Auto merge of #138601 - RalfJung:wasm-abi-fcw, r=alexcrichton

add FCW to warn about wasm ABI transition

See https://github.com/rust-lang/rust/issues/122532 for context: the "C" ABI on wasm32-unk-unk will change. The goal of this lint is to warn about any function definition and calls whose behavior will be affected by the change. My understanding is the following:
- scalar arguments are fine
  - including 128 bit types, they get passed as two `i64` arguments in both ABIs
- `repr(C)` structs (recursively) wrapping a single scalar argument are fine (unless they have extra padding due to over-alignment attributes)
- all return values are fine

`@bjorn3` `@alexcrichton` `@Manishearth` is that correct?

I am making this a "show up in future compat reports" lint to maximize the chances people become aware of this. OTOH this likely means warnings for most users of Diplomat so maybe we shouldn't do this?

IIUC, wasm-bindgen should be unaffected by this lint as they only pass scalar types as arguments.

Tracking issue: https://github.com/rust-lang/rust/issues/138762
Transition plan blog post: https://github.com/rust-lang/blog.rust-lang.org/pull/1531

try-job: dist-various-2

3 files changed, 8 insertions, 2 deletions

diff --git a/compiler/rustc_target/src/callconv/mod.rs b/compiler/rustc_target/src/callconv/mod.rs
index 6d0ee3c7ee5..a52b2b76bc1 100644
--- a/compiler/rustc_target/src/callconv/mod.rs
+++ b/compiler/rustc_target/src/callconv/mod.rs
@@ -705,7 +705,7 @@ impl<'a, Ty> FnAbi<'a, Ty> {
             "xtensa" => xtensa::compute_abi_info(cx, self),
             "riscv32" | "riscv64" => riscv::compute_abi_info(cx, self),
             "wasm32" => {
-                if spec.os == "unknown" && cx.wasm_c_abi_opt() == WasmCAbi::Legacy {
+                if spec.os == "unknown" && matches!(cx.wasm_c_abi_opt(), WasmCAbi::Legacy { .. }) {
                     wasm::compute_wasm_abi_info(self)
                 } else {
                     wasm::compute_c_abi_info(cx, self)
diff --git a/compiler/rustc_target/src/callconv/wasm.rs b/compiler/rustc_target/src/callconv/wasm.rs
index 364a6551131..881168c98c3 100644
--- a/compiler/rustc_target/src/callconv/wasm.rs
+++ b/compiler/rustc_target/src/callconv/wasm.rs
@@ -10,6 +10,9 @@ where
     if val.layout.is_aggregate() {
         if let Some(unit) = val.layout.homogeneous_aggregate(cx).ok().and_then(|ha| ha.unit()) {
             let size = val.layout.size;
+            // This size check also catches over-aligned scalars as `size` will be rounded up to a
+            // multiple of the alignment, and the default alignment of all scalar types on wasm
+            // equals their size.
             if unit.size == size {
                 val.cast_to(unit);
                 return true;
diff --git a/compiler/rustc_target/src/spec/mod.rs b/compiler/rustc_target/src/spec/mod.rs
index 263646d8347..7234d1dc63e 100644
--- a/compiler/rustc_target/src/spec/mod.rs
+++ b/compiler/rustc_target/src/spec/mod.rs
@@ -2234,7 +2234,10 @@ pub enum WasmCAbi {
     /// Spec-compliant C ABI.
     Spec,
     /// Legacy ABI. Which is non-spec-compliant.
-    Legacy,
+    Legacy {
+        /// Indicates whether the `wasm_c_abi` lint should be emitted.
+        with_lint: bool,
+    },
 }
 
 pub trait HasWasmCAbiOpt {