Escape trailing backslashes in build script env vars

cargo/cargo_build_script_runner/lib.rs

@@ -125,7 +125,7 @@ impl BuildScriptOutput {
         v.iter()
             .filter_map(|x| {
                 if let BuildScriptOutput::Env(env) = x {
-                    Some(Self::redact_exec_root(env, exec_root))
+                    Some(Self::escape_for_serializing(Self::redact_exec_root(env, exec_root)))
                 } else {
                     None
                 }
@@ -143,7 +143,7 @@ impl BuildScriptOutput {
                     Some(format!(
                         "{}{}",
                         prefix,
-                        Self::redact_exec_root(env, exec_root)
+                        Self::escape_for_serializing(Self::redact_exec_root(env, exec_root))
                     ))
                 } else {
                     None
@@ -176,6 +176,19 @@ impl BuildScriptOutput {
     fn redact_exec_root(value: &str, exec_root: &str) -> String {
         value.replace(exec_root, "${pwd}")
     }
+
+    // The process-wrapper treats trailing backslashes as escapes for following newlines.
+    // If the env var ends with a backslash (and accordingly doesn't have a following newline),
+    // escape it so that it doesn't get turned into a newline by the process-wrapper.
+    //
+    // Note that this code doesn't handle newlines in strings - that's because Cargo treats build
+    // script output as single-line-oriented, so stops processing at the end of a line regardless.
+    fn escape_for_serializing(mut value: String) -> String {
+        if value.ends_with('\\') {
+            value.push('\\');
+        }
+        value
+    }
 }
 
 #[cfg(test)]

test/build_env/src/build.rs

@@ -1,4 +1,5 @@
 fn main() {
     println!("cargo:rustc-env=CARGO_PKG_NAME_FROM_BUILD_SCRIPT={}", env!("CARGO_PKG_NAME"));
     println!("cargo:rustc-env=CARGO_CRATE_NAME_FROM_BUILD_SCRIPT={}", env!("CARGO_CRATE_NAME"));
+    println!("cargo:rustc-env=HAS_TRAILING_SLASH=foo\\");
 }

test/build_env/tests/cargo.rs

@@ -4,4 +4,5 @@ fn cargo_env_vars() {
     assert_eq!(env!("CARGO_CRATE_NAME"), "cargo_env_vars_test");
     assert_eq!(env!("CARGO_PKG_NAME_FROM_BUILD_SCRIPT"), "cargo_build_script_env-vars");
     assert_eq!(env!("CARGO_CRATE_NAME_FROM_BUILD_SCRIPT"), "cargo_build_script_env_vars");
+    assert_eq!(env!("HAS_TRAILING_SLASH"), "foo\\");
 }