Auto merge of rust-lang#75677 - cuviper:shrink-towel, r=Mark-Simulacrum

Don't panic in Vec::shrink_to_fit

We can help the compiler see that `Vec::shrink_to_fit` will never reach the panic case in `RawVec::shrink_to_fit`, just by guarding the call only for cases where the capacity is strictly greater. A capacity less than the length is only possible through an unsafe call to `set_len`, which would break the `Vec` invariants, so `shrink_to_fit` can just ignore that.

This removes the panicking code from the examples in both rust-lang#71861 and rust-lang#75636.

Author: bors

alloc/src/vec.rs

@@ -622,7 +622,10 @@ impl<T> Vec<T> {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn shrink_to_fit(&mut self) {
-        if self.capacity() != self.len {
+        // The capacity is never less than the length, and there's nothing to do when
+        // they are equal, so we can avoid the panic case in `RawVec::shrink_to_fit`
+        // by only calling it with a greater capacity.
+        if self.capacity() > self.len {
             self.buf.shrink_to_fit(self.len);
         }
     }