failure` is enabled by default
 
 ```
 error[type-assertion-failure]: Argument does not have asserted type `Never`
-  --> src/mdtest_snippet.py:16:5
+  --> src/mdtest_snippet.py:29:5
    |
-15 |     assert_never(any_)  # error: [type-assertion-failure]
-16 |     assert_never(unknown)  # error: [type-assertion-failure]
+28 | def _(unknown: Unknown):
+29 |     assert_never(unknown)  # error: [type-assertion-failure]
    |     ^^^^^^^^^^^^^-------^
    |                  |
    |                  Inferred type of argument is `Unknown`