Fall back to rustc's default value for crt-static/static_flag

We do this by storing the default target features in `TargetInfo`.

dev-tools/gen-target-info/src/main.rs

@@ -25,6 +25,7 @@ fn generate_target_mapping(f: &mut File, target_specs: &RustcTargetSpecs) -> std
         let os = spec.os.as_deref().unwrap_or("none");
         let env = spec.env.as_deref().unwrap_or("");
         let abi = spec.abi.as_deref().unwrap_or("");
+        let features = spec.cfgs.target_features.join(",");
 
         let unversioned_llvm_target = if spec.llvm_target.contains("apple") {
             // Remove deployment target information from LLVM target triples (we
@@ -84,6 +85,15 @@ fn generate_target_mapping(f: &mut File, target_specs: &RustcTargetSpecs) -> std
             f,
             "            unversioned_llvm_target: {unversioned_llvm_target:?},"
         )?;
+        // NOTE: Features are generated from nightly versions, which will
+        // result in unstable values being output here as well. That is
+        // probably desirable since:
+        // 1. They're only used when `cc` is used outside a build script, and
+        //    then we can't do feature detection, so we have to pick either
+        //    the stable or the nightly representation.
+        // 2. The nightly representation is much more feature-ful, and `cc`'s
+        //    conversion is going to be best-effort anyhow.
+        writeln!(f, "            features: {features:?},")?;
         writeln!(f, "        }},")?;
         writeln!(f, "    ),")?;
     }

dev-tools/gen-target-info/src/target_specs.rs

@@ -37,8 +37,7 @@ pub struct RustcTargetSpecs(
 );
 
 /// Potentially useful values from:
-/// https://doc.rust-lang.org/reference/conditional-compilation.html
-///
+/// <https://doc.rust-lang.org/reference/conditional-compilation.html>
 /// That are not directly / easily exposed in `TargetSpec`.
 #[derive(Debug, Default)]
 pub struct Cfgs {

src/lib.rs

@@ -793,6 +793,10 @@ impl Build {
     /// When enabled on systems that support dynamic linking, this prevents
     /// linking with the shared libraries.
     ///
+    /// If not specified, this falls back to:
+    /// - `-Ctarget-features=+crt-static` when compiling in a build script.
+    /// - A target-specific default.
+    ///
     /// # Example
     ///
     /// ```no_run
@@ -1301,6 +1305,9 @@ impl Build {
     /// 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.
+    ///
+    /// If not specified, this falls back to `-Ctarget-features=+crt-static`
+    /// when compiling in a build script.
     pub fn static_crt(&mut self, static_crt: bool) -> &mut Build {
         self.static_crt = Some(static_crt);
         self
@@ -1944,18 +1951,10 @@ 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");
-                        let features = features.as_deref().unwrap_or_default();
-                        if features.to_string_lossy().contains("crt-static") {
-                            "-MT"
-                        } else {
-                            "-MD"
-                        }
-                    }
+                let crt_flag = if self.static_crt.unwrap_or_else(|| target.crt_static()) {
+                    "-MT"
+                } else {
+                    "-MD"
                 };
                 cmd.push_cc_arg(crt_flag.into());
 
@@ -2142,12 +2141,8 @@ impl Build {
                     cmd.args.push("-finput-charset=utf-8".into());
                 }
 
-                if self.static_flag.is_none() {
-                    let features = self.getenv("CARGO_CFG_TARGET_FEATURE");
-                    let features = features.as_deref().unwrap_or_default();
-                    if features.to_string_lossy().contains("crt-static") {
-                        cmd.args.push("-static".into());
-                    }
+                if self.static_flag.is_none() && target.crt_static() {
+                    cmd.args.push("-static".into());
                 }
 
                 // armv7 targets get to use armv7 instructions

src/target.rs

@@ -43,6 +43,16 @@ pub(crate) struct TargetInfo<'a> {
     ///
     /// This is the same as the value of `cfg!(target_abi)`.
     pub abi: &'a str,
+    /// The set of target features, separated by commas.
+    /// See <https://doc.rust-lang.org/reference/attributes/codegen.html#the-target_feature-attribute>
+    ///
+    /// This is the same as the value of `CARGO_CFG_TARGET_FEATURE`, and
+    /// can be overwritten by the user with `-Ctarget-feature=...`.
+    ///
+    /// This means it also includes the special feature `crt-static`.
+    /// NOTE: The default value is not available to build scripts, see:
+    /// <https://github.com/rust-lang/cargo/issues/14778>
+    features: &'a str,
     /// The unversioned LLVM/Clang target triple.
     unversioned_llvm_target: &'a str,
 }
@@ -66,6 +76,13 @@ impl FromStr for TargetInfo<'_> {
     }
 }
 
+impl TargetInfo<'_> {
+    /// See <https://doc.rust-lang.org/reference/linkage.html#static-and-dynamic-c-runtimes>
+    pub(crate) fn crt_static(&self) -> bool {
+        self.features.contains("crt-static")
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use std::str::FromStr;