Mark {f32,f64}::{next_up,next_down,midpoint} inline

Most float functions are marked `#[inline]` so any float symbols used by
these functions only need to be provided if the function itself is used.
RFL recently noticed that `next_up`, `next_down`, and `midpoint` for
`f32` and `f64` are not inline, which causes linker errors when building
with certain configurations [1].

Add the missing attributes so the symbols should no longer be required.

Cc: Gary Guo <gary@garyguo.net>
Reported-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/all/20240806150619.192882-1-ojeda@kernel.org/ [1]

core/src/num/f32.rs

@@ -797,6 +797,7 @@ impl f32 {
     /// [`INFINITY`]: Self::INFINITY
     /// [`MIN`]: Self::MIN
     /// [`MAX`]: Self::MAX
+    #[inline]
     #[unstable(feature = "float_next_up_down", issue = "91399")]
     #[rustc_const_unstable(feature = "float_next_up_down", issue = "91399")]
     pub const fn next_up(self) -> Self {
@@ -845,6 +846,7 @@ impl f32 {
     /// [`INFINITY`]: Self::INFINITY
     /// [`MIN`]: Self::MIN
     /// [`MAX`]: Self::MAX
+    #[inline]
     #[unstable(feature = "float_next_up_down", issue = "91399")]
     #[rustc_const_unstable(feature = "float_next_up_down", issue = "91399")]
     pub const fn next_down(self) -> Self {
@@ -1042,6 +1044,7 @@ impl f32 {
     /// assert_eq!(1f32.midpoint(4.0), 2.5);
     /// assert_eq!((-5.5f32).midpoint(8.0), 1.25);
     /// ```
+    #[inline]
     #[unstable(feature = "num_midpoint", issue = "110840")]
     pub fn midpoint(self, other: f32) -> f32 {
         cfg_if! {

core/src/num/f64.rs

@@ -805,6 +805,7 @@ impl f64 {
     /// [`INFINITY`]: Self::INFINITY
     /// [`MIN`]: Self::MIN
     /// [`MAX`]: Self::MAX
+    #[inline]
     #[unstable(feature = "float_next_up_down", issue = "91399")]
     #[rustc_const_unstable(feature = "float_next_up_down", issue = "91399")]
     pub const fn next_up(self) -> Self {
@@ -853,6 +854,7 @@ impl f64 {
     /// [`INFINITY`]: Self::INFINITY
     /// [`MIN`]: Self::MIN
     /// [`MAX`]: Self::MAX
+    #[inline]
     #[unstable(feature = "float_next_up_down", issue = "91399")]
     #[rustc_const_unstable(feature = "float_next_up_down", issue = "91399")]
     pub const fn next_down(self) -> Self {
@@ -1051,6 +1053,7 @@ impl f64 {
     /// assert_eq!(1f64.midpoint(4.0), 2.5);
     /// assert_eq!((-5.5f64).midpoint(8.0), 1.25);
     /// ```
+    #[inline]
     #[unstable(feature = "num_midpoint", issue = "110840")]
     pub fn midpoint(self, other: f64) -> f64 {
         const LO: f64 = f64::MIN_POSITIVE * 2.;