set the correct rustc and cargo even for tests invoking parse_inner

src/bootstrap/src/core/config/config.rs

@@ -1189,19 +1189,7 @@ impl Config {
     pub fn parse(args: &[String]) -> Config {
         #[cfg(test)]
         fn get_toml(_: &Path) -> TomlConfig {
-            let mut default = TomlConfig::default();
-
-            // When configuring bootstrap for tests, make sure to set the rustc and Cargo to the
-            // same ones used to call the tests. If we don't do that, bootstrap will use its own
-            // detection logic to find a suitable rustc and Cargo, which doesn't work when the
-            // caller is specìfying a custom local rustc or Cargo in their config.toml.
-            default.build = Some(Build {
-                rustc: std::env::var_os("RUSTC").map(|v| v.into()),
-                cargo: std::env::var_os("CARGO").map(|v| v.into()),
-                ..Build::default()
-            });
-
-            default
+            TomlConfig::default()
         }
 
         #[cfg(not(test))]
@@ -1341,6 +1329,18 @@ impl Config {
             TomlConfig::default()
         };
 
+        #[cfg(test)]
+        {
+            // When configuring bootstrap for tests, make sure to set the rustc and Cargo to the
+            // same ones used to call the tests (if custom ones are not defined in the toml). If we
+            // don't do that, bootstrap will use its own detection logic to find a suitable rustc
+            // and Cargo, which doesn't work when the caller is specìfying a custom local rustc or
+            // Cargo in their config.toml.
+            let build = toml.build.get_or_insert_default();
+            build.rustc = build.rustc.or(std::env::var_os("RUSTC").into());
+            build.cargo = build.cargo.or(std::env::var_os("CARGO").into());
+        }
+
         if let Some(include) = &toml.profile {
             // Allows creating alias for profile names, allowing
             // profiles to be renamed while maintaining back compatibility