fix(idl_gen_rust): Fix lifetime warning added in Rust 1.89 (#8709)

Rust 1.89 added a new lifetime-related warning:
<https://blog.rust-lang.org/2025/08/07/Rust-1.89.0/#mismatched-lifetime-syntaxes-lint>

The Rust code generator currently emits code which trips this warning. This very small PR
fixes the issue for the relevant generated functions and for the Rust flexbuffers code.

Fixes #8705

rust/flexbuffers/src/builder/map.rs

@@ -43,7 +43,7 @@ impl<'a> MapBuilder<'a> {
     ///
     /// This will panic (in debug mode) if `key` contains internal nulls.
     #[inline]
-    pub fn start_vector(&mut self, key: &str) -> VectorBuilder {
+    pub fn start_vector(&mut self, key: &str) -> VectorBuilder<'_> {
         // Push the key that refers to this nested vector.
         self.builder.push_key(key);
         // Nested vector.
@@ -55,7 +55,7 @@ impl<'a> MapBuilder<'a> {
     ///
     /// This will panic (in debug mode) if `key` contains internal nulls.
     #[inline]
-    pub fn start_map(&mut self, key: &str) -> MapBuilder {
+    pub fn start_map(&mut self, key: &str) -> MapBuilder<'_> {
         // Push the key that refers to this nested vector.
         self.builder.push_key(key);
         // Nested map.

rust/flexbuffers/src/builder/vector.rs

@@ -36,13 +36,13 @@ impl<'a> VectorBuilder<'a> {
     }
     /// Starts a nested vector that will be pushed onto this vector when it is dropped.
     #[inline]
-    pub fn start_vector(&mut self) -> VectorBuilder {
+    pub fn start_vector(&mut self) -> VectorBuilder<'_> {
         let start = Some(self.builder.values.len());
         VectorBuilder { builder: self.builder, start }
     }
     /// Starts a nested map that will be pushed onto this vector when it is dropped.
     #[inline]
-    pub fn start_map(&mut self) -> MapBuilder {
+    pub fn start_map(&mut self) -> MapBuilder<'_> {
         let start = Some(self.builder.values.len());
         MapBuilder { builder: self.builder, start }
     }