[msvc] Don't use -M* flags.

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 by
relying on default MSVC behaviour that allows linking
with either VCCRT version, static or dynamic...

Author: dot-asm

src/lib.rs

@@ -976,9 +976,20 @@ impl Build {
         self
     }
 
-    /// Configures whether the /MT flag or the /MD flag will be passed to msvc build tools.
-    ///
-    /// This option defaults to `false`, and affect only msvc targets.
+    /// No-op option retained for backward compatibility.
+    ///
+    /// Originally the method was introduced to harmonize with externally
+    /// enforced `-MT` flag and resolve warnings in Rust bootstrap procedure
+    /// on `*-windows-msvc` targets. It's argued that a more versatile approach
+    /// is to **not** specify any of the `-M*` flags in cc-rs, but rely on
+    /// the default MSVC compiler behaviour or externally enforced flag[s].
+    /// The default behaviour allows forced linking with either VCCRT library,
+    /// static or dynamic, whichever chosen by rustc at link time.
+    ///
+    /// On a related note, if you compile Rust static library for use with an
+    /// external application, it's likely to be appropriate to pass `-Zl` flag
+    /// in order to not interfere with the target application's linking
+    /// procedure.
     pub fn static_crt(&mut self, static_crt: bool) -> &mut Build {
         self.static_crt = Some(static_crt);
         self
@@ -1549,22 +1560,6 @@ 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());
-
                 match &opt_level[..] {
                     // Msvc uses /O1 to enable all optimizations that minimize code size.
                     "z" | "s" | "1" => cmd.push_opt_unless_duplicate("-O1".into()),

tests/test.rs

@@ -377,8 +377,7 @@ 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"));
 }
 
@@ -419,22 +418,6 @@ 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();