Rollup merge of #136083 - bend-n:⃤⃤, r=lcnr

Suggest {to,from}_ne_bytes for transmutations between arrays and integers, etc

implements #136067

Rust has helper methods for many kinds of safe transmutes, for example integer<->bytes. This is a lint against using transmute for these cases.

```rs
fn bytes_at_home(x: [u8; 4]) -> u32 {
   transmute(x)
}

// other examples
transmute::<[u8; 2], u16>();
transmute::<[u8; 8], f64>();
transmute::<u32, [u8; 4]>();
transmute::<char, u32>();
transmute::<u32, char>();
```
It would be handy to suggest `u32::from_ne_bytes(x)`.
This is implemented for `[u8; _]` -> `{float int}`

This also implements the cases:
`fXX` <-> `uXX` = `{from_bits, to_bits}`
`uXX` -> `iXX` via `cast_unsigned` and `cast_signed`
{`char` -> `u32`, `bool` -> `n8`} via `from`
`u32` -> `char` via `from_u32_unchecked` (note: notes `from_u32().unwrap()`) (contested)
`u8` -> `bool` via `==` (debatable)

---
try-job: aarch64-gnu
try-job: test-various

1 files changed, 2 insertions, 6 deletions

diff --git a/compiler/rustc_codegen_gcc/example/example.rs b/compiler/rustc_codegen_gcc/example/example.rs
index 03470b74d0a..888fa89201e 100644
--- a/compiler/rustc_codegen_gcc/example/example.rs
+++ b/compiler/rustc_codegen_gcc/example/example.rs
@@ -1,6 +1,6 @@
 #![feature(no_core, unboxed_closures)]
 #![no_core]
-#![allow(dead_code)]
+#![allow(dead_code, unnecessary_transmutes)]
 
 extern crate mini_core;
 
@@ -11,11 +11,7 @@ fn abc(a: u8) -> u8 {
 }
 
 fn bcd(b: bool, a: u8) -> u8 {
-    if b {
-        a * 2
-    } else {
-        a * 3
-    }
+    if b { a * 2 } else { a * 3 }
 }
 
 fn call() {