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Correct non-ZST too-large-array-size check in MapWindows::new #147891

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Correct non-ZST too-large-array-size check in MapWindows::new #147891

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@sigurd4 sigurd4 commented Oct 19, 2025 •
edited by rustbot
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Happened to look through the source code and found what seems to be a logical error in the check in MapWindows::new. There's supposed to be a check to prevent too large arrays for non-zero-sized types, but it seems to be wrong. Isn't this supposed to be != instead of ==?

Feature iter_map_windows issue #87155

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Correct non-ZST too-large-array-size check in MapWindows::new
785f316 
Happened to look through the source code and found what seems to be a logical error in the check in `MapWindows::new`. There's supposed to be a check to prevent too large arrays for non-zero-sized types, but it seems to be wrong. Isn't this supposed to be `!=` instead of `==`?
@rustbot rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Oct 19, 2025
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rustbot commented Oct 19, 2025

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The job aarch64-gnu-llvm-20-2 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot) 
    iter::adapters::map_windows::test_too_large_array_size

test result: FAILED. 2242 passed; 1 failed; 5 ignored; 0 measured; 0 filtered out; finished in 2.86s

error: test failed, to rerun pass `-p coretests --test coretests`
env -u RUSTC_WRAPPER CARGO_ENCODED_RUSTDOCFLAGS="-Csymbol-mangling-version=v0\u{1f}-Zrandomize-layout\u{1f}-Zunstable-options\u{1f}--check-cfg=cfg(bootstrap)\u{1f}-Dwarnings\u{1f}-Wrustdoc::invalid_codeblock_attributes\u{1f}--crate-version\u{1f}1.92.0-nightly\t(c8e65743b\t2025-10-19)" CARGO_ENCODED_RUSTFLAGS="-Csymbol-mangling-version=v0\u{1f}-Zrandomize-layout\u{1f}-Zunstable-options\u{1f}--check-cfg=cfg(bootstrap)\u{1f}-Zmacro-backtrace\u{1f}-Csplit-debuginfo=off\u{1f}-Clink-arg=-L/usr/lib/llvm-20/lib\u{1f}-Cllvm-args=-import-instr-limit=10\u{1f}-Clink-args=-Wl,-z,origin\u{1f}-Clink-args=-Wl,-rpath,$ORIGIN/../lib\u{1f}-Alinker-messages\u{1f}--cap-lints=allow\u{1f}--cfg\u{1f}randomized_layouts" RUSTC="/checkout/obj/build/aarch64-unknown-linux-gnu/stage2-codegen/cg_clif/dist/rustc-clif" RUSTDOC="/checkout/obj/build/aarch64-unknown-linux-gnu/stage2-codegen/cg_clif/dist/rustdoc-clif" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage0/bin/cargo" "test" "--manifest-path" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2-codegen/cg_clif/build/sysroot_tests/Cargo.toml" "--target-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2-codegen/cg_clif/build/sysroot_tests_target" "--locked" "--target" "aarch64-unknown-linux-gnu" "-p" "coretests" "-p" "alloctests" "--tests" "--" "-q" exited with status ExitStatus(unix_wait_status(25856))
Bootstrap failed while executing `--stage 2 test --skip tests --skip coverage-map --skip coverage-run --skip library --skip tidyselftest`
Command `/checkout/obj/build/aarch64-unknown-linux-gnu/stage0/bin/cargo run --target aarch64-unknown-linux-gnu -Zbinary-dep-depinfo -j 4 -Zroot-dir=/checkout --locked --color always --release --manifest-path /checkout/compiler/rustc_codegen_cranelift/build_system/Cargo.toml -- test --download-dir /checkout/obj/build/cg_clif_download --out-dir /checkout/obj/build/aarch64-unknown-linux-gnu/stage2-codegen/cg_clif --no-unstable-features --use-backend cranelift --sysroot llvm --skip-test testsuite.extended_sysroot [workdir=/checkout/compiler/rustc_codegen_cranelift]` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/test.rs:3689:25
Executed at: src/bootstrap/src/core/build_steps/test.rs:3734:26

Command has failed. Rerun with -v to see more details.
Build completed unsuccessfully in 0:21:26
  local time: Sun Oct 19 21:29:15 UTC 2025
  network time: Sun, 19 Oct 2025 21:29:15 GMT
##[error]Process completed with exit code 1.
##[group]Run echo "disk usage:"

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sigurd4 commented Oct 20, 2025

I may have misunderstood the intention of that check, as to why it only performs the check for ZST. Looking at how the unit test for too large arrays start failing, i assume I've misunderstood.

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joboet commented Oct 20, 2025

For non-ZSTs, a [[T; N]; 2] can only exist as a type if [T; N * 2] can exist, otherwise the result is a monomorphisation error. But for ZSTs, there is no such guarantee, as the size in memory is always zero. Hence it's not always possible to obtain a [T; N * 2] from the [[T; N]; 2], but that is needed by MapWindows. In conclusion, == is the right comparison.

@joboet joboet closed this Oct 20, 2025
@rustbot rustbot removed the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Oct 20, 2025
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