Rollup merge of rust-lang#57729 - pnkfelix:issue-55748-pat-types-are-constraints-on-bindings-too, r=nikomatsakis

extra testing of how NLL handles wildcard type `_`

test that wildcard type `_` is not duplicated by `type Foo<X> = (X, X);` and potentially instantiated at different types when used in type ascriptions in let bindings.

(NLL's handling of this for the type ascription *expression form* is currently broken, or at least differs from what AST-borrowck does. I'll file a separate bug about that. Its not something critical to address since that expression is guarded by `#![feature(type_ascription)]`.)

cc rust-lang#55748

Author: Centril

src/test/ui/nll/user-annotations/issue-55748-pat-types-constrain-bindings.rs

@@ -0,0 +1,70 @@
+// This test is ensuring that type ascriptions on let bindings
+// constrain both:
+//
+// 1. the input expression on the right-hand side (after any potential
+//    coercion, and allowing for covariance), *and*
+//
+// 2. the bindings (if any) nested within the pattern on the left-hand
+//    side (and here, the type-constraint is *invariant*).
+
+#![feature(nll)]
+
+#![allow(dead_code, unused_mut)]
+type PairUncoupled<'a, 'b, T> = (&'a T, &'b T);
+type PairCoupledRegions<'a, T> = (&'a T, &'a T);
+type PairCoupledTypes<T> = (T, T);
+
+fn uncoupled_lhs<'a>(_x: &'a u32, s: &'static u32) -> &'static u32 {
+    let ((mut y, mut _z),): (PairUncoupled<u32>,) = ((s, &_x),); // ok
+    // Above compiling does *not* imply below would compile.
+    // ::std::mem::swap(&mut y, &mut _z);
+    y
+}
+
+fn swap_regions((mut y, mut _z): PairCoupledRegions<u32>) {
+    ::std::mem::swap(&mut y, &mut _z);
+}
+
+fn coupled_regions_lhs<'a>(_x: &'a u32, s: &'static u32) -> &'static u32 {
+    let ((y, _z),): (PairCoupledRegions<u32>,) = ((s, &_x),);
+    // If above line compiled, so should line below ...
+
+    // swap_regions((y, _z));
+
+    // ... but the ascribed type also invalidates this use of `y`
+    y //~ ERROR lifetime may not live long enough
+}
+
+fn swap_types((mut y, mut _z): PairCoupledTypes<&u32>) {
+    ::std::mem::swap(&mut y, &mut _z);
+}
+
+fn coupled_types_lhs<'a>(_x: &'a u32, s: &'static u32) -> &'static u32 {
+    let ((y, _z),): (PairCoupledTypes<&u32>,) = ((s, &_x),);
+    // If above line compiled, so should line below ...
+
+    // swap_types((y, _z));
+
+    // ... but the ascribed type also invalidates this use of `y`
+    y //~ ERROR lifetime may not live long enough
+}
+
+fn swap_wilds((mut y, mut _z): PairCoupledTypes<&u32>) {
+    ::std::mem::swap(&mut y, &mut _z);
+}
+
+fn coupled_wilds_lhs<'a>(_x: &'a u32, s: &'static u32) -> &'static u32 {
+    let ((y, _z),): (PairCoupledTypes<_>,) = ((s, &_x),);
+    // If above line compiled, so should line below
+    // swap_wilds((y, _z));
+
+    // ... but the ascribed type also invalidates this use of `y`
+    y //~ ERROR lifetime may not live long enough
+}
+
+fn main() {
+    uncoupled_lhs(&3, &4);
+    coupled_regions_lhs(&3, &4);
+    coupled_types_lhs(&3, &4);
+    coupled_wilds_lhs(&3, &4);
+}

src/test/ui/nll/user-annotations/issue-55748-pat-types-constrain-bindings.stderr

@@ -0,0 +1,29 @@
+error: lifetime may not live long enough
+  --> $DIR/issue-55748-pat-types-constrain-bindings.rs:35:5
+   |
+LL | fn coupled_regions_lhs<'a>(_x: &'a u32, s: &'static u32) -> &'static u32 {
+   |                        -- lifetime `'a` defined here
+...
+LL |     y
+   |     ^ returning this value requires that `'a` must outlive `'static`
+
+error: lifetime may not live long enough
+  --> $DIR/issue-55748-pat-types-constrain-bindings.rs:49:5
+   |
+LL | fn coupled_types_lhs<'a>(_x: &'a u32, s: &'static u32) -> &'static u32 {
+   |                      -- lifetime `'a` defined here
+...
+LL |     y
+   |     ^ returning this value requires that `'a` must outlive `'static`
+
+error: lifetime may not live long enough
+  --> $DIR/issue-55748-pat-types-constrain-bindings.rs:62:5
+   |
+LL | fn coupled_wilds_lhs<'a>(_x: &'a u32, s: &'static u32) -> &'static u32 {
+   |                      -- lifetime `'a` defined here
+...
+LL |     y
+   |     ^ returning this value requires that `'a` must outlive `'static`
+
+error: aborting due to 3 previous errors
+