Fix ICE when opaque captures a duplicated/invalid lifetime

Author: compiler-errors

compiler/rustc_ast_lowering/src/lib.rs

@@ -1845,11 +1845,11 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             GenericParamKind::Lifetime => {
                 // AST resolution emitted an error on those parameters, so we lower them using
                 // `ParamName::Error`.
+                let ident = self.lower_ident(param.ident);
                 let param_name =
                     if let Some(LifetimeRes::Error) = self.resolver.get_lifetime_res(param.id) {
-                        ParamName::Error
+                        ParamName::Error(ident)
                     } else {
-                        let ident = self.lower_ident(param.ident);
                         ParamName::Plain(ident)
                     };
                 let kind =

compiler/rustc_hir/src/hir.rs

@@ -52,6 +52,13 @@ pub enum ParamName {
     /// Some user-given name like `T` or `'x`.
     Plain(Ident),
 
+    /// Indicates an illegal name was given and an error has been
+    /// reported (so we should squelch other derived errors).
+    ///
+    /// Occurs when, e.g., `'_` is used in the wrong place, or a
+    /// lifetime name is duplicated.
+    Error(Ident),
+
     /// Synthetic name generated when user elided a lifetime in an impl header.
     ///
     /// E.g., the lifetimes in cases like these:
@@ -67,18 +74,13 @@ pub enum ParamName {
     /// where `'f` is something like `Fresh(0)`. The indices are
     /// unique per impl, but not necessarily continuous.
     Fresh,
-
-    /// Indicates an illegal name was given and an error has been
-    /// reported (so we should squelch other derived errors). Occurs
-    /// when, e.g., `'_` is used in the wrong place.
-    Error,
 }
 
 impl ParamName {
     pub fn ident(&self) -> Ident {
         match *self {
-            ParamName::Plain(ident) => ident,
-            ParamName::Fresh | ParamName::Error => Ident::with_dummy_span(kw::UnderscoreLifetime),
+            ParamName::Plain(ident) | ParamName::Error(ident) => ident,
+            ParamName::Fresh => Ident::with_dummy_span(kw::UnderscoreLifetime),
         }
     }
 }

compiler/rustc_hir/src/intravisit.rs

@@ -928,8 +928,8 @@ pub fn walk_generic_param<'v, V: Visitor<'v>>(
 ) -> V::Result {
     try_visit!(visitor.visit_id(param.hir_id));
     match param.name {
-        ParamName::Plain(ident) => try_visit!(visitor.visit_ident(ident)),
-        ParamName::Error | ParamName::Fresh => {}
+        ParamName::Plain(ident) | ParamName::Error(ident) => try_visit!(visitor.visit_ident(ident)),
+        ParamName::Fresh => {}
     }
     match param.kind {
         GenericParamKind::Lifetime { .. } => {}

compiler/rustc_hir_analysis/src/check/wfcheck.rs

@@ -2007,7 +2007,10 @@ fn check_variances_for_type_defn<'tcx>(
         }
 
         match hir_param.name {
-            hir::ParamName::Error => {}
+            hir::ParamName::Error(_) => {
+                // Don't report a bivariance error for a lifetime that isn't
+                // even valid to name.
+            }
             _ => {
                 let has_explicit_bounds = explicitly_bounded_params.contains(&parameter);
                 report_bivariance(tcx, hir_param, has_explicit_bounds, item);

tests/ui/impl-trait/captured-invalid-lifetime.rs

@@ -0,0 +1,18 @@
+// This uses edition 2024 for new lifetime capture rules.
+//@ edition: 2024
+
+// The problem here is that the presence of the opaque which captures all lifetimes in scope
+// means that the duplicated `'a` (which I'll call the dupe) is considered to be *early-bound*
+// since it shows up in the output but not the inputs. This is paired with the fact that we
+// were previously setting the name of the dupe to `'_` in the generic param definition, which
+// means that the identity args for the function were `['a#0, '_#1]` even though the lifetime
+// for the dupe should've been `'a#1`. This difference in symbol meant that NLL couldn't
+// actually match the lifetime against the identity lifeitmes, leading to an ICE.
+
+struct Foo<'a>(&'a ());
+
+impl<'a> Foo<'a> {
+    fn pass<'a>() -> impl Sized {}
+}
+
+fn main() {}