add FCW to warn about wasm ABI transition

compiler/rustc_codegen_ssa/src/mir/naked_asm.rs

@@ -332,7 +332,7 @@ fn wasm_functype<'tcx>(tcx: TyCtxt<'tcx>, fn_abi: &FnAbi<'tcx, Ty<'tcx>>, def_id
     // please also add `wasm32-unknown-unknown` back in `tests/assembly/wasm32-naked-fn.rs`
     // basically the commit introducing this comment should be reverted
     if let PassMode::Pair { .. } = fn_abi.ret.mode {
-        let _ = WasmCAbi::Legacy;
+        let _ = WasmCAbi::Legacy { with_lint: true };
         span_bug!(
             tcx.def_span(def_id),
             "cannot return a pair (the wasm32-unknown-unknown ABI is broken, see https://github.com/rust-lang/rust/issues/115666"
@@ -384,7 +384,7 @@ fn wasm_type<'tcx>(
                 BackendRepr::SimdVector { .. } => "v128",
                 BackendRepr::Memory { .. } => {
                     // FIXME: remove this branch once the wasm32-unknown-unknown ABI is fixed
-                    let _ = WasmCAbi::Legacy;
+                    let _ = WasmCAbi::Legacy { with_lint: true };
                     span_bug!(
                         tcx.def_span(def_id),
                         "cannot use memory args (the wasm32-unknown-unknown ABI is broken, see https://github.com/rust-lang/rust/issues/115666"

compiler/rustc_lint_defs/src/builtin.rs

@@ -143,6 +143,7 @@ declare_lint_pass! {
         UNUSED_VARIABLES,
         USELESS_DEPRECATED,
         WARNINGS,
+        WASM_C_ABI,
         // tidy-alphabetical-end
     ]
 }
@@ -5082,6 +5083,8 @@ declare_lint! {
     /// }
     /// ```
     ///
+    /// This will produce:
+    ///
     /// ```text
     /// warning: ABI error: this function call uses a avx vector type, which is not enabled in the caller
     ///  --> lint_example.rs:18:12
@@ -5125,3 +5128,46 @@ declare_lint! {
         reference: "issue #116558 <https://github.com/rust-lang/rust/issues/116558>",
     };
 }
+
+declare_lint! {
+    /// The `wasm_c_abi` lint detects usage of the `extern "C"` ABI of wasm that is affected
+    /// by a planned ABI change that has the goal of aligning Rust with the standard C ABI
+    /// of this target.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,ignore (needs wasm32-unknown-unknown)
+    /// #[repr(C)]
+    /// struct MyType(i32, i32);
+    ///
+    /// extern "C" my_fun(x: MyType) {}
+    /// ```
+    ///
+    /// This will produce:
+    ///
+    /// ```text
+    /// error: this function function definition is affected by the wasm ABI transition: it passes an argument of non-scalar type `MyType`
+    /// --> $DIR/wasm_c_abi_transition.rs:17:1
+    ///  |
+    ///  | pub extern "C" fn my_fun(_x: MyType) {}
+    ///  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+    ///  |
+    ///  = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+    ///  = note: for more information, see issue #138762 <https://github.com/rust-lang/rust/issues/138762>
+    ///  = help: the "C" ABI Rust uses on wasm32-unknown-unknown will change to align with the standard "C" ABI for this target
+    /// ```
+    ///
+    /// ### Explanation
+    ///
+    /// Rust has historically implemented a non-spec-compliant C ABI on wasm32-unknown-unknown. This
+    /// has caused incompatibilities with other compilers and Wasm targets. In a future version
+    /// of Rust, this will be fixed, and therefore code relying on the non-spec-compliant C ABI will
+    /// stop functioning.
+    pub WASM_C_ABI,
+    Warn,
+    "detects code relying on rustc's non-spec-compliant wasm C ABI",
+    @future_incompatible = FutureIncompatibleInfo {
+        reason: FutureIncompatibilityReason::FutureReleaseErrorReportInDeps,
+        reference: "issue #138762 <https://github.com/rust-lang/rust/issues/138762>",
+    };
+}

compiler/rustc_monomorphize/messages.ftl

@@ -63,4 +63,11 @@ monomorphize_symbol_already_defined = symbol `{$symbol}` is already defined
 monomorphize_unknown_cgu_collection_mode =
     unknown codegen-item collection mode '{$mode}', falling back to 'lazy' mode
 
+monomorphize_wasm_c_abi_transition =
+    this function {$is_call ->
+      [true] call
+      *[false] definition
+    } involves an argument of type `{$ty}` which is affected by the wasm ABI transition
+    .help = the "C" ABI Rust uses on wasm32-unknown-unknown will change to align with the standard "C" ABI for this target
+
 monomorphize_written_to_path = the full type name has been written to '{$path}'

compiler/rustc_monomorphize/src/errors.rs

@@ -103,3 +103,12 @@ pub(crate) struct AbiRequiredTargetFeature<'a> {
     /// Whether this is a problem at a call site or at a declaration.
     pub is_call: bool,
 }
+
+#[derive(LintDiagnostic)]
+#[diag(monomorphize_wasm_c_abi_transition)]
+#[help]
+pub(crate) struct WasmCAbiTransition<'a> {
+    pub ty: Ty<'a>,
+    /// Whether this is a problem at a call site or at a declaration.
+    pub is_call: bool,
+}

compiler/rustc_monomorphize/src/mono_checks/abi_check.rs

@@ -3,11 +3,13 @@
 use rustc_abi::{BackendRepr, RegKind};
 use rustc_hir::CRATE_HIR_ID;
 use rustc_middle::mir::{self, traversal};
-use rustc_middle::ty::{self, Instance, InstanceKind, Ty, TyCtxt};
-use rustc_session::lint::builtin::ABI_UNSUPPORTED_VECTOR_TYPES;
+use rustc_middle::ty::layout::LayoutCx;
+use rustc_middle::ty::{self, Instance, InstanceKind, Ty, TyCtxt, TypingEnv};
+use rustc_session::lint::builtin::{ABI_UNSUPPORTED_VECTOR_TYPES, WASM_C_ABI};
 use rustc_span::def_id::DefId;
 use rustc_span::{DUMMY_SP, Span, Symbol, sym};
-use rustc_target::callconv::{Conv, FnAbi, PassMode};
+use rustc_target::callconv::{ArgAbi, Conv, FnAbi, PassMode};
+use rustc_target::spec::{HasWasmCAbiOpt, WasmCAbi};
 
 use crate::errors;
 
@@ -26,13 +28,15 @@ fn uses_vector_registers(mode: &PassMode, repr: &BackendRepr) -> bool {
 /// for a certain function.
 /// `is_call` indicates whether this is a call-site check or a definition-site check;
 /// this is only relevant for the wording in the emitted error.
-fn do_check_abi<'tcx>(
+fn do_check_simd_vector_abi<'tcx>(
     tcx: TyCtxt<'tcx>,
     abi: &FnAbi<'tcx, Ty<'tcx>>,
     def_id: DefId,
     is_call: bool,
     span: impl Fn() -> Span,
 ) {
+    // We check this on all functions, including those using the "Rust" ABI.
+    // For the "Rust" ABI it would be a bug if the lint ever triggered, but better safe than sorry.
     let feature_def = tcx.sess.target.features_for_correct_vector_abi();
     let codegen_attrs = tcx.codegen_fn_attrs(def_id);
     let have_feature = |feat: Symbol| {
@@ -88,6 +92,61 @@ fn do_check_abi<'tcx>(
     }
 }
 
+/// Determines whether the given argument is passed the same way on the old and new wasm ABIs.
+fn wasm_abi_safe<'tcx>(tcx: TyCtxt<'tcx>, arg: &ArgAbi<'tcx, Ty<'tcx>>) -> bool {
+    if matches!(arg.layout.backend_repr, BackendRepr::Scalar(_)) {
+        return true;
+    }
+
+    // This matches `unwrap_trivial_aggregate` in the wasm ABI logic.
+    if arg.layout.is_aggregate() {
+        let cx = LayoutCx::new(tcx, TypingEnv::fully_monomorphized());
+        if let Some(unit) = arg.layout.homogeneous_aggregate(&cx).ok().and_then(|ha| ha.unit()) {
+            let size = arg.layout.size;
+            // Ensure there's just a single `unit` element in `arg`.
+            if unit.size == size {
+                return true;
+            }
+        }
+    }
+
+    false
+}
+
+/// Warns against usage of `extern "C"` on wasm32-unknown-unknown that is affected by the
+/// ABI transition.
+fn do_check_wasm_abi<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    abi: &FnAbi<'tcx, Ty<'tcx>>,
+    is_call: bool,
+    span: impl Fn() -> Span,
+) {
+    // Only proceed for `extern "C" fn` on wasm32-unknown-unknown (same check as what `adjust_for_foreign_abi` uses to call `compute_wasm_abi_info`),
+    // and only proceed if `wasm_c_abi_opt` indicates we should emit the lint.
+    if !(tcx.sess.target.arch == "wasm32"
+        && tcx.sess.target.os == "unknown"
+        && tcx.wasm_c_abi_opt() == WasmCAbi::Legacy { with_lint: true }
+        && abi.conv == Conv::C)
+    {
+        return;
+    }
+    // Warn against all types whose ABI will change. Return values are not affected by this change.
+    for arg_abi in abi.args.iter() {
+        if wasm_abi_safe(tcx, arg_abi) {
+            continue;
+        }
+        let span = span();
+        tcx.emit_node_span_lint(
+            WASM_C_ABI,
+            CRATE_HIR_ID,
+            span,
+            errors::WasmCAbiTransition { ty: arg_abi.layout.ty, is_call },
+        );
+        // Let's only warn once per function.
+        break;
+    }
+}
+
 /// Checks that the ABI of a given instance of a function does not contain vector-passed arguments
 /// or return values for which the corresponding target feature is not enabled.
 fn check_instance_abi<'tcx>(tcx: TyCtxt<'tcx>, instance: Instance<'tcx>) {
@@ -98,13 +157,10 @@ fn check_instance_abi<'tcx>(tcx: TyCtxt<'tcx>, instance: Instance<'tcx>) {
         // function.
         return;
     };
-    do_check_abi(
-        tcx,
-        abi,
-        instance.def_id(),
-        /*is_call*/ false,
-        || tcx.def_span(instance.def_id()),
-    )
+    do_check_simd_vector_abi(tcx, abi, instance.def_id(), /*is_call*/ false, || {
+        tcx.def_span(instance.def_id())
+    });
+    do_check_wasm_abi(tcx, abi, /*is_call*/ false, || tcx.def_span(instance.def_id()));
 }
 
 /// Checks that a call expression does not try to pass a vector-passed argument which requires a
@@ -141,7 +197,8 @@ fn check_call_site_abi<'tcx>(
         // ABI failed to compute; this will not get through codegen.
         return;
     };
-    do_check_abi(tcx, callee_abi, caller.def_id(), /*is_call*/ true, || span);
+    do_check_simd_vector_abi(tcx, callee_abi, caller.def_id(), /*is_call*/ true, || span);
+    do_check_wasm_abi(tcx, callee_abi, /*is_call*/ true, || span);
 }
 
 fn check_callees_abi<'tcx>(tcx: TyCtxt<'tcx>, instance: Instance<'tcx>, body: &mir::Body<'tcx>) {

compiler/rustc_session/src/options.rs

@@ -1886,7 +1886,8 @@ pub mod parse {
     pub(crate) fn parse_wasm_c_abi(slot: &mut WasmCAbi, v: Option<&str>) -> bool {
         match v {
             Some("spec") => *slot = WasmCAbi::Spec,
-            Some("legacy") => *slot = WasmCAbi::Legacy,
+            // Explicitly setting the `-Z` flag suppresses the lint.
+            Some("legacy") => *slot = WasmCAbi::Legacy { with_lint: false },
             _ => return false,
         }
         true
@@ -2599,7 +2600,7 @@ written to standard error output)"),
         Requires `-Clto[=[fat,yes]]`"),
     wasi_exec_model: Option<WasiExecModel> = (None, parse_wasi_exec_model, [TRACKED],
         "whether to build a wasi command or reactor"),
-    wasm_c_abi: WasmCAbi = (WasmCAbi::Legacy, parse_wasm_c_abi, [TRACKED],
+    wasm_c_abi: WasmCAbi = (WasmCAbi::Legacy { with_lint: true }, parse_wasm_c_abi, [TRACKED],
         "use spec-compliant C ABI for `wasm32-unknown-unknown` (default: legacy)"),
     write_long_types_to_disk: bool = (true, parse_bool, [UNTRACKED],
         "whether long type names should be written to files instead of being printed in errors"),

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)

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;

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 {

library/proc_macro/src/bridge/mod.rs

@@ -7,6 +7,10 @@
 //! Rust ABIs (e.g., stage0/bin/rustc vs stage1/bin/rustc during bootstrap).
 
 #![deny(unsafe_code)]
+// proc_macros anyway don't work on wasm hosts so while both sides of this bridge can
+// be built with different versions of rustc, the wasm ABI changes don't really matter.
+#![cfg_attr(bootstrap, allow(unknown_lints))]
+#![allow(wasm_c_abi)]
 
 use std::hash::Hash;
 use std::ops::{Bound, Range};

tests/ui/abi/compatibility.rs

@@ -62,7 +62,6 @@
 //@[nvptx64] needs-llvm-components: nvptx
 #![feature(no_core, rustc_attrs, lang_items)]
 #![feature(unsized_fn_params, transparent_unions)]
-#![no_std]
 #![no_core]
 #![allow(unused, improper_ctypes_definitions, internal_features)]
 

tests/ui/lint/wasm_c_abi_transition.rs

@@ -0,0 +1,41 @@
+//@ compile-flags: --target wasm32-unknown-unknown
+//@ needs-llvm-components: webassembly
+//@ add-core-stubs
+//@ build-fail
+
+#![feature(no_core)]
+#![no_core]
+#![crate_type = "lib"]
+#![deny(wasm_c_abi)]
+
+extern crate minicore;
+use minicore::*;
+
+pub extern "C" fn my_fun_trivial(_x: i32, _y: f32) {}
+
+#[repr(C)]
+pub struct MyType(i32, i32);
+pub extern "C" fn my_fun(_x: MyType) {} //~ERROR: wasm ABI transition
+//~^WARN: previously accepted
+
+// This one is ABI-safe as it only wraps a single field,
+// and the return type can be anything.
+#[repr(C)]
+pub struct MySafeType(i32);
+pub extern "C" fn my_fun_safe(_x: MySafeType) -> MyType { loop {} }
+
+// This one not ABI-safe due to the alignment.
+#[repr(C, align(16))]
+pub struct MyAlignedType(i32);
+pub extern "C" fn my_fun_aligned(_x: MyAlignedType) {} //~ERROR: wasm ABI transition
+//~^WARN: previously accepted
+
+// Check call-site warning
+extern "C" {
+    fn other_fun(x: MyType);
+}
+
+pub fn call_other_fun(x: MyType) {
+    unsafe { other_fun(x) } //~ERROR: wasm ABI transition
+    //~^WARN: previously accepted
+}