Skip downloading CI rustc if rust.debug-assertions = true

Warn if `download-rustc` used with `rust.debug-assertions` as alt rustc
builds currently do not have rustc debug assertions enabled.

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

@@ -1771,8 +1771,36 @@ impl Config {
                 std_features: std_features_toml,
             } = rust;
 
-            config.download_rustc_commit =
-                config.download_ci_rustc_commit(download_rustc, config.llvm_assertions);
+            // 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 should 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.
+            let debug_assertions_requested = matches!(rustc_debug_assertions_toml, Some(true))
+                || (matches!(debug_toml, Some(true))
+                    && !matches!(rustc_debug_assertions_toml, Some(false)));
+
+            if debug_assertions_requested {
+                if let Some(ref opt) = download_rustc {
+                    if opt.is_string_or_true() {
+                        panic!(
+                            "WARN: currently no CI rustc builds have rustc debug assertions \
+                            enabled. Please either set `rust.debug-assertions` to `false` if you \
+                            want to use download CI rustc or set `rust.download-rustc` to `false`."
+                        );
+                    }
+                }
+            }
+
+            config.download_rustc_commit = config.download_ci_rustc_commit(
+                download_rustc,
+                debug_assertions_requested,
+                config.llvm_assertions,
+            );
 
             debug = debug_toml;
             rustc_debug_assertions = rustc_debug_assertions_toml;
@@ -2778,6 +2806,7 @@ impl Config {
     fn download_ci_rustc_commit(
         &self,
         download_rustc: Option<StringOrBool>,
+        debug_assertions_requested: bool,
         llvm_assertions: bool,
     ) -> Option<String> {
         if !is_download_ci_available(&self.build.triple, llvm_assertions) {
@@ -2786,9 +2815,9 @@ impl Config {
 
         // If `download-rustc` is not set, default to rebuilding.
         let if_unchanged = match download_rustc {
-            // Globally default for `download-rustc` to `false`, because some contributors don't use
+            // Globally default `download-rustc` to `false`, because some contributors don't use
             // profiles for reasons such as:
-            // - They need to seemlessly switch between compiler/library work.
+            // - They need to seamlessly switch between compiler/library work.
             // - They don't want to use compiler profile because they need to override too many
             //   things and it's easier to not use a profile.
             None | Some(StringOrBool::Bool(false)) => return None,
@@ -2849,6 +2878,14 @@ impl Config {
             return None;
         }
 
+        if debug_assertions_requested {
+            println!(
+                "WARN: `rust.debug-assertions = true` will prevent downloading CI rustc as alt CI \
+                rustc is not currently built with debug assertions."
+            );
+            return None;
+        }
+
         Some(commit)
     }
 

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

@@ -447,3 +447,83 @@ 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'
+    "#,
+    );
+
+    // Relies on global default for `download-rustc` to be `false`.
+    let _ = parse(
+        r#"
+    [rust]
+    debug = true
+    "#,
+    );
+
+    let _ = parse(
+        r#"
+    [rust]
+    debug-assertions = true
+    "#,
+    );
+}
+
+#[test]
+fn download_rustc_xor_rustc_debug_assertions_no_debug_assert_ok() {
+    let _ = parse(
+        r#"
+    [rust]
+    debug = true
+    debug-assertions = false
+    download-rustc = true
+    "#,
+    );
+}

src/bootstrap/src/utils/change_tracker.rs

@@ -308,6 +308,6 @@ pub const CONFIG_CHANGE_HISTORY: &[ChangeInfo] = &[
     ChangeInfo {
         change_id: 133068,
         severity: ChangeSeverity::Info,
-        summary: "Reverted `download-rustc` defaults; global default is now `false`, and only use `'if-unchanged'` default for library and tools profile.",
+        summary: "Reverted `download-rustc` defaults; global default is now `false`, and only use `'if-unchanged'` default for library and tools profile. Also fixed that `rust.debug-assertions = true` is incompatible with `rust.download-rustc = true | 'if-unchanged'`",
     },
 ];