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

goldens/rust/basic_generated.rs

@@ -244,7 +244,7 @@ impl core::fmt::Debug for Universe<'_> {
 /// catch every error, or be maximally performant. For the
 /// previous, unchecked, behavior use
 /// `root_as_universe_unchecked`.
-pub fn root_as_universe(buf: &[u8]) -> Result<Universe, flatbuffers::InvalidFlatbuffer> {
+pub fn root_as_universe(buf: &[u8]) -> Result<Universe<'_>, flatbuffers::InvalidFlatbuffer> {
   flatbuffers::root::<Universe>(buf)
 }
 #[inline]
@@ -254,7 +254,7 @@ pub fn root_as_universe(buf: &[u8]) -> Result<Universe, flatbuffers::InvalidFlat
 /// catch every error, or be maximally performant. For the
 /// previous, unchecked, behavior use
 /// `size_prefixed_root_as_universe_unchecked`.
-pub fn size_prefixed_root_as_universe(buf: &[u8]) -> Result<Universe, flatbuffers::InvalidFlatbuffer> {
+pub fn size_prefixed_root_as_universe(buf: &[u8]) -> Result<Universe<'_>, flatbuffers::InvalidFlatbuffer> {
   flatbuffers::size_prefixed_root::<Universe>(buf)
 }
 #[inline]
@@ -287,14 +287,14 @@ pub fn size_prefixed_root_as_universe_with_opts<'b, 'o>(
 /// Assumes, without verification, that a buffer of bytes contains a Universe and returns it.
 /// # Safety
 /// Callers must trust the given bytes do indeed contain a valid `Universe`.
-pub unsafe fn root_as_universe_unchecked(buf: &[u8]) -> Universe {
+pub unsafe fn root_as_universe_unchecked(buf: &[u8]) -> Universe<'_> {
   unsafe { flatbuffers::root_unchecked::<Universe>(buf) }
 }
 #[inline]
 /// Assumes, without verification, that a buffer of bytes contains a size prefixed Universe and returns it.
 /// # Safety
 /// Callers must trust the given bytes do indeed contain a valid size prefixed `Universe`.
-pub unsafe fn size_prefixed_root_as_universe_unchecked(buf: &[u8]) -> Universe {
+pub unsafe fn size_prefixed_root_as_universe_unchecked(buf: &[u8]) -> Universe<'_> {
   unsafe { flatbuffers::size_prefixed_root_unchecked::<Universe>(buf) }
 }
 #[inline]

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 }
     }

samples/rust_generated/my_game/sample/monster_generated.rs

@@ -407,7 +407,7 @@ impl MonsterT {
 /// catch every error, or be maximally performant. For the
 /// previous, unchecked, behavior use
 /// `root_as_monster_unchecked`.
-pub fn root_as_monster(buf: &[u8]) -> Result<Monster, flatbuffers::InvalidFlatbuffer> {
+pub fn root_as_monster(buf: &[u8]) -> Result<Monster<'_>, flatbuffers::InvalidFlatbuffer> {
   flatbuffers::root::<Monster>(buf)
 }
 #[inline]
@@ -417,7 +417,7 @@ pub fn root_as_monster(buf: &[u8]) -> Result<Monster, flatbuffers::InvalidFlatbu
 /// catch every error, or be maximally performant. For the
 /// previous, unchecked, behavior use
 /// `size_prefixed_root_as_monster_unchecked`.
-pub fn size_prefixed_root_as_monster(buf: &[u8]) -> Result<Monster, flatbuffers::InvalidFlatbuffer> {
+pub fn size_prefixed_root_as_monster(buf: &[u8]) -> Result<Monster<'_>, flatbuffers::InvalidFlatbuffer> {
   flatbuffers::size_prefixed_root::<Monster>(buf)
 }
 #[inline]
@@ -450,14 +450,14 @@ pub fn size_prefixed_root_as_monster_with_opts<'b, 'o>(
 /// Assumes, without verification, that a buffer of bytes contains a Monster and returns it.
 /// # Safety
 /// Callers must trust the given bytes do indeed contain a valid `Monster`.
-pub unsafe fn root_as_monster_unchecked(buf: &[u8]) -> Monster {
+pub unsafe fn root_as_monster_unchecked(buf: &[u8]) -> Monster<'_> {
   unsafe { flatbuffers::root_unchecked::<Monster>(buf) }
 }
 #[inline]
 /// Assumes, without verification, that a buffer of bytes contains a size prefixed Monster and returns it.
 /// # Safety
 /// Callers must trust the given bytes do indeed contain a valid size prefixed `Monster`.
-pub unsafe fn size_prefixed_root_as_monster_unchecked(buf: &[u8]) -> Monster {
+pub unsafe fn size_prefixed_root_as_monster_unchecked(buf: &[u8]) -> Monster<'_> {
   unsafe { flatbuffers::size_prefixed_root_unchecked::<Monster>(buf) }
 }
 #[inline]

src/idl_gen_rust.cpp

@@ -2437,7 +2437,7 @@ class RustGenerator : public BaseGenerator {
     code_ += "/// `root_as_{{STRUCT_FN}}_unchecked`.";
     code_ +=
         "pub fn root_as_{{STRUCT_FN}}(buf: &[u8]) "
-        "-> Result<{{STRUCT_TY}}, flatbuffers::InvalidFlatbuffer> {";
+        "-> Result<{{STRUCT_TY}}<'_>, flatbuffers::InvalidFlatbuffer> {";
     code_ += "  flatbuffers::root::<{{STRUCT_TY}}>(buf)";
     code_ += "}";
     code_ += "#[inline]";
@@ -2449,7 +2449,7 @@ class RustGenerator : public BaseGenerator {
     code_ += "/// `size_prefixed_root_as_{{STRUCT_FN}}_unchecked`.";
     code_ +=
         "pub fn size_prefixed_root_as_{{STRUCT_FN}}"
-        "(buf: &[u8]) -> Result<{{STRUCT_TY}}, "
+        "(buf: &[u8]) -> Result<{{STRUCT_TY}}<'_>, "
         "flatbuffers::InvalidFlatbuffer> {";
     code_ += "  flatbuffers::size_prefixed_root::<{{STRUCT_TY}}>(buf)";
     code_ += "}";
@@ -2499,7 +2499,7 @@ class RustGenerator : public BaseGenerator {
         " `{{STRUCT_TY}}`.";
     code_ +=
         "pub unsafe fn root_as_{{STRUCT_FN}}_unchecked"
-        "(buf: &[u8]) -> {{STRUCT_TY}} {";
+        "(buf: &[u8]) -> {{STRUCT_TY}}<'_> {";
     code_ += "  unsafe { flatbuffers::root_unchecked::<{{STRUCT_TY}}>(buf) }";
     code_ += "}";
     code_ += "#[inline]";
@@ -2512,7 +2512,7 @@ class RustGenerator : public BaseGenerator {
         " size prefixed `{{STRUCT_TY}}`.";
     code_ +=
         "pub unsafe fn size_prefixed_root_as_{{STRUCT_FN}}"
-        "_unchecked(buf: &[u8]) -> {{STRUCT_TY}} {";
+        "_unchecked(buf: &[u8]) -> {{STRUCT_TY}}<'_> {";
     code_ +=
         "  unsafe { flatbuffers::size_prefixed_root_unchecked::<{{STRUCT_TY}}>"
         "(buf) }";

tests/arrays_test/my_game/example/array_table_generated.rs

@@ -149,7 +149,7 @@ impl ArrayTableT {
 /// catch every error, or be maximally performant. For the
 /// previous, unchecked, behavior use
 /// `root_as_array_table_unchecked`.
-pub fn root_as_array_table(buf: &[u8]) -> Result<ArrayTable, flatbuffers::InvalidFlatbuffer> {
+pub fn root_as_array_table(buf: &[u8]) -> Result<ArrayTable<'_>, flatbuffers::InvalidFlatbuffer> {
   flatbuffers::root::<ArrayTable>(buf)
 }
 #[inline]
@@ -159,7 +159,7 @@ pub fn root_as_array_table(buf: &[u8]) -> Result<ArrayTable, flatbuffers::Invali
 /// catch every error, or be maximally performant. For the
 /// previous, unchecked, behavior use
 /// `size_prefixed_root_as_array_table_unchecked`.
-pub fn size_prefixed_root_as_array_table(buf: &[u8]) -> Result<ArrayTable, flatbuffers::InvalidFlatbuffer> {
+pub fn size_prefixed_root_as_array_table(buf: &[u8]) -> Result<ArrayTable<'_>, flatbuffers::InvalidFlatbuffer> {
   flatbuffers::size_prefixed_root::<ArrayTable>(buf)
 }
 #[inline]
@@ -192,14 +192,14 @@ pub fn size_prefixed_root_as_array_table_with_opts<'b, 'o>(
 /// Assumes, without verification, that a buffer of bytes contains a ArrayTable and returns it.
 /// # Safety
 /// Callers must trust the given bytes do indeed contain a valid `ArrayTable`.
-pub unsafe fn root_as_array_table_unchecked(buf: &[u8]) -> ArrayTable {
+pub unsafe fn root_as_array_table_unchecked(buf: &[u8]) -> ArrayTable<'_> {
   unsafe { flatbuffers::root_unchecked::<ArrayTable>(buf) }
 }
 #[inline]
 /// Assumes, without verification, that a buffer of bytes contains a size prefixed ArrayTable and returns it.
 /// # Safety
 /// Callers must trust the given bytes do indeed contain a valid size prefixed `ArrayTable`.
-pub unsafe fn size_prefixed_root_as_array_table_unchecked(buf: &[u8]) -> ArrayTable {
+pub unsafe fn size_prefixed_root_as_array_table_unchecked(buf: &[u8]) -> ArrayTable<'_> {
   unsafe { flatbuffers::size_prefixed_root_unchecked::<ArrayTable>(buf) }
 }
 pub const ARRAY_TABLE_IDENTIFIER: &str = "ARRT";

tests/monster_test/my_game/example/monster_generated.rs

@@ -1935,7 +1935,7 @@ impl MonsterT {
 /// catch every error, or be maximally performant. For the
 /// previous, unchecked, behavior use
 /// `root_as_monster_unchecked`.
-pub fn root_as_monster(buf: &[u8]) -> Result<Monster, flatbuffers::InvalidFlatbuffer> {
+pub fn root_as_monster(buf: &[u8]) -> Result<Monster<'_>, flatbuffers::InvalidFlatbuffer> {
   flatbuffers::root::<Monster>(buf)
 }
 #[inline]
@@ -1945,7 +1945,7 @@ pub fn root_as_monster(buf: &[u8]) -> Result<Monster, flatbuffers::InvalidFlatbu
 /// catch every error, or be maximally performant. For the
 /// previous, unchecked, behavior use
 /// `size_prefixed_root_as_monster_unchecked`.
-pub fn size_prefixed_root_as_monster(buf: &[u8]) -> Result<Monster, flatbuffers::InvalidFlatbuffer> {
+pub fn size_prefixed_root_as_monster(buf: &[u8]) -> Result<Monster<'_>, flatbuffers::InvalidFlatbuffer> {
   flatbuffers::size_prefixed_root::<Monster>(buf)
 }
 #[inline]
@@ -1978,14 +1978,14 @@ pub fn size_prefixed_root_as_monster_with_opts<'b, 'o>(
 /// Assumes, without verification, that a buffer of bytes contains a Monster and returns it.
 /// # Safety
 /// Callers must trust the given bytes do indeed contain a valid `Monster`.
-pub unsafe fn root_as_monster_unchecked(buf: &[u8]) -> Monster {
+pub unsafe fn root_as_monster_unchecked(buf: &[u8]) -> Monster<'_> {
   unsafe { flatbuffers::root_unchecked::<Monster>(buf) }
 }
 #[inline]
 /// Assumes, without verification, that a buffer of bytes contains a size prefixed Monster and returns it.
 /// # Safety
 /// Callers must trust the given bytes do indeed contain a valid size prefixed `Monster`.
-pub unsafe fn size_prefixed_root_as_monster_unchecked(buf: &[u8]) -> Monster {
+pub unsafe fn size_prefixed_root_as_monster_unchecked(buf: &[u8]) -> Monster<'_> {
   unsafe { flatbuffers::size_prefixed_root_unchecked::<Monster>(buf) }
 }
 pub const MONSTER_IDENTIFIER: &str = "MONS";

tests/monster_test_serialize/my_game/example/monster_generated.rs

@@ -2162,7 +2162,7 @@ impl MonsterT {
 /// catch every error, or be maximally performant. For the
 /// previous, unchecked, behavior use
 /// `root_as_monster_unchecked`.
-pub fn root_as_monster(buf: &[u8]) -> Result<Monster, flatbuffers::InvalidFlatbuffer> {
+pub fn root_as_monster(buf: &[u8]) -> Result<Monster<'_>, flatbuffers::InvalidFlatbuffer> {
   flatbuffers::root::<Monster>(buf)
 }
 #[inline]
@@ -2172,7 +2172,7 @@ pub fn root_as_monster(buf: &[u8]) -> Result<Monster, flatbuffers::InvalidFlatbu
 /// catch every error, or be maximally performant. For the
 /// previous, unchecked, behavior use
 /// `size_prefixed_root_as_monster_unchecked`.
-pub fn size_prefixed_root_as_monster(buf: &[u8]) -> Result<Monster, flatbuffers::InvalidFlatbuffer> {
+pub fn size_prefixed_root_as_monster(buf: &[u8]) -> Result<Monster<'_>, flatbuffers::InvalidFlatbuffer> {
   flatbuffers::size_prefixed_root::<Monster>(buf)
 }
 #[inline]
@@ -2205,14 +2205,14 @@ pub fn size_prefixed_root_as_monster_with_opts<'b, 'o>(
 /// Assumes, without verification, that a buffer of bytes contains a Monster and returns it.
 /// # Safety
 /// Callers must trust the given bytes do indeed contain a valid `Monster`.
-pub unsafe fn root_as_monster_unchecked(buf: &[u8]) -> Monster {
+pub unsafe fn root_as_monster_unchecked(buf: &[u8]) -> Monster<'_> {
   unsafe { flatbuffers::root_unchecked::<Monster>(buf) }
 }
 #[inline]
 /// Assumes, without verification, that a buffer of bytes contains a size prefixed Monster and returns it.
 /// # Safety
 /// Callers must trust the given bytes do indeed contain a valid size prefixed `Monster`.
-pub unsafe fn size_prefixed_root_as_monster_unchecked(buf: &[u8]) -> Monster {
+pub unsafe fn size_prefixed_root_as_monster_unchecked(buf: &[u8]) -> Monster<'_> {
   unsafe { flatbuffers::size_prefixed_root_unchecked::<Monster>(buf) }
 }
 pub const MONSTER_IDENTIFIER: &str = "MONS";

tests/optional_scalars/optional_scalars/scalar_stuff_generated.rs

@@ -951,7 +951,7 @@ impl ScalarStuffT {
 /// catch every error, or be maximally performant. For the
 /// previous, unchecked, behavior use
 /// `root_as_scalar_stuff_unchecked`.
-pub fn root_as_scalar_stuff(buf: &[u8]) -> Result<ScalarStuff, flatbuffers::InvalidFlatbuffer> {
+pub fn root_as_scalar_stuff(buf: &[u8]) -> Result<ScalarStuff<'_>, flatbuffers::InvalidFlatbuffer> {
   flatbuffers::root::<ScalarStuff>(buf)
 }
 #[inline]
@@ -961,7 +961,7 @@ pub fn root_as_scalar_stuff(buf: &[u8]) -> Result<ScalarStuff, flatbuffers::Inva
 /// catch every error, or be maximally performant. For the
 /// previous, unchecked, behavior use
 /// `size_prefixed_root_as_scalar_stuff_unchecked`.
-pub fn size_prefixed_root_as_scalar_stuff(buf: &[u8]) -> Result<ScalarStuff, flatbuffers::InvalidFlatbuffer> {
+pub fn size_prefixed_root_as_scalar_stuff(buf: &[u8]) -> Result<ScalarStuff<'_>, flatbuffers::InvalidFlatbuffer> {
   flatbuffers::size_prefixed_root::<ScalarStuff>(buf)
 }
 #[inline]
@@ -994,14 +994,14 @@ pub fn size_prefixed_root_as_scalar_stuff_with_opts<'b, 'o>(
 /// Assumes, without verification, that a buffer of bytes contains a ScalarStuff and returns it.
 /// # Safety
 /// Callers must trust the given bytes do indeed contain a valid `ScalarStuff`.
-pub unsafe fn root_as_scalar_stuff_unchecked(buf: &[u8]) -> ScalarStuff {
+pub unsafe fn root_as_scalar_stuff_unchecked(buf: &[u8]) -> ScalarStuff<'_> {
   unsafe { flatbuffers::root_unchecked::<ScalarStuff>(buf) }
 }
 #[inline]
 /// Assumes, without verification, that a buffer of bytes contains a size prefixed ScalarStuff and returns it.
 /// # Safety
 /// Callers must trust the given bytes do indeed contain a valid size prefixed `ScalarStuff`.
-pub unsafe fn size_prefixed_root_as_scalar_stuff_unchecked(buf: &[u8]) -> ScalarStuff {
+pub unsafe fn size_prefixed_root_as_scalar_stuff_unchecked(buf: &[u8]) -> ScalarStuff<'_> {
   unsafe { flatbuffers::size_prefixed_root_unchecked::<ScalarStuff>(buf) }
 }
 pub const SCALAR_STUFF_IDENTIFIER: &str = "NULL";