feat: allow usage of turbo without turbo.json (#9149)

### Description

This PR adds the ability to use `turbo` in a monorepo that doesn't have
a `turbo.json`. This feature is currently gated behind
`--experimental-allow-no-turbo-json`/`TURBO_ALLOW_NO_TURBO_JSON`.

A majority of the PR is refactoring the `EngineBuilder` so that it no
longer directly loads `TurboJson`s, but delegates to a
`TurboJsonLoader`. This allows us to use different strategies for
resolving `TurboJson` loads depending on runtime options e.g. single
package mode or task access.

Reviewing this PR is best done by viewing each commit individually.

### Testing Instructions

Unit testing for `turbo.json` loading changes.

Integration test for verifying the new loader is activated with the new
env var/flag.

crates/turborepo-lib/src/cli/mod.rs

@@ -222,6 +222,8 @@ pub struct Args {
     /// should be used.
     #[clap(long, global = true)]
     pub dangerously_disable_package_manager_check: bool,
+    #[clap(long = "experimental-allow-no-turbo-json", hide = true, global = true)]
+    pub allow_no_turbo_json: bool,
     /// Use the `turbo.json` located at the provided path instead of one at the
     /// root of the repository.
     #[clap(long, global = true)]

crates/turborepo-lib/src/commands/mod.rs

@@ -108,6 +108,7 @@ impl CommandBase {
                     .and_then(|args| args.remote_cache_read_only()),
             )
             .with_run_summary(self.args.run_args().and_then(|args| args.summarize()))
+            .with_allow_no_turbo_json(self.args.allow_no_turbo_json.then_some(true))
             .build()
     }
 

crates/turborepo-lib/src/config/env.rs

@@ -38,6 +38,7 @@ const TURBO_MAPPING: &[(&str, &str)] = [
     ("turbo_remote_only", "remote_only"),
     ("turbo_remote_cache_read_only", "remote_cache_read_only"),
     ("turbo_run_summary", "run_summary"),
+    ("turbo_allow_no_turbo_json", "allow_no_turbo_json"),
 ]
 .as_slice();
 
@@ -86,6 +87,7 @@ impl ResolvedConfigurationOptions for EnvVars {
         let remote_only = self.truthy_value("remote_only").flatten();
         let remote_cache_read_only = self.truthy_value("remote_cache_read_only").flatten();
         let run_summary = self.truthy_value("run_summary").flatten();
+        let allow_no_turbo_json = self.truthy_value("allow_no_turbo_json").flatten();
 
         // Process timeout
         let timeout = self
@@ -171,6 +173,7 @@ impl ResolvedConfigurationOptions for EnvVars {
             remote_only,
             remote_cache_read_only,
             run_summary,
+            allow_no_turbo_json,
 
             // Processed numbers
             timeout,
@@ -317,6 +320,7 @@ mod test {
         env.insert("turbo_remote_only".into(), "1".into());
         env.insert("turbo_remote_cache_read_only".into(), "1".into());
         env.insert("turbo_run_summary".into(), "true".into());
+        env.insert("turbo_allow_no_turbo_json".into(), "true".into());
 
         let config = EnvVars::new(&env)
             .unwrap()
@@ -328,6 +332,7 @@ mod test {
         assert!(config.remote_only());
         assert!(config.remote_cache_read_only());
         assert!(config.run_summary());
+        assert!(config.allow_no_turbo_json());
         assert_eq!(turbo_api, config.api_url.unwrap());
         assert_eq!(turbo_login, config.login_url.unwrap());
         assert_eq!(turbo_team, config.team_slug.unwrap());
@@ -365,6 +370,7 @@ mod test {
         env.insert("turbo_remote_only".into(), "".into());
         env.insert("turbo_remote_cache_read_only".into(), "".into());
         env.insert("turbo_run_summary".into(), "".into());
+        env.insert("turbo_allow_no_turbo_json".into(), "".into());
 
         let config = EnvVars::new(&env)
             .unwrap()
@@ -387,6 +393,7 @@ mod test {
         assert!(!config.remote_only());
         assert!(!config.remote_cache_read_only());
         assert!(!config.run_summary());
+        assert!(!config.allow_no_turbo_json());
     }
 
     #[test]

crates/turborepo-lib/src/config/mod.rs

@@ -17,6 +17,7 @@ use thiserror::Error;
 use turbo_json::TurboJsonReader;
 use turbopath::{AbsoluteSystemPath, AbsoluteSystemPathBuf};
 use turborepo_errors::TURBO_SITE;
+use turborepo_repository::package_graph::PackageName;
 
 pub use crate::turbo_json::{RawTurboJson, UIMode};
 use crate::{
@@ -179,6 +180,8 @@ pub enum Error {
         #[source_code]
         text: NamedSource,
     },
+    #[error("Cannot load turbo.json for in {0} single package mode")]
+    InvalidTurboJsonLoad(PackageName),
 }
 
 const DEFAULT_API_URL: &str = "https://vercel.com/api";
@@ -239,6 +242,7 @@ pub struct ConfigurationOptions {
     pub(crate) remote_only: Option<bool>,
     pub(crate) remote_cache_read_only: Option<bool>,
     pub(crate) run_summary: Option<bool>,
+    pub(crate) allow_no_turbo_json: Option<bool>,
 }
 
 #[derive(Default)]
@@ -367,6 +371,10 @@ impl ConfigurationOptions {
             .clone()
             .unwrap_or_else(|| repo_root.join_component(CONFIG_FILE))
     }
+
+    pub fn allow_no_turbo_json(&self) -> bool {
+        self.allow_no_turbo_json.unwrap_or_default()
+    }
 }
 
 // Maps Some("") to None to emulate how Go handles empty strings