move check_ffi_pure to attribute parser

compiler/rustc_attr_parsing/src/attributes/link_attrs.rs

@@ -14,11 +14,11 @@ use super::util::parse_single_integer;
 use crate::attributes::AttributeSafety;
 use crate::attributes::cfg::parse_cfg_entry;
 use crate::session_diagnostics::{
-    AsNeededCompatibility, BundleNeedsStatic, EmptyLinkName, ExportSymbolsNeedsStatic,
-    ImportNameTypeRaw, ImportNameTypeX86, IncompatibleWasmLink, InvalidLinkModifier,
-    InvalidMachoSection, InvalidMachoSectionReason, LinkFrameworkApple, LinkOrdinalOutOfRange,
-    LinkRequiresName, MultipleModifiers, NullOnLinkSection, RawDylibNoNul, RawDylibOnlyWindows,
-    WholeArchiveNeedsStatic,
+    AsNeededCompatibility, BothFfiConstAndPure, BundleNeedsStatic, EmptyLinkName,
+    ExportSymbolsNeedsStatic, ImportNameTypeRaw, ImportNameTypeX86, IncompatibleWasmLink,
+    InvalidLinkModifier, InvalidMachoSection, InvalidMachoSectionReason, LinkFrameworkApple,
+    LinkOrdinalOutOfRange, LinkRequiresName, MultipleModifiers, NullOnLinkSection, RawDylibNoNul,
+    RawDylibOnlyWindows, UnusedMultiple, WholeArchiveNeedsStatic,
 };
 
 pub(crate) struct LinkNameParser;
@@ -540,12 +540,41 @@ impl NoArgsAttributeParser for FfiConstParser {
     const CREATE: fn(Span) -> AttributeKind = |_| AttributeKind::FfiConst;
 }
 
-pub(crate) struct FfiPureParser;
-impl NoArgsAttributeParser for FfiPureParser {
-    const PATH: &[Symbol] = &[sym::ffi_pure];
+#[derive(Default)]
+pub(crate) struct FfiPureParser {
+    span: Option<Span>,
+}
+
+impl AttributeParser for FfiPureParser {
+    const ATTRIBUTES: AcceptMapping<Self> =
+        &[(&[sym::ffi_pure], template!(Word), |this, cx, args| {
+            let Some(()) = cx.expect_no_args(args) else {
+                return;
+            };
+
+            if let Some(earlier) = this.span {
+                let span = cx.attr_span;
+                cx.emit_err(UnusedMultiple { this: span, other: earlier, name: sym::ffi_pure });
+            } else {
+                this.span = Some(cx.attr_span);
+            }
+        })];
     const SAFETY: AttributeSafety = AttributeSafety::Unsafe { unsafe_since: None };
     const ALLOWED_TARGETS: AllowedTargets = AllowedTargets::AllowList(&[Allow(Target::ForeignFn)]);
-    const CREATE: fn(Span) -> AttributeKind = AttributeKind::FfiPure;
+
+    fn finalize(self, cx: &FinalizeContext<'_, '_>) -> Option<AttributeKind> {
+        let attr_span = self.span?;
+
+        // `#[ffi_const]` functions cannot be `#[ffi_pure]`.
+        for other_attr in cx.all_attrs {
+            if other_attr.word_is(sym::ffi_const) {
+                cx.emit_err(BothFfiConstAndPure { attr_span });
+                break;
+            }
+        }
+
+        Some(AttributeKind::FfiPure(attr_span))
+    }
 }
 
 pub(crate) struct RustcStdInternalSymbolParser;

compiler/rustc_attr_parsing/src/context.rs

@@ -135,6 +135,7 @@ attribute_parsers!(
         ConfusablesParser,
         ConstStabilityParser,
         DocParser,
+        FfiPureParser,
         MacroUseParser,
         NakedParser,
         OnConstParser,
@@ -234,7 +235,6 @@ attribute_parsers!(
         Single<WithoutArgs<DefaultLibAllocatorParser>>,
         Single<WithoutArgs<ExportStableParser>>,
         Single<WithoutArgs<FfiConstParser>>,
-        Single<WithoutArgs<FfiPureParser>>,
         Single<WithoutArgs<FundamentalParser>>,
         Single<WithoutArgs<LoopMatchParser>>,
         Single<WithoutArgs<MacroEscapeParser>>,

compiler/rustc_attr_parsing/src/session_diagnostics.rs

@@ -1156,3 +1156,12 @@ pub(crate) enum InvalidMachoSectionReason {
     #[note("section name `{$section}` is longer than 16 bytes")]
     SectionTooLong { section: String },
 }
+
+#[derive(Diagnostic)]
+#[diag("`#[ffi_const]` function cannot be `#[ffi_pure]`", code = E0757)]
+#[note("`#[ffi_pure]` is redundant here because `#[ffi_const]` provides stronger guarantees")]
+pub(crate) struct BothFfiConstAndPure {
+    #[primary_span]
+    #[suggestion("remove `#[ffi_pure]`", code = "", applicability = "maybe-incorrect")]
+    pub attr_span: Span,
+}

compiler/rustc_passes/src/check_attr.rs

@@ -174,9 +174,6 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
                 Attribute::Parsed(AttributeKind::NonExhaustive(attr_span)) => {
                     self.check_non_exhaustive(*attr_span, span, target, item)
                 }
-                &Attribute::Parsed(AttributeKind::FfiPure(attr_span)) => {
-                    self.check_ffi_pure(attr_span, attrs)
-                }
                 Attribute::Parsed(AttributeKind::MayDangle(attr_span)) => {
                     self.check_may_dangle(hir_id, *attr_span)
                 }
@@ -229,6 +226,7 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
                     | AttributeKind::ExportStable
                     | AttributeKind::Feature(..)
                     | AttributeKind::FfiConst
+                    | AttributeKind::FfiPure(..)
                     | AttributeKind::Fundamental
                     | AttributeKind::Ignore { .. }
                     | AttributeKind::InstructionSet(..)
@@ -1090,13 +1088,6 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
         }
     }
 
-    fn check_ffi_pure(&self, attr_span: Span, attrs: &[Attribute]) {
-        if find_attr!(attrs, FfiConst) {
-            // `#[ffi_const]` functions cannot be `#[ffi_pure]`
-            self.dcx().emit_err(errors::BothFfiConstAndPure { attr_span });
-        }
-    }
-
     /// Checks if `#[may_dangle]` is applied to a lifetime or type generic parameter in `Drop` impl.
     fn check_may_dangle(&self, hir_id: HirId, attr_span: Span) {
         if let hir::Node::GenericParam(param) = self.tcx.hir_node(hir_id)

compiler/rustc_passes/src/errors.rs

@@ -171,13 +171,6 @@ pub(crate) struct DocMaskedNotExternCrateSelf {
     pub item_span: Span,
 }
 
-#[derive(Diagnostic)]
-#[diag("`#[ffi_const]` function cannot be `#[ffi_pure]`", code = E0757)]
-pub(crate) struct BothFfiConstAndPure {
-    #[primary_span]
-    pub attr_span: Span,
-}
-
 #[derive(Diagnostic)]
 #[diag("attribute should be applied to an `extern` block with non-Rust ABI")]
 #[warning(

tests/ui/ffi-attrs/ffi_const2.stderr

@@ -2,7 +2,9 @@ error[E0757]: `#[ffi_const]` function cannot be `#[ffi_pure]`
   --> $DIR/ffi_const2.rs:4:5
    |
 LL |     #[unsafe(ffi_pure)]
-   |     ^^^^^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^^^^^ help: remove `#[ffi_pure]`
+   |
+   = note: `#[ffi_pure]` is redundant here because `#[ffi_const]` provides stronger guarantees
 
 error: aborting due to 1 previous error