Auto merge of #117919 - daxpedda:wasm-c-abi, r=wesleywiser

Introduce perma-unstable `wasm-c-abi` flag

Now that `wasm-bindgen` v0.2.88 supports the spec-compliant C ABI, the idea is to switch to that in a future version of Rust. In the meantime it would be good to let people test and play around with it.

This PR introduces a new perma-unstable `-Zwasm-c-abi` compiler flag, which switches to the new spec-compliant C ABI when targeting `wasm32-unknown-unknown`.

Alternatively, we could also stabilize this and then deprecate it when we switch. I will leave this to the Rust maintainers to decide.

This is a companion PR to #117918, but they could be merged independently.
MCP: rust-lang/compiler-team#703
Tracking issue: #122532

Author: bors

src/builder.rs

@@ -31,7 +31,7 @@ use rustc_span::Span;
 use rustc_target::abi::{
     self, call::FnAbi, Align, HasDataLayout, Size, TargetDataLayout, WrappingRange,
 };
-use rustc_target::spec::{HasTargetSpec, Target};
+use rustc_target::spec::{HasTargetSpec, HasWasmCAbiOpt, Target, WasmCAbi};
 
 use crate::common::{type_is_pointer, SignType, TypeReflection};
 use crate::context::CodegenCx;
@@ -2352,6 +2352,12 @@ impl<'tcx> HasTargetSpec for Builder<'_, '_, 'tcx> {
     }
 }
 
+impl<'tcx> HasWasmCAbiOpt for Builder<'_, '_, 'tcx> {
+    fn wasm_c_abi_opt(&self) -> WasmCAbi {
+        self.cx.wasm_c_abi_opt()
+    }
+}
+
 pub trait ToGccComp {
     fn to_gcc_comparison(&self) -> ComparisonOp;
 }

src/context.rs

@@ -20,7 +20,7 @@ use rustc_span::{source_map::respan, Span};
 use rustc_target::abi::{
     call::FnAbi, HasDataLayout, PointeeInfo, Size, TargetDataLayout, VariantIdx,
 };
-use rustc_target::spec::{HasTargetSpec, Target, TlsModel};
+use rustc_target::spec::{HasTargetSpec, HasWasmCAbiOpt, Target, TlsModel, WasmCAbi};
 
 use crate::callee::get_fn;
 use crate::common::SignType;
@@ -557,6 +557,12 @@ impl<'gcc, 'tcx> HasTargetSpec for CodegenCx<'gcc, 'tcx> {
     }
 }
 
+impl<'gcc, 'tcx> HasWasmCAbiOpt for CodegenCx<'gcc, 'tcx> {
+    fn wasm_c_abi_opt(&self) -> WasmCAbi {
+        self.tcx.sess.opts.unstable_opts.wasm_c_abi
+    }
+}
+
 impl<'gcc, 'tcx> LayoutOfHelpers<'tcx> for CodegenCx<'gcc, 'tcx> {
     type LayoutOfResult = TyAndLayout<'tcx>;