also test fastcall

Author: RalfJung

compiler/rustc_target/src/spec/mod.rs

@@ -2813,11 +2813,15 @@ impl Target {
             Abi::EfiApi if self.arch == "x86_64" => Abi::Win64 { unwind: false },
             Abi::EfiApi => Abi::C { unwind: false },
 
-            // See commentary in `is_abi_supported`.
+            // See commentary in `is_abi_supported`: we map these to "C" on targets
+            // where they do not make sense.
             Abi::Stdcall { .. } | Abi::Thiscall { .. } if self.arch == "x86" => abi,
             Abi::Stdcall { unwind } | Abi::Thiscall { unwind } => Abi::C { unwind },
-            Abi::Fastcall { .. } if self.arch == "x86" => abi,
-            Abi::Vectorcall { .. } if ["x86", "x86_64"].contains(&&self.arch[..]) => abi,
+            Abi::Fastcall { .. } | Abi::Vectorcall { .. }
+                if ["x86", "x86_64"].contains(&&self.arch[..]) =>
+            {
+                abi
+            }
             Abi::Fastcall { unwind } | Abi::Vectorcall { unwind } => Abi::C { unwind },
 
             // The Windows x64 calling convention we use for `extern "Rust"`

tests/codegen/abi-win64-zst.rs

@@ -1,3 +1,5 @@
+//@ compile-flags: -Z merge-functions=disabled
+
 //@ revisions: linux
 //@[linux] compile-flags: --target x86_64-unknown-linux-gnu
 //@[linux] needs-llvm-components: x86
@@ -31,6 +33,12 @@ extern "sysv64" fn pass_zst_sysv64(_: ()) {}
 #[no_mangle]
 extern "vectorcall" fn pass_zst_vectorcall(_: ()) {}
 
+// windows-gnu: define x86_fastcallcc void @pass_zst_fastcall()
+// windows-msvc: define x86_fastcallcc void @pass_zst_fastcall()
+#[no_mangle]
+#[cfg(windows)] // "fastcall" is not valid on 64bit Linux
+extern "fastcall" fn pass_zst_fastcall(_: ()) {}
+
 // For `extern "C"` functions, ZST are *not* ignored on windows-gnu.
 // That's likely an accident on their side but the ABI is what it is.