Make download-rustc incompatible with rust.debug-assertions for now

Alt rustc builds currently do not have rustc debug assertions enabled.

src/bootstrap/src/core/config/config.rs

@@ -1767,6 +1767,29 @@ impl Config {
                 std_features: std_features_toml,
             } = rust;
 
+            // Before something like [Enable debug assertions on alt
+            // builds](https://github.com/rust-lang/rust/pull/131077) lands, alt rustc builds do
+            // *not* have rustc debug assertions enabled. We must not download an CI alt rustc if we
+            // need rustc to have debug assertions (e.g. for crashes test suite).
+            //
+            // Note that `rust.debug = true` (currently) implies `rust.debug-assertions = true`!
+            //
+            // This relies also on the fact that global default for `download-rustc` will be `false`
+            // if it's not explicitly set.
+            if matches!(rustc_debug_assertions, Some(true))
+                || (matches!(debug, Some(true)) && rustc_debug_assertions.is_none())
+            {
+                if let Some(ref opt) = download_rustc {
+                    if opt.is_string_or_true() {
+                        panic!(
+                            "ERROR: currently no CI rustc builds have rustc debug assertions\
+                            enabled. Please either set `rust.debug-assertions` to `false` or set \
+                            `rust.download-rustc` to `false`."
+                        );
+                    }
+                }
+            }
+
             config.download_rustc_commit =
                 config.download_ci_rustc_commit(download_rustc, config.llvm_assertions);
 

src/bootstrap/src/core/config/tests.rs

@@ -447,3 +447,68 @@ fn check_rustc_if_unchanged_paths() {
         assert!(config.src.join(p).exists(), "{p} doesn't exist.");
     }
 }
+
+#[test]
+#[should_panic]
+fn download_rustc_xor_rustc_debug_assertions_both_true() {
+    parse(
+        r#"
+    [rust]
+    debug-assertions = true
+    download-rustc = true
+    "#,
+    );
+}
+
+#[test]
+#[should_panic]
+fn download_rustc_xor_rustc_debug_assertions_debug_download_unchanged() {
+    parse(
+        r#"
+    [rust]
+    debug-assertions = true
+    download-rustc = 'if-unchanged'
+    "#,
+    );
+}
+
+#[test]
+#[should_panic]
+fn download_rustc_xor_rustc_debug_assertions_debug_true() {
+    parse(
+        r#"
+    [rust]
+    debug = true
+    download-rustc = true
+    "#,
+    );
+}
+
+#[test]
+fn download_rustc_xor_rustc_debug_assertions_only_download_rustc() {
+    let _ = parse(
+        r#"
+    [rust]
+    download-rustc = true
+    "#,
+    );
+
+    let _ = parse(
+        r#"
+    [rust]
+    download-rustc = 'if-unchanged'
+    "#,
+    );
+}
+
+#[test]
+fn download_rustc_xor_rustc_debug_assertions_no_debug_assert_ok() {
+    let _ = parse(
+        r#"
+    [rust]
+    debug = true
+    debug-assertions = false
+    download-rustc = true
+    "#,
+    );
+}