Add test of current behavior (infer free region within closure body)

src/test/ui/regions/issue-56537-closure-uses-region-from-container.rs

@@ -0,0 +1,74 @@
+// This is a collection of examples where a function's formal
+// parameter has an explicit lifetime and a closure within that
+// function returns that formal parameter. The closure's return type,
+// to be correctly inferred, needs to include the lifetime introduced
+// by the function.
+//
+// This works today, which precludes changing things so that closures
+// follow the same lifetime-elision rules used elsehwere. See
+// rust-lang/rust#56537
+
+// compile-pass
+// We are already testing NLL explicitly via the revision system below.
+// ignore-compare-mode-nll
+
+// revisions: ll nll migrate
+//[ll] compile-flags:-Zborrowck=ast
+//[nll] compile-flags:-Zborrowck=mir -Z two-phase-borrows
+//[migrate] compile-flags:-Zborrowck=migrate -Z two-phase-borrows
+
+fn willy_no_annot<'w>(p: &'w str, q: &str) -> &'w str {
+    let free_dumb = |_x| { p }; // no type annotation at all
+    let hello = format!("Hello");
+    free_dumb(&hello)
+}
+
+fn willy_ret_type_annot<'w>(p: &'w str, q: &str) -> &'w str {
+    let free_dumb = |_x| -> &str { p }; // type annotation on the return type
+    let hello = format!("Hello");
+    free_dumb(&hello)
+}
+
+fn willy_ret_region_annot<'w>(p: &'w str, q: &str) -> &'w str {
+    let free_dumb = |_x| -> &'w str { p }; // type+region annotation on return type
+    let hello = format!("Hello");
+    free_dumb(&hello)
+}
+
+fn willy_arg_type_ret_type_annot<'w>(p: &'w str, q: &str) -> &'w str {
+    let free_dumb = |_x: &str| -> &str { p }; // type annotation on arg and return types
+    let hello = format!("Hello");
+    free_dumb(&hello)
+}
+
+fn willy_arg_type_ret_region_annot<'w>(p: &'w str, q: &str) -> &'w str {
+    let free_dumb = |_x: &str| -> &'w str { p }; // fully annotated
+    let hello = format!("Hello");
+    free_dumb(&hello)
+}
+
+fn main() {
+    let world = format!("World");
+    let w1: &str = {
+        let hello = format!("He11o");
+        willy_no_annot(&world, &hello)
+    };
+    let w2: &str = {
+        let hello = format!("He22o");
+        willy_ret_type_annot(&world, &hello)
+    };
+    let w3: &str = {
+        let hello = format!("He33o");
+        willy_ret_region_annot(&world, &hello)
+    };
+    let w4: &str = {
+        let hello = format!("He44o");
+        willy_arg_type_ret_type_annot(&world, &hello)
+    };
+    let w5: &str = {
+        let hello = format!("He55o");
+        willy_arg_type_ret_region_annot(&world, &hello)
+    };
+    assert_eq!((w1, w2, w3, w4, w5),
+               ("World","World","World","World","World"));
+}