[red-knot] Add support for unpacking for target

Author: dhruvmanila

crates/red_knot_python_semantic/resources/mdtest/unpacking.md

@@ -472,3 +472,85 @@ def _(arg: tuple[int, str] | Iterable):
     reveal_type(a)  # revealed: int | bytes
     reveal_type(b)  # revealed: str | bytes
 ```
+
+## For statement
+
+### Same types
+
+```py
+def _(arg: tuple[tuple[int, int], tuple[int, int]]):
+    for a, b in arg:
+        reveal_type(a)  # revealed: int
+        reveal_type(b)  # revealed: int
+```
+
+### Mixed types (1)
+
+```py
+def _(arg: tuple[tuple[int, int], tuple[int, str]]):
+    for a, b in arg:
+        reveal_type(a)  # revealed: int
+        reveal_type(b)  # revealed: int | str
+```
+
+### Mixed types (2)
+
+```py
+def _(arg: tuple[tuple[int, str], tuple[str, int]]):
+    for a, b in arg:
+        reveal_type(a)  # revealed: int | str
+        reveal_type(b)  # revealed: str | int
+```
+
+### Mixed types (3)
+
+```py
+def _(arg: tuple[tuple[int, int, int], tuple[int, str, bytes], tuple[int, int, str]]):
+    for a, b, c in arg:
+        reveal_type(a)  # revealed: int
+        reveal_type(b)  # revealed: int | str
+        reveal_type(c)  # revealed: int | bytes | str
+```
+
+### Same literal types
+
+```py
+for a, b in ((1, 2), (3, 4)):
+    reveal_type(a)  # revealed: Literal[1, 3]
+    reveal_type(b)  # revealed: Literal[2, 4]
+```
+
+### Mixed literal types
+
+```py
+for a, b in ((1, 2), ("a", "b")):
+    reveal_type(a)  # revealed: Literal[1] | Literal["a"]
+    reveal_type(b)  # revealed: Literal[2] | Literal["b"]
+```
+
+### Mixed literals (2)
+
+```py
+# error: "Object of type `Literal[1]` is not iterable"
+# error: "Object of type `Literal[2]` is not iterable"
+# error: "Object of type `Literal[4]` is not iterable"
+for a, b in (1, 2, (3, "a"), 4, (5, "b"), "c"):
+    reveal_type(a)  # revealed: Unknown | Literal[3, 5] | LiteralString
+    reveal_type(b)  # revealed: Unknown | Literal["a", "b"]
+```
+
+### Custom iterator
+
+```py
+class Iterator:
+    def __next__(self) -> tuple[int, int]:
+        return (1, 2)
+
+class Iterable:
+    def __iter__(self) -> Iterator:
+        return Iterator()
+
+for a, b in Iterable():
+    reveal_type(a)  # revealed: int
+    reveal_type(b)  # revealed: int
+```

crates/red_knot_python_semantic/src/semantic_index/builder.rs

@@ -26,7 +26,7 @@ use crate::semantic_index::symbol::{
 };
 use crate::semantic_index::use_def::{FlowSnapshot, UseDefMapBuilder};
 use crate::semantic_index::SemanticIndex;
-use crate::unpack::Unpack;
+use crate::unpack::{Unpack, UnpackValue};
 use crate::Db;
 
 use super::constraint::{Constraint, ConstraintNode, PatternConstraint};
@@ -726,7 +726,7 @@ where
                                     unsafe {
                                         AstNodeRef::new(self.module.clone(), target)
                                     },
-                                    value,
+                                    UnpackValue::Assign(value),
                                     countme::Count::default(),
                                 )),
                             })
@@ -909,16 +909,45 @@ where
                     orelse,
                 },
             ) => {
-                self.add_standalone_expression(iter);
+                debug_assert_eq!(&self.current_assignments, &[]);
+
+                let iter_expr = self.add_standalone_expression(iter);
                 self.visit_expr(iter);
 
                 let pre_loop = self.flow_snapshot();
                 let saved_break_states = std::mem::take(&mut self.loop_break_states);
 
-                debug_assert_eq!(&self.current_assignments, &[]);
-                self.push_assignment(for_stmt.into());
+                let current_assignment = match &**target {
+                    ast::Expr::List(_) | ast::Expr::Tuple(_) => Some(CurrentAssignment::For {
+                        node: for_stmt,
+                        first: true,
+                        unpack: Some(Unpack::new(
+                            self.db,
+                            self.file,
+                            self.current_scope(),
+                            #[allow(unsafe_code)]
+                            unsafe {
+                                AstNodeRef::new(self.module.clone(), target)
+                            },
+                            UnpackValue::Iterable(iter_expr),
+                            countme::Count::default(),
+                        )),
+                    }),
+                    ast::Expr::Name(_) => Some(CurrentAssignment::For {
+                        node: for_stmt,
+                        unpack: None,
+                        first: false,
+                    }),
+                    _ => None,
+                };
+
+                if let Some(current_assignment) = current_assignment {
+                    self.push_assignment(current_assignment);
+                }
                 self.visit_expr(target);
-                self.pop_assignment();
+                if current_assignment.is_some() {
+                    self.pop_assignment();
+                }
 
                 // TODO: Definitions created by loop variables
                 // (and definitions created inside the body)
@@ -1136,12 +1165,18 @@ where
                         Some(CurrentAssignment::AugAssign(aug_assign)) => {
                             self.add_definition(symbol, aug_assign);
                         }
-                        Some(CurrentAssignment::For(node)) => {
+                        Some(CurrentAssignment::For {
+                            node,
+                            first,
+                            unpack,
+                        }) => {
                             self.add_definition(
                                 symbol,
                                 ForStmtDefinitionNodeRef {
+                                    unpack,
+                                    first,
                                     iterable: &node.iter,
-                                    target: name_node,
+                                    name: name_node,
                                     is_async: node.is_async,
                                 },
                             );
@@ -1177,7 +1212,9 @@ where
                     }
                 }
 
-                if let Some(CurrentAssignment::Assign { first, .. }) = self.current_assignment_mut()
+                if let Some(
+                    CurrentAssignment::Assign { first, .. } | CurrentAssignment::For { first, .. },
+                ) = self.current_assignment_mut()
                 {
                     *first = false;
                 }
@@ -1391,7 +1428,11 @@ enum CurrentAssignment<'a> {
     },
     AnnAssign(&'a ast::StmtAnnAssign),
     AugAssign(&'a ast::StmtAugAssign),
-    For(&'a ast::StmtFor),
+    For {
+        node: &'a ast::StmtFor,
+        first: bool,
+        unpack: Option<Unpack<'a>>,
+    },
     Named(&'a ast::ExprNamed),
     Comprehension {
         node: &'a ast::Comprehension,
@@ -1415,12 +1456,6 @@ impl<'a> From<&'a ast::StmtAugAssign> for CurrentAssignment<'a> {
     }
 }
 
-impl<'a> From<&'a ast::StmtFor> for CurrentAssignment<'a> {
-    fn from(value: &'a ast::StmtFor) -> Self {
-        Self::For(value)
-    }
-}
-
 impl<'a> From<&'a ast::ExprNamed> for CurrentAssignment<'a> {
     fn from(value: &'a ast::ExprNamed) -> Self {
         Self::Named(value)

crates/red_knot_python_semantic/src/semantic_index/definition.rs

@@ -225,8 +225,10 @@ pub(crate) struct WithItemDefinitionNodeRef<'a> {
 
 #[derive(Copy, Clone, Debug)]
 pub(crate) struct ForStmtDefinitionNodeRef<'a> {
+    pub(crate) unpack: Option<Unpack<'a>>,
     pub(crate) iterable: &'a ast::Expr,
-    pub(crate) target: &'a ast::ExprName,
+    pub(crate) name: &'a ast::ExprName,
+    pub(crate) first: bool,
     pub(crate) is_async: bool,
 }
 
@@ -298,12 +300,16 @@ impl<'db> DefinitionNodeRef<'db> {
                 DefinitionKind::AugmentedAssignment(AstNodeRef::new(parsed, augmented_assignment))
             }
             DefinitionNodeRef::For(ForStmtDefinitionNodeRef {
+                unpack,
                 iterable,
-                target,
+                name,
+                first,
                 is_async,
             }) => DefinitionKind::For(ForStmtDefinitionKind {
+                target: TargetKind::from(unpack),
                 iterable: AstNodeRef::new(parsed.clone(), iterable),
-                target: AstNodeRef::new(parsed, target),
+                name: AstNodeRef::new(parsed, name),
+                first,
                 is_async,
             }),
             DefinitionNodeRef::Comprehension(ComprehensionDefinitionNodeRef {
@@ -382,10 +388,12 @@ impl<'db> DefinitionNodeRef<'db> {
             Self::AnnotatedAssignment(node) => node.into(),
             Self::AugmentedAssignment(node) => node.into(),
             Self::For(ForStmtDefinitionNodeRef {
+                unpack: _,
                 iterable: _,
-                target,
+                name,
+                first: _,
                 is_async: _,
-            }) => target.into(),
+            }) => name.into(),
             Self::Comprehension(ComprehensionDefinitionNodeRef { target, .. }) => target.into(),
             Self::VariadicPositionalParameter(node) => node.into(),
             Self::VariadicKeywordParameter(node) => node.into(),
@@ -452,7 +460,7 @@ pub enum DefinitionKind<'db> {
     Assignment(AssignmentDefinitionKind<'db>),
     AnnotatedAssignment(AstNodeRef<ast::StmtAnnAssign>),
     AugmentedAssignment(AstNodeRef<ast::StmtAugAssign>),
-    For(ForStmtDefinitionKind),
+    For(ForStmtDefinitionKind<'db>),
     Comprehension(ComprehensionDefinitionKind),
     VariadicPositionalParameter(AstNodeRef<ast::Parameter>),
     VariadicKeywordParameter(AstNodeRef<ast::Parameter>),
@@ -477,7 +485,7 @@ impl Ranged for DefinitionKind<'_> {
             DefinitionKind::Assignment(assignment) => assignment.name().range(),
             DefinitionKind::AnnotatedAssignment(assign) => assign.target.range(),
             DefinitionKind::AugmentedAssignment(aug_assign) => aug_assign.target.range(),
-            DefinitionKind::For(for_stmt) => for_stmt.target().range(),
+            DefinitionKind::For(for_stmt) => for_stmt.name().range(),
             DefinitionKind::Comprehension(comp) => comp.target().range(),
             DefinitionKind::VariadicPositionalParameter(parameter) => parameter.name.range(),
             DefinitionKind::VariadicKeywordParameter(parameter) => parameter.name.range(),
@@ -665,22 +673,32 @@ impl WithItemDefinitionKind {
 }
 
 #[derive(Clone, Debug)]
-pub struct ForStmtDefinitionKind {
+pub struct ForStmtDefinitionKind<'db> {
+    target: TargetKind<'db>,
     iterable: AstNodeRef<ast::Expr>,
-    target: AstNodeRef<ast::ExprName>,
+    name: AstNodeRef<ast::ExprName>,
+    first: bool,
     is_async: bool,
 }
 
-impl ForStmtDefinitionKind {
+impl<'db> ForStmtDefinitionKind<'db> {
     pub(crate) fn iterable(&self) -> &ast::Expr {
         self.iterable.node()
     }
 
-    pub(crate) fn target(&self) -> &ast::ExprName {
-        self.target.node()
+    pub(crate) fn target(&self) -> TargetKind<'db> {
+        self.target
     }
 
-    pub(crate) fn is_async(&self) -> bool {
+    pub(crate) fn name(&self) -> &ast::ExprName {
+        self.name.node()
+    }
+
+    pub(crate) const fn is_first(&self) -> bool {
+        self.first
+    }
+
+    pub(crate) const fn is_async(&self) -> bool {
         self.is_async
     }
 }
@@ -756,12 +774,6 @@ impl From<&ast::StmtAugAssign> for DefinitionNodeKey {
     }
 }
 
-impl From<&ast::StmtFor> for DefinitionNodeKey {
-    fn from(value: &ast::StmtFor) -> Self {
-        Self(NodeKey::from_node(value))
-    }
-}
-
 impl From<&ast::Parameter> for DefinitionNodeKey {
     fn from(node: &ast::Parameter) -> Self {
         Self(NodeKey::from_node(node))