Adjusted the suggestion text.

src/cargo/core/compiler/custom_build.rs

@@ -737,10 +737,12 @@ impl BuildOutput {
  let new_syntax_added_in = RustVersion::from_str("1.77.0")?;
  if !new_syntax_added_in.is_compatible_with(msrv.as_partial()) {
  let old_syntax_suggestion = if has_reserved_prefix(flag) {
- format!("Consider using the old `cargo:` syntax in front of `{flag}`.\n")
+ format!(
+ "Switch to the old `cargo:{flag}` syntax (note the single colon).\n"
+ )
  } else if flag.starts_with("metadata=") {
  let old_format_flag = flag.strip_prefix("metadata=").unwrap();
- format!("Consider using the old `cargo:{old_format_flag}` syntax instead of `cargo::{flag}` (note the single colon).\n")
+ format!("Switch to the old `cargo:{old_format_flag}` syntax instead of `cargo::{flag}` (note the single colon).\n")
  } else {
  String::new()
  };

tests/testsuite/build_script.rs

@@ -5501,7 +5501,7 @@ fn test_new_syntax_with_old_msrv() {
 [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:foo=bar` syntax instead of `cargo::metadata=foo=bar` (note the single colon).
+Switch to the old `cargo:foo=bar` syntax instead of `cargo::metadata=foo=bar` (note the single colon).
 See https://doc.rust-lang.org/cargo/reference/build-scripts.html#outputs-of-the-build-script \
 for more information about build script outputs.
 ",
@@ -5542,7 +5542,7 @@ fn test_new_syntax_with_old_msrv_and_reserved_prefix() {
 [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=cfg(foo)`.
+Switch to the old `cargo:rustc-check-cfg=cfg(foo)` syntax (note the single colon).
 See https://doc.rust-lang.org/cargo/reference/build-scripts.html#outputs-of-the-build-script \
 for more information about build script outputs.
 ",