Rollup merge of rust-lang#132260 - Zalathar:type-safe-cast, r=compiler-errors

cg_llvm: Use a type-safe helper to cast `&str` and `&[u8]` to `*const c_char`

In `rustc_codegen_llvm` there are many uses of `.as_ptr().cast()` to convert a string or byte-slice to `*const c_char`, which then gets passed through FFI.

This works, but is fragile, because there's nothing constraining the pointer cast to actually be from `u8` to `c_char`. If the original value changes to something else that has an `as_ptr` method, or the context changes to expect something other than `c_char`, the cast will silently do the wrong thing.

By making the cast more explicit via a helper method, we can be sure that it will either perform the intended cast, or fail at compile time.

Author: workingjubilee

compiler/rustc_codegen_llvm/src/allocator.rs

@@ -7,6 +7,7 @@ use rustc_middle::bug;
 use rustc_middle::ty::TyCtxt;
 use rustc_session::config::{DebugInfo, OomStrategy};
 
+use crate::common::AsCCharPtr;
 use crate::llvm::{self, Context, False, Module, True, Type};
 use crate::{ModuleLlvm, attributes, debuginfo};
 
@@ -76,14 +77,14 @@ pub(crate) unsafe fn codegen(
     unsafe {
         // __rust_alloc_error_handler_should_panic
         let name = OomStrategy::SYMBOL;
-        let ll_g = llvm::LLVMRustGetOrInsertGlobal(llmod, name.as_ptr().cast(), name.len(), i8);
+        let ll_g = llvm::LLVMRustGetOrInsertGlobal(llmod, name.as_c_char_ptr(), name.len(), i8);
         llvm::set_visibility(ll_g, llvm::Visibility::from_generic(tcx.sess.default_visibility()));
         let val = tcx.sess.opts.unstable_opts.oom.should_panic();
         let llval = llvm::LLVMConstInt(i8, val as u64, False);
         llvm::LLVMSetInitializer(ll_g, llval);
 
         let name = NO_ALLOC_SHIM_IS_UNSTABLE;
-        let ll_g = llvm::LLVMRustGetOrInsertGlobal(llmod, name.as_ptr().cast(), name.len(), i8);
+        let ll_g = llvm::LLVMRustGetOrInsertGlobal(llmod, name.as_c_char_ptr(), name.len(), i8);
         llvm::set_visibility(ll_g, llvm::Visibility::from_generic(tcx.sess.default_visibility()));
         let llval = llvm::LLVMConstInt(i8, 0, False);
         llvm::LLVMSetInitializer(ll_g, llval);
@@ -115,7 +116,7 @@ fn create_wrapper_function(
         );
         let llfn = llvm::LLVMRustGetOrInsertFunction(
             llmod,
-            from_name.as_ptr().cast(),
+            from_name.as_c_char_ptr(),
             from_name.len(),
             ty,
         );
@@ -137,7 +138,7 @@ fn create_wrapper_function(
         }
 
         let callee =
-            llvm::LLVMRustGetOrInsertFunction(llmod, to_name.as_ptr().cast(), to_name.len(), ty);
+            llvm::LLVMRustGetOrInsertFunction(llmod, to_name.as_c_char_ptr(), to_name.len(), ty);
         if let Some(no_return) = no_return {
             // -> ! DIFlagNoReturn
             attributes::apply_to_llfn(callee, llvm::AttributePlace::Function, &[no_return]);

compiler/rustc_codegen_llvm/src/asm.rs

@@ -15,7 +15,7 @@ use smallvec::SmallVec;
 use tracing::debug;
 
 use crate::builder::Builder;
-use crate::common::Funclet;
+use crate::common::{AsCCharPtr, Funclet};
 use crate::context::CodegenCx;
 use crate::type_::Type;
 use crate::type_of::LayoutLlvmExt;
@@ -420,7 +420,7 @@ impl<'tcx> AsmCodegenMethods<'tcx> for CodegenCx<'_, 'tcx> {
         unsafe {
             llvm::LLVMAppendModuleInlineAsm(
                 self.llmod,
-                template_str.as_ptr().cast(),
+                template_str.as_c_char_ptr(),
                 template_str.len(),
             );
         }
@@ -458,14 +458,14 @@ pub(crate) fn inline_asm_call<'ll>(
     let fty = bx.cx.type_func(&argtys, output);
     unsafe {
         // Ask LLVM to verify that the constraints are well-formed.
-        let constraints_ok = llvm::LLVMRustInlineAsmVerify(fty, cons.as_ptr().cast(), cons.len());
+        let constraints_ok = llvm::LLVMRustInlineAsmVerify(fty, cons.as_c_char_ptr(), cons.len());
         debug!("constraint verification result: {:?}", constraints_ok);
         if constraints_ok {
             let v = llvm::LLVMRustInlineAsm(
                 fty,
-                asm.as_ptr().cast(),
+                asm.as_c_char_ptr(),
                 asm.len(),
-                cons.as_ptr().cast(),
+                cons.as_c_char_ptr(),
                 cons.len(),
                 volatile,
                 alignstack,

compiler/rustc_codegen_llvm/src/back/lto.rs

@@ -25,6 +25,7 @@ use tracing::{debug, info};
 use crate::back::write::{
     self, CodegenDiagnosticsStage, DiagnosticHandlers, bitcode_section_name, save_temp_bitcode,
 };
+use crate::common::AsCCharPtr;
 use crate::errors::{
     DynamicLinkingWithLTO, LlvmError, LtoBitcodeFromRlib, LtoDisallowed, LtoDylib, LtoProcMacro,
 };
@@ -604,7 +605,7 @@ pub(crate) fn run_pass_manager(
     unsafe {
         if !llvm::LLVMRustHasModuleFlag(
             module.module_llvm.llmod(),
-            "LTOPostLink".as_ptr().cast(),
+            "LTOPostLink".as_c_char_ptr(),
             11,
         ) {
             llvm::LLVMRustAddModuleFlagU32(

compiler/rustc_codegen_llvm/src/back/write.rs

@@ -34,6 +34,7 @@ use crate::back::owned_target_machine::OwnedTargetMachine;
 use crate::back::profiling::{
     LlvmSelfProfiler, selfprofile_after_pass_callback, selfprofile_before_pass_callback,
 };
+use crate::common::AsCCharPtr;
 use crate::errors::{
     CopyBitcode, FromLlvmDiag, FromLlvmOptimizationDiag, LlvmError, UnknownCompression,
     WithLlvmError, WriteBytecode,
@@ -596,9 +597,9 @@ pub(crate) unsafe fn llvm_optimize(
             llvm_selfprofiler,
             selfprofile_before_pass_callback,
             selfprofile_after_pass_callback,
-            extra_passes.as_ptr().cast(),
+            extra_passes.as_c_char_ptr(),
             extra_passes.len(),
-            llvm_plugins.as_ptr().cast(),
+            llvm_plugins.as_c_char_ptr(),
             llvm_plugins.len(),
         )
     };
@@ -1042,7 +1043,7 @@ unsafe fn embed_bitcode(
             llvm::LLVMSetInitializer(llglobal, llconst);
 
             let section = bitcode_section_name(cgcx);
-            llvm::LLVMSetSection(llglobal, section.as_ptr().cast());
+            llvm::LLVMSetSection(llglobal, section.as_c_char_ptr());
             llvm::set_linkage(llglobal, llvm::Linkage::PrivateLinkage);
             llvm::LLVMSetGlobalConstant(llglobal, llvm::True);
 
@@ -1066,9 +1067,9 @@ unsafe fn embed_bitcode(
             // We need custom section flags, so emit module-level inline assembly.
             let section_flags = if cgcx.is_pe_coff { "n" } else { "e" };
             let asm = create_section_with_flags_asm(".llvmbc", section_flags, bitcode);
-            llvm::LLVMAppendModuleInlineAsm(llmod, asm.as_ptr().cast(), asm.len());
+            llvm::LLVMAppendModuleInlineAsm(llmod, asm.as_c_char_ptr(), asm.len());
             let asm = create_section_with_flags_asm(".llvmcmd", section_flags, cmdline.as_bytes());
-            llvm::LLVMAppendModuleInlineAsm(llmod, asm.as_ptr().cast(), asm.len());
+            llvm::LLVMAppendModuleInlineAsm(llmod, asm.as_c_char_ptr(), asm.len());
         }
     }
 }

compiler/rustc_codegen_llvm/src/common.rs

@@ -392,3 +392,21 @@ pub(crate) fn get_dllimport<'tcx>(
     tcx.native_library(id)
         .and_then(|lib| lib.dll_imports.iter().find(|di| di.name.as_str() == name))
 }
+
+/// Extension trait for explicit casts to `*const c_char`.
+pub(crate) trait AsCCharPtr {
+    /// Equivalent to `self.as_ptr().cast()`, but only casts to `*const c_char`.
+    fn as_c_char_ptr(&self) -> *const c_char;
+}
+
+impl AsCCharPtr for str {
+    fn as_c_char_ptr(&self) -> *const c_char {
+        self.as_ptr().cast()
+    }
+}
+
+impl AsCCharPtr for [u8] {
+    fn as_c_char_ptr(&self) -> *const c_char {
+        self.as_ptr().cast()
+    }
+}

compiler/rustc_codegen_llvm/src/consts.rs

@@ -19,7 +19,7 @@ use rustc_target::abi::{
 };
 use tracing::{debug, instrument, trace};
 
-use crate::common::CodegenCx;
+use crate::common::{AsCCharPtr, CodegenCx};
 use crate::errors::{
     InvalidMinimumAlignmentNotPowerOfTwo, InvalidMinimumAlignmentTooLarge, SymbolAlreadyDefined,
 };
@@ -400,7 +400,7 @@ impl<'ll> CodegenCx<'ll, '_> {
 
                 let new_g = llvm::LLVMRustGetOrInsertGlobal(
                     self.llmod,
-                    name.as_ptr().cast(),
+                    name.as_c_char_ptr(),
                     name.len(),
                     val_llty,
                 );
@@ -451,7 +451,7 @@ impl<'ll> CodegenCx<'ll, '_> {
                 if let Some(section) = attrs.link_section {
                     let section = llvm::LLVMMDStringInContext2(
                         self.llcx,
-                        section.as_str().as_ptr().cast(),
+                        section.as_str().as_c_char_ptr(),
                         section.as_str().len(),
                     );
                     assert!(alloc.provenance().ptrs().is_empty());
@@ -462,7 +462,7 @@ impl<'ll> CodegenCx<'ll, '_> {
                     let bytes =
                         alloc.inspect_with_uninit_and_ptr_outside_interpreter(0..alloc.len());
                     let alloc =
-                        llvm::LLVMMDStringInContext2(self.llcx, bytes.as_ptr().cast(), bytes.len());
+                        llvm::LLVMMDStringInContext2(self.llcx, bytes.as_c_char_ptr(), bytes.len());
                     let data = [section, alloc];
                     let meta = llvm::LLVMMDNodeInContext2(self.llcx, data.as_ptr(), data.len());
                     let val = llvm::LLVMMetadataAsValue(self.llcx, meta);

compiler/rustc_codegen_llvm/src/context.rs

@@ -29,6 +29,7 @@ use smallvec::SmallVec;
 
 use crate::back::write::to_llvm_code_model;
 use crate::callee::get_fn;
+use crate::common::AsCCharPtr;
 use crate::debuginfo::metadata::apply_vcall_visibility_metadata;
 use crate::llvm::{Metadata, MetadataType};
 use crate::type_::Type;
@@ -231,7 +232,7 @@ pub(crate) unsafe fn create_module<'ll>(
     // If we're normalizing integers with CFI, ensure LLVM generated functions do the same.
     // See https://github.com/llvm/llvm-project/pull/104826
     if sess.is_sanitizer_cfi_normalize_integers_enabled() {
-        let cfi_normalize_integers = c"cfi-normalize-integers".as_ptr().cast();
+        let cfi_normalize_integers = c"cfi-normalize-integers".as_ptr();
         unsafe {
             llvm::LLVMRustAddModuleFlagU32(
                 llmod,
@@ -268,7 +269,7 @@ pub(crate) unsafe fn create_module<'ll>(
         let pfe =
             PatchableFunctionEntry::from_config(sess.opts.unstable_opts.patchable_function_entry);
         if pfe.prefix() > 0 {
-            let kcfi_offset = c"kcfi-offset".as_ptr().cast();
+            let kcfi_offset = c"kcfi-offset".as_ptr();
             unsafe {
                 llvm::LLVMRustAddModuleFlagU32(
                     llmod,
@@ -429,7 +430,7 @@ pub(crate) unsafe fn create_module<'ll>(
     let name_metadata = unsafe {
         llvm::LLVMMDStringInContext2(
             llcx,
-            rustc_producer.as_ptr().cast(),
+            rustc_producer.as_c_char_ptr(),
             rustc_producer.as_bytes().len(),
         )
     };
@@ -453,7 +454,7 @@ pub(crate) unsafe fn create_module<'ll>(
                 llmod,
                 llvm::LLVMModFlagBehavior::Error,
                 c"target-abi".as_ptr(),
-                llvm_abiname.as_ptr().cast(),
+                llvm_abiname.as_c_char_ptr(),
                 llvm_abiname.len(),
             );
         }
@@ -474,7 +475,7 @@ pub(crate) unsafe fn create_module<'ll>(
             // We already checked this during option parsing
             _ => unreachable!(),
         };
-        unsafe { llvm::LLVMRustAddModuleFlagU32(llmod, behavior, key.as_ptr().cast(), *value) }
+        unsafe { llvm::LLVMRustAddModuleFlagU32(llmod, behavior, key.as_c_char_ptr(), *value) }
     }
 
     llmod

compiler/rustc_codegen_llvm/src/coverageinfo/mod.rs

@@ -14,7 +14,7 @@ use rustc_target::abi::Size;
 use tracing::{debug, instrument};
 
 use crate::builder::Builder;
-use crate::common::CodegenCx;
+use crate::common::{AsCCharPtr, CodegenCx};
 use crate::coverageinfo::map_data::FunctionCoverageCollector;
 use crate::llvm;
 
@@ -236,7 +236,7 @@ fn create_pgo_func_name_var<'ll, 'tcx>(
     unsafe {
         llvm::LLVMRustCoverageCreatePGOFuncNameVar(
             llfn,
-            mangled_fn_name.as_ptr().cast(),
+            mangled_fn_name.as_c_char_ptr(),
             mangled_fn_name.len(),
         )
     }
@@ -248,7 +248,7 @@ pub(crate) fn write_filenames_section_to_buffer<'a>(
 ) {
     let (pointers, lengths) = filenames
         .into_iter()
-        .map(|s: &str| (s.as_ptr().cast(), s.len()))
+        .map(|s: &str| (s.as_c_char_ptr(), s.len()))
         .unzip::<_, _, Vec<_>, Vec<_>>();
 
     unsafe {
@@ -291,7 +291,7 @@ pub(crate) fn write_mapping_to_buffer(
 }
 
 pub(crate) fn hash_bytes(bytes: &[u8]) -> u64 {
-    unsafe { llvm::LLVMRustCoverageHashByteArray(bytes.as_ptr().cast(), bytes.len()) }
+    unsafe { llvm::LLVMRustCoverageHashByteArray(bytes.as_c_char_ptr(), bytes.len()) }
 }
 
 pub(crate) fn mapping_version() -> u32 {