Improves Rust code generation (#8564)

* Fixes checks for serde features in flexbuffers crate

* Removes unused MapReaderIndexer use statement

* Fixes warning about nightly cfg usage

Enabling a cfg attribute through cargo::rustc-cfg in build.rs should be coupled with a cargo::rust-check-cfg value so that the compiler knows about the custom cfg. See: https://doc.rust-lang.org/rustc/check-cfg/cargo-specifics.html#cargorustc-check-cfg-for-buildrsbuild-script.

* Migrates usage of deprecated float constants

This update fixes a compiler warning from use of the old constants.

Constants like EPSILON are now directly on the float primitives (e.g. f32::EPSILON) rather than in the f32 module (std::f32::EPSILON).

The new constants have existed since 1.43.0, which appears to be below the MSRV for the flatbuffers crate.

* Fixes incorrect key in flatbuffers Cargo.toml

The old code was using package.rust, which triggered a warning about an unused key:

warning: flatbuffers/rust/flatbuffers/Cargo.toml: unused manifest key: package.rust

The correct key for specifying MSRV is rust-version. See: https://doc.rust-lang.org/cargo/reference/rust-version.html#rust-version.

---------

Co-authored-by: Wouter van Oortmerssen <aardappel@gmail.com>

rust/flexbuffers/src/reader/de.rs

@@ -160,7 +160,7 @@ impl<'de> VariantAccess<'de> for Reader<&'de [u8]> {
 impl<'de> Deserializer<'de> for Reader<&'de [u8]> {
     type Error = DeserializationError;
     fn is_human_readable(&self) -> bool {
-        cfg!(deserialize_human_readable)
+        cfg!(feature = "deserialize_human_readable")
     }
 
     fn deserialize_any<V>(self, visitor: V) -> Result<V::Value, Self::Error>

rust/flexbuffers/src/reader/mod.rs

@@ -26,7 +26,7 @@ mod serialize;
 mod vector;
 pub use de::DeserializationError;
 pub use iter::ReaderIterator;
-pub use map::{MapReader, MapReaderIndexer};
+pub use map::MapReader;
 pub use vector::VectorReader;
 
 /// All the possible errors when reading a flexbuffer.