Add UI test to ensure no more allocations pre-main

tests/ui/runtime/aborting-alloc.rs

@@ -0,0 +1,26 @@
+//! Helper for 'no-allocation-before-main'.
+//!
+//! This also contains a meta-test to make sure that the AbortingAllocator does indeed abort.
+//!
+//! -Cprefer-dynamic=no is required as otherwise #[global_allocator] does nothing.
+//@ run-fail
+//@ compile-flags: -Cprefer-dynamic=no
+
+pub struct AbortingAllocator;
+
+unsafe impl std::alloc::GlobalAlloc for AbortingAllocator {
+    unsafe fn alloc(&self, _: std::alloc::Layout) -> *mut u8 {
+        std::process::abort()
+    }
+
+    unsafe fn dealloc(&self, _: *mut u8, _: std::alloc::Layout) {
+        std::process::abort()
+    }
+}
+
+#[global_allocator]
+static ALLOCATOR: AbortingAllocator = AbortingAllocator;
+
+fn main() {
+    std::hint::black_box(String::from("An allocation"));
+}

tests/ui/runtime/no-allocation-before-main.rs

@@ -0,0 +1,13 @@
+//! Tests that a program with no body does not allocate.
+//!
+//! The initial runtime should not allocate for performance/binary size reasons.
+//!
+//! -Cprefer-dynamic=no is required as otherwise #[global_allocator] does nothing.
+//@ run-pass
+//@ compile-flags: -Cprefer-dynamic=no
+
+#[allow(dead_code)]
+#[path = "aborting-alloc.rs"]
+mod aux;
+
+fn main() {}