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>

tests/rust_usage_test/bin/flatbuffers_alloc_check.rs

@@ -142,10 +142,10 @@ fn main() {
             // We know the bits should be exactly equal here but compilers may
             // optimize floats in subtle ways so we're playing it safe and using
             // epsilon comparison
-            assert!((pos.x() - 1.0f32).abs() < std::f32::EPSILON);
-            assert!((pos.y() - 2.0f32).abs() < std::f32::EPSILON);
-            assert!((pos.z() - 3.0f32).abs() < std::f32::EPSILON);
-            assert!((pos.test1() - 3.0f64).abs() < std::f64::EPSILON);
+            assert!((pos.x() - 1.0f32).abs() < f32::EPSILON);
+            assert!((pos.y() - 2.0f32).abs() < f32::EPSILON);
+            assert!((pos.z() - 3.0f32).abs() < f32::EPSILON);
+            assert!((pos.test1() - 3.0f64).abs() < f64::EPSILON);
             assert_eq!(pos.test2(), my_game::example::Color::Green);
             let pos_test3 = pos.test3();
             assert_eq!(pos_test3.a(), 5i16);