Add target to use LLVM wasm backend

The new target is wasm32-experimental-emscripten. Adds a new
configuration option to opt in to building experimental LLVM backends
such as the WebAssembly backend. The target name was chosen to be
similar to the existing wasm32-unknown-emscripten target so that the
build and tests would work with minimal other code changes. When/if the
new target replaces the old target, simply renaming it should just work.

src/bootstrap/config.rs

@@ -60,6 +60,7 @@ pub struct Config {
     pub llvm_static_stdcpp: bool,
     pub llvm_link_shared: bool,
     pub llvm_targets: Option<String>,
+    pub llvm_experimental_targets: Option<String>,
     pub llvm_link_jobs: Option<u32>,
     pub llvm_clean_rebuild: bool,
 
@@ -189,6 +190,7 @@ struct Llvm {
     version_check: Option<bool>,
     static_libstdcpp: Option<bool>,
     targets: Option<String>,
+    experimental_targets: Option<String>,
     link_jobs: Option<u32>,
     clean_rebuild: Option<bool>,
 }
@@ -350,6 +352,7 @@ impl Config {
             set(&mut config.llvm_static_stdcpp, llvm.static_libstdcpp);
             set(&mut config.llvm_clean_rebuild, llvm.clean_rebuild);
             config.llvm_targets = llvm.targets.clone();
+            config.llvm_experimental_targets = llvm.experimental_targets.clone();
             config.llvm_link_jobs = llvm.link_jobs;
         }
 

src/bootstrap/config.toml.example

@@ -53,6 +53,14 @@
 # Rust team and file an issue if you need assistance in porting!
 #targets = "X86;ARM;AArch64;Mips;PowerPC;SystemZ;JSBackend;MSP430;Sparc;NVPTX;Hexagon"
 
+# LLVM experimental targets to build support for. These targets are specified in
+# the same format as above, but since these targets are experimental, they are
+# not built by default and the experimental Rust compilation targets that depend
+# on them will not work unless the user opts in to building them. Possible
+# experimental LLVM targets include WebAssembly for the
+# wasm32-experimental-emscripten Rust target.
+#experimental-targets = ""
+
 # Cap the number of parallel linker invocations when compiling LLVM.
 # This can be useful when building LLVM with debug info, which significantly
 # increases the size of binaries and consequently the memory required by

src/bootstrap/native.rs

@@ -86,6 +86,11 @@ pub fn llvm(build: &Build, target: &str) {
         None => "X86;ARM;AArch64;Mips;PowerPC;SystemZ;JSBackend;MSP430;Sparc;NVPTX;Hexagon",
     };
 
+    let llvm_exp_targets = match build.config.llvm_experimental_targets {
+        Some(ref s) => s,
+        None => "",
+    };
+
     let assertions = if build.config.llvm_assertions {"ON"} else {"OFF"};
 
     cfg.target(target)
@@ -94,6 +99,7 @@ pub fn llvm(build: &Build, target: &str) {
        .profile(profile)
        .define("LLVM_ENABLE_ASSERTIONS", assertions)
        .define("LLVM_TARGETS_TO_BUILD", llvm_targets)
+       .define("LLVM_EXPERIMENTAL_TARGETS_TO_BUILD", llvm_exp_targets)
        .define("LLVM_INCLUDE_EXAMPLES", "OFF")
        .define("LLVM_INCLUDE_TESTS", "OFF")
        .define("LLVM_INCLUDE_DOCS", "OFF")