Improve estimation of capacity in Vec::from_iter

Closes #48994

1 files changed, 8 insertions, 2 deletions

diff --git a/library/alloc/src/vec/spec_from_iter_nested.rs b/library/alloc/src/vec/spec_from_iter_nested.rs
index 948cf044197..f915ebb86e5 100644
--- a/library/alloc/src/vec/spec_from_iter_nested.rs
+++ b/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);
                 }