add enable-warnings flag for llvm, and disable it by default.

config.example.toml

@@ -146,6 +146,9 @@ changelog-seen = 2
 # Whether to build the clang compiler.
 #clang = false
 
+# Whether to enable llvm compilation warnings.
+#enable-warnings = false
+
 # Custom CMake defines to set when building LLVM.
 #build-config = {}
 

src/bootstrap/CHANGELOG.md

@@ -25,6 +25,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 - If you have Rust already installed, `x.py` will now infer the host target
   from the default rust toolchain. [#78513](https://github.com/rust-lang/rust/pull/78513)
 - Add options for enabling overflow checks, one for std (`overflow-checks-std`) and one for everything else (`overflow-checks`). Both default to false.
+- Add llvm option `enable-warnings` to have control on llvm compilation warnings. Default to false.
 
 
 ## [Version 2] - 2020-09-25

src/bootstrap/config.rs

@@ -133,6 +133,7 @@ pub struct Config {
     pub llvm_allow_old_toolchain: bool,
     pub llvm_polly: bool,
     pub llvm_clang: bool,
+    pub llvm_enable_warnings: bool,
     pub llvm_from_ci: bool,
     pub llvm_build_config: HashMap<String, String>,
 
@@ -688,6 +689,7 @@ define_config! {
         allow_old_toolchain: Option<bool> = "allow-old-toolchain",
         polly: Option<bool> = "polly",
         clang: Option<bool> = "clang",
+        enable_warnings: Option<bool> = "enable-warnings",
         download_ci_llvm: Option<StringOrBool> = "download-ci-llvm",
         build_config: Option<HashMap<String, String>> = "build-config",
     }
@@ -1184,6 +1186,7 @@ impl Config {
             config.llvm_allow_old_toolchain = llvm.allow_old_toolchain.unwrap_or(false);
             config.llvm_polly = llvm.polly.unwrap_or(false);
             config.llvm_clang = llvm.clang.unwrap_or(false);
+            config.llvm_enable_warnings = llvm.enable_warnings.unwrap_or(false);
             config.llvm_build_config = llvm.build_config.clone().unwrap_or(Default::default());
 
             let asserts = llvm_assertions.unwrap_or(false);

src/bootstrap/defaults/config.codegen.toml

@@ -7,6 +7,8 @@ compiler-docs = true
 # This enables debug-assertions in LLVM,
 # catching logic errors in codegen much earlier in the process.
 assertions = true
+# enable warnings during the llvm compilation
+enable-warnings = true
 
 [rust]
 # This enables `RUSTC_LOG=debug`, avoiding confusing situations

src/bootstrap/native.rs

@@ -304,6 +304,7 @@ impl Step for Llvm {
         let assertions = if builder.config.llvm_assertions { "ON" } else { "OFF" };
         let plugins = if builder.config.llvm_plugins { "ON" } else { "OFF" };
         let enable_tests = if builder.config.llvm_tests { "ON" } else { "OFF" };
+        let enable_warnings = if builder.config.llvm_enable_warnings { "ON" } else { "OFF" };
 
         cfg.out_dir(&out_dir)
             .profile(profile)
@@ -321,7 +322,8 @@ impl Step for Llvm {
             .define("LLVM_ENABLE_Z3_SOLVER", "OFF")
             .define("LLVM_PARALLEL_COMPILE_JOBS", builder.jobs().to_string())
             .define("LLVM_TARGET_ARCH", target_native.split('-').next().unwrap())
-            .define("LLVM_DEFAULT_TARGET_TRIPLE", target_native);
+            .define("LLVM_DEFAULT_TARGET_TRIPLE", target_native)
+            .define("LLVM_ENABLE_WARNINGS", enable_warnings);
 
         // Parts of our test suite rely on the `FileCheck` tool, which is built by default in
         // `build/$TARGET/llvm/build/bin` is but *not* then installed to `build/$TARGET/llvm/bin`.
@@ -483,11 +485,6 @@ impl Step for Llvm {
             cfg.define(key, val);
         }
 
-        // FIXME: we don't actually need to build all LLVM tools and all LLVM
-        //        libraries here, e.g., we just want a few components and a few
-        //        tools. Figure out how to filter them down and only build the right
-        //        tools and libs on all platforms.
-
         if builder.config.dry_run() {
             return res;
         }