explain that the size_of constant also serves to avoid optimizing away 'unused' size_of calls#157587
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…y 'unused' size_of calls
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explain that the size_of constant also serves to avoid optimizing away 'unused' size_of calls
During a discussion at the all-hands I got worried that we might do uncanny optimizations on code like this which would lead to that code actually compiling
```rust
#![crate_type = "lib"]
#[unsafe(no_mangle)] // ensure this gets monomorphized
pub fn f() {
assert_valid_type::<[u32; 1 << 62]>();
}
pub fn assert_valid_type<T>() {
std::mem::size_of::<T>();
}
```
We do something like that for [even more cursed similar code](https://play.rust-lang.org/?version=nightly&mode=release&edition=2021&gist=41b84c5e705c9d86a05a919e2135e2d3).
But turns out that for `size_of`, we are good! Ever since that was made a constant, the required_consts system ensures that we do indeed evaluate the constant even if we do not use its results.
r? @oli-obk
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…uwer Rollup of 13 pull requests Successful merges: - #147302 (asm! support for the Xtensa architecture) - #148820 (Add very basic "comptime" fn implementation) - #157299 (Fix unstable diagnostics in tests) - #143511 (Improve TLS codegen by marking the panic/init path as cold) - #154608 (Add `_value` API for number literals in proc-macro) - #156762 (xfs support in `test_rename_directory_to_non_empty_directory`) - #157300 (Relax test requirements for consistency) - #157383 (tests: codegen-llvm: Ignore BPF targets in c-variadic-opt) - #157413 (fix: don't suggest .into_iter() for .cloned()/.copied() on non-reference Option) - #157578 (Fix diagnostics for non-exhaustive destructuring assignments (#157553)) - #157587 (explain that the size_of constant also serves to avoid optimizing away 'unused' size_of calls) - #157596 (test: remove ineffective link-extern-crate-with-drop-type test) - #157602 (rustdoc: Remove unnecessary fast path)
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Rollup merge of #157587 - RalfJung:size-of-no-dce, r=oli-obk explain that the size_of constant also serves to avoid optimizing away 'unused' size_of calls During a discussion at the all-hands I got worried that we might do uncanny optimizations on code like this which would lead to that code actually compiling ```rust #![crate_type = "lib"] #[unsafe(no_mangle)] // ensure this gets monomorphized pub fn f() { assert_valid_type::<[u32; 1 << 62]>(); } pub fn assert_valid_type<T>() { std::mem::size_of::<T>(); } ``` We do something like that for [even more cursed similar code](https://play.rust-lang.org/?version=nightly&mode=release&edition=2021&gist=41b84c5e705c9d86a05a919e2135e2d3). But turns out that for `size_of`, we are good! Ever since that was made a constant, the required_consts system ensures that we do indeed evaluate the constant even if we do not use its results. r? @oli-obk
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During a discussion at the all-hands I got worried that we might do uncanny optimizations on code like this which would lead to that code actually compiling
We do something like that for even more cursed similar code.
But turns out that for
size_of, we are good! Ever since that was made a constant, the required_consts system ensures that we do indeed evaluate the constant even if we do not use its results.r? @oli-obk