Auto merge of #86164 - FabianWolff:issue-86053, r=davidtwco

Handle C-variadic arguments properly when reporting region errors

This pull request fixes #86053. The issue is that for a C-variadic function
```rust
#![feature(c_variadic)]
unsafe extern "C" fn foo(_: (), ...) {}
```
`foo`'s signature will contain only the first parameter (and have `c_variadic` set to `true`), whereas its body has a second argument (a `hir::Pat` for the `...`).

The code for reporting region errors iterates over the body's parameters and tries to fetch the corresponding parameter from the signature; this causes an out-of-bounds ICE for the `...` (though not in the example above, because there are no region errors to report).

I have simply restricted the iteration over the body parameters to exclude `...`, which is fine because `...` cannot cause a region error.

1 files changed, 34 insertions, 25 deletions

diff --git a/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/util.rs b/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/util.rs
index 17a56046a5c..8dcdd4b149e 100644
--- a/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/util.rs
+++ b/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/util.rs
@@ -56,33 +56,42 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
         let fn_decl = hir.fn_decl_by_hir_id(owner_id).unwrap();
         let poly_fn_sig = self.tcx().fn_sig(id);
         let fn_sig = self.tcx().liberate_late_bound_regions(id, poly_fn_sig);
-        body.params.iter().enumerate().find_map(|(index, param)| {
-            // May return None; sometimes the tables are not yet populated.
-            let ty = fn_sig.inputs()[index];
-            let mut found_anon_region = false;
-            let new_param_ty = self.tcx().fold_regions(ty, &mut false, |r, _| {
-                if *r == *anon_region {
-                    found_anon_region = true;
-                    replace_region
+        body.params
+            .iter()
+            .take(if fn_sig.c_variadic {
+                fn_sig.inputs().len()
+            } else {
+                assert_eq!(fn_sig.inputs().len(), body.params.len());
+                body.params.len()
+            })
+            .enumerate()
+            .find_map(|(index, param)| {
+                // May return None; sometimes the tables are not yet populated.
+                let ty = fn_sig.inputs()[index];
+                let mut found_anon_region = false;
+                let new_param_ty = self.tcx().fold_regions(ty, &mut false, |r, _| {
+                    if *r == *anon_region {
+                        found_anon_region = true;
+                        replace_region
+                    } else {
+                        r
+                    }
+                });
+                if found_anon_region {
+                    let ty_hir_id = fn_decl.inputs[index].hir_id;
+                    let param_ty_span = hir.span(ty_hir_id);
+                    let is_first = index == 0;
+                    Some(AnonymousParamInfo {
+                        param,
+                        param_ty: new_param_ty,
+                        param_ty_span,
+                        bound_region,
+                        is_first,
+                    })
                 } else {
-                    r
+                    None
                 }
-            });
-            if found_anon_region {
-                let ty_hir_id = fn_decl.inputs[index].hir_id;
-                let param_ty_span = hir.span(ty_hir_id);
-                let is_first = index == 0;
-                Some(AnonymousParamInfo {
-                    param,
-                    param_ty: new_param_ty,
-                    param_ty_span,
-                    bound_region,
-                    is_first,
-                })
-            } else {
-                None
-            }
-        })
+            })
     }
 
     pub(super) fn future_return_type(