Add LLVM attributes in batches instead of individually

This should improve performance.

compiler/rustc_codegen_llvm/src/attributes.rs

@@ -7,53 +7,75 @@ use rustc_codegen_ssa::traits::*;
 use rustc_data_structures::small_c_str::SmallCStr;
 use rustc_hir::def_id::DefId;
 use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags;
-use rustc_middle::ty::layout::HasTyCtxt;
 use rustc_middle::ty::{self, TyCtxt};
 use rustc_session::config::OptLevel;
-use rustc_session::Session;
 use rustc_target::spec::abi::Abi;
 use rustc_target::spec::{FramePointer, SanitizerSet, StackProbeType, StackProtector};
+use smallvec::SmallVec;
 
 use crate::attributes;
 use crate::llvm::AttributePlace::Function;
-use crate::llvm::{self, Attribute};
+use crate::llvm::{self, Attribute, AttributeKind, AttributePlace};
 use crate::llvm_util;
 pub use rustc_attr::{InlineAttr, InstructionSetAttr, OptimizeAttr};
 
 use crate::context::CodegenCx;
 use crate::value::Value;
 
-/// Mark LLVM function to use provided inline heuristic.
-#[inline]
-fn inline<'ll>(cx: &CodegenCx<'ll, '_>, val: &'ll Value, inline: InlineAttr) {
-    use self::InlineAttr::*;
-    match inline {
-        Hint => Attribute::InlineHint.apply_llfn(Function, val),
-        Always => Attribute::AlwaysInline.apply_llfn(Function, val),
-        Never => {
-            if cx.tcx().sess.target.arch != "amdgpu" {
-                Attribute::NoInline.apply_llfn(Function, val);
-            }
-        }
-        None => {}
-    };
+pub fn apply_to_llfn(llfn: &Value, idx: AttributePlace, attrs: &[&Attribute]) {
+    if !attrs.is_empty() {
+        llvm::AddFunctionAttributes(llfn, idx, attrs);
+    }
 }
 
-/// Apply LLVM sanitize attributes.
+pub fn remove_from_llfn(llfn: &Value, idx: AttributePlace, attrs: &[AttributeKind]) {
+    if !attrs.is_empty() {
+        llvm::RemoveFunctionAttributes(llfn, idx, attrs);
+    }
+}
+
+pub fn apply_to_callsite(callsite: &Value, idx: AttributePlace, attrs: &[&Attribute]) {
+    if !attrs.is_empty() {
+        llvm::AddCallSiteAttributes(callsite, idx, attrs);
+    }
+}
+
+/// Get LLVM attribute for the provided inline heuristic.
 #[inline]
-pub fn sanitize<'ll>(cx: &CodegenCx<'ll, '_>, no_sanitize: SanitizerSet, llfn: &'ll Value) {
+fn inline_attr<'ll>(cx: &CodegenCx<'ll, '_>, inline: InlineAttr) -> Option<&'ll Attribute> {
+    match inline {
+        InlineAttr::Hint => Some(AttributeKind::InlineHint.create_attr(cx.llcx)),
+        InlineAttr::Always => Some(AttributeKind::AlwaysInline.create_attr(cx.llcx)),
+        InlineAttr::Never => {
+            if cx.sess().target.arch != "amdgpu" {
+                Some(AttributeKind::NoInline.create_attr(cx.llcx))
+            } else {
+                None
+            }
+        }
+        InlineAttr::None => None,
+    }
+}
+
+/// Get LLVM sanitize attributes.
+#[inline]
+pub fn sanitize_attrs<'ll>(
+    cx: &CodegenCx<'ll, '_>,
+    no_sanitize: SanitizerSet,
+) -> SmallVec<impl smallvec::Array<Item = &'ll Attribute>> {
+    let mut attrs = SmallVec::<[_; 4]>::new();
     let enabled = cx.tcx.sess.opts.debugging_opts.sanitizer - no_sanitize;
     if enabled.contains(SanitizerSet::ADDRESS) {
-        llvm::Attribute::SanitizeAddress.apply_llfn(Function, llfn);
+        attrs.push(llvm::AttributeKind::SanitizeAddress.create_attr(cx.llcx));
     }
     if enabled.contains(SanitizerSet::MEMORY) {
-        llvm::Attribute::SanitizeMemory.apply_llfn(Function, llfn);
+        attrs.push(llvm::AttributeKind::SanitizeMemory.create_attr(cx.llcx));
     }
     if enabled.contains(SanitizerSet::THREAD) {
-        llvm::Attribute::SanitizeThread.apply_llfn(Function, llfn);
+        attrs.push(llvm::AttributeKind::SanitizeThread.create_attr(cx.llcx));
     }
     if enabled.contains(SanitizerSet::HWADDRESS) {
-        llvm::Attribute::SanitizeHWAddress.apply_llfn(Function, llfn);
+        attrs.push(llvm::AttributeKind::SanitizeHWAddress.create_attr(cx.llcx));
     }
     if enabled.contains(SanitizerSet::MEMTAG) {
         // Check to make sure the mte target feature is actually enabled.
@@ -66,26 +88,21 @@ pub fn sanitize<'ll>(cx: &CodegenCx<'ll, '_>, no_sanitize: SanitizerSet, llfn: &
             sess.err("`-Zsanitizer=memtag` requires `-Ctarget-feature=+mte`");
         }
 
-        llvm::Attribute::SanitizeMemTag.apply_llfn(Function, llfn);
+        attrs.push(llvm::AttributeKind::SanitizeMemTag.create_attr(cx.llcx));
     }
+    attrs
 }
 
 /// Tell LLVM to emit or not emit the information necessary to unwind the stack for the function.
 #[inline]
-pub fn emit_uwtable(val: &Value) {
+pub fn uwtable_attr(llcx: &llvm::Context) -> &Attribute {
     // NOTE: We should determine if we even need async unwind tables, as they
     // take have more overhead and if we can use sync unwind tables we
     // probably should.
-    llvm::EmitUWTableAttr(val, true);
+    llvm::CreateUWTableAttr(llcx, true)
 }
 
-/// Tell LLVM if this function should be 'naked', i.e., skip the epilogue and prologue.
-#[inline]
-fn naked(val: &Value, is_naked: bool) {
-    Attribute::Naked.toggle_llfn(Function, val, is_naked);
-}
-
-pub fn set_frame_pointer_type<'ll>(cx: &CodegenCx<'ll, '_>, llfn: &'ll Value) {
+pub fn frame_pointer_type_attr<'ll>(cx: &CodegenCx<'ll, '_>) -> Option<&'ll Attribute> {
     let mut fp = cx.sess().target.frame_pointer;
     // "mcount" function relies on stack pointer.
     // See <https://sourceware.org/binutils/docs/gprof/Implementation.html>.
@@ -96,19 +113,14 @@ pub fn set_frame_pointer_type<'ll>(cx: &CodegenCx<'ll, '_>, llfn: &'ll Value) {
     let attr_value = match fp {
         FramePointer::Always => cstr!("all"),
         FramePointer::NonLeaf => cstr!("non-leaf"),
-        FramePointer::MayOmit => return,
+        FramePointer::MayOmit => return None,
     };
-    llvm::AddFunctionAttrStringValue(
-        llfn,
-        llvm::AttributePlace::Function,
-        cstr!("frame-pointer"),
-        attr_value,
-    );
+    Some(llvm::CreateAttrStringValue(cx.llcx, cstr!("frame-pointer"), attr_value))
 }
 
 /// Tell LLVM what instrument function to insert.
 #[inline]
-fn set_instrument_function<'ll>(cx: &CodegenCx<'ll, '_>, llfn: &'ll Value) {
+fn instrument_function_attr<'ll>(cx: &CodegenCx<'ll, '_>) -> Option<&'ll Attribute> {
     if cx.sess().instrument_mcount() {
         // Similar to `clang -pg` behavior. Handled by the
         // `post-inline-ee-instrument` LLVM pass.
@@ -117,16 +129,17 @@ fn set_instrument_function<'ll>(cx: &CodegenCx<'ll, '_>, llfn: &'ll Value) {
         // See test/CodeGen/mcount.c in clang.
         let mcount_name = CString::new(cx.sess().target.mcount.as_str().as_bytes()).unwrap();
 
-        llvm::AddFunctionAttrStringValue(
-            llfn,
-            llvm::AttributePlace::Function,
+        Some(llvm::CreateAttrStringValue(
+            cx.llcx,
             cstr!("instrument-function-entry-inlined"),
             &mcount_name,
-        );
+        ))
+    } else {
+        None
     }
 }
 
-fn set_probestack<'ll>(cx: &CodegenCx<'ll, '_>, llfn: &'ll Value) {
+fn probestack_attr<'ll>(cx: &CodegenCx<'ll, '_>) -> Option<&'ll Attribute> {
     // Currently stack probes seem somewhat incompatible with the address
     // sanitizer and thread sanitizer. With asan we're already protected from
     // stack overflow anyway so we don't really need stack probes regardless.
@@ -137,107 +150,105 @@ fn set_probestack<'ll>(cx: &CodegenCx<'ll, '_>, llfn: &'ll Value) {
         .sanitizer
         .intersects(SanitizerSet::ADDRESS | SanitizerSet::THREAD)
     {
-        return;
+        return None;
     }
 
     // probestack doesn't play nice either with `-C profile-generate`.
     if cx.sess().opts.cg.profile_generate.enabled() {
-        return;
+        return None;
     }
 
     // probestack doesn't play nice either with gcov profiling.
     if cx.sess().opts.debugging_opts.profile {
-        return;
+        return None;
     }
 
     let attr_value = match cx.sess().target.stack_probes {
-        StackProbeType::None => None,
+        StackProbeType::None => return None,
         // Request LLVM to generate the probes inline. If the given LLVM version does not support
         // this, no probe is generated at all (even if the attribute is specified).
-        StackProbeType::Inline => Some(cstr!("inline-asm")),
+        StackProbeType::Inline => cstr!("inline-asm"),
         // Flag our internal `__rust_probestack` function as the stack probe symbol.
         // This is defined in the `compiler-builtins` crate for each architecture.
-        StackProbeType::Call => Some(cstr!("__rust_probestack")),
+        StackProbeType::Call => cstr!("__rust_probestack"),
         // Pick from the two above based on the LLVM version.
         StackProbeType::InlineOrCall { min_llvm_version_for_inline } => {
             if llvm_util::get_version() < min_llvm_version_for_inline {
-                Some(cstr!("__rust_probestack"))
+                cstr!("__rust_probestack")
             } else {
-                Some(cstr!("inline-asm"))
+                cstr!("inline-asm")
             }
         }
     };
-    if let Some(attr_value) = attr_value {
-        llvm::AddFunctionAttrStringValue(
-            llfn,
-            llvm::AttributePlace::Function,
-            cstr!("probe-stack"),
-            attr_value,
-        );
-    }
+    Some(llvm::CreateAttrStringValue(cx.llcx, cstr!("probe-stack"), attr_value))
 }
 
-fn set_stackprotector<'ll>(cx: &CodegenCx<'ll, '_>, llfn: &'ll Value) {
+fn stackprotector_attr<'ll>(cx: &CodegenCx<'ll, '_>) -> Option<&'ll Attribute> {
     let sspattr = match cx.sess().stack_protector() {
-        StackProtector::None => return,
-        StackProtector::All => Attribute::StackProtectReq,
-        StackProtector::Strong => Attribute::StackProtectStrong,
-        StackProtector::Basic => Attribute::StackProtect,
+        StackProtector::None => return None,
+        StackProtector::All => AttributeKind::StackProtectReq,
+        StackProtector::Strong => AttributeKind::StackProtectStrong,
+        StackProtector::Basic => AttributeKind::StackProtect,
     };
 
-    sspattr.apply_llfn(Function, llfn)
+    Some(sspattr.create_attr(cx.llcx))
 }
 
-pub fn apply_target_cpu_attr<'ll>(cx: &CodegenCx<'ll, '_>, llfn: &'ll Value) {
+pub fn target_cpu_attr<'ll>(cx: &CodegenCx<'ll, '_>) -> &'ll Attribute {
     let target_cpu = SmallCStr::new(llvm_util::target_cpu(cx.tcx.sess));
-    llvm::AddFunctionAttrStringValue(
-        llfn,
-        llvm::AttributePlace::Function,
-        cstr!("target-cpu"),
-        target_cpu.as_c_str(),
-    );
+    llvm::CreateAttrStringValue(cx.llcx, cstr!("target-cpu"), target_cpu.as_c_str())
 }
 
-pub fn apply_tune_cpu_attr<'ll>(cx: &CodegenCx<'ll, '_>, llfn: &'ll Value) {
-    if let Some(tune) = llvm_util::tune_cpu(cx.tcx.sess) {
+pub fn tune_cpu_attr<'ll>(cx: &CodegenCx<'ll, '_>) -> Option<&'ll Attribute> {
+    llvm_util::tune_cpu(cx.tcx.sess).map(|tune| {
         let tune_cpu = SmallCStr::new(tune);
-        llvm::AddFunctionAttrStringValue(
-            llfn,
-            llvm::AttributePlace::Function,
-            cstr!("tune-cpu"),
-            tune_cpu.as_c_str(),
-        );
-    }
+        llvm::CreateAttrStringValue(cx.llcx, cstr!("tune-cpu"), tune_cpu.as_c_str())
+    })
 }
 
-/// Sets the `NonLazyBind` LLVM attribute on a given function,
-/// assuming the codegen options allow skipping the PLT.
-pub fn non_lazy_bind<'ll>(sess: &Session, llfn: &'ll Value) {
+/// Get the `NonLazyBind` LLVM attribute,
+/// if the codegen options allow skipping the PLT.
+pub fn non_lazy_bind_attr<'ll>(cx: &CodegenCx<'ll, '_>) -> Option<&'ll Attribute> {
     // Don't generate calls through PLT if it's not necessary
-    if !sess.needs_plt() {
-        Attribute::NonLazyBind.apply_llfn(Function, llfn);
+    if !cx.sess().needs_plt() {
+        Some(AttributeKind::NonLazyBind.create_attr(cx.llcx))
+    } else {
+        None
     }
 }
 
-pub(crate) fn default_optimisation_attrs<'ll>(sess: &Session, llfn: &'ll Value) {
-    match sess.opts.optimize {
+/// Returns attributes to remove and to add, respectively,
+/// to set the default optimizations attrs on a function.
+#[inline]
+pub(crate) fn default_optimisation_attrs<'ll>(
+    cx: &CodegenCx<'ll, '_>,
+) -> (
+    // Attributes to remove
+    SmallVec<impl smallvec::Array<Item = AttributeKind>>,
+    // Attributes to add
+    SmallVec<impl smallvec::Array<Item = &'ll Attribute>>,
+) {
+    let mut to_remove = SmallVec::<[_; 3]>::new();
+    let mut to_add = SmallVec::<[_; 2]>::new();
+    match cx.sess().opts.optimize {
         OptLevel::Size => {
-            llvm::Attribute::MinSize.unapply_llfn(Function, llfn);
-            llvm::Attribute::OptimizeForSize.apply_llfn(Function, llfn);
-            llvm::Attribute::OptimizeNone.unapply_llfn(Function, llfn);
+            to_remove.push(llvm::AttributeKind::MinSize);
+            to_add.push(llvm::AttributeKind::OptimizeForSize.create_attr(cx.llcx));
+            to_remove.push(llvm::AttributeKind::OptimizeNone);
         }
         OptLevel::SizeMin => {
-            llvm::Attribute::MinSize.apply_llfn(Function, llfn);
-            llvm::Attribute::OptimizeForSize.apply_llfn(Function, llfn);
-            llvm::Attribute::OptimizeNone.unapply_llfn(Function, llfn);
+            to_add.push(llvm::AttributeKind::MinSize.create_attr(cx.llcx));
+            to_add.push(llvm::AttributeKind::OptimizeForSize.create_attr(cx.llcx));
+            to_remove.push(llvm::AttributeKind::OptimizeNone);
         }
         OptLevel::No => {
-            llvm::Attribute::MinSize.unapply_llfn(Function, llfn);
-            llvm::Attribute::OptimizeForSize.unapply_llfn(Function, llfn);
-            llvm::Attribute::OptimizeNone.unapply_llfn(Function, llfn);
+            to_remove.push(llvm::AttributeKind::MinSize);
+            to_remove.push(llvm::AttributeKind::OptimizeForSize);
+            to_remove.push(llvm::AttributeKind::OptimizeNone);
         }
         _ => {}
     }
+    (to_remove, to_add)
 }
 
 /// Composite function which sets LLVM attributes for function depending on its AST (`#[attribute]`)
@@ -249,30 +260,35 @@ pub fn from_fn_attrs<'ll, 'tcx>(
 ) {
     let codegen_fn_attrs = cx.tcx.codegen_fn_attrs(instance.def_id());
 
+    let mut to_remove = SmallVec::<[_; 4]>::new();
+    let mut to_add = SmallVec::<[_; 16]>::new();
+
     match codegen_fn_attrs.optimize {
         OptimizeAttr::None => {
-            default_optimisation_attrs(cx.tcx.sess, llfn);
+            let (to_remove_opt, to_add_opt) = default_optimisation_attrs(cx);
+            to_remove.extend(to_remove_opt);
+            to_add.extend(to_add_opt);
         }
         OptimizeAttr::Speed => {
-            llvm::Attribute::MinSize.unapply_llfn(Function, llfn);
-            llvm::Attribute::OptimizeForSize.unapply_llfn(Function, llfn);
-            llvm::Attribute::OptimizeNone.unapply_llfn(Function, llfn);
+            to_remove.push(llvm::AttributeKind::MinSize);
+            to_remove.push(llvm::AttributeKind::OptimizeForSize);
+            to_remove.push(llvm::AttributeKind::OptimizeNone);
         }
         OptimizeAttr::Size => {
-            llvm::Attribute::MinSize.apply_llfn(Function, llfn);
-            llvm::Attribute::OptimizeForSize.apply_llfn(Function, llfn);
-            llvm::Attribute::OptimizeNone.unapply_llfn(Function, llfn);
+            to_add.push(llvm::AttributeKind::MinSize.create_attr(cx.llcx));
+            to_add.push(llvm::AttributeKind::OptimizeForSize.create_attr(cx.llcx));
+            to_remove.push(llvm::AttributeKind::OptimizeNone);
         }
     }
 
-    let inline_attr = if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::NAKED) {
+    let inline = if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::NAKED) {
         InlineAttr::Never
     } else if codegen_fn_attrs.inline == InlineAttr::None && instance.def.requires_inline(cx.tcx) {
         InlineAttr::Hint
     } else {
         codegen_fn_attrs.inline
     };
-    inline(cx, llfn, inline_attr);
+    to_add.extend(inline_attr(cx, inline));
 
     // The `uwtable` attribute according to LLVM is:
     //
@@ -291,52 +307,54 @@ pub fn from_fn_attrs<'ll, 'tcx>(
     // You can also find more info on why Windows always requires uwtables here:
     //      https://bugzilla.mozilla.org/show_bug.cgi?id=1302078
     if cx.sess().must_emit_unwind_tables() {
-        attributes::emit_uwtable(llfn);
+        to_add.push(uwtable_attr(cx.llcx));
     }
 
     if cx.sess().opts.debugging_opts.profile_sample_use.is_some() {
-        llvm::AddFunctionAttrString(llfn, Function, cstr!("use-sample-profile"));
+        to_add.push(llvm::CreateAttrString(cx.llcx, cstr!("use-sample-profile")));
     }
 
     // FIXME: none of these three functions interact with source level attributes.
-    set_frame_pointer_type(cx, llfn);
-    set_instrument_function(cx, llfn);
-    set_probestack(cx, llfn);
-    set_stackprotector(cx, llfn);
+    to_add.extend(frame_pointer_type_attr(cx));
+    to_add.extend(instrument_function_attr(cx));
+    to_add.extend(probestack_attr(cx));
+    to_add.extend(stackprotector_attr(cx));
 
     if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::COLD) {
-        Attribute::Cold.apply_llfn(Function, llfn);
+        to_add.push(AttributeKind::Cold.create_attr(cx.llcx));
     }
     if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::FFI_RETURNS_TWICE) {
-        Attribute::ReturnsTwice.apply_llfn(Function, llfn);
+        to_add.push(AttributeKind::ReturnsTwice.create_attr(cx.llcx));
     }
     if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::FFI_PURE) {
-        Attribute::ReadOnly.apply_llfn(Function, llfn);
+        to_add.push(AttributeKind::ReadOnly.create_attr(cx.llcx));
     }
     if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::FFI_CONST) {
-        Attribute::ReadNone.apply_llfn(Function, llfn);
+        to_add.push(AttributeKind::ReadNone.create_attr(cx.llcx));
     }
     if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::NAKED) {
-        naked(llfn, true);
+        to_add.push(AttributeKind::Naked.create_attr(cx.llcx));
     }
     if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::ALLOCATOR) {
-        Attribute::NoAlias.apply_llfn(llvm::AttributePlace::ReturnValue, llfn);
+        // apply to return place instead of function (unlike all other attributes applied in this function)
+        let no_alias = AttributeKind::NoAlias.create_attr(cx.llcx);
+        attributes::apply_to_llfn(llfn, AttributePlace::ReturnValue, &[no_alias]);
     }
     if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::CMSE_NONSECURE_ENTRY) {
-        llvm::AddFunctionAttrString(llfn, Function, cstr!("cmse_nonsecure_entry"));
+        to_add.push(llvm::CreateAttrString(cx.llcx, cstr!("cmse_nonsecure_entry")));
     }
     if let Some(align) = codegen_fn_attrs.alignment {
         llvm::set_alignment(llfn, align as usize);
     }
-    sanitize(cx, codegen_fn_attrs.no_sanitize, llfn);
+    to_add.extend(sanitize_attrs(cx, codegen_fn_attrs.no_sanitize));
 
     // Always annotate functions with the target-cpu they are compiled for.
     // Without this, ThinLTO won't inline Rust functions into Clang generated
     // functions (because Clang annotates functions this way too).
-    apply_target_cpu_attr(cx, llfn);
+    to_add.push(target_cpu_attr(cx));
     // tune-cpu is only conveyed through the attribute for our purpose.
     // The target doesn't care; the subtarget reads our attribute.
-    apply_tune_cpu_attr(cx, llfn);
+    to_add.extend(tune_cpu_attr(cx));
 
     let function_features =
         codegen_fn_attrs.target_features.iter().map(|f| f.as_str()).collect::<Vec<&str>>();
@@ -379,22 +397,12 @@ pub fn from_fn_attrs<'ll, 'tcx>(
         // If this function is an import from the environment but the wasm
         // import has a specific module/name, apply them here.
         if let Some(module) = wasm_import_module(cx.tcx, instance.def_id()) {
-            llvm::AddFunctionAttrStringValue(
-                llfn,
-                llvm::AttributePlace::Function,
-                cstr!("wasm-import-module"),
-                &module,
-            );
+            to_add.push(llvm::CreateAttrStringValue(cx.llcx, cstr!("wasm-import-module"), &module));
 
             let name =
                 codegen_fn_attrs.link_name.unwrap_or_else(|| cx.tcx.item_name(instance.def_id()));
             let name = CString::new(name.as_str()).unwrap();
-            llvm::AddFunctionAttrStringValue(
-                llfn,
-                llvm::AttributePlace::Function,
-                cstr!("wasm-import-name"),
-                &name,
-            );
+            to_add.push(llvm::CreateAttrStringValue(cx.llcx, cstr!("wasm-import-name"), &name));
         }
 
         // The `"wasm"` abi on wasm targets automatically enables the
@@ -414,13 +422,11 @@ pub fn from_fn_attrs<'ll, 'tcx>(
         global_features.extend(function_features.into_iter());
         let features = global_features.join(",");
         let val = CString::new(features).unwrap();
-        llvm::AddFunctionAttrStringValue(
-            llfn,
-            llvm::AttributePlace::Function,
-            cstr!("target-features"),
-            &val,
-        );
+        to_add.push(llvm::CreateAttrStringValue(cx.llcx, cstr!("target-features"), &val));
     }
+
+    attributes::remove_from_llfn(llfn, Function, &to_remove);
+    attributes::apply_to_llfn(llfn, Function, &to_add);
 }
 
 fn wasm_import_module(tcx: TyCtxt<'_>, id: DefId) -> Option<CString> {