handle ci-rustc incompatible options during config parse

This change ensures that `config.toml` does not use CI rustc incompatible
options when CI rustc is enabled. This is necessary because some options
can change compiler's behavior in certain scenarios.

The list may not be complete, but should be a good first step as it's better than nothing!

Signed-off-by: onur-ozkan <work@onurozkan.dev>

Author: onur-ozkan

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

@@ -38,6 +38,24 @@ macro_rules! check_ci_llvm {
     };
 }
 
+macro_rules! err_incompatible_ci_rustc_option {
+    ($name:expr, $option_name:literal) => {
+        assert!(
+            $name.is_none(),
+            "ERROR: Setting `rust.{}` is incompatible with `rust.download-rustc`.",
+            $option_name
+        );
+    };
+}
+
+macro_rules! warn_incompatible_ci_rustc_option {
+    ($name:expr, $option_name:literal) => {
+        if $name.is_some() {
+            println!("WARNING: `rust.{}` has no effect with `rust.download-rustc`.", $option_name);
+        }
+    };
+}
+
 #[derive(Clone, Default)]
 pub enum DryRun {
     /// This isn't a dry run.
@@ -1605,11 +1623,56 @@ impl Config {
             } = rust;
 
             is_user_configured_rust_channel = channel.is_some();
-            set(&mut config.channel, channel);
+            set(&mut config.channel, channel.clone());
 
             config.download_rustc_commit = config.download_ci_rustc_commit(download_rustc);
 
-            // FIXME: handle download-rustc incompatible options.
+            // There are two kinds of checks for CI rustc incompatible options:
+            //    1. Checking an option that may change the compiler behaviour/output.
+            //    2. Checking an option that have no effect on the compiler behaviour/output.
+            //
+            // If the option belongs to the first category, we call `err_incompatible_ci_rustc_option` macro
+            // for a hard error; otherwise, we just print a warning with `warn_incompatible_ci_rustc_option` macro.
+            if config.download_rustc_commit.is_some() {
+                err_incompatible_ci_rustc_option!(optimize_toml, "optimize");
+                err_incompatible_ci_rustc_option!(debug_toml, "debug");
+                err_incompatible_ci_rustc_option!(codegen_units, "codegen-units");
+                err_incompatible_ci_rustc_option!(codegen_units_std, "codegen-units-std");
+                err_incompatible_ci_rustc_option!(debug_assertions_toml, "debug-assertions");
+                err_incompatible_ci_rustc_option!(
+                    debug_assertions_std_toml,
+                    "debug-assertions-std"
+                );
+                err_incompatible_ci_rustc_option!(debug_logging, "debug-logging");
+                err_incompatible_ci_rustc_option!(overflow_checks_toml, "overflow-checks");
+                err_incompatible_ci_rustc_option!(overflow_checks_std_toml, "overflow-checks-std");
+                err_incompatible_ci_rustc_option!(debuginfo_level_toml, "debuginfo-level");
+                err_incompatible_ci_rustc_option!(
+                    debuginfo_level_rustc_toml,
+                    "debuginfo-level-rustc"
+                );
+                err_incompatible_ci_rustc_option!(debuginfo_level_std_toml, "debuginfo-level-std");
+                err_incompatible_ci_rustc_option!(split_debuginfo, "split-debuginfo");
+                err_incompatible_ci_rustc_option!(backtrace, "backtrace");
+                err_incompatible_ci_rustc_option!(default_linker, "default-linker");
+                err_incompatible_ci_rustc_option!(rpath, "rpath");
+                err_incompatible_ci_rustc_option!(strip, "strip");
+                err_incompatible_ci_rustc_option!(frame_pointers, "frame-pointers");
+                err_incompatible_ci_rustc_option!(stack_protector, "stack-protector");
+                err_incompatible_ci_rustc_option!(lld_mode, "use-lld");
+                err_incompatible_ci_rustc_option!(llvm_tools, "llvm-tools");
+                err_incompatible_ci_rustc_option!(llvm_bitcode_linker, "llvm-bitcode-linker");
+                err_incompatible_ci_rustc_option!(jemalloc, "jemalloc");
+                err_incompatible_ci_rustc_option!(control_flow_guard, "control-flow-guard");
+                err_incompatible_ci_rustc_option!(ehcont_guard, "ehcont-guard");
+                err_incompatible_ci_rustc_option!(lto, "lto");
+                err_incompatible_ci_rustc_option!(validate_mir_opts, "validate-mir-opts");
+
+                warn_incompatible_ci_rustc_option!(channel, "channel");
+                warn_incompatible_ci_rustc_option!(description, "description");
+                warn_incompatible_ci_rustc_option!(incremental, "incremental");
+                warn_incompatible_ci_rustc_option!(parallel_compiler, "parallel-compiler");
+            }
 
             debug = debug_toml;
             debug_assertions = debug_assertions_toml;