Introduce perma-unstable wasm-c-abi flag

compiler/rustc_interface/src/tests.rs

@@ -830,6 +830,7 @@ fn test_unstable_options_tracking_hash() {
     tracked!(use_ctors_section, Some(true));
     tracked!(verify_llvm_ir, true);
     tracked!(virtual_function_elimination, true);
+    tracked!(wasm_c_abi, true);
     tracked!(wasi_exec_model, Some(WasiExecModel::Reactor));
     // tidy-alphabetical-end
 

compiler/rustc_middle/src/ty/layout.rs

@@ -15,6 +15,7 @@ use rustc_span::symbol::{sym, Symbol};
 use rustc_span::{ErrorGuaranteed, Span, DUMMY_SP};
 use rustc_target::abi::call::FnAbi;
 use rustc_target::abi::*;
+use rustc_target::spec::HasWasmCAbiOpt;
 use rustc_target::spec::{abi::Abi as SpecAbi, HasTargetSpec, PanicStrategy, Target};
 
 use std::cmp;
@@ -552,6 +553,12 @@ impl<'tcx> HasTargetSpec for TyCtxt<'tcx> {
     }
 }
 
+impl<'tcx> HasWasmCAbiOpt for TyCtxt<'tcx> {
+    fn wasm_c_abi_opt(&self) -> bool {
+        self.sess.opts.unstable_opts.wasm_c_abi
+    }
+}
+
 impl<'tcx> HasTyCtxt<'tcx> for TyCtxt<'tcx> {
     #[inline]
     fn tcx(&self) -> TyCtxt<'tcx> {
@@ -597,6 +604,12 @@ impl<'tcx, T: HasTargetSpec> HasTargetSpec for LayoutCx<'tcx, T> {
     }
 }
 
+impl<'tcx, T: HasWasmCAbiOpt> HasWasmCAbiOpt for LayoutCx<'tcx, T> {
+    fn wasm_c_abi_opt(&self) -> bool {
+        self.tcx.wasm_c_abi_opt()
+    }
+}
+
 impl<'tcx, T: HasTyCtxt<'tcx>> HasTyCtxt<'tcx> for LayoutCx<'tcx, T> {
     fn tcx(&self) -> TyCtxt<'tcx> {
         self.tcx.tcx()

compiler/rustc_session/src/options.rs

@@ -1931,6 +1931,8 @@ written to standard error output)"),
     virtual_function_elimination: bool = (false, parse_bool, [TRACKED],
         "enables dead virtual function elimination optimization. \
         Requires `-Clto[=[fat,yes]]`"),
+    wasm_c_abi: bool = (false, parse_bool, [TRACKED],
+        "use spec-compliant C ABI for `wasm32-unknown-unknown` (default: no)"),
     wasi_exec_model: Option<WasiExecModel> = (None, parse_wasi_exec_model, [TRACKED],
         "whether to build a wasi command or reactor"),
     write_long_types_to_disk: bool = (true, parse_bool, [UNTRACKED],

compiler/rustc_target/src/abi/call/mod.rs

@@ -1,6 +1,6 @@
 use crate::abi::{self, Abi, Align, FieldsShape, Size};
 use crate::abi::{HasDataLayout, TyAbiInterface, TyAndLayout};
-use crate::spec::{self, HasTargetSpec};
+use crate::spec::{self, HasTargetSpec, HasWasmCAbiOpt};
 use rustc_span::Symbol;
 use std::fmt;
 use std::str::FromStr;
@@ -764,7 +764,7 @@ impl<'a, Ty> FnAbi<'a, Ty> {
     ) -> Result<(), AdjustForForeignAbiError>
     where
         Ty: TyAbiInterface<'a, C> + Copy,
-        C: HasDataLayout + HasTargetSpec,
+        C: HasDataLayout + HasTargetSpec + HasWasmCAbiOpt,
     {
         if abi == spec::abi::Abi::X86Interrupt {
             if let Some(arg) = self.args.first_mut() {
@@ -821,7 +821,7 @@ impl<'a, Ty> FnAbi<'a, Ty> {
             "sparc" => sparc::compute_abi_info(cx, self),
             "sparc64" => sparc64::compute_abi_info(cx, self),
             "nvptx64" => {
-                if cx.target_spec().adjust_abi(abi) == spec::abi::Abi::PtxKernel {
+                if cx.target_spec().adjust_abi(cx, abi) == spec::abi::Abi::PtxKernel {
                     nvptx64::compute_ptx_kernel_abi_info(cx, self)
                 } else {
                     nvptx64::compute_abi_info(self)
@@ -830,7 +830,7 @@ impl<'a, Ty> FnAbi<'a, Ty> {
             "hexagon" => hexagon::compute_abi_info(self),
             "riscv32" | "riscv64" => riscv::compute_abi_info(cx, self),
             "wasm32" | "wasm64" => {
-                if cx.target_spec().adjust_abi(abi) == spec::abi::Abi::Wasm {
+                if cx.target_spec().adjust_abi(cx, abi) == spec::abi::Abi::Wasm {
                     wasm::compute_wasm_abi_info(self)
                 } else {
                     wasm::compute_c_abi_info(cx, self)

compiler/rustc_target/src/spec/mod.rs

@@ -1830,6 +1830,10 @@ impl HasTargetSpec for Target {
     }
 }
 
+pub trait HasWasmCAbiOpt {
+    fn wasm_c_abi_opt(&self) -> bool;
+}
+
 type StaticCow<T> = Cow<'static, T>;
 
 /// Optional aspects of a target specification.
@@ -2441,9 +2445,18 @@ impl DerefMut for Target {
 
 impl Target {
     /// Given a function ABI, turn it into the correct ABI for this target.
-    pub fn adjust_abi(&self, abi: Abi) -> Abi {
+    pub fn adjust_abi<C>(&self, cx: &C, abi: Abi) -> Abi
+    where
+        C: HasWasmCAbiOpt,
+    {
         match abi {
-            Abi::C { .. } => self.default_adjusted_cabi.unwrap_or(abi),
+            Abi::C { .. } => {
+                if self.arch == "wasm32" && self.os == "unknown" && cx.wasm_c_abi_opt() {
+                    abi
+                } else {
+                    self.default_adjusted_cabi.unwrap_or(abi)
+                }
+            }
             Abi::System { unwind } if self.is_like_windows && self.arch == "x86" => {
                 Abi::Stdcall { unwind }
             }

compiler/rustc_ty_utils/src/abi.rs

@@ -169,7 +169,7 @@ fn fn_sig_for_fn_abi<'tcx>(
 #[inline]
 fn conv_from_spec_abi(tcx: TyCtxt<'_>, abi: SpecAbi) -> Conv {
     use rustc_target::spec::abi::Abi::*;
-    match tcx.sess.target.adjust_abi(abi) {
+    match tcx.sess.target.adjust_abi(&tcx, abi) {
         RustIntrinsic | PlatformIntrinsic | Rust | RustCall => Conv::Rust,
 
         // This is intentionally not using `Conv::Cold`, as that has to preserve

src/doc/unstable-book/src/compiler-flags/wasm-c-abi.md

@@ -0,0 +1,10 @@
+# `wasm-c-abi`
+
+This option controls whether Rust uses the spec-compliant C ABI when compiling
+for the `wasm32-unknown-unknown` target.
+
+This makes it possible to be ABI-compatible with all other spec-compliant Wasm
+like Rusts `wasm32-wasi`.
+
+This compiler flag is perma-unstable, as it will be enabled by default in the
+future with no option to fall back to the old non-spec-compliant ABI.