Enable varargs support for calling conventions other than C or cdecl

compiler/rustc_feature/src/active.rs

@@ -390,6 +390,9 @@ declare_features! (
     (active, exclusive_range_pattern, "1.11.0", Some(37854), None),
     /// Allows exhaustive pattern matching on types that contain uninhabited types.
     (active, exhaustive_patterns, "1.13.0", Some(51085), None),
+    /// Allows using `efiapi`, `sysv64` and `win64` as calling convention
+    /// for functions with varargs.
+    (active, extended_varargs_abi_support, "1.65.0", Some(100189), None),
     /// Allows defining `extern type`s.
     (active, extern_types, "1.23.0", Some(43467), None),
     /// Allows the use of `#[ffi_const]` on foreign functions.

compiler/rustc_hir_analysis/src/lib.rs

@@ -106,7 +106,7 @@ use rustc_middle::middle;
 use rustc_middle::ty::query::Providers;
 use rustc_middle::ty::{self, Ty, TyCtxt};
 use rustc_middle::util;
-use rustc_session::config::EntryFnType;
+use rustc_session::{config::EntryFnType, parse::feature_err};
 use rustc_span::{symbol::sym, Span, DUMMY_SP};
 use rustc_target::spec::abi::Abi;
 use rustc_trait_selection::traits::error_reporting::TypeErrCtxtExt as _;
@@ -118,20 +118,40 @@ use astconv::AstConv;
 use bounds::Bounds;
 
 fn require_c_abi_if_c_variadic(tcx: TyCtxt<'_>, decl: &hir::FnDecl<'_>, abi: Abi, span: Span) {
-    match (decl.c_variadic, abi) {
-        // The function has the correct calling convention, or isn't a "C-variadic" function.
-        (false, _) | (true, Abi::C { .. }) | (true, Abi::Cdecl { .. }) => {}
-        // The function is a "C-variadic" function with an incorrect calling convention.
-        (true, _) => {
-            let mut err = struct_span_err!(
-                tcx.sess,
+    const ERROR_HEAD: &str = "C-variadic function must have a compatible calling convention";
+    const CONVENTIONS_UNSTABLE: &str = "`C`, `cdecl`, `win64`, `sysv64` or `efiapi`";
+    const CONVENTIONS_STABLE: &str = "`C` or `cdecl`";
+    const UNSTABLE_EXPLAIN: &str =
+        "using calling conventions other than `C` or `cdecl` for varargs functions is unstable";
+
+    if !decl.c_variadic || matches!(abi, Abi::C { .. } | Abi::Cdecl { .. }) {
+        return;
+    }
+
+    let extended_abi_support = tcx.features().extended_varargs_abi_support;
+    let conventions = match (extended_abi_support, abi.supports_varargs()) {
+        // User enabled additional ABI support for varargs and function ABI matches those ones.
+        (true, true) => return,
+
+        // Using this ABI would be ok, if the feature for additional ABI support was enabled.
+        // Return CONVENTIONS_STABLE, because we want the other error to look the same.
+        (false, true) => {
+            feature_err(
+                &tcx.sess.parse_sess,
+                sym::extended_varargs_abi_support,
                 span,
-                E0045,
-                "C-variadic function must have C or cdecl calling convention"
-            );
-            err.span_label(span, "C-variadics require C or cdecl calling convention").emit();
+                UNSTABLE_EXPLAIN,
+            )
+            .emit();
+            CONVENTIONS_STABLE
         }
-    }
+
+        (false, false) => CONVENTIONS_STABLE,
+        (true, false) => CONVENTIONS_UNSTABLE,
+    };
+
+    let mut err = struct_span_err!(tcx.sess, span, E0045, "{}, like {}", ERROR_HEAD, conventions);
+    err.span_label(span, ERROR_HEAD).emit();
 }
 
 fn require_same_types<'tcx>(

compiler/rustc_span/src/symbol.rs

@@ -694,6 +694,7 @@ symbols! {
         export_name,
         expr,
         extended_key_value_attributes,
+        extended_varargs_abi_support,
         extern_absolute_paths,
         extern_crate_item_prelude,
         extern_crate_self,

compiler/rustc_target/src/spec/abi.rs

@@ -40,6 +40,28 @@ pub enum Abi {
     RustCold,
 }
 
+impl Abi {
+    pub fn supports_varargs(self) -> bool {
+        // * C and Cdecl obviously support varargs.
+        // * C can be based on SysV64 or Win64, so they must support varargs.
+        // * EfiApi is based on Win64 or C, so it also supports it.
+        //
+        // * Stdcall does not, because it would be impossible for the callee to clean
+        //   up the arguments. (callee doesn't know how many arguments are there)
+        // * Same for Fastcall, Vectorcall and Thiscall.
+        // * System can become Stdcall, so is also a no-no.
+        // * Other calling conventions are related to hardware or the compiler itself.
+        match self {
+            Self::C { .. }
+            | Self::Cdecl { .. }
+            | Self::Win64 { .. }
+            | Self::SysV64 { .. }
+            | Self::EfiApi => true,
+            _ => false,
+        }
+    }
+}
+
 #[derive(Copy, Clone)]
 pub struct AbiData {
     abi: Abi,

src/doc/unstable-book/src/language-features/extended-varargs-abi-support.md

@@ -0,0 +1,10 @@
+# `extended_varargs_abi_support`
+
+The tracking issue for this feature is: [#100189]
+
+[#100189]: /~https://github.com/rust-lang/rust/issues/100189
+
+------------------------
+
+This feature adds the possibility of using `sysv64`, `win64` or `efiapi` calling
+conventions on functions with varargs.

src/test/ui/c-variadic/feature-gate-extended_varargs_abi_support.rs

@@ -0,0 +1,19 @@
+#![feature(abi_efiapi)]
+
+fn efiapi(f: extern "efiapi" fn(usize, ...)) {
+    //~^ ERROR: C-variadic function must have a compatible calling convention, like `C` or `cdecl`
+    //~^^ ERROR: using calling conventions other than `C` or `cdecl` for varargs functions is unstable
+    f(22, 44);
+}
+fn sysv(f: extern "sysv64" fn(usize, ...)) {
+    //~^ ERROR: C-variadic function must have a compatible calling convention, like `C` or `cdecl`
+    //~^^ ERROR: using calling conventions other than `C` or `cdecl` for varargs functions is unstable
+    f(22, 44);
+}
+fn win(f: extern "win64" fn(usize, ...)) {
+    //~^ ERROR: C-variadic function must have a compatible calling convention, like `C` or `cdecl`
+    //~^^ ERROR: using calling conventions other than `C` or `cdecl` for varargs functions is unstable
+    f(22, 44);
+}
+
+fn main() {}

src/test/ui/c-variadic/feature-gate-extended_varargs_abi_support.stderr

@@ -0,0 +1,49 @@
+error[E0658]: using calling conventions other than `C` or `cdecl` for varargs functions is unstable
+  --> $DIR/feature-gate-extended_varargs_abi_support.rs:3:14
+   |
+LL | fn efiapi(f: extern "efiapi" fn(usize, ...)) {
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #100189 </~https://github.com/rust-lang/rust/issues/100189> for more information
+   = help: add `#![feature(extended_varargs_abi_support)]` to the crate attributes to enable
+
+error[E0045]: C-variadic function must have a compatible calling convention, like `C` or `cdecl`
+  --> $DIR/feature-gate-extended_varargs_abi_support.rs:3:14
+   |
+LL | fn efiapi(f: extern "efiapi" fn(usize, ...)) {
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ C-variadic function must have a compatible calling convention
+
+error[E0658]: using calling conventions other than `C` or `cdecl` for varargs functions is unstable
+  --> $DIR/feature-gate-extended_varargs_abi_support.rs:8:12
+   |
+LL | fn sysv(f: extern "sysv64" fn(usize, ...)) {
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #100189 </~https://github.com/rust-lang/rust/issues/100189> for more information
+   = help: add `#![feature(extended_varargs_abi_support)]` to the crate attributes to enable
+
+error[E0045]: C-variadic function must have a compatible calling convention, like `C` or `cdecl`
+  --> $DIR/feature-gate-extended_varargs_abi_support.rs:8:12
+   |
+LL | fn sysv(f: extern "sysv64" fn(usize, ...)) {
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ C-variadic function must have a compatible calling convention
+
+error[E0658]: using calling conventions other than `C` or `cdecl` for varargs functions is unstable
+  --> $DIR/feature-gate-extended_varargs_abi_support.rs:13:11
+   |
+LL | fn win(f: extern "win64" fn(usize, ...)) {
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #100189 </~https://github.com/rust-lang/rust/issues/100189> for more information
+   = help: add `#![feature(extended_varargs_abi_support)]` to the crate attributes to enable
+
+error[E0045]: C-variadic function must have a compatible calling convention, like `C` or `cdecl`
+  --> $DIR/feature-gate-extended_varargs_abi_support.rs:13:11
+   |
+LL | fn win(f: extern "win64" fn(usize, ...)) {
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ C-variadic function must have a compatible calling convention
+
+error: aborting due to 6 previous errors
+
+Some errors have detailed explanations: E0045, E0658.
+For more information about an error, try `rustc --explain E0045`.

src/test/ui/c-variadic/variadic-ffi-1.rs

@@ -6,7 +6,9 @@
 trait Sized { }
 
 extern "stdcall" {
-    fn printf(_: *const u8, ...); //~ ERROR: variadic function must have C or cdecl calling
+    fn printf(_: *const u8, ...);
+    //~^ ERROR: C-variadic function must have a compatible calling convention,
+    // like C, cdecl, win64, sysv64 or efiapi
 }
 
 extern "C" {

src/test/ui/c-variadic/variadic-ffi-1.stderr

@@ -1,17 +1,17 @@
-error[E0045]: C-variadic function must have C or cdecl calling convention
+error[E0045]: C-variadic function must have a compatible calling convention, like `C` or `cdecl`
   --> $DIR/variadic-ffi-1.rs:9:5
    |
 LL |     fn printf(_: *const u8, ...);
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ C-variadics require C or cdecl calling convention
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ C-variadic function must have a compatible calling convention
 
 error[E0060]: this function takes at least 2 arguments but 0 arguments were supplied
-  --> $DIR/variadic-ffi-1.rs:20:9
+  --> $DIR/variadic-ffi-1.rs:22:9
    |
 LL |         foo();
    |         ^^^-- two arguments of type `isize` and `u8` are missing
    |
 note: function defined here
-  --> $DIR/variadic-ffi-1.rs:13:8
+  --> $DIR/variadic-ffi-1.rs:15:8
    |
 LL |     fn foo(f: isize, x: u8, ...);
    |        ^^^
@@ -21,13 +21,13 @@ LL |         foo(/* isize */, /* u8 */);
    |            ~~~~~~~~~~~~~~~~~~~~~~~
 
 error[E0060]: this function takes at least 2 arguments but 1 argument was supplied
-  --> $DIR/variadic-ffi-1.rs:21:9
+  --> $DIR/variadic-ffi-1.rs:23:9
    |
 LL |         foo(1);
    |         ^^^--- an argument of type `u8` is missing
    |
 note: function defined here
-  --> $DIR/variadic-ffi-1.rs:13:8
+  --> $DIR/variadic-ffi-1.rs:15:8
    |
 LL |     fn foo(f: isize, x: u8, ...);
    |        ^^^
@@ -37,7 +37,7 @@ LL |         foo(1, /* u8 */);
    |            ~~~~~~~~~~~~~
 
 error[E0308]: mismatched types
-  --> $DIR/variadic-ffi-1.rs:23:56
+  --> $DIR/variadic-ffi-1.rs:25:56
    |
 LL |         let x: unsafe extern "C" fn(f: isize, x: u8) = foo;
    |                -------------------------------------   ^^^ expected non-variadic fn, found variadic function
@@ -48,7 +48,7 @@ LL |         let x: unsafe extern "C" fn(f: isize, x: u8) = foo;
                  found fn item `unsafe extern "C" fn(_, _, ...) {foo}`
 
 error[E0308]: mismatched types
-  --> $DIR/variadic-ffi-1.rs:24:54
+  --> $DIR/variadic-ffi-1.rs:26:54
    |
 LL |         let y: extern "C" fn(f: isize, x: u8, ...) = bar;
    |                -----------------------------------   ^^^ expected variadic fn, found non-variadic function
@@ -59,37 +59,37 @@ LL |         let y: extern "C" fn(f: isize, x: u8, ...) = bar;
                  found fn item `extern "C" fn(_, _) {bar}`
 
 error[E0617]: can't pass `f32` to variadic function
-  --> $DIR/variadic-ffi-1.rs:26:19
+  --> $DIR/variadic-ffi-1.rs:28:19
    |
 LL |         foo(1, 2, 3f32);
    |                   ^^^^ help: cast the value to `c_double`: `3f32 as c_double`
 
 error[E0617]: can't pass `bool` to variadic function
-  --> $DIR/variadic-ffi-1.rs:27:19
+  --> $DIR/variadic-ffi-1.rs:29:19
    |
 LL |         foo(1, 2, true);
    |                   ^^^^ help: cast the value to `c_int`: `true as c_int`
 
 error[E0617]: can't pass `i8` to variadic function
-  --> $DIR/variadic-ffi-1.rs:28:19
+  --> $DIR/variadic-ffi-1.rs:30:19
    |
 LL |         foo(1, 2, 1i8);
    |                   ^^^ help: cast the value to `c_int`: `1i8 as c_int`
 
 error[E0617]: can't pass `u8` to variadic function
-  --> $DIR/variadic-ffi-1.rs:29:19
+  --> $DIR/variadic-ffi-1.rs:31:19
    |
 LL |         foo(1, 2, 1u8);
    |                   ^^^ help: cast the value to `c_uint`: `1u8 as c_uint`
 
 error[E0617]: can't pass `i16` to variadic function
-  --> $DIR/variadic-ffi-1.rs:30:19
+  --> $DIR/variadic-ffi-1.rs:32:19
    |
 LL |         foo(1, 2, 1i16);
    |                   ^^^^ help: cast the value to `c_int`: `1i16 as c_int`
 
 error[E0617]: can't pass `u16` to variadic function
-  --> $DIR/variadic-ffi-1.rs:31:19
+  --> $DIR/variadic-ffi-1.rs:33:19
    |
 LL |         foo(1, 2, 1u16);
    |                   ^^^^ help: cast the value to `c_uint`: `1u16 as c_uint`

src/test/ui/c-variadic/variadic-ffi-2.rs

@@ -1,7 +1,20 @@
 // ignore-arm stdcall isn't supported
+#![feature(extended_varargs_abi_support)]
+#![feature(abi_efiapi)]
 
 fn baz(f: extern "stdcall" fn(usize, ...)) {
-    //~^ ERROR: variadic function must have C or cdecl calling convention
+    //~^ ERROR: C-variadic function must have a compatible calling convention,
+    // like C, cdecl, win64, sysv64 or efiapi
+    f(22, 44);
+}
+
+fn sysv(f: extern "sysv64" fn(usize, ...)) {
+    f(22, 44);
+}
+fn win(f: extern "win64" fn(usize, ...)) {
+    f(22, 44);
+}
+fn efiapi(f: extern "efiapi" fn(usize, ...)) {
     f(22, 44);
 }
 

src/test/ui/c-variadic/variadic-ffi-2.stderr

@@ -1,8 +1,8 @@
-error[E0045]: C-variadic function must have C or cdecl calling convention
-  --> $DIR/variadic-ffi-2.rs:3:11
+error[E0045]: C-variadic function must have a compatible calling convention, like `C`, `cdecl`, `win64`, `sysv64` or `efiapi`
+  --> $DIR/variadic-ffi-2.rs:5:11
    |
 LL | fn baz(f: extern "stdcall" fn(usize, ...)) {
-   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ C-variadics require C or cdecl calling convention
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ C-variadic function must have a compatible calling convention
 
 error: aborting due to previous error
 

src/test/ui/error-codes/E0045.stderr

@@ -1,8 +1,8 @@
-error[E0045]: C-variadic function must have C or cdecl calling convention
+error[E0045]: C-variadic function must have a compatible calling convention, like `C` or `cdecl`
   --> $DIR/E0045.rs:1:17
    |
 LL | extern "Rust" { fn foo(x: u8, ...); }
-   |                 ^^^^^^^^^^^^^^^^^^^ C-variadics require C or cdecl calling convention
+   |                 ^^^^^^^^^^^^^^^^^^^ C-variadic function must have a compatible calling convention
 
 error: aborting due to previous error