Docs: Compile-time bounds check in index expression

[johannhof]

The reference was claiming all vectors all bounds-checked at run-time, when constant vectors are usually checked at compile-time.

For the changed example see http://is.gd/28ak9E

Also fixed a minor grammar issue.

[rust-highfive]

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[bluss]

Consider the case of a runtime value for the index and a fixed size array, then it's checked at run time. Not sure how to explain that in a concise manner.

[johannhof]

Right, good catch. I'll try to rephrase.

[steveklabnik]

yes, http://is.gd/SUUj9T shows this off.

[johannhof]

Updated. Should explain most cases now.

[bluss]

great. This is all about rust semantics right. I mean, the optimizing part of the compilation may find that it always is out of bounds, and compile that call to panic in statically, but we don't emit any warning or otherwise even notice that :-)

[johannhof]

r? @steveklabnik

reassigning ❤️

[steveklabnik]

@bors: r+ rollup

[bors]

📌 Commit 295b62d has been approved by steveklabnik

[steveklabnik]

Excellent, thanks so much!

[steveklabnik]

Oh, and you should check in over at #25196, as this means you get to be in AUTHORS. :)

[johannhof]

yay :D