Auto merge of rust-lang#92138 - AngelicosPhosphoros:try_smarter_vec_from_iter_48994_2, r=Mark-Simulacrum

Improve capacity estimation in Vec::from_iter

Iterates on the attempt made in rust-lang#53086.

Closes rust-lang#48994

Author: bors

library/alloc/src/raw_vec.rs

@@ -108,7 +108,7 @@ impl<T, A: Allocator> RawVec<T, A> {
     //   to round up a request of less than 8 bytes to at least 8 bytes.
     // - 4 if elements are moderate-sized (<= 1 KiB).
     // - 1 otherwise, to avoid wasting too much space for very short Vecs.
-    const MIN_NON_ZERO_CAP: usize = if mem::size_of::<T>() == 1 {
+    pub(crate) const MIN_NON_ZERO_CAP: usize = if mem::size_of::<T>() == 1 {
         8
     } else if mem::size_of::<T>() <= 1024 {
         4

library/alloc/src/vec/spec_from_iter_nested.rs

@@ -1,5 +1,8 @@
+use core::cmp;
 use core::iter::TrustedLen;
-use core::ptr::{self};
+use core::ptr;
+
+use crate::raw_vec::RawVec;
 
 use super::{SpecExtend, Vec};
 
@@ -24,8 +27,11 @@ where
             None => return Vec::new(),
             Some(element) => {
                 let (lower, _) = iterator.size_hint();
-                let mut vector = Vec::with_capacity(lower.saturating_add(1));
+                let initial_capacity =
+                    cmp::max(RawVec::<T>::MIN_NON_ZERO_CAP, lower.saturating_add(1));
+                let mut vector = Vec::with_capacity(initial_capacity);
                 unsafe {
+                    // SAFETY: We requested capacity at least 1
                     ptr::write(vector.as_mut_ptr(), element);
                     vector.set_len(1);
                 }