Rollup merge of rust-lang#120475 - Nilstrieb:cargo-build-my-a-, r=michaelwoerister

Improve error message when `cargo build` is used to build the compiler

Inspired by rust-lang#76446.

Doing it for `core` is probably higher value but also way harder because tools like cargo or rustc-build-sysroot would need to be fixed first, which I don't feel like doing.

Author: GuillaumeGomez

compiler/rustc_macros/build.rs

@@ -0,0 +1,16 @@
+fn main() {
+    println!("cargo:rerun-if-changed=build.rs");
+    println!("cargo:rerun-if-env-changed=RUSTC_BOOTSTRAP");
+    if !std::env::var("RUSTC_BOOTSTRAP").is_ok() {
+        eprintln!(
+            "error: you are attempting to build the compiler without going through bootstrap"
+        );
+        eprintln!(
+            "help: see https://rustc-dev-guide.rust-lang.org/building/how-to-build-and-run.html for how to build the compiler"
+        );
+        eprintln!(
+            "help: if you know what you're doing, set the RUSTC_BOOTSTRAP environment variable to any value"
+        );
+        panic!("wrong command used for building");
+    }
+}