use download-rustc="if-unchanged" as a global default

src/bootstrap/defaults/config.dist.toml

@@ -11,6 +11,7 @@ extended = true
 # Most users installing from source want to build all parts of the project from source.
 [llvm]
 download-ci-llvm = false
+
 [rust]
 # We have several defaults in bootstrap that depend on whether the channel is `dev` (e.g. `omit-git-hash` and `download-ci-llvm`).
 # Make sure they don't get set when installing from source.

src/bootstrap/defaults/config.library.toml

@@ -8,9 +8,6 @@ bench-stage = 0
 [rust]
 # This greatly increases the speed of rebuilds, especially when there are only minor changes. However, it makes the initial build slightly slower.
 incremental = true
-# Download rustc from CI instead of building it from source.
-# For stage > 1 builds, this cuts compile times significantly when there are no changes on "compiler" tree.
-download-rustc = "if-unchanged"
 # Make the compiler and standard library faster to build, at the expense of a ~20% runtime slowdown.
 lto = "off"
 

src/bootstrap/defaults/config.tools.toml

@@ -3,11 +3,6 @@
 [rust]
 # This greatly increases the speed of rebuilds, especially when there are only minor changes. However, it makes the initial build slightly slower.
 incremental = true
-# Download rustc from CI instead of building it from source.
-# For stage > 1 builds, this cuts compile times significantly when there are no changes on "compiler" tree.
-# Using these defaults will download the stage2 compiler (see `download-rustc`
-# setting) and the stage2 toolchain should therefore be used for these defaults.
-download-rustc = "if-unchanged"
 
 [build]
 # Document with the in-tree rustdoc by default, since `download-rustc` makes it quick to compile.

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

@@ -1663,10 +1663,26 @@ impl Config {
         let mut debuginfo_level_tools = None;
         let mut debuginfo_level_tests = None;
         let mut optimize = None;
-        let mut omit_git_hash = None;
         let mut lld_enabled = None;
         let mut std_features = None;
 
+        let default = config.channel == "dev";
+        config.omit_git_hash = toml.rust.as_ref().and_then(|r| r.omit_git_hash).unwrap_or(default);
+
+        config.rust_info = GitInfo::new(config.omit_git_hash, &config.src);
+        config.cargo_info = GitInfo::new(config.omit_git_hash, &config.src.join("src/tools/cargo"));
+        config.rust_analyzer_info =
+            GitInfo::new(config.omit_git_hash, &config.src.join("src/tools/rust-analyzer"));
+        config.clippy_info =
+            GitInfo::new(config.omit_git_hash, &config.src.join("src/tools/clippy"));
+        config.miri_info = GitInfo::new(config.omit_git_hash, &config.src.join("src/tools/miri"));
+        config.rustfmt_info =
+            GitInfo::new(config.omit_git_hash, &config.src.join("src/tools/rustfmt"));
+        config.enzyme_info =
+            GitInfo::new(config.omit_git_hash, &config.src.join("src/tools/enzyme"));
+        config.in_tree_llvm_info = GitInfo::new(false, &config.src.join("src/llvm-project"));
+        config.in_tree_gcc_info = GitInfo::new(false, &config.src.join("src/gcc"));
+
         let mut is_user_configured_rust_channel = false;
 
         if let Some(rust) = toml.rust {
@@ -1697,7 +1713,7 @@ impl Config {
                 verbose_tests,
                 optimize_tests,
                 codegen_tests,
-                omit_git_hash: omit_git_hash_toml,
+                omit_git_hash: _, // already handled above
                 dist_src,
                 save_toolstates,
                 codegen_backends,
@@ -1748,7 +1764,6 @@ impl Config {
             std_features = std_features_toml;
 
             optimize = optimize_toml;
-            omit_git_hash = omit_git_hash_toml;
             config.rust_new_symbol_mangling = new_symbol_mangling;
             set(&mut config.rust_optimize_tests, optimize_tests);
             set(&mut config.codegen_tests, codegen_tests);
@@ -1824,24 +1839,6 @@ impl Config {
 
         config.reproducible_artifacts = flags.reproducible_artifact;
 
-        // rust_info must be set before is_ci_llvm_available() is called.
-        let default = config.channel == "dev";
-        config.omit_git_hash = omit_git_hash.unwrap_or(default);
-        config.rust_info = GitInfo::new(config.omit_git_hash, &config.src);
-
-        config.cargo_info = GitInfo::new(config.omit_git_hash, &config.src.join("src/tools/cargo"));
-        config.rust_analyzer_info =
-            GitInfo::new(config.omit_git_hash, &config.src.join("src/tools/rust-analyzer"));
-        config.clippy_info =
-            GitInfo::new(config.omit_git_hash, &config.src.join("src/tools/clippy"));
-        config.miri_info = GitInfo::new(config.omit_git_hash, &config.src.join("src/tools/miri"));
-        config.rustfmt_info =
-            GitInfo::new(config.omit_git_hash, &config.src.join("src/tools/rustfmt"));
-        config.enzyme_info =
-            GitInfo::new(config.omit_git_hash, &config.src.join("src/tools/enzyme"));
-        config.in_tree_llvm_info = GitInfo::new(false, &config.src.join("src/llvm-project"));
-        config.in_tree_gcc_info = GitInfo::new(false, &config.src.join("src/gcc"));
-
         // We need to override `rust.channel` if it's manually specified when using the CI rustc.
         // This is because if the compiler uses a different channel than the one specified in config.toml,
         // tests may fail due to using a different channel than the one used by the compiler during tests.
@@ -2759,9 +2756,19 @@ impl Config {
 
         // If `download-rustc` is not set, default to rebuilding.
         let if_unchanged = match download_rustc {
-            None | Some(StringOrBool::Bool(false)) => return None,
+            None => self.rust_info.is_managed_git_subrepository(),
+            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" => {
+                if !self.rust_info.is_managed_git_subrepository() {
+                    println!(
+                        "ERROR: `download-rustc=if-unchanged` is only compatible with Git managed sources."
+                    );
+                    crate::exit!(1);
+                }
+
+                true
+            }
             Some(StringOrBool::String(other)) => {
                 panic!("unrecognized option for download-rustc: {other}")
             }
@@ -2788,7 +2795,7 @@ impl Config {
                     }
                     println!("ERROR: could not find commit hash for downloading rustc");
                     println!("HELP: maybe your repository history is too shallow?");
-                    println!("HELP: consider disabling `download-rustc`");
+                    println!("HELP: consider setting `rust.download-rustc=false` in config.toml");
                     println!("HELP: or fetch enough history to include one upstream commit");
                     crate::exit!(1);
                 }

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

@@ -135,6 +135,7 @@ change-id = 0
 [rust]
 lto = "off"
 deny-warnings = true
+download-rustc=false
 
 [build]
 gdb = "foo"
@@ -200,6 +201,8 @@ runner = "x86_64-runner"
             .collect(),
         "setting dictionary value"
     );
+    assert!(!config.llvm_from_ci);
+    assert!(!config.download_rustc());
 }
 
 #[test]