Move CallPath into its own module (#3847)

crates/ruff_python_ast/src/call_path.rs

@@ -0,0 +1,63 @@
+use rustpython_parser::ast::{Expr, ExprKind};
+use smallvec::smallvec;
+
+/// A representation of a qualified name, like `typing.List`.
+pub type CallPath<'a> = smallvec::SmallVec<[&'a str; 8]>;
+
+fn collect_call_path_inner<'a>(expr: &'a Expr, parts: &mut CallPath<'a>) -> bool {
+    match &expr.node {
+        ExprKind::Attribute { value, attr, .. } => {
+            if collect_call_path_inner(value, parts) {
+                parts.push(attr);
+                true
+            } else {
+                false
+            }
+        }
+        ExprKind::Name { id, .. } => {
+            parts.push(id);
+            true
+        }
+        _ => false,
+    }
+}
+
+/// Convert an `Expr` to its [`CallPath`] segments (like `["typing", "List"]`).
+pub fn collect_call_path(expr: &Expr) -> CallPath {
+    let mut segments = smallvec![];
+    collect_call_path_inner(expr, &mut segments);
+    segments
+}
+
+/// Convert an `Expr` to its call path (like `List`, or `typing.List`).
+pub fn compose_call_path(expr: &Expr) -> Option<String> {
+    let call_path = collect_call_path(expr);
+    if call_path.is_empty() {
+        None
+    } else {
+        Some(format_call_path(&call_path))
+    }
+}
+
+/// Format a call path for display.
+pub fn format_call_path(call_path: &[&str]) -> String {
+    if call_path
+        .first()
+        .expect("Unable to format empty call path")
+        .is_empty()
+    {
+        call_path[1..].join(".")
+    } else {
+        call_path.join(".")
+    }
+}
+
+/// Split a fully-qualified name (like `typing.List`) into (`typing`, `List`).
+pub fn to_call_path(target: &str) -> CallPath {
+    if target.contains('.') {
+        target.split('.').collect()
+    } else {
+        // Special-case: for builtins, return `["", "int"]` instead of `["int"]`.
+        smallvec!["", target]
+    }
+}