Fix weird implicit dependency between rustllvm and rustc_codegen_llvm

rustllvm relies on the `LLVMRustStringWriteImpl` symbol existing, but
this symbol was previously defined in a *downstream* crate
(rustc_codegen_llvm, which depends on rustc_llvm.

While this somehow worked under the old 'separate bootstrap step for
codegen' scheme, it meant that rustc_llvm could not actually be built by
itself, since it relied linking to the downstream rustc_codegen_llvm
crate.

Now that librustc_codegen_llvm is just a normal crate, we actually try
to build a standalone rustc_llvm when we run tests. This commit moves
`LLVMRustStringWriteImpl` into rustc_llvm (technically the rustllvm
directory, which has its contents built by rustc_llvm). This ensures
that we can build each crate in the graph by itself, without requiring
that any downstream crates be linked in as well.

Cargo.lock

@@ -3672,6 +3672,7 @@ version = "0.0.0"
 dependencies = [
  "build_helper",
  "cc",
+ "libc",
 ]
 
 [[package]]

src/librustc_codegen_llvm/llvm/mod.rs

@@ -10,11 +10,11 @@
 
 use std::str::FromStr;
 use std::string::FromUtf8Error;
-use std::slice;
 use std::ffi::CStr;
 use std::cell::RefCell;
-use libc::{c_uint, c_char, size_t};
+use libc::c_uint;
 use rustc_data_structures::small_c_str::SmallCStr;
+use rustc_llvm::RustString;
 
 pub mod archive_ro;
 pub mod diagnostic;
@@ -81,21 +81,6 @@ fn from_str(s: &str) -> Result<Self, Self::Err> {
     }
 }
 
-#[repr(C)]
-pub struct RustString {
-    bytes: RefCell<Vec<u8>>,
-}
-
-/// Appending to a Rust string -- used by RawRustStringOstream.
-#[no_mangle]
-pub unsafe extern "C" fn LLVMRustStringWriteImpl(sr: &RustString,
-                                                 ptr: *const c_char,
-                                                 size: size_t) {
-    let slice = slice::from_raw_parts(ptr as *const u8, size as usize);
-
-    sr.bytes.borrow_mut().extend_from_slice(slice);
-}
-
 pub fn SetInstructionCallConv(instr: &'a Value, cc: CallConv) {
     unsafe {
         LLVMSetInstructionCallConv(instr, cc as c_uint);

src/librustc_llvm/Cargo.toml

@@ -13,6 +13,9 @@ path = "lib.rs"
 static-libstdcpp = []
 emscripten = []
 
+[dependencies]
+libc = "0.2"
+
 [build-dependencies]
 build_helper = { path = "../build_helper" }
 cc = "1.0.1"

src/librustc_llvm/lib.rs

@@ -5,6 +5,26 @@
 
 // NOTE: This crate only exists to allow linking on mingw targets.
 
+use std::cell::RefCell;
+use std::slice;
+use libc::{c_char, size_t};
+
+
+#[repr(C)]
+pub struct RustString {
+    pub bytes: RefCell<Vec<u8>>,
+}
+
+/// Appending to a Rust string -- used by RawRustStringOstream.
+#[no_mangle]
+pub unsafe extern "C" fn LLVMRustStringWriteImpl(sr: &RustString,
+                                                 ptr: *const c_char,
+                                                 size: size_t) {
+    let slice = slice::from_raw_parts(ptr as *const u8, size as usize);
+
+    sr.bytes.borrow_mut().extend_from_slice(slice);
+}
+
 /// Initialize targets enabled by the build script via `cfg(llvm_component = "...")`.
 /// N.B., this function can't be moved to `rustc_codegen_llvm` because of the `cfg`s.
 pub fn initialize_available_targets() {