[red-knot] Reachability analysis

Author: sharkdp

crates/red_knot_python_semantic/resources/mdtest/statically_known_branches.md

@@ -1502,13 +1502,14 @@ if True:
 from module import symbol
 ```
 
-## Unsupported features
+## Unreachable code
 
-We do not support full unreachable code analysis yet. We also raise diagnostics from
-statically-known to be false branches:
+A closely related feature is the ability to detect unreachable code. For example, we do not emit a
+diagnostic here:
 
 ```py
 if False:
-    # error: [unresolved-reference]
     x
 ```
+
+See [unreachable.md](unreachable.md) for more tests on this topic.

crates/red_knot_python_semantic/resources/mdtest/unreachable.md

@@ -1,9 +1,16 @@
 # Unreachable code
 
+This document describes our approach to handling unreachable code. There are two aspects to this.
+One is to detect and mark blocks of code that are unreachable. This is useful for notifying the
+user, as it can often be indicative of an error. The second aspect of this is to make sure that we
+do not emit (incorrect) diagnostics in unreachable code.
+
 ## Detecting unreachable code
 
 In this section, we look at various scenarios how sections of code can become unreachable. We should
-eventually introduce a new diagnostic that would detect unreachable code.
+eventually introduce a new diagnostic that would detect unreachable code. In an editor/LSP context,
+there are ways to 'gray out' sections of code, which is helpful for blocks of code that are not
+'dead' code, but inactive under certain conditions, like platform-specific code.
 
 ### Terminal statements
 
@@ -85,21 +92,21 @@ def f():
     print("unreachable")
 ```
 
-## Python version and platform checks
+### Python version and platform checks
 
 It is common to have code that is specific to a certain Python version or platform. This case is
 special because whether or not the code is reachable depends on externally configured constants. And
 if we are checking for a set of parameters that makes one of these branches unreachable, that is
 likely not something that the user wants to be warned about, because there are probably other sets
 of parameters that make the branch reachable.
 
-### `sys.version_info` branches
+#### `sys.version_info` branches
 
 Consider the following example. If we check with a Python version lower than 3.11, the import
 statement is unreachable. If we check with a Python version equal to or greater than 3.11, the
 import statement is definitely reachable. We should not emit any diagnostics in either case.
 
-#### Checking with Python version 3.10
+##### Checking with Python version 3.10
 
 ```toml
 [environment]
@@ -115,7 +122,7 @@ if sys.version_info >= (3, 11):
     from typing import Self
 ```
 
-#### Checking with Python version 3.12
+##### Checking with Python version 3.12
 
 ```toml
 [environment]
@@ -129,12 +136,12 @@ if sys.version_info >= (3, 11):
     from typing import Self
 ```
 
-### `sys.platform` branches
+#### `sys.platform` branches
 
 The problem is even more pronounced with `sys.platform` branches, since we don't necessarily have
 the platform information available.
 
-#### Checking with platform `win32`
+##### Checking with platform `win32`
 
 ```toml
 [environment]
@@ -148,7 +155,7 @@ if sys.platform == "win32":
     sys.getwindowsversion()
 ```
 
-#### Checking with platform `linux`
+##### Checking with platform `linux`
 
 ```toml
 [environment]
@@ -164,13 +171,21 @@ if sys.platform == "win32":
     sys.getwindowsversion()
 ```
 
-#### Checking without a specified platform
+##### Checking with platform set to `all`
 
 ```toml
 [environment]
-# python-platform not specified
+python-platform = "all"
 ```
 
+If `python-platform` is set to `all`, we treat the platform as unspecified. This means that we do
+not infer a literal type like `Literal["win32"]` for `sys.platform`, but instead fall back to
+`LiteralString` (the `typeshed` annotation for `sys.platform`). This means that we can not
+statically determine the truthiness of a branch like `sys.platform == "win32"`.
+
+See <https://github.com/astral-sh/ruff/issues/16983#issuecomment-2777146188> for a plan on how this
+could be improved.
+
 ```py
 import sys
 
@@ -180,11 +195,13 @@ if sys.platform == "win32":
     sys.getwindowsversion()
 ```
 
-#### Checking with platform set to `all`
+##### Checking without a specified platform
+
+If `python-platform` is not specified, we currently default to `all`:
 
 ```toml
 [environment]
-python-platform = "all"
+# python-platform not specified
 ```
 
 ```py
@@ -196,9 +213,29 @@ if sys.platform == "win32":
     sys.getwindowsversion()
 ```
 
-## No false positive diagnostics in unreachable code
+## No (incorrect) diagnostics in unreachable code
+
+```toml
+[environment]
+python-version = "3.10"
+```
+
+In this section, we demonstrate that we do not emit (incorrect) diagnostics in unreachable sections
+of code.
+
+It could be argued that no diagnostics at all should be emitted in unreachable code. The reasoning
+is that any issues inside the unreachable section would not cause problems at runtime. And type
+checking the unreachable code under the assumption that it *is* reachable might lead to false
+positives (see the "Global constants" example below).
 
-In this section, we make sure that we do not emit false positive diagnostics in unreachable code.
+On the other hand, it could be argued that code like `1 + "a"` is incorrect, no matter if it is
+reachable or not. Some developers like to use things like early `return` statements while debugging,
+and for this use case, it is helpful to still see some diagnostics in unreachable sections.
+
+We currently follow the second approach, but we do not attempt to provide the full set of
+diagnostics in unreachable sections. In fact, we silence a certain category of diagnostics
+(`unresolved-reference`, `unresolved-attribute`, …), in order to avoid *incorrect* diagnostics. In
+the future, we may revisit this decision.
 
 ### Use of variables in unreachable code
 
@@ -225,19 +262,17 @@ def outer():
     x = 1
 
     def inner():
-        return x  # Name `x` used when not defined
+        reveal_type(x)  # revealed: Unknown
     while True:
         pass
 ```
 
-## No diagnostics in unreachable code
-
-In general, no diagnostics should be emitted in unreachable code. The reasoning is that any issues
-inside the unreachable section would not cause problems at runtime. And type checking the
-unreachable code under the assumption that it *is* reachable might lead to false positives:
+### Global constants
 
 ```py
-FEATURE_X_ACTIVATED = False
+from typing import Literal
+
+FEATURE_X_ACTIVATED: Literal[False] = False
 
 if FEATURE_X_ACTIVATED:
     def feature_x():
@@ -248,7 +283,166 @@ def f():
         # Type checking this particular section as if it were reachable would
         # lead to a false positive, so we should not emit diagnostics here.
 
-        # TODO: no error should be emitted here
-        # error: [unresolved-reference]
         feature_x()
 ```
+
+### Exhaustive check of syntactic constructs
+
+We include some more examples here to make sure that silencing of diagnostics works for
+syntactically different cases. To test this, we use `ExceptionGroup`, which is only available in
+Python 3.11 and later. We have set the Python version to 3.10 for this whole section, to have
+`match` statements available, but not `ExceptionGroup`.
+
+To start, we make sure that we do not emit a diagnostic in this simple case:
+
+```py
+import sys
+
+if sys.version_info >= (3, 11):
+    ExceptionGroup  # no error here
+```
+
+Similarly, if we negate the logic, we also emit no error:
+
+```py
+if sys.version_info < (3, 11):
+    pass
+else:
+    ExceptionGroup  # no error here
+```
+
+This also works for more complex `if`-`elif`-`else` chains:
+
+```py
+if sys.version_info >= (3, 13):
+    ExceptionGroup  # no error here
+elif sys.version_info >= (3, 12):
+    ExceptionGroup  # no error here
+elif sys.version_info >= (3, 11):
+    ExceptionGroup  # no error here
+elif sys.version_info >= (3, 10):
+    pass
+else:
+    pass
+```
+
+The same works for ternary expressions:
+
+```py
+class ExceptionGroupPolyfill: ...
+
+MyExceptionGroup1 = ExceptionGroup if sys.version_info >= (3, 11) else ExceptionGroupPolyfill
+MyExceptionGroup1 = ExceptionGroupPolyfill if sys.version_info < (3, 11) else ExceptionGroup
+```
+
+Due to short-circuiting, this also works for Boolean operators:
+
+```py
+sys.version_info >= (3, 11) and ExceptionGroup
+sys.version_info < (3, 11) or ExceptionGroup
+```
+
+And in `match` statements:
+
+```py
+reveal_type(sys.version_info.minor)  # revealed: Literal[10]
+
+match sys.version_info.minor:
+    case 13:
+        ExceptionGroup
+    case 12:
+        ExceptionGroup
+    case 11:
+        ExceptionGroup
+    case _:
+        pass
+```
+
+Terminal statements can also lead to unreachable code:
+
+```py
+def f():
+    if sys.version_info < (3, 11):
+        raise RuntimeError("this code only works for Python 3.11+")
+
+    ExceptionGroup
+```
+
+Finally, not that anyone would ever use it, but it also works for `while` loops:
+
+```py
+while sys.version_info >= (3, 11):
+    ExceptionGroup
+```
+
+### Silencing errors for actually unknown symbols
+
+We currently also silence diagnostics for symbols that are not actually defined anywhere. It is
+conceivable that this could be improved, but is not a priority for now.
+
+```py
+if False:
+    does_not_exist
+
+def f():
+    return
+    does_not_exist
+```
+
+### Attributes
+
+When attribute expressions appear in unreachable code, we should not emit `unresolved-attribute`
+diagnostics:
+
+```py
+import sys
+import builtins
+
+if sys.version_info >= (3, 11):
+    # TODO
+    # error: [unresolved-attribute]
+    builtins.ExceptionGroup
+```
+
+### Imports
+
+When import statements appear in unreachable code, we should not emit `unresolved-import`
+diagnostics:
+
+```py
+import sys
+
+if sys.version_info >= (3, 11):
+    # TODO
+    # error: [unresolved-import]
+    from builtins import ExceptionGroup
+
+    # TODO
+    # error: [unresolved-import]
+    import builtins.ExceptionGroup
+
+    # See https://docs.python.org/3/whatsnew/3.11.html#new-modules
+
+    # TODO
+    # error: [unresolved-import]
+    import tomllib
+
+    # TODO
+    # error: [unresolved-import]
+    import wsgiref.types
+```
+
+### Emit diagnostics for definitely wrong code
+
+Even though the expressions in the snippet below are unreachable, we still emit diagnostics for
+them:
+
+```py
+if False:
+    1 + "a"  # error: [unsupported-operator]
+
+def f():
+    return
+
+    1 / 0  # error: [division-by-zero]
+```