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/flatbuffers/Cargo.toml

@@ -9,7 +9,7 @@ homepage = "https://google.github.io/flatbuffers/"
 repository = "https://github.com/google/flatbuffers"
 keywords = ["flatbuffers", "serialization", "zero-copy"]
 categories = ["encoding", "data-structures", "memory-management"]
-rust = "1.51"
+rust-version = "1.51"
 
 [features]
 default = ["std"]

rust/flatbuffers/build.rs

@@ -1,4 +1,4 @@
-use rustc_version::{version_meta, Channel};
+use rustc_version::{Channel, version_meta};
 
 fn main() {
     let version_meta = version_meta().unwrap();
@@ -6,6 +6,7 @@ fn main() {
     // To use nightly features we declare this and then we can use
     // #[cfg(nightly)]
     // for nightly only features
+    println!("cargo:rustc-check-cfg=cfg(nightly)");
     if version_meta.channel == Channel::Nightly {
         println!("cargo:rustc-cfg=nightly")
     }