Rust: Fix lifetime in union _as_ accessors (#5140)

* Fix lifetime in union _as_ accessors

In the accessors for union field, the return value is implicitly taking the lifetime of &self.
This is irrelevant and prevents usages of the returned value, because it is needlessly bound to the parent field lifetime.

This patch makes the return value inherit the lifetime of the data, like other methods do.

src/idl_gen_rust.cpp

@@ -1331,8 +1331,8 @@ class RustGenerator : public BaseGenerator {
 
         code_ += "  #[inline]";
         code_ += "  #[allow(non_snake_case)]";
-        code_ += "  pub fn {{FIELD_NAME}}_as_{{U_ELEMENT_NAME}}(&'a self) -> "
-                 "Option<{{U_ELEMENT_TABLE_TYPE}}> {";
+        code_ += "  pub fn {{FIELD_NAME}}_as_{{U_ELEMENT_NAME}}(&self) -> "
+                 "Option<{{U_ELEMENT_TABLE_TYPE}}<'a>> {";
         code_ += "    if self.{{FIELD_NAME}}_type() == {{U_ELEMENT_ENUM_TYPE}} {";
         code_ += "      self.{{FIELD_NAME}}().map(|u| "
                  "{{U_ELEMENT_TABLE_TYPE}}::init_from_table(u))";

tests/monster_test_generated.rs

@@ -1259,7 +1259,7 @@ impl<'a> Monster<'a> {
   }
   #[inline]
   #[allow(non_snake_case)]
-  pub fn test_as_monster(&'a self) -> Option<Monster> {
+  pub fn test_as_monster(&self) -> Option<Monster<'a>> {
     if self.test_type() == Any::Monster {
       self.test().map(|u| Monster::init_from_table(u))
     } else {
@@ -1269,7 +1269,7 @@ impl<'a> Monster<'a> {
 
   #[inline]
   #[allow(non_snake_case)]
-  pub fn test_as_test_simple_table_with_enum(&'a self) -> Option<TestSimpleTableWithEnum> {
+  pub fn test_as_test_simple_table_with_enum(&self) -> Option<TestSimpleTableWithEnum<'a>> {
     if self.test_type() == Any::TestSimpleTableWithEnum {
       self.test().map(|u| TestSimpleTableWithEnum::init_from_table(u))
     } else {
@@ -1279,7 +1279,7 @@ impl<'a> Monster<'a> {
 
   #[inline]
   #[allow(non_snake_case)]
-  pub fn test_as_my_game_example_2_monster(&'a self) -> Option<super::example_2::Monster> {
+  pub fn test_as_my_game_example_2_monster(&self) -> Option<super::example_2::Monster<'a>> {
     if self.test_type() == Any::MyGame_Example2_Monster {
       self.test().map(|u| super::example_2::Monster::init_from_table(u))
     } else {
@@ -1289,7 +1289,7 @@ impl<'a> Monster<'a> {
 
   #[inline]
   #[allow(non_snake_case)]
-  pub fn any_unique_as_m(&'a self) -> Option<Monster> {
+  pub fn any_unique_as_m(&self) -> Option<Monster<'a>> {
     if self.any_unique_type() == AnyUniqueAliases::M {
       self.any_unique().map(|u| Monster::init_from_table(u))
     } else {
@@ -1299,7 +1299,7 @@ impl<'a> Monster<'a> {
 
   #[inline]
   #[allow(non_snake_case)]
-  pub fn any_unique_as_t(&'a self) -> Option<TestSimpleTableWithEnum> {
+  pub fn any_unique_as_t(&self) -> Option<TestSimpleTableWithEnum<'a>> {
     if self.any_unique_type() == AnyUniqueAliases::T {
       self.any_unique().map(|u| TestSimpleTableWithEnum::init_from_table(u))
     } else {
@@ -1309,7 +1309,7 @@ impl<'a> Monster<'a> {
 
   #[inline]
   #[allow(non_snake_case)]
-  pub fn any_unique_as_m2(&'a self) -> Option<super::example_2::Monster> {
+  pub fn any_unique_as_m2(&self) -> Option<super::example_2::Monster<'a>> {
     if self.any_unique_type() == AnyUniqueAliases::M2 {
       self.any_unique().map(|u| super::example_2::Monster::init_from_table(u))
     } else {
@@ -1319,7 +1319,7 @@ impl<'a> Monster<'a> {
 
   #[inline]
   #[allow(non_snake_case)]
-  pub fn any_ambiguous_as_m1(&'a self) -> Option<Monster> {
+  pub fn any_ambiguous_as_m1(&self) -> Option<Monster<'a>> {
     if self.any_ambiguous_type() == AnyAmbiguousAliases::M1 {
       self.any_ambiguous().map(|u| Monster::init_from_table(u))
     } else {
@@ -1329,7 +1329,7 @@ impl<'a> Monster<'a> {
 
   #[inline]
   #[allow(non_snake_case)]
-  pub fn any_ambiguous_as_m2(&'a self) -> Option<Monster> {
+  pub fn any_ambiguous_as_m2(&self) -> Option<Monster<'a>> {
     if self.any_ambiguous_type() == AnyAmbiguousAliases::M2 {
       self.any_ambiguous().map(|u| Monster::init_from_table(u))
     } else {
@@ -1339,7 +1339,7 @@ impl<'a> Monster<'a> {
 
   #[inline]
   #[allow(non_snake_case)]
-  pub fn any_ambiguous_as_m3(&'a self) -> Option<Monster> {
+  pub fn any_ambiguous_as_m3(&self) -> Option<Monster<'a>> {
     if self.any_ambiguous_type() == AnyAmbiguousAliases::M3 {
       self.any_ambiguous().map(|u| Monster::init_from_table(u))
     } else {