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.

cc-test/build.rs

@@ -57,10 +57,10 @@ 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") {
+        // 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");
@@ -91,6 +91,26 @@ 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_registry.rs

@@ -488,11 +488,24 @@ mod impl_ {
             .join("bin")
             .join(&format!("Host{}", host))
             .join(&host.to_lowercase());
-        let lib_path = path.join("lib").join(&target);
+        let lib_path = path
+            .join(if use_spectre_mitigated_libs() {
+                r"lib\spectre"
+            } else {
+                "lib"
+            })
+            .join(&target);
         let include_path = path.join("include");
         Some((bin_path, host_dylib_path, lib_path, include_path))
     }
 
+    fn use_spectre_mitigated_libs() -> bool {
+        match env::var("VSCMD_ARG_VCVARS_SPECTRE") {
+            Ok(s) => s == "spectre",
+            _ => false,
+        }
+    }
+
     fn atl_paths(target: &str, path: &Path) -> Option<(PathBuf, PathBuf)> {
         let atl_path = path.join("atlmfc");
         let sub = lib_subdir(target)?;