Update speaker notes for arrays.md with clarification on runtime check optimization #2781
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According to the error I got ``` Compiling playground v0.0.1 (/playground) error: this operation will panic at runtime --> src/main.rs:3:5 | 3 | a[7] = 0; | ^^^^ index out of bounds: the length is 5 but the index is 7 | = note: `#[deny(unconditional_panic)]` on by default error: could not compile `playground` (bin "playground") due to 1 previous error ``` Array accesses seem to be checked at compile time.
src/tuples-and-arrays/arrays.md
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| @@ -27,8 +27,8 @@ fn main() { | |||
| later. | |||
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| - Try accessing an out-of-bounds array element. Array accesses are checked at | |||
| runtime. Rust can usually optimize these checks away, and they can be avoided | |||
| using unsafe Rust. | |||
| compile time. Rust can usually optimize these checks away, and they can be | |||
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The original meaning was correct, and important. In more sophisticated cases than this simple example, Rust may very well leave the runtime bounds-checks in place, and these can result in a substantial performance overhead above unchecked languages like C++.
It's difficult to demonstrate this in a short code sample because the checks are often optimized away at compile time -- in this case to an unconditional panic. I suspect black_box could be useful to avoid this optimization. Do you want to try adjusting the example so that it shows the runtime bounds check?
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Ah I see what you mean. I tried using black_box but it seems to not prevent the compile-time safety checks. I changed the out-of-bound index to be returned from a function and add that example to below the note. PTAL. Let me know what you think.
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Nice! The speaker note still says "compile time" though. It is also a good idea to add a sentence about how the optimization is so good that it's hard to give an example of runtime checks failing, and here requires a function to generate the index.
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Oops. PTAL again.
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(just needs formatting fixed) |
daivinhtran
changed the title
According to the error I got from trying out-of-bound index, array accesses seem to be checked at compile time.