Extend the build directive syntax with cargo::metadata=

Author: 0xPoe

src/cargo/core/compiler/custom_build.rs

@@ -643,28 +643,55 @@ impl BuildOutput {
                 Ok(line) => line.trim(),
                 Err(..) => continue,
             };
-            let mut iter = line.splitn(2, ':');
+            let mut new_syntax = true;
+            let mut iter = line.splitn(2, "::");
             if iter.next() != Some("cargo") {
-                // skip this line since it doesn't start with "cargo:"
-                continue;
+                // For backwards compatibility, we also accept lines that start with "cargo:".
+                new_syntax = false;
+                iter = line.splitn(2, ":");
+                if iter.next() != Some("cargo") {
+                    // skip this line since it doesn't start with "cargo:" or "cargo::".
+                    continue;
+                }
             }
             let data = match iter.next() {
                 Some(val) => val,
                 None => continue,
             };
 
-            // getting the `key=value` part of the line
-            let mut iter = data.splitn(2, '=');
+            // getting the `key=value` part of the line]
+            let n = if new_syntax {
+                // cargo::metadata=key=value
+                3
+            } else {
+                // cargo:key=value
+                2
+            };
+            let mut iter = data.splitn(n, '=');
+            if new_syntax {
+                if iter.next() != Some("metadata") {
+                    // skip this line since it doesn't start with "cargo::metadata=".
+                    bail!("invalid output in {}: `{}`\n\
+                    Expected a line with `cargo::metadata=key=value` but it did not start with `cargo::metadata=`.\n\
+                    See https://doc.rust-lang.org/cargo/reference/build-scripts.html#outputs-of-the-build-script \
+                    for more information about build script outputs.", whence, line);
+                }
+            }
             let key = iter.next();
             let value = iter.next();
             let (key, value) = match (key, value) {
                 (Some(a), Some(b)) => (a, b.trim_end()),
-                // Line started with `cargo:` but didn't match `key=value`.
+                // Line started with `cargo:` or `cargo::metadata=` but didn't match `key=value`.
                 _ => bail!("invalid output in {}: `{}`\n\
-                    Expected a line with `cargo:key=value` with an `=` character, \
-                    but none was found.\n\
+                    Expected a line with `{}` but it did not match `key=value`.\n\
                     See https://doc.rust-lang.org/cargo/reference/build-scripts.html#outputs-of-the-build-script \
-                    for more information about build script outputs.", whence, line),
+                    for more information about build script outputs.", whence, line,
+                    if new_syntax {
+                        "cargo::metadata=key=value"
+                    } else {
+                        "cargo:key=value"
+                    }
+                ),
             };
 
             // This will rewrite paths if the target directory has been moved.