Old syntax suggestion.

src/cargo/core/compiler/custom_build.rs

@@ -725,13 +725,26 @@ impl BuildOutput {
         fn check_minimum_supported_rust_version_for_new_syntax(
             pkg_descr: &str,
             msrv: &Option<RustVersion>,
+            key: &str,
         ) -> CargoResult<()> {
             if let Some(msrv) = msrv {
                 let new_syntax_added_in = RustVersion::from_str("1.77.0")?;
                 if !new_syntax_added_in.is_compatible_with(msrv.as_partial()) {
+                    let prefix = format!("{key}=");
+
+                    let old_syntax_suggestion = RESERVED_PREFIXES
+                        .contains(&&*prefix)
+                        .then(|| {
+                            format!(
+                                "Consider using the old `cargo:` syntax in front of `{prefix}`.\n"
+                            )
+                        })
+                        .unwrap_or_default();
+
                     bail!(
                         "the `cargo::` syntax for build script output instructions was added in \
                         Rust 1.77.0, but the minimum supported Rust version of `{pkg_descr}` is {msrv}.\n\
+                        {old_syntax_suggestion}\
                         {DOCS_LINK_SUGGESTION}"
                     );
                 }
@@ -793,9 +806,10 @@ impl BuildOutput {
             };
             let mut old_syntax = false;
             let (key, value) = if let Some(data) = line.strip_prefix("cargo::") {
-                check_minimum_supported_rust_version_for_new_syntax(pkg_descr, msrv)?;
                 // For instance, `cargo::rustc-flags=foo` or `cargo::metadata=foo=bar`.
-                parse_directive(whence.as_str(), line, data, old_syntax)?
+                let (key, value) = parse_directive(whence.as_str(), line, data, old_syntax)?;
+                check_minimum_supported_rust_version_for_new_syntax(pkg_descr, msrv, key)?;
+                (key, value)
             } else if let Some(data) = line.strip_prefix("cargo:") {
                 old_syntax = true;
                 // For instance, `cargo:rustc-flags=foo`.

tests/testsuite/build_script.rs

@@ -5508,6 +5508,47 @@ for more information about build script outputs.
         .run();
 }
 
+#[cargo_test]
+fn test_new_syntax_with_old_msrv_and_reserved_prefix() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.5.0"
+                edition = "2015"
+                authors = []
+                build = "build.rs"
+                rust-version = "1.60.0"
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .file(
+            "build.rs",
+            r#"
+                fn main() {
+                    println!("cargo::rustc-check-cfg=cfg(foo)");
+                }
+            "#,
+        )
+        .build();
+
+    p.cargo("build")
+        .with_status(101)
+        .with_stderr_contains(
+            "\
+[COMPILING] foo [..]
+error: the `cargo::` syntax for build script output instructions was added in Rust 1.77.0, \
+but the minimum supported Rust version of `foo v0.5.0 ([ROOT]/foo)` is 1.60.0.
+Consider using the old `cargo:` syntax in front of `rustc-check-cfg=`.
+See https://doc.rust-lang.org/cargo/reference/build-scripts.html#outputs-of-the-build-script \
+for more information about build script outputs.
+",
+        )
+        .run();
+}
+
 #[cargo_test]
 fn test_new_syntax_with_compatible_partial_msrv() {
     let p = project()