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perf(windows): delay-load system DLLs the proxies don't use #5161

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Merged
ChrisDenton merged 3 commits into from
Oct 28, 2026
Merged

perf(windows): delay-load system DLLs the proxies don't use #5161

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baf0369
perf(windows): delay-load system DLLs the proxies don't use
azchohfi Oct 22, 2026
958542d
build(windows): don't build test and bench harnesses for rustup-init
azchohfi Oct 22, 2026
68cc7a1
build(windows): skip the startup delay-load list in test builds
azchohfi Oct 27, 2026
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16 changes: 16 additions & 0 deletions build.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -70,6 +70,22 @@ fn main() {
for dll in delay_load_dlls {
println!("cargo::rustc-link-arg-bin=rustup-init=/delayload:{dll}.dll");
}
// Also delay load system DLLs that the proxies never call. Every `cargo`,
// `rustc` etc. started through rustup is a separate process on Windows, and
// each of these DLLs (with what it pulls in, such as gdi32 for user32) costs
// start-up time when it is loaded eagerly.
//
// Builds with the `test` feature (CI's test runs) skip this: they also link
// the binary's empty test and bench harnesses, which don't import these
// DLLs, so `/WX` would turn the unused `/delayload` into a link error.
if env::var_os("CARGO_FEATURE_TEST").is_none() {
let delay_load_for_startup = [
"user32", "shell32", "ole32", "oleaut32", "crypt32", "secur32", "ws2_32",
];
for dll in delay_load_for_startup {
println!("cargo::rustc-link-arg-bin=rustup-init=/delayload:{dll}.dll");
}
}
// When using delayload, it's necessary to also link delayimp.lib
// https://learn.microsoft.com/en-us/cpp/build/reference/dependentloadflag?view=msvc-170
println!("cargo::rustc-link-arg-bin=rustup-init=delayimp.lib");
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