fix(es/compat): Handle nullish in fn expr scope (#7980)

**Related issue:**

  - Closes: #7977

crates/swc/tests/fixture/issues-7xxx/7977/input/.swcrc

@@ -0,0 +1,26 @@
+{
+    "$schema": "https://json.schemastore.org/swcrc",
+    "jsc": {
+        "parser": {
+            "syntax": "typescript",
+            "tsx": false,
+            "dynamicImport": true
+        },
+        "transform": {
+            "react": {
+                "runtime": "automatic"
+            }
+        },
+        "baseUrl": "./",
+        "target": "es2019",
+        "keepClassNames": true,
+        "minify": {
+            "compress": false
+        }
+    },
+    "isModule": true,
+    "module": {
+        "type": "es6"
+    },
+    "minify": false
+}

crates/swc/tests/fixture/issues-7xxx/7977/input/index.ts

@@ -0,0 +1 @@
+const foo = () => baz() ?? qux;

crates/swc/tests/fixture/issues-7xxx/7977/output/index.ts

@@ -0,0 +1,4 @@
+const foo = ()=>{
+    var _baz;
+    return (_baz = baz()) !== null && _baz !== void 0 ? _baz : qux;
+};

crates/swc_ecma_transforms_compat/src/es2020/nullish_coalescing.rs

@@ -1,7 +1,7 @@
 use std::mem::take;
 
 use serde::Deserialize;
-use swc_common::{util::take::Take, Span, DUMMY_SP};
+use swc_common::{util::take::Take, Span, Spanned, DUMMY_SP};
 use swc_ecma_ast::*;
 use swc_ecma_utils::{alias_if_required, undefined, StmtLike};
 use swc_ecma_visit::{as_folder, noop_visit_mut_type, Fold, VisitMut, VisitMutWith};
@@ -192,6 +192,35 @@ impl VisitMut for NullishCoalescing {
             _ => {}
         }
     }
+
+    fn visit_mut_block_stmt_or_expr(&mut self, n: &mut BlockStmtOrExpr) {
+        let vars = self.vars.take();
+        n.visit_mut_children_with(self);
+
+        if !self.vars.is_empty() {
+            if let BlockStmtOrExpr::Expr(expr) = n {
+                // expr
+                // { var decl = init; return expr; }
+                let span = expr.span();
+                let stmts = vec![
+                    VarDecl {
+                        span: DUMMY_SP,
+                        kind: VarDeclKind::Var,
+                        decls: self.vars.take(),
+                        declare: false,
+                    }
+                    .into(),
+                    Stmt::Return(ReturnStmt {
+                        span: DUMMY_SP,
+                        arg: Some(expr.take()),
+                    }),
+                ];
+                *n = BlockStmtOrExpr::BlockStmt(BlockStmt { span, stmts });
+            }
+        }
+
+        self.vars = vars;
+    }
 }
 
 #[tracing::instrument(level = "info", skip_all)]