Check for whether index is out of bound during compilation failed unexpectedly #124889
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This repros without the copying method. This panics at runtime: #[derive(Debug)]
struct Arr<T, const N: usize>([T; N]);
fn main() {
let arr = Arr(*b"Hello, world");
println!("{:?}", arr.0[20]);
// let inner = arr.0;
// println!("{:?}", inner[20]);
}While this gives a compiler error: #[derive(Debug)]
struct Arr<T, const N: usize>([T; N]);
fn main() {
let arr = Arr(*b"Hello, world");
// println!("{:?}", arr.0[20]);
let inner = arr.0;
println!("{:?}", inner[20]);
} |
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Interestingly this panics: struct Arr([u8; 12]);
fn main() {
let arr = Arr(*b"Hello, world");
// Panics
println!("{:?}", arr.0[20]);But this (without the struct Arr([u8; 12]);
fn main() {
let arr = Arr(*b"Hello, world");
// Compiler errror
arr.0[20];
}Expanding the macros for the former println call (into HIR) gives: ::std::io::_print(format_arguments::new_v1(&["", "\n"],
&[format_argument::new_debug(&arr.0[20])]));
};I guessed that the reference before struct Arr([u8; 12]);
fn main() {
let arr = Arr(*b"Hello, world");
// Panics
(&arr.0)[20];
}In fact, this happens even if you remove the enclosing struct: fn main() {
let arr: [u8; 12] = *b"Hello, world";
// Panic
(&arr)[20];fn main() {
let arr: [u8; 12] = *b"Hello, world";
// Compile error
arr[20];
}So it appears that the issue is that the lint to check whether a constant index into a constant array or slice will be out of bounds occurs with arrays but not with slices. I don't know if this is a bug or not. |
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@rustbot label +A-lint |
rustbot
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jieyouxu
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here's another example pub fn returns_slice() -> [(); 1] {
[()]
}
pub struct SliceStruct(pub [(); 1]);
pub fn built_for_this() -> SliceStruct {
SliceStruct([()])
}
fn main() {
// compiler error
returns_slice()[1];
// panics
built_for_this().0[1];
// compiler error
SliceStruct([()]).0[1];
} |
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Fixed by #129517: https://rust.godbolt.org/z/KWK8c5Mr8 |
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I tried these two codes:
I expected to see this happen:
When running
cargo build,"index out of bounds: the length is 12 but the index is 20" should be indicated for both codes
Instead, this happened:
Only Code 2 had it
Meta
rustc --version --verbose:Backtrace
Building:
Running(Code 1):
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