Correct the helper name and the tip message

src/bin/cargo/commands/build.rs

@@ -30,7 +30,7 @@ pub fn cli() -> Command {
         )
         .arg_features()
         .arg_release("Build artifacts in release mode, with optimizations")
-        .arg_unsupported_mode("debug", "build", "release")
+        .arg_redundant_default_mode("debug", "build", "release")
         .arg_profile("Build artifacts with the specified profile")
         .arg_parallel()
         .arg_target_triple("Build for the target triple")

src/bin/cargo/commands/install.rs

@@ -83,7 +83,7 @@ pub fn cli() -> Command {
             "debug",
             "Build in debug mode (with the 'dev' profile) instead of release mode",
         ))
-        .arg_unsupported_mode("release", "install", "debug")
+        .arg_redundant_default_mode("release", "install", "debug")
         .arg_profile("Install artifacts with the specified profile")
         .arg_target_triple("Build for the target triple")
         .arg_target_dir()

src/cargo/util/command_prelude.rs

@@ -112,17 +112,15 @@ pub trait CommandExt: Sized {
         self._arg(flag("keep-going", "").value_parser(value_parser).hide(true))
     }
 
-    fn arg_unsupported_mode(
+    fn arg_redundant_default_mode(
         self,
-        want: &'static str,
+        default_mode: &'static str,
         command: &'static str,
-        actual: &'static str,
+        supported_mode: &'static str,
     ) -> Self {
-        let msg = format!(
-            "There is no `--{want}` for `cargo {command}`. Only `--{actual}` is supported."
-        );
+        let msg = format!("`--{default_mode}` is the default for `cargo {command}`; instead `--{supported_mode}` is supported");
         let value_parser = UnknownArgumentValueParser::suggest(msg);
-        self._arg(flag(want, "").value_parser(value_parser).hide(true))
+        self._arg(flag(default_mode, "").value_parser(value_parser).hide(true))
     }
 
     fn arg_targets_all(

tests/testsuite/build.rs

@@ -136,7 +136,7 @@ fn incremental_config() {
 }
 
 #[cargo_test]
-fn cargo_compile_with_unsupported_mode() {
+fn cargo_compile_with_redundant_default_mode() {
     let p = project()
         .file("Cargo.toml", &basic_bin_manifest("foo"))
         .file("src/foo.rs", &main_file(r#""i am foo""#, &[]))
@@ -147,7 +147,7 @@ fn cargo_compile_with_unsupported_mode() {
             "\
 error: unexpected argument '--debug' found
 
-  tip: There is no `--debug` for `cargo build`. Only `--release` is supported.
+  tip: `--debug` is the default for `cargo build`; instead `--release` is supported
 
 Usage: cargo[EXE] build [OPTIONS]
 

tests/testsuite/install.rs

@@ -2445,15 +2445,15 @@ fn ambiguous_registry_vs_local_package() {
 }
 
 #[cargo_test]
-fn install_with_unsupported_mode() {
+fn install_with_redundant_default_mode() {
     pkg("foo", "0.0.1");
 
     cargo_process("install foo --release")
         .with_stderr(
             "\
 error: unexpected argument '--release' found
 
-  tip: There is no `--release` for `cargo install`. Only `--debug` is supported.
+  tip: `--release` is the default for `cargo install`; instead `--debug` is supported
 
 Usage: cargo[EXE] install [OPTIONS] [crate]...