Add PrintTAFn flag for targeted type analysis printing

[KMJ-007]

Summary

This PR adds a new PrintTAFn flag to the -Z autodiff option that allows printing type analysis information for a specific function, rather than all functions.

Changes

New Flag

• Added PrintTAFn=<function_name> option to -Z autodiff
• Usage: -Z autodiff=Enable,PrintTAFn=my_function_name

Implementation Details

• Rust side: Added PrintTAFn(String) variant to AutoDiff enum
• Parser: Updated parse_autodiff to handle PrintTAFn=<function_name> syntax with proper error handling
• FFI: Added set_print_type_fun function to interface with Enzyme's FunctionToAnalyze command line option
• Documentation: Updated help text and documentation for the new flag

Files Modified

• compiler/rustc_session/src/config.rs: Added PrintTAFn(String) variant
• compiler/rustc_session/src/options.rs: Updated parser and help text (now shows PrintTAFn in the list)
• compiler/rustc_codegen_llvm/src/llvm/enzyme_ffi.rs: Added FFI function and static variable
• compiler/rustc_codegen_llvm/src/back/lto.rs: Added handling for new flag
• src/doc/rustc-dev-guide/src/autodiff/flags.md: Updated documentation
• src/doc/unstable-book/src/compiler-flags/autodiff.md: Updated documentation

Testing

The flag can be tested with:

rustc +enzyme -Z autodiff=Enable,PrintTAFn=square test.rs

This will print type analysis information only for the function named "square" instead of all functions.

Error Handling

The parser includes proper error handling:

• Missing argument: PrintTAFn without =<function_name> will show an error
• Unknown options: Invalid autodiff options will be reported

r? @ZuseZ4

[rustbot]

Some changes occurred in compiler/rustc_codegen_llvm/src/llvm/enzyme_ffi.rs

cc @ZuseZ4

The rustc-dev-guide subtree was changed. If this PR only touches the dev guide consider submitting a PR directly to rust-lang/rustc-dev-guide otherwise thank you for updating the dev guide with your changes.

cc @BoxyUwU, @jieyouxu, @Kobzol

Some changes occurred in src/doc/unstable-book/src/compiler-flags/autodiff.md

cc @ZuseZ4

[rust-log-analyzer]

The job mingw-check-tidy failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

[TIMING] core::build_steps::tool::Tidy { compiler: Compiler { stage: 0, host: x86_64-unknown-linux-gnu, forced_compiler: false }, target: x86_64-unknown-linux-gnu } -- 0.000
fmt check
Diff in /checkout/compiler/rustc_codegen_llvm/src/llvm/enzyme_ffi.rs:57:
 
 #[cfg(llvm_enzyme)]
 pub(crate) mod Enzyme_AD {
-    use libc::c_void;
     use std::ffi::{CString, c_char};
+
+    use libc::c_void;
 
     unsafe extern "C" {
         pub(crate) fn EnzymeSetCLBool(arg1: *mut ::std::os::raw::c_void, arg2: u8);
fmt: checked 6077 files
Build completed unsuccessfully in 0:00:44
  local time: Fri Jun 20 20:59:03 UTC 2025
  network time: Fri, 20 Jun 2025 20:59:03 GMT
##[error]Process completed with exit code 1.