Rollup merge of rust-lang#126906 - GrigorenkoPV:fixme-split_at_first,…

… r=Mark-Simulacrum

Small fixme in core now that split_first has no codegen issues

rust-lang#109328 (comment)

BTW, I have a crate implementing exactly this kind of an iterator: https://github.com/GrigorenkoPV/head-tail-iter and I was wondering if it would be worthwhile to try and make an ACP for it to get it included in std (or maybe itertools). My only doubt is that it kinda incentives writing O(n^2) algorithms and is not the hard to replace with a `while let` loop (just as in this PR).

core/src/num/dec2flt/common.rs

@@ -39,9 +39,7 @@ impl ByteSlice for [u8] {
     fn parse_digits(&self, mut func: impl FnMut(u8)) -> &Self {
         let mut s = self;
 
-        // FIXME: Can't use s.split_first() here yet,
-        // see https://github.com/rust-lang/rust/issues/109328
-        while let [c, s_next @ ..] = s {
+        while let Some((c, s_next)) = s.split_first() {
             let c = c.wrapping_sub(b'0');
             if c < 10 {
                 func(c);

core/src/num/dec2flt/parse.rs

@@ -51,9 +51,7 @@ fn try_parse_19digits(s_ref: &mut &[u8], x: &mut u64) {
     let mut s = *s_ref;
 
     while *x < MIN_19DIGIT_INT {
-        // FIXME: Can't use s.split_first() here yet,
-        // see https://github.com/rust-lang/rust/issues/109328
-        if let [c, s_next @ ..] = s {
+        if let Some((c, s_next)) = s.split_first() {
             let digit = c.wrapping_sub(b'0');
 
             if digit < 10 {