Create a rust_python_ast crate (#3370)

This PR productionizes @MichaReiser's suggestion in https://github.com/charliermarsh/ruff/issues/1820#issuecomment-1440204423, by creating a separate crate for the `ast` module (`rust_python_ast`). This will enable us to further split up the `ruff` crate, as we'll be able to create (e.g.) separate sub-linter crates that have access to these common AST utilities.

This was mostly a straightforward copy (with adjustments to module imports), as the few dependencies that _did_ require modifications were handled in #3366, #3367, and #3368.

crates/ruff_python_ast/src/branch_detection.rs

@@ -0,0 +1,110 @@
+use std::cmp::Ordering;
+
+use rustc_hash::FxHashMap;
+use rustpython_parser::ast::ExcepthandlerKind::ExceptHandler;
+use rustpython_parser::ast::{Stmt, StmtKind};
+
+use crate::types::RefEquality;
+
+/// Return the common ancestor of `left` and `right` below `stop`, or `None`.
+fn common_ancestor<'a>(
+    left: &'a RefEquality<'a, Stmt>,
+    right: &'a RefEquality<'a, Stmt>,
+    stop: Option<&'a RefEquality<'a, Stmt>>,
+    depths: &'a FxHashMap<RefEquality<'a, Stmt>, usize>,
+    child_to_parent: &'a FxHashMap<RefEquality<'a, Stmt>, RefEquality<'a, Stmt>>,
+) -> Option<&'a RefEquality<'a, Stmt>> {
+    if let Some(stop) = stop {
+        if left == stop || right == stop {
+            return None;
+        }
+    }
+    if left == right {
+        return Some(left);
+    }
+
+    let left_depth = depths.get(left)?;
+    let right_depth = depths.get(right)?;
+    match left_depth.cmp(right_depth) {
+        Ordering::Less => common_ancestor(
+            left,
+            child_to_parent.get(right)?,
+            stop,
+            depths,
+            child_to_parent,
+        ),
+        Ordering::Equal => common_ancestor(
+            child_to_parent.get(left)?,
+            child_to_parent.get(right)?,
+            stop,
+            depths,
+            child_to_parent,
+        ),
+        Ordering::Greater => common_ancestor(
+            child_to_parent.get(left)?,
+            right,
+            stop,
+            depths,
+            child_to_parent,
+        ),
+    }
+}
+
+/// Return the alternative branches for a given node.
+fn alternatives<'a>(stmt: &'a RefEquality<'a, Stmt>) -> Vec<Vec<RefEquality<'a, Stmt>>> {
+    match &stmt.node {
+        StmtKind::If { body, .. } => vec![body.iter().map(RefEquality).collect()],
+        StmtKind::Try {
+            body,
+            handlers,
+            orelse,
+            ..
+        }
+        | StmtKind::TryStar {
+            body,
+            handlers,
+            orelse,
+            ..
+        } => vec![body.iter().chain(orelse.iter()).map(RefEquality).collect()]
+            .into_iter()
+            .chain(handlers.iter().map(|handler| {
+                let ExceptHandler { body, .. } = &handler.node;
+                body.iter().map(RefEquality).collect()
+            }))
+            .collect(),
+        _ => vec![],
+    }
+}
+
+/// Return `true` if `stmt` is a descendent of any of the nodes in `ancestors`.
+fn descendant_of<'a>(
+    stmt: &RefEquality<'a, Stmt>,
+    ancestors: &[RefEquality<'a, Stmt>],
+    stop: &RefEquality<'a, Stmt>,
+    depths: &FxHashMap<RefEquality<'a, Stmt>, usize>,
+    child_to_parent: &FxHashMap<RefEquality<'a, Stmt>, RefEquality<'a, Stmt>>,
+) -> bool {
+    ancestors.iter().any(|ancestor| {
+        common_ancestor(stmt, ancestor, Some(stop), depths, child_to_parent).is_some()
+    })
+}
+
+/// Return `true` if `left` and `right` are on different branches of an `if` or
+/// `try` statement.
+pub fn different_forks<'a>(
+    left: &RefEquality<'a, Stmt>,
+    right: &RefEquality<'a, Stmt>,
+    depths: &FxHashMap<RefEquality<'a, Stmt>, usize>,
+    child_to_parent: &FxHashMap<RefEquality<'a, Stmt>, RefEquality<'a, Stmt>>,
+) -> bool {
+    if let Some(ancestor) = common_ancestor(left, right, None, depths, child_to_parent) {
+        for items in alternatives(ancestor) {
+            let l = descendant_of(left, &items, ancestor, depths, child_to_parent);
+            let r = descendant_of(right, &items, ancestor, depths, child_to_parent);
+            if l ^ r {
+                return true;
+            }
+        }
+    }
+    false
+}