use if-unchanged only when ci rustc is available

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

Author: onur-ozkan

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

@@ -22,6 +22,7 @@ use crate::core::build_steps::compile::CODEGEN_BACKEND_PREFIX;
 use crate::core::build_steps::llvm;
 pub use crate::core::config::flags::Subcommand;
 use crate::core::config::flags::{Color, Flags, Warnings};
+use crate::core::download::is_download_ci_available;
 use crate::utils::cache::{INTERNER, Interned};
 use crate::utils::channel::{self, GitInfo};
 use crate::utils::helpers::{self, exe, output, t};
@@ -1627,9 +1628,11 @@ impl Config {
             config.mandir = mandir.map(PathBuf::from);
         }
 
+        config.llvm_assertions =
+            toml.llvm.as_ref().map_or(false, |llvm| llvm.assertions.unwrap_or(false));
+
         // Store off these values as options because if they're not provided
         // we'll infer default values for them later
-        let mut llvm_assertions = None;
         let mut llvm_tests = None;
         let mut llvm_enzyme = None;
         let mut llvm_plugins = None;
@@ -1712,7 +1715,8 @@ impl Config {
             is_user_configured_rust_channel = channel.is_some();
             set(&mut config.channel, channel.clone());
 
-            config.download_rustc_commit = config.download_ci_rustc_commit(download_rustc);
+            config.download_rustc_commit =
+                config.download_ci_rustc_commit(download_rustc, config.llvm_assertions);
 
             debug = debug_toml;
             debug_assertions = debug_assertions_toml;
@@ -1848,7 +1852,7 @@ impl Config {
                 optimize: optimize_toml,
                 thin_lto,
                 release_debuginfo,
-                assertions,
+                assertions: _,
                 tests,
                 enzyme,
                 plugins,
@@ -1882,7 +1886,6 @@ impl Config {
                 Some(StringOrBool::Bool(false)) | None => {}
             }
             set(&mut config.ninja_in_file, ninja);
-            llvm_assertions = assertions;
             llvm_tests = tests;
             llvm_enzyme = enzyme;
             llvm_plugins = plugins;
@@ -1911,8 +1914,8 @@ impl Config {
             config.llvm_enable_warnings = enable_warnings.unwrap_or(false);
             config.llvm_build_config = build_config.clone().unwrap_or(Default::default());
 
-            let asserts = llvm_assertions.unwrap_or(false);
-            config.llvm_from_ci = config.parse_download_ci_llvm(download_ci_llvm, asserts);
+            config.llvm_from_ci =
+                config.parse_download_ci_llvm(download_ci_llvm, config.llvm_assertions);
 
             if config.llvm_from_ci {
                 let warn = |option: &str| {
@@ -2080,7 +2083,6 @@ impl Config {
         // Now that we've reached the end of our configuration, infer the
         // default values for all options that we haven't otherwise stored yet.
 
-        config.llvm_assertions = llvm_assertions.unwrap_or(false);
         config.llvm_tests = llvm_tests.unwrap_or(false);
         config.llvm_enzyme = llvm_enzyme.unwrap_or(false);
         config.llvm_plugins = llvm_plugins.unwrap_or(false);
@@ -2711,12 +2713,18 @@ impl Config {
     }
 
     /// Returns the commit to download, or `None` if we shouldn't download CI artifacts.
-    fn download_ci_rustc_commit(&self, download_rustc: Option<StringOrBool>) -> Option<String> {
+    fn download_ci_rustc_commit(
+        &self,
+        download_rustc: Option<StringOrBool>,
+        llvm_assertions: bool,
+    ) -> Option<String> {
         // If `download-rustc` is not set, default to rebuilding.
         let if_unchanged = match download_rustc {
             None | Some(StringOrBool::Bool(false)) => return None,
             Some(StringOrBool::Bool(true)) => false,
-            Some(StringOrBool::String(s)) if s == "if-unchanged" => true,
+            Some(StringOrBool::String(s)) if s == "if-unchanged" => {
+                is_download_ci_available(&self.build.triple, llvm_assertions)
+            }
             Some(StringOrBool::String(other)) => {
                 panic!("unrecognized option for download-rustc: {other}")
             }

src/bootstrap/src/core/download.rs

@@ -832,3 +832,43 @@ fn path_is_dylib(path: &Path) -> bool {
     // The .so is not necessarily the extension, it might be libLLVM.so.18.1
     path.to_str().map_or(false, |path| path.contains(".so"))
 }
+
+/// Checks whether the CI rustc is available for the given target triple.
+pub(crate) fn is_download_ci_available(target_triple: &str, llvm_assertions: bool) -> bool {
+    // All tier 1 targets and tier 2 targets with host tools.
+    const SUPPORTED_PLATFORMS: &[&str] = &[
+        "aarch64-apple-darwin",
+        "aarch64-pc-windows-msvc",
+        "aarch64-unknown-linux-gnu",
+        "aarch64-unknown-linux-musl",
+        "arm-unknown-linux-gnueabi",
+        "arm-unknown-linux-gnueabihf",
+        "armv7-unknown-linux-gnueabihf",
+        "i686-pc-windows-gnu",
+        "i686-pc-windows-msvc",
+        "i686-unknown-linux-gnu",
+        "loongarch64-unknown-linux-gnu",
+        "powerpc-unknown-linux-gnu",
+        "powerpc64-unknown-linux-gnu",
+        "powerpc64le-unknown-linux-gnu",
+        "riscv64gc-unknown-linux-gnu",
+        "s390x-unknown-linux-gnu",
+        "x86_64-apple-darwin",
+        "x86_64-pc-windows-gnu",
+        "x86_64-pc-windows-msvc",
+        "x86_64-unknown-freebsd",
+        "x86_64-unknown-illumos",
+        "x86_64-unknown-linux-gnu",
+        "x86_64-unknown-linux-musl",
+        "x86_64-unknown-netbsd",
+    ];
+
+    const SUPPORTED_PLATFORMS_WITH_ASSERTIONS: &[&str] =
+        &["x86_64-unknown-linux-gnu", "x86_64-pc-windows-msvc"];
+
+    if llvm_assertions {
+        SUPPORTED_PLATFORMS_WITH_ASSERTIONS.contains(&target_triple)
+    } else {
+        SUPPORTED_PLATFORMS.contains(&target_triple)
+    }
+}