Rollup merge of #144232 - xacrimon:explicit-tail-call, r=WaffleLapkin

Implement support for `become` and explicit tail call codegen for the LLVM backend

This PR implements codegen of explicit tail calls via `become` in `rustc_codegen_ssa` and support within the LLVM backend. Completes a task on (https://github.com/rust-lang/rust/issues/112788). This PR implements all the necessary bits to make explicit tail calls usable, other backends have received stubs for now and will ICE if you use `become` on them. I suspect there is some bikeshedding to be done on how we should go about implementing this for other backends, but it should be relatively straightforward for GCC after this is merged.

During development I also put together a POC bytecode VM based on tail call dispatch to test these changes out and analyze the codegen to make sure it generates expected assembly. That is available [here](https://github.com/xacrimon/tcvm).

3 files changed, 21 insertions, 0 deletions

diff --git a/compiler/rustc_codegen_gcc/messages.ftl b/compiler/rustc_codegen_gcc/messages.ftl
index a70ac08f01a..b9b77b7d18c 100644
--- a/compiler/rustc_codegen_gcc/messages.ftl
+++ b/compiler/rustc_codegen_gcc/messages.ftl
@@ -4,3 +4,5 @@ codegen_gcc_unwinding_inline_asm =
 codegen_gcc_copy_bitcode = failed to copy bitcode to object file: {$err}
 
 codegen_gcc_lto_bitcode_from_rlib = failed to get bitcode from object file for LTO ({$gcc_err})
+
+codegen_gcc_explicit_tail_calls_unsupported = explicit tail calls with the 'become' keyword are not implemented in the GCC backend
diff --git a/compiler/rustc_codegen_gcc/src/builder.rs b/compiler/rustc_codegen_gcc/src/builder.rs
index a4ec4bf8dea..4aee211e2ef 100644
--- a/compiler/rustc_codegen_gcc/src/builder.rs
+++ b/compiler/rustc_codegen_gcc/src/builder.rs
@@ -34,6 +34,7 @@ use rustc_target::spec::{HasTargetSpec, HasX86AbiOpt, Target, X86Abi};
 
 use crate::common::{SignType, TypeReflection, type_is_pointer};
 use crate::context::CodegenCx;
+use crate::errors;
 use crate::intrinsic::llvm;
 use crate::type_of::LayoutGccExt;
 
@@ -1742,6 +1743,20 @@ impl<'a, 'gcc, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'gcc, 'tcx> {
         call
     }
 
+    fn tail_call(
+        &mut self,
+        _llty: Self::Type,
+        _fn_attrs: Option<&CodegenFnAttrs>,
+        _fn_abi: &FnAbi<'tcx, Ty<'tcx>>,
+        _llfn: Self::Value,
+        _args: &[Self::Value],
+        _funclet: Option<&Self::Funclet>,
+        _instance: Option<Instance<'tcx>>,
+    ) {
+        // FIXME: implement support for explicit tail calls like rustc_codegen_llvm.
+        self.tcx.dcx().emit_fatal(errors::ExplicitTailCallsUnsupported);
+    }
+
     fn zext(&mut self, value: RValue<'gcc>, dest_typ: Type<'gcc>) -> RValue<'gcc> {
         // FIXME(antoyo): this does not zero-extend.
         self.gcc_int_cast(value, dest_typ)
diff --git a/compiler/rustc_codegen_gcc/src/errors.rs b/compiler/rustc_codegen_gcc/src/errors.rs
index 0aa16bd88b4..b252c39c0c0 100644
--- a/compiler/rustc_codegen_gcc/src/errors.rs
+++ b/compiler/rustc_codegen_gcc/src/errors.rs
@@ -19,3 +19,7 @@ pub(crate) struct CopyBitcode {
 pub(crate) struct LtoBitcodeFromRlib {
     pub gcc_err: String,
 }
+
+#[derive(Diagnostic)]
+#[diag(codegen_gcc_explicit_tail_calls_unsupported)]
+pub(crate) struct ExplicitTailCallsUnsupported;