Auto merge of #9625 - hi-rustin:rustin-msg, r=ehuss

Include toolchain specification in error message

close https://github.com/rust-lang/rustup/issues/2795

Author: bors

src/cargo/core/compiler/standard_lib.rs

@@ -200,11 +200,19 @@ fn detect_sysroot_src_path(target_data: &RustcTargetData<'_>) -> CargoResult<Pat
         .join("rust");
     let lock = src_path.join("Cargo.lock");
     if !lock.exists() {
-        anyhow::bail!(
+        let msg = format!(
             "{:?} does not exist, unable to build with the standard \
              library, try:\n        rustup component add rust-src",
             lock
         );
+        match env::var("RUSTUP_TOOLCHAIN") {
+            Ok(rustup_toolchain) => {
+                anyhow::bail!("{} --toolchain {}", msg, rustup_toolchain);
+            }
+            Err(_) => {
+                anyhow::bail!(msg);
+            }
+        }
     }
     Ok(src_path)
 }