Rollup merge of #113780 - dtolnay:printkindpath, r=b-naber

Support `--print KIND=PATH` command line syntax

As is already done for `--emit KIND=PATH` and `-L KIND=PATH`.

In the discussion of #110785, it was pointed out that `--print KIND=PATH` is nicer than trying to apply the single global `-o` path to `--print`'s output, because in general there can be multiple print requests within a single rustc invocation, and anyway `-o` would already be used for a different meaning in the case of `link-args` and `native-static-libs`.

I am interested in using `--print cfg=PATH` in Buck2. Currently Buck2 works around the lack of support for `--print KIND=PATH` by [indirecting through a Python wrapper script](https://github.com/facebook/buck2/blob/d43cf3a51a31f00be2c2248e78271b0fef0452b4/prelude/rust/tools/get_rustc_cfg.py) to redirect rustc's stdout into the location dictated by the build system.

From skimming Cargo's usages of `--print`, it definitely seems like it would benefit from `--print KIND=PATH` too. Currently it is working around the lack of this by inserting `--crate-name=___ --print=crate-name` so that it can look for a line containing `___` as a delimiter between the 2 other `--print` informations it actually cares about. This is commented as a "HACK" and "abuse". https://github.com/rust-lang/cargo/blob/31eda6f7c360d9911f853b3014e057db61238f3e/src/cargo/core/compiler/build_context/target_info.rs#L242 (FYI `@weihanglo` as you dealt with this recently in https://github.com/rust-lang/cargo/pull/11633.)

Mentioning reviewers active in #110785: `@fee1-dead` `@jyn514` `@bjorn3`

1 files changed, 22 insertions, 10 deletions

diff --git a/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp b/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp
index eb3d67e720f..e5fb6b0953f 100644
--- a/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp
+++ b/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp
@@ -1,5 +1,6 @@
 #include <stdio.h>
 
+#include <iomanip>
 #include <vector>
 #include <set>
 
@@ -306,44 +307,55 @@ static size_t getLongestEntryLength(ArrayRef<KV> Table) {
   return MaxLen;
 }
 
-extern "C" void LLVMRustPrintTargetCPUs(LLVMTargetMachineRef TM, const char* TargetCPU) {
+using PrintBackendInfo = void(void*, const char* Data, size_t Len);
+
+extern "C" void LLVMRustPrintTargetCPUs(LLVMTargetMachineRef TM,
+                                        const char* TargetCPU,
+                                        PrintBackendInfo Print,
+                                        void* Out) {
   const TargetMachine *Target = unwrap(TM);
   const MCSubtargetInfo *MCInfo = Target->getMCSubtargetInfo();
   const Triple::ArchType HostArch = Triple(sys::getDefaultTargetTriple()).getArch();
   const Triple::ArchType TargetArch = Target->getTargetTriple().getArch();
 
+  std::ostringstream Buf;
+
 #if LLVM_VERSION_GE(17, 0)
   const ArrayRef<SubtargetSubTypeKV> CPUTable = MCInfo->getAllProcessorDescriptions();
 #elif defined(LLVM_RUSTLLVM)
   const ArrayRef<SubtargetSubTypeKV> CPUTable = MCInfo->getCPUTable();
 #else
-  printf("Full target CPU help is not supported by this LLVM version.\n\n");
+  Buf << "Full target CPU help is not supported by this LLVM version.\n\n";
   SubtargetSubTypeKV TargetCPUKV = { TargetCPU, {{}}, {{}} };
   const ArrayRef<SubtargetSubTypeKV> CPUTable = TargetCPUKV;
 #endif
   unsigned MaxCPULen = getLongestEntryLength(CPUTable);
 
-  printf("Available CPUs for this target:\n");
+  Buf << "Available CPUs for this target:\n";
   // Don't print the "native" entry when the user specifies --target with a
   // different arch since that could be wrong or misleading.
   if (HostArch == TargetArch) {
     MaxCPULen = std::max(MaxCPULen, (unsigned) std::strlen("native"));
     const StringRef HostCPU = sys::getHostCPUName();
-    printf("    %-*s - Select the CPU of the current host (currently %.*s).\n",
-      MaxCPULen, "native", (int)HostCPU.size(), HostCPU.data());
+    Buf << "    " << std::left << std::setw(MaxCPULen) << "native"
+        << " - Select the CPU of the current host "
+           "(currently " << HostCPU.str() << ").\n";
   }
   for (auto &CPU : CPUTable) {
     // Compare cpu against current target to label the default
     if (strcmp(CPU.Key, TargetCPU) == 0) {
-      printf("    %-*s - This is the default target CPU"
-      " for the current build target (currently %s).",
-        MaxCPULen, CPU.Key, Target->getTargetTriple().str().c_str());
+      Buf << "    " << std::left << std::setw(MaxCPULen) << CPU.Key
+          << " - This is the default target CPU for the current build target "
+             "(currently " << Target->getTargetTriple().str() << ").";
     }
     else {
-      printf("    %-*s", MaxCPULen, CPU.Key);
+      Buf << "    " << CPU.Key;
     }
-    printf("\n");
+    Buf << "\n";
   }
+
+  const auto &BufString = Buf.str();
+  Print(Out, BufString.data(), BufString.size());
 }
 
 extern "C" size_t LLVMRustGetTargetFeaturesCount(LLVMTargetMachineRef TM) {