Restore existing bindings when unbinding caught exceptions (#5256)

## Summary

In the latest release, we made some improvements to the semantic model,
but our modifications to exception-unbinding are causing some
false-positives. For example:

```py
try:
    v = 3
except ImportError as v:
    print(v)
else:
    print(v)
```

In the latest release, we started unbinding `v` after the `except`
handler. (We used to restore the existing binding, the `v = 3`, but this
was quite complicated.) Because we don't have full branch analysis, we
can't then know that `v` is still bound in the `else` branch.

The solution here modifies `resolve_read` to skip-lookup when hitting
unbound exceptions. So when store the "unbind" for `except ImportError
as v`, we save the binding that it shadowed `v = 3`, and skip to that.

Closes #5249.

Closes #5250.

Author: charliermarsh

crates/ruff/src/checkers/ast/mod.rs

@@ -3852,6 +3852,9 @@ where
                             );
                         }
 
+                        // Store the existing binding, if any.
+                        let existing_id = self.semantic.lookup(name);
+
                         // Add the bound exception name to the scope.
                         let binding_id = self.add_binding(
                             name,
@@ -3862,14 +3865,6 @@ where
 
                         walk_except_handler(self, except_handler);
 
-                        // Remove it from the scope immediately after.
-                        self.add_binding(
-                            name,
-                            range,
-                            BindingKind::UnboundException,
-                            BindingFlags::empty(),
-                        );
-
                         // If the exception name wasn't used in the scope, emit a diagnostic.
                         if !self.semantic.is_used(binding_id) {
                             if self.enabled(Rule::UnusedVariable) {
@@ -3889,6 +3884,13 @@ where
                                 self.diagnostics.push(diagnostic);
                             }
                         }
+
+                        self.add_binding(
+                            name,
+                            range,
+                            BindingKind::UnboundException(existing_id),
+                            BindingFlags::empty(),
+                        );
                     }
                     None => walk_except_handler(self, except_handler),
                 }
@@ -4236,7 +4238,7 @@ impl<'a> Checker<'a> {
             let shadowed = &self.semantic.bindings[shadowed_id];
             if !matches!(
                 shadowed.kind,
-                BindingKind::Builtin | BindingKind::Deletion | BindingKind::UnboundException,
+                BindingKind::Builtin | BindingKind::Deletion | BindingKind::UnboundException(_),
             ) {
                 let references = shadowed.references.clone();
                 let is_global = shadowed.is_global();

crates/ruff/src/renamer.rs

@@ -251,7 +251,7 @@ impl Renamer {
             | BindingKind::ClassDefinition
             | BindingKind::FunctionDefinition
             | BindingKind::Deletion
-            | BindingKind::UnboundException => {
+            | BindingKind::UnboundException(_) => {
                 Some(Edit::range_replacement(target.to_string(), binding.range))
             }
         }

crates/ruff/src/rules/pyflakes/mod.rs

@@ -353,9 +353,59 @@ mod tests {
             except Exception as x:
                 pass
 
+            # No error here, though it should arguably be an F821 error. `x` will
+            # be unbound after the `except` block (assuming an exception is raised
+            # and caught).
             print(x)
             "#,
-        "print_after_shadowing_except"
+        "print_in_body_after_shadowing_except"
+    )]
+    #[test_case(
+        r#"
+        def f():
+            x = 1
+
+            try:
+                1 / 0
+            except ValueError as x:
+                pass
+            except ImportError as x:
+                pass
+
+            # No error here, though it should arguably be an F821 error. `x` will
+            # be unbound after the `except` block (assuming an exception is raised
+            # and caught).
+            print(x)
+            "#,
+        "print_in_body_after_double_shadowing_except"
+    )]
+    #[test_case(
+        r#"
+        def f():
+            try:
+                x = 3
+            except ImportError as x:
+                print(x)
+            else:
+                print(x)
+            "#,
+        "print_in_try_else_after_shadowing_except"
+    )]
+    #[test_case(
+        r#"
+        def f():
+            list = [1, 2, 3]
+
+            for e in list:
+                if e % 2 == 0:
+                    try:
+                        pass
+                    except Exception as e:
+                        print(e)
+                else:
+                    print(e)
+            "#,
+        "print_in_if_else_after_shadowing_except"
     )]
     #[test_case(
         r#"
@@ -366,6 +416,79 @@ mod tests {
             "#,
         "double_del"
     )]
+    #[test_case(
+        r#"
+        x = 1
+
+        def f():
+            try:
+                pass
+            except ValueError as x:
+                pass
+
+            # This should resolve to the `x` in `x = 1`.
+            print(x)
+            "#,
+        "load_after_unbind_from_module_scope"
+    )]
+    #[test_case(
+        r#"
+        x = 1
+
+        def f():
+            try:
+                pass
+            except ValueError as x:
+                pass
+
+            try:
+                pass
+            except ValueError as x:
+                pass
+
+            # This should resolve to the `x` in `x = 1`.
+            print(x)
+            "#,
+        "load_after_multiple_unbinds_from_module_scope"
+    )]
+    #[test_case(
+        r#"
+        x = 1
+
+        def f():
+            try:
+                pass
+            except ValueError as x:
+                pass
+
+            def g():
+                try:
+                    pass
+                except ValueError as x:
+                    pass
+
+                # This should resolve to the `x` in `x = 1`.
+                print(x)
+            "#,
+        "load_after_unbind_from_nested_module_scope"
+    )]
+    #[test_case(
+        r#"
+        class C:
+            x = 1
+
+            def f():
+                try:
+                    pass
+                except ValueError as x:
+                    pass
+
+                # This should raise an F821 error, rather than resolving to the
+                # `x` in `x = 1`.
+                print(x)
+            "#,
+        "load_after_unbind_from_class_scope"
+    )]
     fn contents(contents: &str, snapshot: &str) {
         let diagnostics = test_snippet(contents, &Settings::for_rules(&Linter::Pyflakes));
         assert_messages!(snapshot, diagnostics);

crates/ruff/src/rules/pyflakes/snapshots/ruff__rules__pyflakes__tests__load_after_multiple_unbinds_from_module_scope.snap

@@ -0,0 +1,44 @@
+---
+source: crates/ruff/src/rules/pyflakes/mod.rs
+---
+<filename>:7:26: F841 [*] Local variable `x` is assigned to but never used
+  |
+5 |     try:
+6 |         pass
+7 |     except ValueError as x:
+  |                          ^ F841
+8 |         pass
+  |
+  = help: Remove assignment to unused variable `x`
+
+ℹ Fix
+4 4 | def f():
+5 5 |     try:
+6 6 |         pass
+7   |-    except ValueError as x:
+  7 |+    except ValueError:
+8 8 |         pass
+9 9 | 
+10 10 |     try:
+
+<filename>:12:26: F841 [*] Local variable `x` is assigned to but never used
+   |
+10 |     try:
+11 |         pass
+12 |     except ValueError as x:
+   |                          ^ F841
+13 |         pass
+   |
+   = help: Remove assignment to unused variable `x`
+
+ℹ Fix
+9  9  | 
+10 10 |     try:
+11 11 |         pass
+12    |-    except ValueError as x:
+   12 |+    except ValueError:
+13 13 |         pass
+14 14 | 
+15 15 |     # This should resolve to the `x` in `x = 1`.
+
+

crates/ruff/src/rules/pyflakes/snapshots/ruff__rules__pyflakes__tests__load_after_unbind_from_class_scope.snap

@@ -0,0 +1,32 @@
+---
+source: crates/ruff/src/rules/pyflakes/mod.rs
+---
+<filename>:8:30: F841 [*] Local variable `x` is assigned to but never used
+  |
+6 |         try:
+7 |             pass
+8 |         except ValueError as x:
+  |                              ^ F841
+9 |             pass
+  |
+  = help: Remove assignment to unused variable `x`
+
+ℹ Fix
+5 5 |     def f():
+6 6 |         try:
+7 7 |             pass
+8   |-        except ValueError as x:
+  8 |+        except ValueError:
+9 9 |             pass
+10 10 | 
+11 11 |         # This should raise an F821 error, rather than resolving to the
+
+<filename>:13:15: F821 Undefined name `x`
+   |
+11 |         # This should raise an F821 error, rather than resolving to the
+12 |         # `x` in `x = 1`.
+13 |         print(x)
+   |               ^ F821
+   |
+
+

crates/ruff/src/rules/pyflakes/snapshots/ruff__rules__pyflakes__tests__load_after_unbind_from_module_scope.snap

@@ -0,0 +1,24 @@
+---
+source: crates/ruff/src/rules/pyflakes/mod.rs
+---
+<filename>:7:26: F841 [*] Local variable `x` is assigned to but never used
+  |
+5 |     try:
+6 |         pass
+7 |     except ValueError as x:
+  |                          ^ F841
+8 |         pass
+  |
+  = help: Remove assignment to unused variable `x`
+
+ℹ Fix
+4 4 | def f():
+5 5 |     try:
+6 6 |         pass
+7   |-    except ValueError as x:
+  7 |+    except ValueError:
+8 8 |         pass
+9 9 | 
+10 10 |     # This should resolve to the `x` in `x = 1`.
+
+

crates/ruff/src/rules/pyflakes/snapshots/ruff__rules__pyflakes__tests__load_after_unbind_from_nested_module_scope.snap

@@ -0,0 +1,44 @@
+---
+source: crates/ruff/src/rules/pyflakes/mod.rs
+---
+<filename>:7:26: F841 [*] Local variable `x` is assigned to but never used
+  |
+5 |     try:
+6 |         pass
+7 |     except ValueError as x:
+  |                          ^ F841
+8 |         pass
+  |
+  = help: Remove assignment to unused variable `x`
+
+ℹ Fix
+4 4 | def f():
+5 5 |     try:
+6 6 |         pass
+7   |-    except ValueError as x:
+  7 |+    except ValueError:
+8 8 |         pass
+9 9 | 
+10 10 |     def g():
+
+<filename>:13:30: F841 [*] Local variable `x` is assigned to but never used
+   |
+11 |         try:
+12 |             pass
+13 |         except ValueError as x:
+   |                              ^ F841
+14 |             pass
+   |
+   = help: Remove assignment to unused variable `x`
+
+ℹ Fix
+10 10 |     def g():
+11 11 |         try:
+12 12 |             pass
+13    |-        except ValueError as x:
+   13 |+        except ValueError:
+14 14 |             pass
+15 15 | 
+16 16 |         # This should resolve to the `x` in `x = 1`.
+
+

crates/ruff/src/rules/pyflakes/snapshots/ruff__rules__pyflakes__tests__print_in_body_after_double_shadowing_except.snap

@@ -0,0 +1,45 @@
+---
+source: crates/ruff/src/rules/pyflakes/mod.rs
+---
+<filename>:7:26: F841 [*] Local variable `x` is assigned to but never used
+  |
+5 |     try:
+6 |         1 / 0
+7 |     except ValueError as x:
+  |                          ^ F841
+8 |         pass
+9 |     except ImportError as x:
+  |
+  = help: Remove assignment to unused variable `x`
+
+ℹ Fix
+4 4 | 
+5 5 |     try:
+6 6 |         1 / 0
+7   |-    except ValueError as x:
+  7 |+    except ValueError:
+8 8 |         pass
+9 9 |     except ImportError as x:
+10 10 |         pass
+
+<filename>:9:27: F841 [*] Local variable `x` is assigned to but never used
+   |
+ 7 |     except ValueError as x:
+ 8 |         pass
+ 9 |     except ImportError as x:
+   |                           ^ F841
+10 |         pass
+   |
+   = help: Remove assignment to unused variable `x`
+
+ℹ Fix
+6  6  |         1 / 0
+7  7  |     except ValueError as x:
+8  8  |         pass
+9     |-    except ImportError as x:
+   9  |+    except ImportError:
+10 10 |         pass
+11 11 | 
+12 12 |     # No error here, though it should arguably be an F821 error. `x` will
+
+