Rollup merge of rust-lang#135139 - c410-f3r:8-years-rfc, r=jhpratt

[generic_assert] Constify methods used by the formatting system

cc rust-lang#44838

Starts the "constification" of all the elements required to allow the execution of the formatting system in constant environments.

```rust
const _: () = { panic!("{:?}", 1i32); };
```

Further stuff is blocked by rust-lang#133999.

Author: jhpratt

core/src/fmt/mod.rs

@@ -596,7 +596,7 @@ impl<'a> Arguments<'a> {
     /// When using the format_args!() macro, this function is used to generate the
     /// Arguments structure.
     #[inline]
-    pub fn new_v1<const P: usize, const A: usize>(
+    pub const fn new_v1<const P: usize, const A: usize>(
         pieces: &'a [&'static str; P],
         args: &'a [rt::Argument<'a>; A],
     ) -> Arguments<'a> {
@@ -612,7 +612,7 @@ impl<'a> Arguments<'a> {
     /// 2. Every `rt::Placeholder::position` value within `fmt` must be a valid index of `args`.
     /// 3. Every `rt::Count::Param` within `fmt` must contain a valid index of `args`.
     #[inline]
-    pub fn new_v1_formatted(
+    pub const fn new_v1_formatted(
         pieces: &'a [&'static str],
         args: &'a [rt::Argument<'a>],
         fmt: &'a [rt::Placeholder],

core/src/fmt/rt.rs

@@ -96,12 +96,12 @@ pub struct Argument<'a> {
 #[rustc_diagnostic_item = "ArgumentMethods"]
 impl Argument<'_> {
     #[inline]
-    fn new<'a, T>(x: &'a T, f: fn(&T, &mut Formatter<'_>) -> Result) -> Argument<'a> {
+    const fn new<'a, T>(x: &'a T, f: fn(&T, &mut Formatter<'_>) -> Result) -> Argument<'a> {
         Argument {
             // INVARIANT: this creates an `ArgumentType<'a>` from a `&'a T` and
             // a `fn(&T, ...)`, so the invariant is maintained.
             ty: ArgumentType::Placeholder {
-                value: NonNull::from(x).cast(),
+                value: NonNull::from_ref(x).cast(),
                 // SAFETY: function pointers always have the same layout.
                 formatter: unsafe { mem::transmute(f) },
                 _lifetime: PhantomData,
@@ -150,7 +150,7 @@ impl Argument<'_> {
         Self::new(x, UpperExp::fmt)
     }
     #[inline]
-    pub fn from_usize(x: &usize) -> Argument<'_> {
+    pub const fn from_usize(x: &usize) -> Argument<'_> {
         Argument { ty: ArgumentType::Count(*x) }
     }
 
@@ -181,7 +181,7 @@ impl Argument<'_> {
     }
 
     #[inline]
-    pub(super) fn as_usize(&self) -> Option<usize> {
+    pub(super) const fn as_usize(&self) -> Option<usize> {
         match self.ty {
             ArgumentType::Count(count) => Some(count),
             ArgumentType::Placeholder { .. } => None,
@@ -199,7 +199,7 @@ impl Argument<'_> {
     /// println!("{f}");
     /// ```
     #[inline]
-    pub fn none() -> [Self; 0] {
+    pub const fn none() -> [Self; 0] {
         []
     }
 }
@@ -216,7 +216,7 @@ impl UnsafeArg {
     /// See documentation where `UnsafeArg` is required to know when it is safe to
     /// create and use `UnsafeArg`.
     #[inline]
-    pub unsafe fn new() -> Self {
+    pub const unsafe fn new() -> Self {
         Self { _private: () }
     }
 }