Do not ICE on indirect fn pointers

Fixes #452.

Author: LegNeato

crates/rustc_codegen_spirv/src/linker/simple_passes.rs

@@ -237,10 +237,18 @@ pub fn check_fragment_insts(sess: &Session, module: &Module) -> super::Result<()
         let mut any_err = None;
         for inst in module.functions[index].all_inst_iter() {
             if inst.class.opcode == Op::FunctionCall {
-                let callee = func_id_to_idx[&inst.operands[0].unwrap_id_ref()];
-                let callee_had_err =
-                    visit(sess, module, visited, stack, names, callee, func_id_to_idx).err();
-                any_err = any_err.or(callee_had_err);
+                let callee_id = inst.operands[0].unwrap_id_ref();
+                if let Some(&callee) = func_id_to_idx.get(&callee_id) {
+                    if let Err(e) =
+                        visit(sess, module, visited, stack, names, callee, func_id_to_idx)
+                    {
+                        any_err = any_err.or(Some(e));
+                    }
+                } else {
+                    // Indirect or external callee: nothing to traverse.
+                    // Keep scanning this function for forbidden fragment ops.
+                    continue;
+                }
             }
             if matches!(
                 inst.class.opcode,

tests/compiletests/ui/lang/issue-452.rs

@@ -0,0 +1,18 @@
+// build-fail
+
+use spirv_std::spirv;
+
+#[derive(Clone, Copy)]
+struct Position(u32);
+
+fn use_cmp(cmp: fn(&Position) -> u32) {
+    let a = Position(0);
+    let b = Position(1);
+
+    let _ = if cmp(&a) <= cmp(&b) { a } else { b };
+}
+
+#[spirv(compute(threads(1)))]
+pub fn main() {
+    use_cmp(|p| p.0);
+}

tests/compiletests/ui/lang/issue-452.stderr

@@ -0,0 +1,47 @@
+error: function pointer types are not allowed
+  --> $DIR/issue-452.rs:8:1
+   |
+LL | fn use_cmp(cmp: fn(&Position) -> u32) {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: used by unnamed global (%10)
+note: used from within `issue_452::use_cmp`
+  --> $DIR/issue-452.rs:8:4
+   |
+LL | fn use_cmp(cmp: fn(&Position) -> u32) {
+   |    ^^^^^^^
+note: called by `issue_452::main`
+  --> $DIR/issue-452.rs:17:5
+   |
+LL |     use_cmp(|p| p.0);
+   |     ^^^^^^^^^^^^^^^^
+note: called by `main`
+  --> $DIR/issue-452.rs:16:8
+   |
+LL | pub fn main() {
+   |        ^^^^
+
+error: indirect calls are not supported in SPIR-V
+  --> $DIR/issue-452.rs:12:27
+   |
+LL |     let _ = if cmp(&a) <= cmp(&b) { a } else { b };
+   |                           ^^^^^^^
+   |
+note: used from within `issue_452::use_cmp`
+  --> $DIR/issue-452.rs:8:4
+   |
+LL | fn use_cmp(cmp: fn(&Position) -> u32) {
+   |    ^^^^^^^
+note: called by `issue_452::main`
+  --> $DIR/issue-452.rs:17:5
+   |
+LL |     use_cmp(|p| p.0);
+   |     ^^^^^^^^^^^^^^^^
+note: called by `main`
+  --> $DIR/issue-452.rs:16:8
+   |
+LL | pub fn main() {
+   |        ^^^^
+
+error: aborting due to 2 previous errors
+