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-rw-r--r--compiler/rustc_ast_lowering/src/lib.rs9
-rw-r--r--compiler/rustc_hir_pretty/src/lib.rs8
-rw-r--r--compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs40
-rw-r--r--compiler/rustc_middle/src/hir/map.rs3
-rw-r--r--compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs5
-rw-r--r--tests/pretty/hir-fn-params.pp38
-rw-r--r--tests/pretty/hir-fn-params.rs34
-rw-r--r--tests/ui/typeck/cyclic_type_ice.stderr2
8 files changed, 115 insertions, 24 deletions
diff --git a/compiler/rustc_ast_lowering/src/lib.rs b/compiler/rustc_ast_lowering/src/lib.rs
index c7d37e2704d..df671cf4b86 100644
--- a/compiler/rustc_ast_lowering/src/lib.rs
+++ b/compiler/rustc_ast_lowering/src/lib.rs
@@ -1516,7 +1516,14 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     fn lower_fn_params_to_names(&mut self, decl: &FnDecl) -> &'hir [Ident] {
         self.arena.alloc_from_iter(decl.inputs.iter().map(|param| match param.pat.kind {
             PatKind::Ident(_, ident, _) => self.lower_ident(ident),
-            _ => Ident::new(kw::Empty, self.lower_span(param.pat.span)),
+            PatKind::Wild => 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))
+            }
         }))
     }
 
diff --git a/compiler/rustc_hir_pretty/src/lib.rs b/compiler/rustc_hir_pretty/src/lib.rs
index 3067766fb4d..1409310339a 100644
--- a/compiler/rustc_hir_pretty/src/lib.rs
+++ b/compiler/rustc_hir_pretty/src/lib.rs
@@ -2148,9 +2148,11 @@ impl<'a> State<'a> {
                 s.print_implicit_self(&decl.implicit_self);
             } else {
                 if let Some(arg_name) = arg_names.get(i) {
-                    s.word(arg_name.to_string());
-                    s.word(":");
-                    s.space();
+                    if arg_name.name != kw::Empty {
+                        s.word(arg_name.to_string());
+                        s.word(":");
+                        s.space();
+                    }
                 } else if let Some(body_id) = body_id {
                     s.ann.nested(s, Nested::BodyParamPat(body_id, i));
                     s.word(":");
diff --git a/compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs b/compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs
index b8517701667..96d0a0fc6de 100644
--- a/compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs
+++ b/compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs
@@ -19,7 +19,7 @@ use rustc_middle::ty::visit::TypeVisitableExt;
 use rustc_middle::ty::{self, IsSuggestable, Ty, TyCtxt};
 use rustc_middle::{bug, span_bug};
 use rustc_session::Session;
-use rustc_span::{DUMMY_SP, Ident, Span, Symbol, kw, sym};
+use rustc_span::{DUMMY_SP, Ident, Span, kw, sym};
 use rustc_trait_selection::error_reporting::infer::{FailureCode, ObligationCauseExt};
 use rustc_trait_selection::infer::InferCtxtExt;
 use rustc_trait_selection::traits::{self, ObligationCauseCode, ObligationCtxt, SelectionContext};
@@ -2679,7 +2679,7 @@ 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(|&param| FnParam::Name(param))).collect(),
                     generics,
                 ))
             }
@@ -2710,23 +2710,14 @@ impl<'tcx> Visitor<'tcx> for FindClosureArg<'tcx> {
 #[derive(Clone, Copy)]
 enum FnParam<'hir> {
     Param(&'hir hir::Param<'hir>),
-    Name(&'hir Ident),
+    Name(Ident),
 }
+
 impl FnParam<'_> {
     fn span(&self) -> Span {
         match self {
-            Self::Param(x) => x.span,
-            Self::Name(x) => x.span,
-        }
-    }
-
-    fn name(&self) -> Option<Symbol> {
-        match self {
-            Self::Param(x) if let hir::PatKind::Binding(_, _, ident, _) = x.pat.kind => {
-                Some(ident.name)
-            }
-            Self::Name(x) if x.name != kw::Empty => Some(x.name),
-            _ => None,
+            Self::Param(param) => param.span,
+            Self::Name(ident) => ident.span,
         }
     }
 
@@ -2734,8 +2725,23 @@ impl FnParam<'_> {
         struct D<'a>(FnParam<'a>, usize);
         impl fmt::Display for D<'_> {
             fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-                if let Some(name) = self.0.name() {
-                    write!(f, "`{name}`")
+                // A "unique" param name is one that (a) exists, and (b) is guaranteed to be unique
+                // among the parameters, i.e. `_` does not count.
+                let unique_name = match self.0 {
+                    FnParam::Param(param)
+                        if let hir::PatKind::Binding(_, _, ident, _) = param.pat.kind =>
+                    {
+                        Some(ident.name)
+                    }
+                    FnParam::Name(ident)
+                        if ident.name != kw::Empty && ident.name != kw::Underscore =>
+                    {
+                        Some(ident.name)
+                    }
+                    _ => None,
+                };
+                if let Some(unique_name) = unique_name {
+                    write!(f, "`{unique_name}`")
                 } else {
                     write!(f, "parameter #{}", self.1 + 1)
                 }
diff --git a/compiler/rustc_middle/src/hir/map.rs b/compiler/rustc_middle/src/hir/map.rs
index 73c0af84a9f..c61c7a4fb02 100644
--- a/compiler/rustc_middle/src/hir/map.rs
+++ b/compiler/rustc_middle/src/hir/map.rs
@@ -281,8 +281,9 @@ impl<'tcx> TyCtxt<'tcx> {
     }
 
     pub fn hir_body_param_names(self, id: BodyId) -> impl Iterator<Item = Ident> {
-        self.hir_body(id).params.iter().map(|arg| match arg.pat.kind {
+        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(),
         })
     }
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 dce85b43df1..9383b82ff3c 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
@@ -1998,7 +1998,10 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                 .iter()
                 .enumerate()
                 .map(|(i, ident)| {
-                    if ident.name.is_empty() || ident.name == kw::SelfLower {
+                    if ident.name.is_empty()
+                        || ident.name == kw::Underscore
+                        || ident.name == kw::SelfLower
+                    {
                         format!("arg{i}")
                     } else {
                         format!("{ident}")
diff --git a/tests/pretty/hir-fn-params.pp b/tests/pretty/hir-fn-params.pp
new file mode 100644
index 00000000000..3799c8a3c3b
--- /dev/null
+++ b/tests/pretty/hir-fn-params.pp
@@ -0,0 +1,38 @@
+#[prelude_import]
+use ::std::prelude::rust_2015::*;
+#[macro_use]
+extern crate std;
+//@ pretty-compare-only
+//@ pretty-mode:hir
+//@ pp-exact:hir-fn-params.pp
+
+// This tests the pretty-printing of various kinds of function parameters.
+
+//---------------------------------------------------------------------------
+// Normal functions and methods.
+
+fn normal_fn(_: u32, a: u32) { }
+
+struct S;
+impl S {
+    fn method(_: u32, a: u32) { }
+}
+
+//---------------------------------------------------------------------------
+// More exotic forms, which get a different pretty-printing path. In the past,
+// anonymous params and `_` params printed incorrectly, e.g. `fn(u32, _: u32)`
+// was printed as `fn(: u32, : u32)`.
+//
+// Ideally we would also test invalid patterns, e.g. `fn(1: u32, &a: u32)`,
+// because they had similar problems. But the pretty-printing tests currently
+// can't contain compile errors.
+
+fn bare_fn(x: fn(u32, _: u32, a: u32)) { }
+
+extern "C" {
+    unsafe fn foreign_fn(_: u32, a: u32);
+}
+
+trait T {
+    fn trait_fn(u32, _: u32, a: u32);
+}
diff --git a/tests/pretty/hir-fn-params.rs b/tests/pretty/hir-fn-params.rs
new file mode 100644
index 00000000000..5ace5289d08
--- /dev/null
+++ b/tests/pretty/hir-fn-params.rs
@@ -0,0 +1,34 @@
+//@ pretty-compare-only
+//@ pretty-mode:hir
+//@ pp-exact:hir-fn-params.pp
+
+// This tests the pretty-printing of various kinds of function parameters.
+
+//---------------------------------------------------------------------------
+// Normal functions and methods.
+
+fn normal_fn(_: u32, a: u32) {}
+
+struct S;
+impl S {
+    fn method(_: u32, a: u32) {}
+}
+
+//---------------------------------------------------------------------------
+// More exotic forms, which get a different pretty-printing path. In the past,
+// anonymous params and `_` params printed incorrectly, e.g. `fn(u32, _: u32)`
+// was printed as `fn(: u32, : u32)`.
+//
+// Ideally we would also test invalid patterns, e.g. `fn(1: u32, &a: u32)`,
+// because they had similar problems. But the pretty-printing tests currently
+// can't contain compile errors.
+
+fn bare_fn(x: fn(u32, _: u32, a: u32)) {}
+
+extern "C" {
+    fn foreign_fn(_: u32, a: u32);
+}
+
+trait T {
+    fn trait_fn(u32, _: u32, a: u32);
+}
diff --git a/tests/ui/typeck/cyclic_type_ice.stderr b/tests/ui/typeck/cyclic_type_ice.stderr
index 4dc02a53c02..645766becbf 100644
--- a/tests/ui/typeck/cyclic_type_ice.stderr
+++ b/tests/ui/typeck/cyclic_type_ice.stderr
@@ -23,7 +23,7 @@ LL |     let f = |_, _| ();
 help: provide the argument
    |
 LL -     f(f);
-LL +     f(/*  */, /*  */);
+LL +     f(/* _ */, /* _ */);
    |
 
 error: aborting due to 2 previous errors
generated by cgit-pink 1.4.1-3-g733a5 (git 2.37.1) at 2026-04-25 06:11:16 +0000