[COMMITTED 12/25] gccrs: Fix ICE accessing empty vector without check

[arthur . cohen]

From: Philip Herron <herron.phi...@googlemail.com>

Fixes #2747

gcc/rust/ChangeLog:

        * typecheck/rust-tyty-subst.cc 
(SubstitutionRef::get_mappings_from_generic_args): fix

gcc/testsuite/ChangeLog:

        * rust/compile/issue-2747.rs: New test.
---
 gcc/rust/typecheck/rust-tyty-subst.cc    |  3 ++-
 gcc/testsuite/rust/compile/issue-2747.rs | 31 ++++++++++++++++++++++++
 2 files changed, 33 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/rust/compile/issue-2747.rs

diff --git a/gcc/rust/typecheck/rust-tyty-subst.cc 
b/gcc/rust/typecheck/rust-tyty-subst.cc
index 0a8340e317d..5a753566d48 100644
--- a/gcc/rust/typecheck/rust-tyty-subst.cc
+++ b/gcc/rust/typecheck/rust-tyty-subst.cc
@@ -660,7 +660,8 @@ SubstitutionRef::get_mappings_from_generic_args (
   if (args.get_type_args ().size () + offs > substitutions.size ())
     {
       rich_location r (line_table, args.get_locus ());
-      r.add_range (substitutions.front ().get_param_locus ());
+      if (!substitutions.empty ())
+       r.add_range (substitutions.front ().get_param_locus ());
 
       rust_error_at (
        r,
diff --git a/gcc/testsuite/rust/compile/issue-2747.rs 
b/gcc/testsuite/rust/compile/issue-2747.rs
new file mode 100644
index 00000000000..a9c09e7372f
--- /dev/null
+++ b/gcc/testsuite/rust/compile/issue-2747.rs
@@ -0,0 +1,31 @@
+#[lang = "sized"]
+pub trait Sized {}
+
+#[lang = "fn_once"]
+pub trait FnOnce<Args> {
+    #[lang = "fn_once_output"]
+    type Output;
+
+    extern "rust-call" fn call_once(self, args: Args) -> Self::Output;
+}
+
+struct Foo<'a, 'b: 'a> {
+    x: &'a i32,
+    y: &'a i32,
+    a: &'b i32,
+    q: &'a [&'b i32],
+}
+
+pub fn test<'x, 'y>(f: Foo<'x, 'y, ()>) {
+    // { dg-error "generic item takes at most 0 type arguments but 1 were 
supplied" "" { target *-*-* } .-1 }
+    let x = 5;
+    let y = 6;
+    let z = 7;
+    type F<'a, 'b> = fn(&'a i32, &'b i32) -> i32;
+    let f = Foo {
+        x: &x,
+        y: &y,
+        a: &z,
+        q: &[&x, &y],
+    };
+}
-- 
2.42.1