[red-knot] add if-statement support to FlowGraph (#11673)

## Summary

Add if-statement support to FlowGraph. This introduces branches and
joins in the graph for the first time.

## Test Plan

Added tests.

crates/red_knot/src/symbols.rs

@@ -184,6 +184,7 @@ pub(crate) enum Definition {
     FunctionDef(TypedNodeKey<ast::StmtFunctionDef>),
     Assignment(TypedNodeKey<ast::StmtAssign>),
     AnnotatedAssignment(TypedNodeKey<ast::StmtAnnAssign>),
+    None,
     // TODO with statements, except handlers, function args...
 }
 
@@ -288,8 +289,8 @@ impl SymbolTable {
         let flow_node_id = self.flow_graph.ast_to_flow[&node_key];
         ReachableDefinitionsIterator {
             table: self,
-            flow_node_id,
             symbol_id,
+            pending: vec![flow_node_id],
         }
     }
 
@@ -545,25 +546,30 @@ where
 #[derive(Debug)]
 pub(crate) struct ReachableDefinitionsIterator<'a> {
     table: &'a SymbolTable,
-    flow_node_id: FlowNodeId,
     symbol_id: SymbolId,
+    pending: Vec<FlowNodeId>,
 }
 
 impl<'a> Iterator for ReachableDefinitionsIterator<'a> {
     type Item = Definition;
 
     fn next(&mut self) -> Option<Self::Item> {
         loop {
-            match &self.table.flow_graph.flow_nodes_by_id[self.flow_node_id] {
-                FlowNode::Start => return None,
+            let flow_node_id = self.pending.pop()?;
+            match &self.table.flow_graph.flow_nodes_by_id[flow_node_id] {
+                FlowNode::Start => return Some(Definition::None),
                 FlowNode::Definition(def_node) => {
                     if def_node.symbol_id == self.symbol_id {
-                        // we found a definition; previous definitions along this path are not
-                        // reachable
-                        self.flow_node_id = FlowGraph::start();
                         return Some(def_node.definition.clone());
                     }
-                    self.flow_node_id = def_node.predecessor;
+                    self.pending.push(def_node.predecessor);
+                }
+                FlowNode::Branch(branch_node) => {
+                    self.pending.push(branch_node.predecessor);
+                }
+                FlowNode::Phi(phi_node) => {
+                    self.pending.push(phi_node.first_predecessor);
+                    self.pending.push(phi_node.second_predecessor);
                 }
             }
         }
@@ -579,15 +585,31 @@ struct FlowNodeId;
 enum FlowNode {
     Start,
     Definition(DefinitionFlowNode),
+    Branch(BranchFlowNode),
+    Phi(PhiFlowNode),
 }
 
+/// A Definition node represents a point in control flow where a symbol is defined
 #[derive(Debug)]
 struct DefinitionFlowNode {
     symbol_id: SymbolId,
     definition: Definition,
     predecessor: FlowNodeId,
 }
 
+/// A Branch node represents a branch in control flow
+#[derive(Debug)]
+struct BranchFlowNode {
+    predecessor: FlowNodeId,
+}
+
+/// A Phi node represents a join point where control flow paths come together
+#[derive(Debug)]
+struct PhiFlowNode {
+    first_predecessor: FlowNodeId,
+    second_predecessor: FlowNodeId,
+}
+
 #[derive(Debug, Default)]
 struct FlowGraph {
     flow_nodes_by_id: IndexVec<FlowNodeId, FlowNode>,
@@ -636,6 +658,10 @@ impl SymbolTableBuilder {
             .add_or_update_symbol(self.cur_scope(), identifier, flags)
     }
 
+    fn new_flow_node(&mut self, node: FlowNode) -> FlowNodeId {
+        self.table.flow_graph.flow_nodes_by_id.push(node)
+    }
+
     fn add_or_update_symbol_with_def(
         &mut self,
         identifier: &str,
@@ -647,15 +673,11 @@ impl SymbolTableBuilder {
             .entry(symbol_id)
             .or_default()
             .push(definition.clone());
-        let new_flow_node_id = self
-            .table
-            .flow_graph
-            .flow_nodes_by_id
-            .push(FlowNode::Definition(DefinitionFlowNode {
-                definition,
-                symbol_id,
-                predecessor: self.current_flow_node(),
-            }));
+        let new_flow_node_id = self.new_flow_node(FlowNode::Definition(DefinitionFlowNode {
+            definition,
+            symbol_id,
+            predecessor: self.current_flow_node(),
+        }));
         self.set_current_flow_node(new_flow_node_id);
         symbol_id
     }
@@ -871,13 +893,127 @@ impl PreorderVisitor<'_> for SymbolTableBuilder {
                 ast::visitor::preorder::walk_stmt(self, stmt);
                 self.current_definition = None;
             }
+            ast::Stmt::If(node) => {
+                // we visit the if "test" condition first regardless
+                self.visit_expr(&node.test);
+
+                // create branch node: does the if test pass or not?
+                let if_branch = self.new_flow_node(FlowNode::Branch(BranchFlowNode {
+                    predecessor: self.current_flow_node(),
+                }));
+
+                // visit the body of the `if` clause
+                self.set_current_flow_node(if_branch);
+                self.visit_body(&node.body);
+
+                // Flow node for the last if/elif condition branch; represents the "no branch
+                // taken yet" possibility (where "taking a branch" means that the condition in an
+                // if or elif evaluated to true and control flow went into that clause).
+                let mut prior_branch = if_branch;
+
+                // Flow node for the state after the prior if/elif/else clause; represents "we have
+                // taken one of the branches up to this point." Initially set to the post-if-clause
+                // state, later will be set to the phi node joining that possible path with the
+                // possibility that we took a later if/elif/else clause instead.
+                let mut post_prior_clause = self.current_flow_node();
+
+                // Flag to mark if the final clause is an "else" -- if so, that means the "match no
+                // clauses" path is not possible, we have to go through one of the clauses.
+                let mut last_branch_is_else = false;
+
+                for clause in &node.elif_else_clauses {
+                    if clause.test.is_some() {
+                        // This is an elif clause. Create a new branch node. Its predecessor is the
+                        // previous branch node, because we can only take one branch in an entire
+                        // if/elif/else chain, so if we take this branch, it can only be because we
+                        // didn't take the previous one.
+                        prior_branch = self.new_flow_node(FlowNode::Branch(BranchFlowNode {
+                            predecessor: prior_branch,
+                        }));
+                        self.set_current_flow_node(prior_branch);
+                    } else {
+                        // This is an else clause. No need to create a branch node; there's no
+                        // branch here, if we haven't taken any previous branch, we definitely go
+                        // into the "else" clause.
+                        self.set_current_flow_node(prior_branch);
+                        last_branch_is_else = true;
+                    }
+                    self.visit_elif_else_clause(clause);
+                    // Update `post_prior_clause` to a new phi node joining the possibility that we
+                    // took any of the previous branches with the possibility that we took the one
+                    // just visited.
+                    post_prior_clause = self.new_flow_node(FlowNode::Phi(PhiFlowNode {
+                        first_predecessor: self.current_flow_node(),
+                        second_predecessor: post_prior_clause,
+                    }));
+                }
+
+                if !last_branch_is_else {
+                    // Final branch was not an "else", which means it's possible we took zero
+                    // branches in the entire if/elif chain, so we need one more phi node to join
+                    // the "no branches taken" possibility.
+                    post_prior_clause = self.new_flow_node(FlowNode::Phi(PhiFlowNode {
+                        first_predecessor: post_prior_clause,
+                        second_predecessor: prior_branch,
+                    }));
+                }
+
+                // Onward, with current flow node set to our final Phi node.
+                self.set_current_flow_node(post_prior_clause);
+            }
             _ => {
                 ast::visitor::preorder::walk_stmt(self, stmt);
             }
         }
     }
 }
 
+impl std::fmt::Display for FlowGraph {
+    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+        writeln!(f, "flowchart TD")?;
+        for (id, node) in self.flow_nodes_by_id.iter_enumerated() {
+            write!(f, "  id{}", id.as_u32())?;
+            match node {
+                FlowNode::Start => writeln!(f, r"[\Start/]")?,
+                FlowNode::Definition(def_node) => {
+                    writeln!(f, r"(Define symbol {})", def_node.symbol_id.as_u32())?;
+                    writeln!(
+                        f,
+                        r"  id{}-->id{}",
+                        def_node.predecessor.as_u32(),
+                        id.as_u32()
+                    )?;
+                }
+                FlowNode::Branch(branch_node) => {
+                    writeln!(f, r"{{Branch}}")?;
+                    writeln!(
+                        f,
+                        r"  id{}-->id{}",
+                        branch_node.predecessor.as_u32(),
+                        id.as_u32()
+                    )?;
+                }
+                FlowNode::Phi(phi_node) => {
+                    writeln!(f, r"((Phi))")?;
+                    writeln!(
+                        f,
+                        r"  id{}-->id{}",
+                        phi_node.second_predecessor.as_u32(),
+                        id.as_u32()
+                    )?;
+                    writeln!(
+                        f,
+                        r"  id{}-->id{}",
+                        phi_node.first_predecessor.as_u32(),
+                        id.as_u32()
+                    )?;
+                }
+            }
+        }
+        Ok(())
+    }
+}
+
 #[derive(Debug, Default)]
 pub struct SymbolTablesStorage(KeyValueCache<FileId, Arc<SymbolTable>>);
 

crates/red_knot/src/types/infer.rs

@@ -79,6 +79,7 @@ pub fn infer_definition_type(
     let file_id = symbol.file_id;
 
     match definition {
+        Definition::None => Ok(Type::Unbound),
         Definition::Import(ImportDefinition {
             module: module_name,
         }) => {
@@ -223,7 +224,7 @@ mod tests {
     use crate::module::{
         resolve_module, set_module_search_paths, ModuleName, ModuleSearchPath, ModuleSearchPathKind,
     };
-    use crate::symbols::{resolve_global_symbol, symbol_table, GlobalSymbolId};
+    use crate::symbols::resolve_global_symbol;
     use crate::types::{infer_symbol_public_type, Type};
     use crate::Name;
 
@@ -399,30 +400,93 @@ mod tests {
     #[test]
     fn resolve_visible_def() -> anyhow::Result<()> {
         let case = create_test()?;
-        let db = &case.db;
 
-        let path = case.src.path().join("a.py");
-        std::fs::write(path, "y = 1; y = 2; x = y")?;
-        let file = resolve_module(db, ModuleName::new("a"))?
-            .expect("module should be found")
-            .path(db)?
-            .file();
-        let symbols = symbol_table(db, file)?;
-        let x_sym = symbols
-            .root_symbol_id_by_name("x")
-            .expect("x symbol should be found");
-
-        let ty = infer_symbol_public_type(
-            db,
-            GlobalSymbolId {
-                file_id: file,
-                symbol_id: x_sym,
-            },
+        write_to_path(&case, "a.py", "y = 1; y = 2; x = y")?;
+
+        assert_public_type(&case, "a", "x", "Literal[2]")
+    }
+
+    #[test]
+    fn join_paths() -> anyhow::Result<()> {
+        let case = create_test()?;
+
+        write_to_path(
+            &case,
+            "a.py",
+            "
+                y = 1
+                y = 2
+                if flag:
+                    y = 3
+                x = y
+            ",
         )?;
 
-        let jar = HasJar::<SemanticJar>::jar(db)?;
-        assert!(matches!(ty, Type::IntLiteral(_)));
-        assert_eq!(format!("{}", ty.display(&jar.type_store)), "Literal[2]");
-        Ok(())
+        assert_public_type(&case, "a", "x", "(Literal[2] | Literal[3])")
+    }
+
+    #[test]
+    fn maybe_unbound() -> anyhow::Result<()> {
+        let case = create_test()?;
+
+        write_to_path(
+            &case,
+            "a.py",
+            "
+                if flag:
+                    y = 1
+                x = y
+            ",
+        )?;
+
+        assert_public_type(&case, "a", "x", "(Unbound | Literal[1])")
+    }
+
+    #[test]
+    fn if_elif_else() -> anyhow::Result<()> {
+        let case = create_test()?;
+
+        write_to_path(
+            &case,
+            "a.py",
+            "
+                y = 1
+                y = 2
+                if flag:
+                    y = 3
+                elif flag2:
+                    y = 4
+                else:
+                    r = y
+                    y = 5
+                    s = y
+                x = y
+            ",
+        )?;
+
+        assert_public_type(&case, "a", "x", "(Literal[3] | Literal[4] | Literal[5])")?;
+        assert_public_type(&case, "a", "r", "Literal[2]")?;
+        assert_public_type(&case, "a", "s", "Literal[5]")
+    }
+
+    #[test]
+    fn if_elif() -> anyhow::Result<()> {
+        let case = create_test()?;
+
+        write_to_path(
+            &case,
+            "a.py",
+            "
+                y = 1
+                y = 2
+                if flag:
+                    y = 3
+                elif flag2:
+                    y = 4
+                x = y
+            ",
+        )?;
+
+        assert_public_type(&case, "a", "x", "(Literal[2] | Literal[3] | Literal[4])")
     }
 }