[msvc] Use -MT -Zl unconditionally.

Since this crate doesn't actually make decisions about
final application linking, it's only appropriate to not
interfere with the process. This can be achieved with
-MT -Zl, because object modules compiled with these flags
can be linked with either VCCRT version, static or dynamic,
debug or release...

For reference, -Zl doesn't affect linking directives set with
in Windows headers to add references to not-so-common libraries
like mfc.lib, windowscodecs.lib to name just a pair.

src/lib.rs

@@ -922,9 +922,9 @@ impl Build {
         self
     }
 
-    /// Configures whether the /MT flag or the /MD flag will be passed to msvc build tools.
+    /// No-op option retained for backward compatibility.
     ///
-    /// This option defaults to `false`, and affect only msvc targets.
+    /// Used to configure whether the /MT flag or the /MD flag will be passed to msvc build tools.
     pub fn static_crt(&mut self, static_crt: bool) -> &mut Build {
         self.static_crt = Some(static_crt);
         self
@@ -1487,21 +1487,14 @@ impl Build {
             ToolFamily::Msvc { .. } => {
                 cmd.push_cc_arg("-nologo".into());
 
-                let crt_flag = match self.static_crt {
-                    Some(true) => "-MT",
-                    Some(false) => "-MD",
-                    None => {
-                        let features = self
-                            .getenv("CARGO_CFG_TARGET_FEATURE")
-                            .unwrap_or(String::new());
-                        if features.contains("crt-static") {
-                            "-MT"
-                        } else {
-                            "-MD"
-                        }
-                    }
-                };
-                cmd.push_cc_arg(crt_flag.into());
+                // Since this crate doesn't actually make decisions about
+                // final application linking, it's only appropriate to not
+                // interfere with the process. This can be achieved with
+                // -MT -Zl, because object modules compiled with these flags
+                // can be linked with either VCCRT version, static or dynamic,
+                // debug or release...
+                cmd.push_cc_arg("-MT".into());
+                cmd.push_cc_arg("-Zl".into());
 
                 match &opt_level[..] {
                     // Msvc uses /O1 to enable all optimizations that minimize code size.

tests/test.rs

@@ -362,8 +362,7 @@ fn msvc_smoke() {
         .must_have("-O2")
         .must_have("foo.c")
         .must_not_have("-Z7")
-        .must_have("-c")
-        .must_have("-MD");
+        .must_have("-c");
     test.cmd(1).must_have(test.td.path().join("foo.o"));
 }
 
@@ -404,22 +403,6 @@ fn msvc_define() {
     test.cmd(0).must_have("-DFOO=bar").must_have("-DBAR");
 }
 
-#[test]
-fn msvc_static_crt() {
-    let test = Test::msvc();
-    test.gcc().static_crt(true).file("foo.c").compile("foo");
-
-    test.cmd(0).must_have("-MT");
-}
-
-#[test]
-fn msvc_no_static_crt() {
-    let test = Test::msvc();
-    test.gcc().static_crt(false).file("foo.c").compile("foo");
-
-    test.cmd(0).must_have("-MD");
-}
-
 #[test]
 fn msvc_no_dash_dash() {
     let test = Test::msvc();