Auto merge of #86267 - ZuseZ4:master, r=nagisa

Allow loading of llvm plugins on nightly

Based on a discussion in  #82734 / with `@wsmoses.`
Mainly moves [this](0149bc4e7e) behind a -Z flag, so it can only be used on nightly,
as requested by `@nagisa` in https://github.com/rust-lang/rust/issues/82734#issuecomment-835863940

This change allows loading of llvm plugins like Enzyme.
Right now it also requires a shared library LLVM build of rustc for symbol resolution.

```rust
// test.rs
extern { fn __enzyme_autodiff(_: usize, ...) -> f64; }

fn square(x : f64) -> f64 {
   return x * x;
}

fn main() {
   unsafe {
      println!("Hello, world {} {}!", square(3.0), __enzyme_autodiff(square as usize, 3.0));
   }
}
```
```
./rustc test.rs -Z llvm-plugins="./LLVMEnzyme-12.so" -C passes="enzyme"
./test
Hello, world 9 6!
```

I will try to figure out how to simplify the usage and get this into stable in a later iteration,
but having this on nightly will already help testing further steps.

compiler/rustc_codegen_llvm/src/llvm_util.rs

@@ -3,13 +3,17 @@
 use libc::c_int;
 use rustc_codegen_ssa::target_features::supported_target_features;
 use rustc_data_structures::fx::FxHashSet;
+use rustc_metadata::dynamic_lib::DynamicLibrary;
 use rustc_middle::bug;
 use rustc_session::config::PrintRequest;
 use rustc_session::Session;
 use rustc_span::symbol::Symbol;
 use rustc_target::spec::{MergeFunctions, PanicStrategy};
 use std::ffi::{CStr, CString};
+use tracing::debug;
 
+use std::mem;
+use std::path::Path;
 use std::ptr;
 use std::slice;
 use std::str;
@@ -129,6 +133,16 @@ fn llvm_arg_to_arg_name(full_arg: &str) -> &str {
 
     llvm::LLVMInitializePasses();
 
+    for plugin in &sess.opts.debugging_opts.llvm_plugins {
+        let path = Path::new(plugin);
+        let res = DynamicLibrary::open(path);
+        match res {
+            Ok(_) => debug!("LLVM plugin loaded succesfully {} ({})", path.display(), plugin),
+            Err(e) => bug!("couldn't load plugin: {}", e),
+        }
+        mem::forget(res);
+    }
+
     rustc_llvm::initialize_available_targets();
 
     llvm::LLVMRustSetLLVMOptions(llvm_args.len() as c_int, llvm_args.as_ptr());

compiler/rustc_interface/src/tests.rs

@@ -715,6 +715,7 @@ macro_rules! tracked {
     tracked!(instrument_coverage, Some(InstrumentCoverage::All));
     tracked!(instrument_mcount, true);
     tracked!(link_only, true);
+    tracked!(llvm_plugins, vec![String::from("plugin_name")]);
     tracked!(merge_functions, Some(MergeFunctions::Disabled));
     tracked!(mir_emit_retag, true);
     tracked!(mir_opt_level, Some(4));

compiler/rustc_session/src/options.rs

@@ -1129,6 +1129,8 @@ mod parse {
         "link native libraries in the linker invocation (default: yes)"),
     link_only: bool = (false, parse_bool, [TRACKED],
         "link the `.rlink` file generated by `-Z no-link` (default: no)"),
+    llvm_plugins: Vec<String> = (Vec::new(), parse_list, [TRACKED],
+        "a list LLVM plugins to enable (space separated)"),
     llvm_time_trace: bool = (false, parse_bool, [UNTRACKED],
         "generate JSON tracing data file from LLVM data (default: no)"),
     ls: bool = (false, parse_bool, [UNTRACKED],