Use `Option<Ident>` for lowered param names.

Parameter patterns are lowered to an `Ident` by
`lower_fn_params_to_names`, which is used when lowering bare function
types, trait methods, and foreign functions. Currently, there are two
exceptional cases where the lowered param can become an empty `Ident`.

- If the incoming pattern is an empty `Ident`. This occurs if the
  parameter is anonymous, e.g. in a bare function type.

- If the incoming pattern is neither an ident nor an underscore. Any
  such parameter will have triggered a compile error (hence the
  `span_delayed_bug`), but lowering still occurs.

This commit replaces these empty `Ident` results with `None`, which
eliminates a number of `kw::Empty` uses, and makes it impossible to fail
to check for these exceptional cases.

Note: the `FIXME` comment in `is_unwrap_or_empty_symbol` is removed. It
actually should have been removed in #138482, the precursor to this PR.
That PR changed the lowering of wild patterns to `_` symbols instead of
empty symbols, which made the mentioned underscore check load-bearing.

17 files changed, 83 insertions, 50 deletions

diff --git a/compiler/rustc_ast_lowering/src/lib.rs b/compiler/rustc_ast_lowering/src/lib.rs
index e24b45c5b19..e08850da4a7 100644
--- a/compiler/rustc_ast_lowering/src/lib.rs
+++ b/compiler/rustc_ast_lowering/src/lib.rs
@@ -1513,16 +1513,22 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         }))
     }
 
-    fn lower_fn_params_to_names(&mut self, decl: &FnDecl) -> &'hir [Ident] {
+    fn lower_fn_params_to_names(&mut self, decl: &FnDecl) -> &'hir [Option<Ident>] {
         self.arena.alloc_from_iter(decl.inputs.iter().map(|param| match param.pat.kind {
-            PatKind::Ident(_, ident, _) => self.lower_ident(ident),
-            PatKind::Wild => Ident::new(kw::Underscore, self.lower_span(param.pat.span)),
+            PatKind::Ident(_, ident, _) => {
+                if ident.name != kw::Empty {
+                    Some(self.lower_ident(ident))
+                } else {
+                    None
+                }
+            }
+            PatKind::Wild => Some(Ident::new(kw::Underscore, self.lower_span(param.pat.span))),
             _ => {
                 self.dcx().span_delayed_bug(
                     param.pat.span,
                     "non-ident/wild param pat must trigger an error",
                 );
-                Ident::new(kw::Empty, self.lower_span(param.pat.span))
+                None
             }
         }))
     }
diff --git a/compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs b/compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs
index 2694a1eda78..978186f76a1 100644
--- a/compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs
+++ b/compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs
@@ -2514,12 +2514,12 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
         let ty::Tuple(params) = tupled_params.kind() else { return };
 
         // Find the first argument with a matching type, get its name
-        let Some((_, this_name)) =
-            params.iter().zip(tcx.hir_body_param_names(closure.body)).find(|(param_ty, name)| {
+        let Some(this_name) = params.iter().zip(tcx.hir_body_param_names(closure.body)).find_map(
+            |(param_ty, name)| {
                 // FIXME: also support deref for stuff like `Rc` arguments
-                param_ty.peel_refs() == local_ty && name != &Ident::empty()
-            })
-        else {
+                if param_ty.peel_refs() == local_ty { name } else { None }
+            },
+        ) else {
             return;
         };
 
@@ -3787,7 +3787,7 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
                 method_args,
                 *fn_span,
                 call_source.from_hir_call(),
-                Some(self.infcx.tcx.fn_arg_names(method_did)[0]),
+                self.infcx.tcx.fn_arg_names(method_did)[0],
             )
         {
             err.note(format!("borrow occurs due to deref coercion to `{deref_target_ty}`"));
diff --git a/compiler/rustc_borrowck/src/diagnostics/mod.rs b/compiler/rustc_borrowck/src/diagnostics/mod.rs
index 208d510db2e..899e145c2c0 100644
--- a/compiler/rustc_borrowck/src/diagnostics/mod.rs
+++ b/compiler/rustc_borrowck/src/diagnostics/mod.rs
@@ -1029,7 +1029,7 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
                 method_args,
                 *fn_span,
                 call_source.from_hir_call(),
-                Some(self.infcx.tcx.fn_arg_names(method_did)[0]),
+                self.infcx.tcx.fn_arg_names(method_did)[0],
             );
 
             return FnSelfUse {
diff --git a/compiler/rustc_hir/src/hir.rs b/compiler/rustc_hir/src/hir.rs
index b5857e359a2..751c379b21a 100644
--- a/compiler/rustc_hir/src/hir.rs
+++ b/compiler/rustc_hir/src/hir.rs
@@ -2949,7 +2949,7 @@ impl<'hir> TraitItem<'hir> {
 #[derive(Debug, Clone, Copy, HashStable_Generic)]
 pub enum TraitFn<'hir> {
     /// No default body in the trait, just a signature.
-    Required(&'hir [Ident]),
+    Required(&'hir [Option<Ident>]),
 
     /// Both signature and body are provided in the trait.
     Provided(BodyId),
@@ -3354,7 +3354,9 @@ pub struct BareFnTy<'hir> {
     pub abi: ExternAbi,
     pub generic_params: &'hir [GenericParam<'hir>],
     pub decl: &'hir FnDecl<'hir>,
-    pub param_names: &'hir [Ident],
+    // `Option` because bare fn parameter names are optional. We also end up
+    // with `None` in some error cases, e.g. invalid parameter patterns.
+    pub param_names: &'hir [Option<Ident>],
 }
 
 #[derive(Debug, Clone, Copy, HashStable_Generic)]
@@ -4335,7 +4337,12 @@ impl ForeignItem<'_> {
 #[derive(Debug, Clone, Copy, HashStable_Generic)]
 pub enum ForeignItemKind<'hir> {
     /// A foreign function.
-    Fn(FnSig<'hir>, &'hir [Ident], &'hir Generics<'hir>),
+    ///
+    /// All argument idents are actually always present (i.e. `Some`), but
+    /// `&[Option<Ident>]` is used because of code paths shared with `TraitFn`
+    /// and `BareFnTy`. The sharing is due to all of these cases not allowing
+    /// arbitrary patterns for parameters.
+    Fn(FnSig<'hir>, &'hir [Option<Ident>], &'hir Generics<'hir>),
     /// A foreign static item (`static ext: u8`).
     Static(&'hir Ty<'hir>, Mutability, Safety),
     /// A foreign type.
diff --git a/compiler/rustc_hir/src/intravisit.rs b/compiler/rustc_hir/src/intravisit.rs
index b79ae1e7cc2..506358341b5 100644
--- a/compiler/rustc_hir/src/intravisit.rs
+++ b/compiler/rustc_hir/src/intravisit.rs
@@ -655,7 +655,9 @@ pub fn walk_foreign_item<'v, V: Visitor<'v>>(
         ForeignItemKind::Fn(ref sig, param_names, ref generics) => {
             try_visit!(visitor.visit_generics(generics));
             try_visit!(visitor.visit_fn_decl(sig.decl));
-            walk_list!(visitor, visit_ident, param_names.iter().copied());
+            for ident in param_names.iter().copied() {
+                visit_opt!(visitor, visit_ident, ident);
+            }
         }
         ForeignItemKind::Static(ref typ, _, _) => {
             try_visit!(visitor.visit_ty_unambig(typ));
@@ -1169,7 +1171,9 @@ pub fn walk_trait_item<'v, V: Visitor<'v>>(
         }
         TraitItemKind::Fn(ref sig, TraitFn::Required(param_names)) => {
             try_visit!(visitor.visit_fn_decl(sig.decl));
-            walk_list!(visitor, visit_ident, param_names.iter().copied());
+            for ident in param_names.iter().copied() {
+                visit_opt!(visitor, visit_ident, ident);
+            }
         }
         TraitItemKind::Fn(ref sig, TraitFn::Provided(body_id)) => {
             try_visit!(visitor.visit_fn(
diff --git a/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs b/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs
index ca820deebdf..84d07c711fa 100644
--- a/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs
+++ b/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs
@@ -1034,7 +1034,13 @@ fn report_trait_method_mismatch<'tcx>(
             let span = tcx
                 .hir_body_param_names(body)
                 .zip(sig.decl.inputs.iter())
-                .map(|(param, ty)| param.span.to(ty.span))
+                .map(|(param_name, ty)| {
+                    if let Some(param_name) = param_name {
+                        param_name.span.to(ty.span)
+                    } else {
+                        ty.span
+                    }
+                })
                 .next()
                 .unwrap_or(impl_err_span);
 
diff --git a/compiler/rustc_hir_analysis/src/hir_ty_lowering/cmse.rs b/compiler/rustc_hir_analysis/src/hir_ty_lowering/cmse.rs
index 5fed2e35287..170500c7a16 100644
--- a/compiler/rustc_hir_analysis/src/hir_ty_lowering/cmse.rs
+++ b/compiler/rustc_hir_analysis/src/hir_ty_lowering/cmse.rs
@@ -49,10 +49,12 @@ pub(crate) fn validate_cmse_abi<'tcx>(
                 Ok(Err(index)) => {
                     // fn(x: u32, u32, u32, u16, y: u16) -> u32,
                     //                           ^^^^^^
-                    let span = bare_fn_ty.param_names[index]
-                        .span
-                        .to(bare_fn_ty.decl.inputs[index].span)
-                        .to(bare_fn_ty.decl.inputs.last().unwrap().span);
+                    let span = if let Some(ident) = bare_fn_ty.param_names[index] {
+                        ident.span.to(bare_fn_ty.decl.inputs[index].span)
+                    } else {
+                        bare_fn_ty.decl.inputs[index].span
+                    }
+                    .to(bare_fn_ty.decl.inputs.last().unwrap().span);
                     let plural = bare_fn_ty.param_names.len() - index != 1;
                     dcx.emit_err(errors::CmseInputsStackSpill { span, plural, abi });
                 }
diff --git a/compiler/rustc_hir_pretty/src/lib.rs b/compiler/rustc_hir_pretty/src/lib.rs
index 98b81dd3def..ddaca89ccf8 100644
--- a/compiler/rustc_hir_pretty/src/lib.rs
+++ b/compiler/rustc_hir_pretty/src/lib.rs
@@ -2,6 +2,7 @@
 //! the definitions in this file have equivalents in `rustc_ast_pretty`.
 
 // tidy-alphabetical-start
+#![feature(let_chains)]
 #![recursion_limit = "256"]
 // tidy-alphabetical-end
 
@@ -898,7 +899,7 @@ impl<'a> State<'a> {
         ident: Ident,
         m: &hir::FnSig<'_>,
         generics: &hir::Generics<'_>,
-        arg_names: &[Ident],
+        arg_names: &[Option<Ident>],
         body_id: Option<hir::BodyId>,
     ) {
         self.print_fn(m.decl, m.header, Some(ident.name), generics, arg_names, body_id);
@@ -2121,7 +2122,7 @@ impl<'a> State<'a> {
         header: hir::FnHeader,
         name: Option<Symbol>,
         generics: &hir::Generics<'_>,
-        arg_names: &[Ident],
+        arg_names: &[Option<Ident>],
         body_id: Option<hir::BodyId>,
     ) {
         self.print_fn_header_info(header);
@@ -2141,7 +2142,7 @@ impl<'a> State<'a> {
                 s.print_implicit_self(&decl.implicit_self);
             } else {
                 if let Some(arg_name) = arg_names.get(i) {
-                    if arg_name.name != kw::Empty {
+                    if let Some(arg_name) = arg_name {
                         s.word(arg_name.to_string());
                         s.word(":");
                         s.space();
@@ -2451,7 +2452,7 @@ impl<'a> State<'a> {
         decl: &hir::FnDecl<'_>,
         name: Option<Symbol>,
         generic_params: &[hir::GenericParam<'_>],
-        arg_names: &[Ident],
+        arg_names: &[Option<Ident>],
     ) {
         self.ibox(INDENT_UNIT);
         self.print_formal_generic_params(generic_params);
diff --git a/compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs b/compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs
index 90bdb3c4b37..f4bd7ec701f 100644
--- a/compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs
+++ b/compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs
@@ -1135,7 +1135,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 && self.tcx.def_kind(fn_def_id).is_fn_like()
                 && let self_implicit =
                     matches!(call_expr.kind, hir::ExprKind::MethodCall(..)) as usize
-                && let Some(arg) =
+                && let Some(Some(arg)) =
                     self.tcx.fn_arg_names(fn_def_id).get(expected_idx.as_usize() + self_implicit)
                 && arg.name != kw::SelfLower
             {
@@ -2678,7 +2678,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     params.get(is_method as usize..params.len() - sig.decl.c_variadic as usize)?;
                 debug_assert_eq!(params.len(), fn_inputs.len());
                 Some((
-                    fn_inputs.zip(params.iter().map(|&param| FnParam::Name(param))).collect(),
+                    fn_inputs.zip(params.iter().map(|&ident| FnParam::Name(ident))).collect(),
                     generics,
                 ))
             }
@@ -2709,14 +2709,20 @@ impl<'tcx> Visitor<'tcx> for FindClosureArg<'tcx> {
 #[derive(Clone, Copy)]
 enum FnParam<'hir> {
     Param(&'hir hir::Param<'hir>),
-    Name(Ident),
+    Name(Option<Ident>),
 }
 
 impl FnParam<'_> {
     fn span(&self) -> Span {
         match self {
             Self::Param(param) => param.span,
-            Self::Name(ident) => ident.span,
+            Self::Name(ident) => {
+                if let Some(ident) = ident {
+                    ident.span
+                } else {
+                    DUMMY_SP
+                }
+            }
         }
     }
 
@@ -2733,7 +2739,8 @@ impl FnParam<'_> {
                         Some(ident.name)
                     }
                     FnParam::Name(ident)
-                        if ident.name != kw::Empty && ident.name != kw::Underscore =>
+                        if let Some(ident) = ident
+                            && ident.name != kw::Underscore =>
                     {
                         Some(ident.name)
                     }
diff --git a/compiler/rustc_hir_typeck/src/method/suggest.rs b/compiler/rustc_hir_typeck/src/method/suggest.rs
index cdfae51583b..908c3ee2eb8 100644
--- a/compiler/rustc_hir_typeck/src/method/suggest.rs
+++ b/compiler/rustc_hir_typeck/src/method/suggest.rs
@@ -3766,7 +3766,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                                 {
                                     let self_first_arg = match method {
                                         hir::TraitFn::Required([ident, ..]) => {
-                                            ident.name == kw::SelfLower
+                                            matches!(ident, Some(Ident { name: kw::SelfLower, .. }))
                                         }
                                         hir::TraitFn::Provided(body_id) => {
                                             self.tcx.hir_body(*body_id).params.first().is_some_and(
diff --git a/compiler/rustc_lint/src/nonstandard_style.rs b/compiler/rustc_lint/src/nonstandard_style.rs
index 715e3506ab8..752636ccaf0 100644
--- a/compiler/rustc_lint/src/nonstandard_style.rs
+++ b/compiler/rustc_lint/src/nonstandard_style.rs
@@ -424,7 +424,9 @@ impl<'tcx> LateLintPass<'tcx> for NonSnakeCase {
         if let hir::TraitItemKind::Fn(_, hir::TraitFn::Required(pnames)) = item.kind {
             self.check_snake_case(cx, "trait method", &item.ident);
             for param_name in pnames {
-                self.check_snake_case(cx, "variable", param_name);
+                if let Some(param_name) = param_name {
+                    self.check_snake_case(cx, "variable", param_name);
+                }
             }
         }
     }
diff --git a/compiler/rustc_metadata/src/rmeta/decoder.rs b/compiler/rustc_metadata/src/rmeta/decoder.rs
index e60904eebeb..24248e8dde3 100644
--- a/compiler/rustc_metadata/src/rmeta/decoder.rs
+++ b/compiler/rustc_metadata/src/rmeta/decoder.rs
@@ -1318,7 +1318,7 @@ impl<'a> CrateMetadataRef<'a> {
             .expect("argument names not encoded for a function")
             .decode((self, sess))
             .nth(0)
-            .is_some_and(|ident| ident.name == kw::SelfLower)
+            .is_some_and(|ident| matches!(ident, Some(Ident { name: kw::SelfLower, .. })))
     }
 
     fn get_associated_item_or_field_def_ids(self, id: DefIndex) -> impl Iterator<Item = DefId> {
diff --git a/compiler/rustc_metadata/src/rmeta/mod.rs b/compiler/rustc_metadata/src/rmeta/mod.rs
index 5536c93f84a..dc453b1e747 100644
--- a/compiler/rustc_metadata/src/rmeta/mod.rs
+++ b/compiler/rustc_metadata/src/rmeta/mod.rs
@@ -443,7 +443,7 @@ define_tables! {
     rendered_const: Table<DefIndex, LazyValue<String>>,
     rendered_precise_capturing_args: Table<DefIndex, LazyArray<PreciseCapturingArgKind<Symbol, Symbol>>>,
     asyncness: Table<DefIndex, ty::Asyncness>,
-    fn_arg_names: Table<DefIndex, LazyArray<Ident>>,
+    fn_arg_names: Table<DefIndex, LazyArray<Option<Ident>>>,
     coroutine_kind: Table<DefIndex, hir::CoroutineKind>,
     coroutine_for_closure: Table<DefIndex, RawDefId>,
     coroutine_by_move_body_def_id: Table<DefIndex, RawDefId>,
diff --git a/compiler/rustc_middle/src/hir/map.rs b/compiler/rustc_middle/src/hir/map.rs
index 2e589150d3e..f17efab81ec 100644
--- a/compiler/rustc_middle/src/hir/map.rs
+++ b/compiler/rustc_middle/src/hir/map.rs
@@ -280,11 +280,11 @@ impl<'tcx> TyCtxt<'tcx> {
         })
     }
 
-    pub fn hir_body_param_names(self, id: BodyId) -> impl Iterator<Item = Ident> {
+    pub fn hir_body_param_names(self, id: BodyId) -> impl Iterator<Item = Option<Ident>> {
         self.hir_body(id).params.iter().map(|param| match param.pat.kind {
-            PatKind::Binding(_, _, ident, _) => ident,
-            PatKind::Wild => Ident::new(kw::Underscore, param.pat.span),
-            _ => Ident::empty(),
+            PatKind::Binding(_, _, ident, _) => Some(ident),
+            PatKind::Wild => Some(Ident::new(kw::Underscore, param.pat.span)),
+            _ => None,
         })
     }
 
diff --git a/compiler/rustc_middle/src/query/mod.rs b/compiler/rustc_middle/src/query/mod.rs
index 94a5a3769a3..527c18addbe 100644
--- a/compiler/rustc_middle/src/query/mod.rs
+++ b/compiler/rustc_middle/src/query/mod.rs
@@ -1410,7 +1410,7 @@ rustc_queries! {
         desc { |tcx| "computing target features for inline asm of `{}`", tcx.def_path_str(def_id) }
     }
 
-    query fn_arg_names(def_id: DefId) -> &'tcx [rustc_span::Ident] {
+    query fn_arg_names(def_id: DefId) -> &'tcx [Option<rustc_span::Ident>] {
         desc { |tcx| "looking up function parameter names for `{}`", tcx.def_path_str(def_id) }
         separate_provide_extern
     }
diff --git a/compiler/rustc_resolve/src/late/diagnostics.rs b/compiler/rustc_resolve/src/late/diagnostics.rs
index bcfcc8000c7..3d666055a94 100644
--- a/compiler/rustc_resolve/src/late/diagnostics.rs
+++ b/compiler/rustc_resolve/src/late/diagnostics.rs
@@ -2217,12 +2217,11 @@ impl<'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {
                                     .delegation_fn_sigs
                                     .get(&def_id)
                                     .is_some_and(|sig| sig.has_self),
-                                None => self
-                                    .r
-                                    .tcx
-                                    .fn_arg_names(def_id)
-                                    .first()
-                                    .is_some_and(|ident| ident.name == kw::SelfLower),
+                                None => {
+                                    self.r.tcx.fn_arg_names(def_id).first().is_some_and(|&ident| {
+                                        matches!(ident, Some(Ident { name: kw::SelfLower, .. }))
+                                    })
+                                }
                             };
                             if has_self {
                                 return Some(AssocSuggestion::MethodWithSelf { called });
diff --git a/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs b/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
index 393d175ea4c..dc8022b95c3 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
@@ -1992,13 +1992,12 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                 .iter()
                 .enumerate()
                 .map(|(i, ident)| {
-                    if ident.name.is_empty()
-                        || ident.name == kw::Underscore
-                        || ident.name == kw::SelfLower
+                    if let Some(ident) = ident
+                        && !matches!(ident, Ident { name: kw::Underscore | kw::SelfLower, .. })
                     {
-                        format!("arg{i}")
-                    } else {
                         format!("{ident}")
+                    } else {
+                        format!("arg{i}")
                     }
                 })
                 .collect();