Improve boxed docs

[GuillaumeGomez]

Fixes#29343.

r? @steveklabnik

[apasel422]

It would be better if this example didn't demonstrate code that is likely to cause a segfault. The typical use case for this is when you start off with a Box, convert it to a raw pointer with into_raw, and then need to restore it.

[GuillaumeGomez]

I see. Just by curiosity: why would it likely cause a segfault in here?

[apasel422]

The Box ends up being a pointer to the stack, and is then freed when the
Box is dropped.

On Saturday, July 9, 2016, Guillaume Gomez notifications@github.com wrote:

In src/liballoc/boxed.rs
#34740 (comment):

@@ -249,6 +249,14 @@ impl<T: ?Sized> Box {
/// This function is unsafe because improper use may lead to
/// memory problems. For example, a double-free may occur if the
/// function is called twice on the same raw pointer.

• ///
• /// # Examples
• ///
• /// ```
• /// let mut x = 5;
• /// let ptr = &mut x as *mut i32;
• /// let b = unsafe { Box::from_raw(ptr) };

I see. Just by curiosity: why would it likely cause a segfault in here?

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[GuillaumeGomez]

Thanks!

[GuillaumeGomez]

Updated.

[frewsxcv]

if let?

[GuillaumeGomez]

@frewsxcv: Thanks!

[steveklabnik]

@bors: r+ rollup

[bors]

📌 Commit 1ef7bdc has been approved by steveklabnik