Move naked function ABI check to its own lint

This check was previously categorized under the lint named
`UNSUPPORTED_NAKED_FUNCTIONS`. That lint is future incompatible and will
be turned into an error in a future release. However, as defined in the
Constrained Naked Functions RFC, this check should only be a warning.
This is because it is possible for a naked function to be implemented in
such a way that it does not break even the undefined ABI. For example, a
`jmp` to a `const`.

Therefore, this patch defines a new lint named
`UNDEFINED_NAKED_FUNCTION_ABI` which contains just this single check.
Unlike `UNSUPPORTED_NAKED_FUNCTIONS`, `UNDEFINED_NAKED_FUNCTION_ABI`
will not be converted to an error in the future.

rust-lang/rfcs#2774
rust-lang/rfcs#2972

compiler/rustc_lint_defs/src/builtin.rs

@@ -2691,6 +2691,38 @@ declare_lint! {
     "detects deprecation attributes with no effect",
 }
 
+declare_lint! {
+    /// The `undefined_naked_function_abi` lint detects naked function definitions that
+    /// either do not specify an ABI or specify the Rust ABI.
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// #![feature(naked_functions)]
+    /// #![feature(asm)]
+    ///
+    /// #[naked]
+    /// pub fn default_abi() -> u32 {
+    ///     unsafe { asm!("", options(noreturn)); }
+    /// }
+    ///
+    /// #[naked]
+    /// pub extern "Rust" fn rust_abi() -> u32 {
+    ///     unsafe { asm!("", options(noreturn)); }
+    /// }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// The Rust ABI is currently undefined. Therefore, naked functions should
+    /// specify a non-Rust ABI.
+    pub UNDEFINED_NAKED_FUNCTION_ABI,
+    Warn,
+    "undefined naked function ABI"
+}
+
 declare_lint! {
     /// The `unsupported_naked_functions` lint detects naked function
     /// definitions that are unsupported but were previously accepted.
@@ -2701,7 +2733,7 @@ declare_lint! {
     /// #![feature(naked_functions)]
     ///
     /// #[naked]
-    /// pub fn f() -> u32 {
+    /// pub extern "sysv64" fn f() -> u32 {
     ///     42
     /// }
     /// ```

compiler/rustc_passes/src/naked_functions.rs

@@ -7,6 +7,7 @@ use rustc_hir::intravisit::{ErasedMap, FnKind, NestedVisitorMap, Visitor};
 use rustc_hir::{ExprKind, HirId, InlineAsmOperand, StmtKind};
 use rustc_middle::ty::query::Providers;
 use rustc_middle::ty::TyCtxt;
+use rustc_session::lint::builtin::UNDEFINED_NAKED_FUNCTION_ABI;
 use rustc_session::lint::builtin::UNSUPPORTED_NAKED_FUNCTIONS;
 use rustc_span::symbol::sym;
 use rustc_span::Span;
@@ -87,7 +88,7 @@ fn check_inline(tcx: TyCtxt<'_>, hir_id: HirId, attrs: &[Attribute]) {
 /// Checks that function uses non-Rust ABI.
 fn check_abi(tcx: TyCtxt<'_>, hir_id: HirId, abi: Abi, fn_ident_span: Span) {
     if abi == Abi::Rust {
-        tcx.struct_span_lint_hir(UNSUPPORTED_NAKED_FUNCTIONS, hir_id, fn_ident_span, |lint| {
+        tcx.struct_span_lint_hir(UNDEFINED_NAKED_FUNCTION_ABI, hir_id, fn_ident_span, |lint| {
             lint.build("Rust ABI is unsupported in naked functions").emit();
         });
     }

src/test/ui/asm/naked-functions.rs

@@ -134,14 +134,12 @@ unsafe extern "C" fn invalid_options_continued() {
 #[naked]
 pub unsafe fn default_abi() {
     //~^ WARN Rust ABI is unsupported in naked functions
-    //~| WARN this was previously accepted
     asm!("", options(noreturn));
 }
 
 #[naked]
 pub unsafe extern "Rust" fn rust_abi() {
     //~^ WARN Rust ABI is unsupported in naked functions
-    //~| WARN this was previously accepted
     asm!("", options(noreturn));
 }
 

src/test/ui/asm/naked-functions.stderr

@@ -284,20 +284,16 @@ warning: Rust ABI is unsupported in naked functions
 LL | pub unsafe fn default_abi() {
    |               ^^^^^^^^^^^
    |
-   = 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 #32408 <https://github.com/rust-lang/rust/issues/32408>
+   = note: `#[warn(undefined_naked_function_abi)]` on by default
 
 warning: Rust ABI is unsupported in naked functions
-  --> $DIR/naked-functions.rs:142:29
+  --> $DIR/naked-functions.rs:141:29
    |
 LL | pub unsafe extern "Rust" fn rust_abi() {
    |                             ^^^^^^^^
-   |
-   = 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 #32408 <https://github.com/rust-lang/rust/issues/32408>
 
 warning: naked functions cannot be inlined
-  --> $DIR/naked-functions.rs:177:1
+  --> $DIR/naked-functions.rs:175:1
    |
 LL | #[inline]
    | ^^^^^^^^^
@@ -306,7 +302,7 @@ LL | #[inline]
    = note: for more information, see issue #32408 <https://github.com/rust-lang/rust/issues/32408>
 
 warning: naked functions cannot be inlined
-  --> $DIR/naked-functions.rs:185:1
+  --> $DIR/naked-functions.rs:183:1
    |
 LL | #[inline(always)]
    | ^^^^^^^^^^^^^^^^^
@@ -315,7 +311,7 @@ LL | #[inline(always)]
    = note: for more information, see issue #32408 <https://github.com/rust-lang/rust/issues/32408>
 
 warning: naked functions cannot be inlined
-  --> $DIR/naked-functions.rs:193:1
+  --> $DIR/naked-functions.rs:191:1
    |
 LL | #[inline(never)]
    | ^^^^^^^^^^^^^^^^
@@ -324,7 +320,7 @@ LL | #[inline(never)]
    = note: for more information, see issue #32408 <https://github.com/rust-lang/rust/issues/32408>
 
 warning: naked functions cannot be inlined
-  --> $DIR/naked-functions.rs:201:1
+  --> $DIR/naked-functions.rs:199:1
    |
 LL | #[inline]
    | ^^^^^^^^^
@@ -333,7 +329,7 @@ LL | #[inline]
    = note: for more information, see issue #32408 <https://github.com/rust-lang/rust/issues/32408>
 
 warning: naked functions cannot be inlined
-  --> $DIR/naked-functions.rs:204:1
+  --> $DIR/naked-functions.rs:202:1
    |
 LL | #[inline(always)]
    | ^^^^^^^^^^^^^^^^^
@@ -342,7 +338,7 @@ LL | #[inline(always)]
    = note: for more information, see issue #32408 <https://github.com/rust-lang/rust/issues/32408>
 
 warning: naked functions cannot be inlined
-  --> $DIR/naked-functions.rs:207:1
+  --> $DIR/naked-functions.rs:205:1
    |
 LL | #[inline(never)]
    | ^^^^^^^^^^^^^^^^