Specify what happens when casting fat pointers more specifically

[CAD97]

[zulip discussion]

[scottmcm]

@CAD97 Do you perhaps have a topic link for the discussion? The link seems to just be to the UCG stream in general.

[joshtriplett]

r=me with an example added.

[CAD97]

...oops, will grab when I find it again :)

[CAD97]

https://rust-lang.zulipchat.com/#narrow/stream/136281-t-lang.2Fwg-unsafe-code-guidelines/topic/Casting.20unsized.20to.20sized.20pointer

[CAD97]

@joshtriplett I finally got around to adding that example 🙃 Is this still good to r+? (Well, after I add the proper uses.)

[CAD97]

That mess of commits shall be a lesson not to try to fix things through GitHub's app; rebased to fix that

[CAD97]

error in ../src/expressions/operator-expr.md: #![feature] attributes are not allowed

Well never mind then

[CAD97]

This is just not my day for writing coherent code is it. The example should finally be accurate now...

[traviscross]

We reviewed this today in the lang-docs call, and this seems to have been superseded by:

• Clarify semantics of the various pointer to pointer casts #1451

...which was merged. So we'll go ahead and close this.