refactor: Introduce crates folder (#2088)

This PR introduces a new `crates` directory and moves all "product" crates into that folder. 

Part of #2059.

crates/ruff/src/docstrings/extraction.rs

@@ -0,0 +1,82 @@
+//! Extract docstrings from an AST.
+
+use rustpython_ast::{Constant, Expr, ExprKind, Stmt, StmtKind};
+
+use crate::docstrings::definition::{Definition, DefinitionKind, Documentable};
+use crate::visibility::{Modifier, VisibleScope};
+
+/// Extract a docstring from a function or class body.
+pub fn docstring_from(suite: &[Stmt]) -> Option<&Expr> {
+    let stmt = suite.first()?;
+    let StmtKind::Expr { value } = &stmt.node else {
+        return None;
+    };
+    if !matches!(
+        &value.node,
+        ExprKind::Constant {
+            value: Constant::Str(_),
+            ..
+        }
+    ) {
+        return None;
+    }
+    Some(value)
+}
+
+/// Extract a `Definition` from the AST node defined by a `Stmt`.
+pub fn extract<'a>(
+    scope: &VisibleScope,
+    stmt: &'a Stmt,
+    body: &'a [Stmt],
+    kind: &Documentable,
+) -> Definition<'a> {
+    let expr = docstring_from(body);
+    match kind {
+        Documentable::Function => match scope {
+            VisibleScope {
+                modifier: Modifier::Module,
+                ..
+            } => Definition {
+                kind: DefinitionKind::Function(stmt),
+                docstring: expr,
+            },
+            VisibleScope {
+                modifier: Modifier::Class,
+                ..
+            } => Definition {
+                kind: DefinitionKind::Method(stmt),
+                docstring: expr,
+            },
+            VisibleScope {
+                modifier: Modifier::Function,
+                ..
+            } => Definition {
+                kind: DefinitionKind::NestedFunction(stmt),
+                docstring: expr,
+            },
+        },
+        Documentable::Class => match scope {
+            VisibleScope {
+                modifier: Modifier::Module,
+                ..
+            } => Definition {
+                kind: DefinitionKind::Class(stmt),
+                docstring: expr,
+            },
+            VisibleScope {
+                modifier: Modifier::Class,
+                ..
+            } => Definition {
+                kind: DefinitionKind::NestedClass(stmt),
+                docstring: expr,
+            },
+            VisibleScope {
+                modifier: Modifier::Function,
+                ..
+            } => Definition {
+                kind: DefinitionKind::NestedClass(stmt),
+                docstring: expr,
+            },
+        },
+    }
+}