Fallback to top-level config.toml if not present in current directory

This also preserves the behavior where x.py will only give a hard error on a missing config file
if it was configured through `--config` or RUST_BOOTSTRAP_CONFIG.
It also removes the top-level fallback for everything except the default path.

src/bootstrap/bootstrap.py

@@ -1233,16 +1233,18 @@ def bootstrap(help_triggered):
     build.verbose = args.verbose
     build.clean = args.clean
 
-    # Read from `--config`, then `RUST_BOOTSTRAP_CONFIG`, then fallback to `config.toml` (if it
-    # exists).
+    # Read from `--config`, then `RUST_BOOTSTRAP_CONFIG`, then `./config.toml`,
+    # then `config.toml` in the root directory.
     toml_path = args.config or os.getenv('RUST_BOOTSTRAP_CONFIG')
-    if not toml_path and os.path.exists('config.toml'):
+    using_default_path = toml_path is None
+    if using_default_path:
         toml_path = 'config.toml'
-
-    if toml_path:
         if not os.path.exists(toml_path):
             toml_path = os.path.join(build.rust_root, toml_path)
 
+    # Give a hard error if `--config` or `RUST_BOOTSTRAP_CONFIG` are set to a missing path,
+    # but not if `config.toml` hasn't been created.
+    if not using_default_path or os.path.exists(toml_path):
         with open(toml_path) as config:
             build.config_toml = config.read()
 

src/bootstrap/config.rs

@@ -647,7 +647,8 @@ impl Config {
         let get_toml = |file: &Path| {
             use std::process;
 
-            let contents = t!(fs::read_to_string(file), "`include` config not found");
+            let contents =
+                t!(fs::read_to_string(file), format!("config file {} not found", file.display()));
             match toml::from_str(&contents) {
                 Ok(table) => table,
                 Err(err) => {
@@ -657,14 +658,24 @@ impl Config {
             }
         };
 
-        // check --config first, then `$RUST_BOOTSTRAP_CONFIG` first, then `config.toml`
+        // Read from `--config`, then `RUST_BOOTSTRAP_CONFIG`, then `./config.toml`, then `config.toml` in the root directory.
         let toml_path = flags
             .config
             .clone()
-            .or_else(|| env::var_os("RUST_BOOTSTRAP_CONFIG").map(PathBuf::from))
-            .unwrap_or_else(|| PathBuf::from("config.toml"));
-        let mut toml =
-            if toml_path.exists() { get_toml(&toml_path) } else { TomlConfig::default() };
+            .or_else(|| env::var_os("RUST_BOOTSTRAP_CONFIG").map(PathBuf::from));
+        let using_default_path = toml_path.is_none();
+        let mut toml_path = toml_path.unwrap_or_else(|| PathBuf::from("config.toml"));
+        if using_default_path && !toml_path.exists() {
+            toml_path = config.src.join(toml_path);
+        }
+
+        // Give a hard error if `--config` or `RUST_BOOTSTRAP_CONFIG` are set to a missing path,
+        // but not if `config.toml` hasn't been created.
+        let mut toml = if !using_default_path || toml_path.exists() {
+            get_toml(&toml_path)
+        } else {
+            TomlConfig::default()
+        };
 
         if let Some(include) = &toml.profile {
             let mut include_path = config.src.clone();