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

Cargo.toml

@@ -163,6 +163,16 @@ wildcard_imports = "warn"
 name = "rustup"
 path = "src/lib.rs"
 
+[[bin]]
+name = "rustup-init"
+path = "src/bin/rustup-init.rs"
+# The binary has no unit tests or benchmarks. Its empty test and bench harnesses
+# would otherwise be linked with the binary's delay-load list (see `build.rs`) but
+# without the code that imports those DLLs, which `/WX` turns into a link error
+# (LNK4199).
+test = false
+bench = false
+
 [profile.release]
 codegen-units = 1
 lto = true

build.rs

@@ -70,6 +70,16 @@ 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.
+    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");