Improve sqlx prepare invocation error

sqlx-cli/src/prepare.rs

@@ -41,8 +41,7 @@ pub async fn run(
     connect_opts: ConnectOpts,
     cargo_args: Vec<String>,
 ) -> anyhow::Result<()> {
-    let cargo = env::var_os("CARGO")
-        .context("failed to get value of `CARGO`; `prepare` subcommand may only be invoked as `cargo sqlx prepare`")?;
+    let cargo = cargo_command(env::var_os("CARGO"))?;
 
     anyhow::ensure!(
         Path::new("Cargo.toml").exists(),
@@ -68,6 +67,13 @@ hint: This command only works in the manifest directory of a Cargo package or wo
     }
 }
 
+fn cargo_command(cargo: Option<OsString>) -> anyhow::Result<OsString> {
+    cargo.context(
+        "the `prepare` subcommand must be invoked as `cargo sqlx prepare`; \
+         running `sqlx prepare` directly is not supported",
+    )
+}
+
 async fn prepare(ctx: &PrepareCtx<'_>) -> anyhow::Result<()> {
     if ctx.connect_opts.database_url.is_some() {
         check_backend(ctx.config, &ctx.connect_opts).await?;
@@ -372,6 +378,16 @@ mod tests {
     use super::*;
     use std::assert_eq;
 
+    #[test]
+    fn missing_cargo_environment_explains_prepare_invocation() {
+        let error = cargo_command(None).expect_err("missing CARGO should fail");
+
+        assert_eq!(
+            error.to_string(),
+            "the `prepare` subcommand must be invoked as `cargo sqlx prepare`; running `sqlx prepare` directly is not supported"
+        );
+    }
+
     #[test]
     fn minimal_project_recompile_action_works() -> anyhow::Result<()> {
         let sample_metadata_path = Path::new("tests")