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

2 files changed, 33 insertions, 6 deletions

diff --git a/compiler/rustc_target/src/abi/call/mod.rs b/compiler/rustc_target/src/abi/call/mod.rs
index f4967b94e93..930edfe1f0d 100644
--- a/compiler/rustc_target/src/abi/call/mod.rs
+++ b/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;
@@ -782,7 +782,7 @@ 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() {
@@ -839,7 +839,9 @@ 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, self.c_variadic) == spec::abi::Abi::PtxKernel {
+                if cx.target_spec().adjust_abi(cx, abi, self.c_variadic)
+                    == spec::abi::Abi::PtxKernel
+                {
                     nvptx64::compute_ptx_kernel_abi_info(cx, self)
                 } else {
                     nvptx64::compute_abi_info(self)
@@ -848,7 +850,7 @@ 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, self.c_variadic) == spec::abi::Abi::Wasm {
+                if cx.target_spec().adjust_abi(cx, abi, self.c_variadic) == spec::abi::Abi::Wasm {
                     wasm::compute_wasm_abi_info(self)
                 } else {
                     wasm::compute_c_abi_info(cx, self)
diff --git a/compiler/rustc_target/src/spec/mod.rs b/compiler/rustc_target/src/spec/mod.rs
index 3b6e15fad46..c42e818167c 100644
--- a/compiler/rustc_target/src/spec/mod.rs
+++ b/compiler/rustc_target/src/spec/mod.rs
@@ -1807,6 +1807,19 @@ impl HasTargetSpec for Target {
     }
 }
 
+/// Which C ABI to use for `wasm32-unknown-unknown`.
+#[derive(Debug, Copy, Clone, Hash, PartialEq, Eq)]
+pub enum WasmCAbi {
+    /// Spec-compliant C ABI.
+    Spec,
+    /// Legacy ABI. Which is non-spec-compliant.
+    Legacy,
+}
+
+pub trait HasWasmCAbiOpt {
+    fn wasm_c_abi_opt(&self) -> WasmCAbi;
+}
+
 type StaticCow<T> = Cow<'static, T>;
 
 /// Optional aspects of a target specification.
@@ -2417,9 +2430,21 @@ 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, c_variadic: bool) -> Abi {
+    pub fn adjust_abi<C>(&self, cx: &C, abi: Abi, c_variadic: bool) -> 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() == WasmCAbi::Spec
+                {
+                    abi
+                } else {
+                    self.default_adjusted_cabi.unwrap_or(abi)
+                }
+            }
 
             // On Windows, `extern "system"` behaves like msvc's `__stdcall`.
             // `__stdcall` only applies on x86 and on non-variadic functions: