SSR: Fix for path resolution of locals

It seems that Semantics::scope, if given a statement node, won't resolve
locals that were defined in the current scope, only in parent scopes.
Not sure if this is intended / expected behavior, but we work around it
for now by finding another nearby node to use as the scope (e.g. the
expression inside the EXPR_STMT).

Author: davidlattimore

crates/ra_ssr/src/resolving.rs

@@ -152,6 +152,7 @@ impl<'db> ResolutionScope<'db> {
             .left_biased()
             .map(|token| token.parent())
             .unwrap_or_else(|| file.syntax().clone());
+        let node = pick_node_for_resolution(node);
         let scope = sema.scope(&node);
         ResolutionScope {
             scope,
@@ -185,6 +186,33 @@ impl<'db> ResolutionScope<'db> {
     }
 }
 
+/// Returns a suitable node for resolving paths in the current scope. If we create a scope based on
+/// a statement node, then we can't resolve local variables that were defined in the current scope
+/// (only in parent scopes). So we find another node, ideally a child of the statement where local
+/// variable resolution is permitted.
+fn pick_node_for_resolution(node: SyntaxNode) -> SyntaxNode {
+    match node.kind() {
+        SyntaxKind::EXPR_STMT => {
+            if let Some(n) = node.first_child() {
+                mark::hit!(cursor_after_semicolon);
+                return n;
+            }
+        }
+        SyntaxKind::LET_STMT | SyntaxKind::BIND_PAT => {
+            if let Some(next) = node.next_sibling() {
+                return pick_node_for_resolution(next);
+            }
+        }
+        SyntaxKind::NAME => {
+            if let Some(parent) = node.parent() {
+                return pick_node_for_resolution(parent);
+            }
+        }
+        _ => {}
+    }
+    node
+}
+
 /// Returns whether `path` or any of its qualifiers contains type arguments.
 fn path_contains_type_arguments(path: Option<ast::Path>) -> bool {
     if let Some(path) = path {

crates/ra_ssr/src/tests.rs

@@ -885,3 +885,40 @@ fn ufcs_matches_method_call() {
         "#]],
     );
 }
+
+#[test]
+fn replace_local_variable_reference() {
+    // The pattern references a local variable `foo` in the block containing the cursor. We should
+    // only replace references to this variable `foo`, not other variables that just happen to have
+    // the same name.
+    mark::check!(cursor_after_semicolon);
+    assert_ssr_transform(
+        "foo + $a ==>> $a - foo",
+        r#"
+            fn bar1() -> i32 {
+                let mut res = 0;
+                let foo = 5;
+                res += foo + 1;
+                let foo = 10;
+                res += foo + 2;<|>
+                res += foo + 3;
+                let foo = 15;
+                res += foo + 4;
+                res
+            }
+            "#,
+        expect![[r#"
+            fn bar1() -> i32 {
+                let mut res = 0;
+                let foo = 5;
+                res += foo + 1;
+                let foo = 10;
+                res += 2 - foo;
+                res += 3 - foo;
+                let foo = 15;
+                res += foo + 4;
+                res
+            }
+        "#]],
+    )
+}