Rollup merge of #142809 - KMJ-007:ad-type-analysis-flag, r=ZuseZ4

Add PrintTAFn flag for targeted type analysis printing

## 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:
```bash
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```

2 files changed, 18 insertions, 3 deletions

diff --git a/compiler/rustc_session/src/config.rs b/compiler/rustc_session/src/config.rs
index c62e4ac30ea..73bb0471c22 100644
--- a/compiler/rustc_session/src/config.rs
+++ b/compiler/rustc_session/src/config.rs
@@ -227,13 +227,15 @@ pub enum CoverageLevel {
 }
 
 /// The different settings that the `-Z autodiff` flag can have.
-#[derive(Clone, Copy, PartialEq, Hash, Debug)]
+#[derive(Clone, PartialEq, Hash, Debug)]
 pub enum AutoDiff {
     /// Enable the autodiff opt pipeline
     Enable,
 
     /// Print TypeAnalysis information
     PrintTA,
+    /// Print TypeAnalysis information for a specific function
+    PrintTAFn(String),
     /// Print ActivityAnalysis Information
     PrintAA,
     /// Print Performance Warnings from Enzyme
diff --git a/compiler/rustc_session/src/options.rs b/compiler/rustc_session/src/options.rs
index 232531dc673..ecd82c0cc01 100644
--- a/compiler/rustc_session/src/options.rs
+++ b/compiler/rustc_session/src/options.rs
@@ -725,7 +725,7 @@ mod desc {
     pub(crate) const parse_list: &str = "a space-separated list of strings";
     pub(crate) const parse_list_with_polarity: &str =
         "a comma-separated list of strings, with elements beginning with + or -";
-    pub(crate) const parse_autodiff: &str = "a comma separated list of settings: `Enable`, `PrintSteps`, `PrintTA`, `PrintAA`, `PrintPerf`, `PrintModBefore`, `PrintModAfter`, `PrintModFinal`, `PrintPasses`, `NoPostopt`, `LooseTypes`, `Inline`";
+    pub(crate) const parse_autodiff: &str = "a comma separated list of settings: `Enable`, `PrintSteps`, `PrintTA`, `PrintTAFn`, `PrintAA`, `PrintPerf`, `PrintModBefore`, `PrintModAfter`, `PrintModFinal`, `PrintPasses`, `NoPostopt`, `LooseTypes`, `Inline`";
     pub(crate) const parse_comma_list: &str = "a comma-separated list of strings";
     pub(crate) const parse_opt_comma_list: &str = parse_comma_list;
     pub(crate) const parse_number: &str = "a number";
@@ -1365,9 +1365,22 @@ pub mod parse {
         let mut v: Vec<&str> = v.split(",").collect();
         v.sort_unstable();
         for &val in v.iter() {
-            let variant = match val {
+            // Split each entry on '=' if it has an argument
+            let (key, arg) = match val.split_once('=') {
+                Some((k, a)) => (k, Some(a)),
+                None => (val, None),
+            };
+
+            let variant = match key {
                 "Enable" => AutoDiff::Enable,
                 "PrintTA" => AutoDiff::PrintTA,
+                "PrintTAFn" => {
+                    if let Some(fun) = arg {
+                        AutoDiff::PrintTAFn(fun.to_string())
+                    } else {
+                        return false;
+                    }
+                }
                 "PrintAA" => AutoDiff::PrintAA,
                 "PrintPerf" => AutoDiff::PrintPerf,
                 "PrintSteps" => AutoDiff::PrintSteps,