Make -Z new-llvm-pass-manager an Option<bool>

To allow it to have an LLVM version dependent default.

compiler/rustc_codegen_llvm/src/back/write.rs

@@ -410,7 +410,7 @@ fn get_pgo_use_path(config: &ModuleConfig) -> Option<CString> {
 
 pub(crate) fn should_use_new_llvm_pass_manager(config: &ModuleConfig) -> bool {
     // The new pass manager is disabled by default.
-    config.new_llvm_pass_manager
+    config.new_llvm_pass_manager.unwrap_or(false)
 }
 
 pub(crate) unsafe fn optimize_with_new_llvm_pass_manager(

compiler/rustc_codegen_ssa/src/back/write.rs

@@ -108,7 +108,7 @@ pub struct ModuleConfig {
     pub vectorize_slp: bool,
     pub merge_functions: bool,
     pub inline_threshold: Option<u32>,
-    pub new_llvm_pass_manager: bool,
+    pub new_llvm_pass_manager: Option<bool>,
     pub emit_lifetime_markers: bool,
 }
 

compiler/rustc_interface/src/tests.rs

@@ -709,7 +709,7 @@ fn test_debugging_options_tracking_hash() {
     tracked!(mir_emit_retag, true);
     tracked!(mir_opt_level, Some(4));
     tracked!(mutable_noalias, Some(true));
-    tracked!(new_llvm_pass_manager, true);
+    tracked!(new_llvm_pass_manager, Some(true));
     tracked!(no_codegen, true);
     tracked!(no_generate_arange_section, true);
     tracked!(no_link, true);

compiler/rustc_session/src/options.rs

@@ -1112,7 +1112,7 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
         "MIR optimization level (0-4; default: 1 in non optimized builds and 2 in optimized builds)"),
     mutable_noalias: Option<bool> = (None, parse_opt_bool, [TRACKED],
         "emit noalias metadata for mutable references (default: yes for LLVM >= 12, otherwise no)"),
-    new_llvm_pass_manager: bool = (false, parse_bool, [TRACKED],
+    new_llvm_pass_manager: Option<bool> = (None, parse_opt_bool, [TRACKED],
         "use new LLVM pass manager (default: no)"),
     nll_facts: bool = (false, parse_bool, [UNTRACKED],
         "dump facts from NLL analysis into side files (default: no)"),