Don't give a hard error on RUSTC_BOOTSTRAP=crate_name

Author: jyn514

src/cargo/core/compiler/custom_build.rs

@@ -589,7 +589,15 @@ impl BuildOutput {
                         // to set RUSTC_BOOTSTRAP.
                         // If this is a nightly build, setting RUSTC_BOOTSTRAP wouldn't affect the
                         // behavior, so still only give a warning.
-                        if nightly_features_allowed {
+                        // NOTE: cargo only allows nightly features on RUSTC_BOOTSTRAP=1, but we
+                        // want setting any value of RUSTC_BOOTSTRAP to downgrade this to a warning
+                        // (so that `RUSTC_BOOTSTRAP=pkg_name` will work)
+                        let rustc_bootstrap_allows = |name: &str| {
+                            std::env::var("RUSTC_BOOTSTRAP").map_or(false, |var| {
+                                var.split(',').any(|s| s == name)
+                            })
+                        };
+                        if nightly_features_allowed || rustc_bootstrap_allows(&*pkg_name) {
                             warnings.push(format!("Cannot set `RUSTC_BOOTSTRAP={}` from {}.\n\
                                 note: Crates cannot set `RUSTC_BOOTSTRAP` themselves, as doing so would subvert the stability guarantees of Rust for your project.",
                                 val, whence

tests/testsuite/build_script_env.rs

@@ -129,4 +129,16 @@ fn rustc_bootstrap() {
         .masquerade_as_nightly_cargo()
         .with_stderr_contains("warning: Cannot set `RUSTC_BOOTSTRAP=1` [..]")
         .run();
+    // RUSTC_BOOTSTRAP set to the name of the crate
+    p.cargo("build")
+        .env("RUSTC_BOOTSTRAP", "foo")
+        .with_stderr_contains("warning: Cannot set `RUSTC_BOOTSTRAP=1` [..]")
+        .run();
+    // RUSTC_BOOTSTRAP set to some random value
+    p.cargo("build")
+        .env("RUSTC_BOOTSTRAP", "bar")
+        .with_stderr_contains("error: Cannot set `RUSTC_BOOTSTRAP=1` [..]")
+        .with_stderr_contains("help: [..] set the environment variable `RUSTC_BOOTSTRAP=foo` [..]")
+        .with_status(101)
+        .run();
 }