Forbid setting RUSTC_BOOTSTRAP from build scripts

src/cargo/core/compiler/custom_build.rs

@@ -549,6 +549,9 @@ impl BuildOutput {
         let name = iter.next();
         let val = iter.next();
         match (name, val) {
+            (Some("RUSTC_BOOTSTRAP"), _) => {
+                failure::bail!("Build scripts are forbidden from setting RUSTC_BOOTSTRAP")
+            }
             (Some(n), Some(v)) => Ok((n.to_owned(), v.to_owned())),
             _ => failure::bail!("Variable rustc-env has no value in {}: {}", whence, value),
         }

tests/testsuite/build_script.rs

@@ -3696,3 +3696,35 @@ fn optional_build_dep_and_required_normal_dep() {
         )
         .run();
 }
+
+#[test]
+fn say_no_to_bootstrap() {
+    project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [project]
+                name = "foo"
+                version = "0.0.0"
+            "#,
+        )
+        .file(
+            "build.rs",
+            r#"
+            fn main() {
+                println!("cargo:rustc-env=RUSTC_BOOTSTRAP=1");
+            }
+        "#,
+        )
+        .file("src/lib.rs", "")
+        .build()
+        .cargo("build -v")
+        .with_status(101)
+        .with_stderr("\
+[COMPILING] foo v0.0.0 ([..])
+[RUNNING] `rustc --crate-name build_script_build build.rs --color never --crate-type bin [..]`
+[RUNNING] `[..]/build-script-build`
+error: Build scripts are forbidden from setting RUSTC_BOOTSTRAP",
+        )
+        .run();
+}