Add [gcc] download-ci-gcc option

Author: Kobzol

Diff for: config.example.toml

@@ -167,6 +167,11 @@
 # Tweaking how GCC is compiled
 # =============================================================================
 [gcc]
+# Download GCC from CI instead of building it locally.
+# Note that this will attempt to download GCC even if there are local
+# modifications to the `src/gcc` submodule.
+# Currently, this is only supported for the `x86_64-unknown-linux-gnu` target.
+# download-ci-gcc = false
 
 # =============================================================================
 # General build configuration options

Diff for: src/bootstrap/src/core/config/config.rs

@@ -171,6 +171,17 @@ impl LldMode {
     }
 }
 
+/// Determines how will GCC be provided.
+#[derive(Default, Clone)]
+pub enum GccCiMode {
+    /// Build GCC from the local `src/gcc` submodule.
+    #[default]
+    BuildLocally,
+    /// Try to download GCC from CI.
+    /// If it is not available on CI, it will be built locally instead.
+    DownloadFromCi,
+}
+
 /// Global configuration for the entire build and/or bootstrap.
 ///
 /// This structure is parsed from `config.toml`, and some of the fields are inferred from `git` or build-time parameters.
@@ -283,6 +294,9 @@ pub struct Config {
     pub llvm_ldflags: Option<String>,
     pub llvm_use_libcxx: bool,
 
+    // gcc codegen options
+    pub gcc_ci_mode: GccCiMode,
+
     // rust codegen options
     pub rust_optimize: RustOptimize,
     pub rust_codegen_units: Option<u32>,
@@ -999,7 +1013,9 @@ define_config! {
 
 define_config! {
     /// TOML representation of how the GCC build is configured.
-    struct Gcc {}
+    struct Gcc {
+        download_ci_gcc: Option<bool> = "download-ci-gcc",
+    }
 }
 
 define_config! {
@@ -2143,6 +2159,16 @@ impl Config {
             config.llvm_from_ci = config.parse_download_ci_llvm(None, false);
         }
 
+        if let Some(gcc) = toml.gcc {
+            config.gcc_ci_mode = match gcc.download_ci_gcc {
+                Some(value) => match value {
+                    true => GccCiMode::DownloadFromCi,
+                    false => GccCiMode::BuildLocally,
+                },
+                None => GccCiMode::default(),
+            };
+        }
+
         if let Some(t) = toml.target {
             for (triple, cfg) in t {
                 let mut target = Target::from_triple(&triple);