MSVC: Add support for linking against the "spectre-mitigated" CRT

Issue Details:
Since VS 2017, MSVC has shipped a set of "spectre-mitigated" CRT static libs: https://devblogs.microsoft.com/cppblog/spectre-mitigations-in-msvc/

Typically these are used by opening a VS Command Prompt in "spectre mode" (https://docs.microsoft.com/en-us/cpp/build/building-on-the-command-line?view=msvc-170#vcvarsall-syntax) which then sets the `LIB` environment variable to point to the directory with the spectre-mitigated libs. However, since `cc` builds its own `LIB` environment variable, it uses the non-spectre-mitigated libs even when invoked from a VS Command Prompt in "spectre mode". This causes issues when trying to build a spectre-mitigated binary using Rust, as `rustc` uses `cc` for linking.

Fix Details:
When `cc` detects that the `VSCMD_ARG_VCVARS_SPECTRE` environment variable is set to `spectre` (either by being run from a VS Command Prompt in "spectre mode", or users may explicitly set this themselves), it will use the spectre-mitigated lib directory when building its `LIB` environment variable.

dev-tools/cc-test/build.rs

@@ -83,9 +83,6 @@ fn main() {
         cc::Build::new().file("src/windows.c").compile("windows");
     }
 
-    // Test that the `windows_registry` module will set PATH by looking for
-    // nmake which runs vanilla cl, and then also test it after we remove all
-    // the relevant env vars from our own process.
     if target.contains("msvc") {
         let cc_frontend = if compiler.is_like_msvc() {
             "MSVC"
@@ -95,6 +92,9 @@ fn main() {
             unimplemented!("Unknown compiler that targets msvc but isn't clang-like or msvc-like")
         };
 
+        // Test that the `windows_registry` module will set PATH by looking for
+        // nmake which runs vanilla cl, and then also test it after we remove all
+        // the relevant env vars from our own process.
         let out = out.join("tmp");
         fs::create_dir(&out).unwrap();
         println!("nmake 1");
@@ -130,6 +130,28 @@ fn main() {
         assert!(status.success());
         println!("cargo:rustc-link-lib=msvc");
         println!("cargo:rustc-link-search={}", out.display());
+
+        // Test that the `windows_registry` module detects if we're in a "spectre
+        // mode" VS environment.
+        fn has_spectre(target: &str) -> bool {
+            cc::windows_registry::find_tool(target, "cl.exe")
+                .unwrap()
+                .env()
+                .iter()
+                .any(|(k, v)| (k == "LIB") && v.to_str().unwrap().contains(r"\lib\spectre\"))
+        }
+
+        std::env::set_var("VSCMD_ARG_VCVARS_SPECTRE", "spectre");
+        assert!(
+            has_spectre(&target),
+            "LIB should use spectre-mitigated libs when VSCMD_ARG_VCVARS_SPECTRE is set"
+        );
+
+        std::env::remove_var("VSCMD_ARG_VCVARS_SPECTRE");
+        assert!(
+            !has_spectre(&target),
+            "LIB should not use spectre-mitigated libs when VSCMD_ARG_VCVARS_SPECTRE is not set"
+        );
     }
 
     // This tests whether we  can build a library but not link it to the main

src/windows/find_tools.rs

@@ -575,8 +575,13 @@ mod impl_ {
         // architecture, because it contains dlls like mspdb140.dll compiled for
         // the host architecture.
         let host_dylib_path = host_path.join(host.to_lowercase());
-        let lib_path = path.join("lib").join(target);
-        let alt_lib_path = (target == "arm64ec").then(|| path.join("lib").join("arm64ec"));
+        let lib_fragment = if use_spectre_mitigated_libs() {
+            r"lib\spectre"
+        } else {
+            "lib"
+        };
+        let lib_path = path.join(lib_fragment).join(target);
+        let alt_lib_path = (target == "arm64ec").then(|| path.join(lib_fragment).join("arm64ec"));
         let include_path = path.join("include");
         Some((
             path,
@@ -625,6 +630,10 @@ mod impl_ {
         Some(version)
     }
 
+    fn use_spectre_mitigated_libs() -> bool {
+        env::var("VSCMD_ARG_VCVARS_SPECTRE").as_deref() == Ok("spectre")
+    }
+
     fn atl_paths(target: TargetArch<'_>, path: &Path) -> Option<(PathBuf, PathBuf)> {
         let atl_path = path.join("atlmfc");
         let sub = lib_subdir(target)?;