Rust: What is the difference between shrink_to_fit and reserve?

shrink_to_fit — reduce capacity. reserve — increase capacity.

shrink_to_fit:

• Reduce capacity.
• Match len.
• Save memory.

reserve:

• Increase capacity.
• Pre-allocate.
• Avoid reallocation.

1let mut v = Vec::with_capacity(100);
2v.push(1);
3v.shrink_to_fit();  // Capacity ~1
4
5let mut v = Vec::new();
6v.reserve(100);  // Capacity >= 100

Key: shrink_to_fit for memory, reserve for performance.