Rollup merge of rust-lang#56479 - mark-i-m:unsat, r=estebank

Better lifetime error message

I propose the following error message as more user-friendly

r? @nikomatsakis

src/librustc_mir/borrow_check/nll/region_infer/error_reporting/mod.rs

@@ -505,7 +505,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
     ) {
         let mut diag = infcx.tcx.sess.struct_span_err(
             span,
-            "unsatisfied lifetime constraints", // FIXME
+            "lifetime may not live long enough"
         );
 
         let counter = &mut 1;

src/test/ui/nll/closure-requirements/escape-argument-callee.stderr

@@ -9,7 +9,7 @@ LL |         let mut closure = expect_sig(|p, y| *p = y);
                for<'r, 's, 't0> extern "rust-call" fn((&ReLateBound(DebruijnIndex(0), BrNamed(crate0:DefIndex(0:0), 'r)) mut &ReLateBound(DebruijnIndex(0), BrNamed(crate0:DefIndex(0:0), 's)) i32, &ReLateBound(DebruijnIndex(0), BrNamed(crate0:DefIndex(0:0), 't0)) i32))
            ]
 
-error: unsatisfied lifetime constraints
+error: lifetime may not live long enough
   --> $DIR/escape-argument-callee.rs:26:45
    |
 LL |         let mut closure = expect_sig(|p, y| *p = y);

src/test/ui/nll/closure-requirements/propagate-approximated-fail-no-postdom.stderr

@@ -16,7 +16,7 @@ LL | |         },
    = note: late-bound region is '_#5r
    = note: late-bound region is '_#6r
 
-error: unsatisfied lifetime constraints
+error: lifetime may not live long enough
   --> $DIR/propagate-approximated-fail-no-postdom.rs:46:13
    |
 LL |         |_outlives1, _outlives2, _outlives3, x, y| {

src/test/ui/nll/closure-requirements/propagate-approximated-ref.rs

@@ -41,10 +41,9 @@ fn demand_y<'x, 'y>(_cell_x: &Cell<&'x u32>, _cell_y: &Cell<&'y u32>, _y: &'y u3
 #[rustc_regions]
 fn supply<'a, 'b>(cell_a: Cell<&'a u32>, cell_b: Cell<&'b u32>) {
     establish_relationships(&cell_a, &cell_b, |_outlives1, _outlives2, x, y| {
-
         // Only works if 'x: 'y:
         demand_y(x, y, x.get())
-        //~^ ERROR unsatisfied lifetime constraints
+        //~^ ERROR lifetime may not live long enough
     });
 }
 

src/test/ui/nll/closure-requirements/propagate-approximated-ref.stderr

@@ -3,10 +3,9 @@ note: External requirements
    |
 LL |       establish_relationships(&cell_a, &cell_b, |_outlives1, _outlives2, x, y| {
    |  _______________________________________________^
-LL | |
 LL | |         // Only works if 'x: 'y:
 LL | |         demand_y(x, y, x.get())
-LL | |         //~^ ERROR unsatisfied lifetime constraints
+LL | |         //~^ ERROR lifetime may not live long enough
 LL | |     });
    | |_____^
    |
@@ -24,17 +23,17 @@ note: No external requirements
    |
 LL | / fn supply<'a, 'b>(cell_a: Cell<&'a u32>, cell_b: Cell<&'b u32>) {
 LL | |     establish_relationships(&cell_a, &cell_b, |_outlives1, _outlives2, x, y| {
-LL | |
 LL | |         // Only works if 'x: 'y:
-...  |
+LL | |         demand_y(x, y, x.get())
+LL | |         //~^ ERROR lifetime may not live long enough
 LL | |     });
 LL | | }
    | |_^
    |
    = note: defining type: DefId(0/0:6 ~ propagate_approximated_ref[317d]::supply[0]) with substs []
 
-error: unsatisfied lifetime constraints
-  --> $DIR/propagate-approximated-ref.rs:46:9
+error: lifetime may not live long enough
+  --> $DIR/propagate-approximated-ref.rs:45:9
    |
 LL | fn supply<'a, 'b>(cell_a: Cell<&'a u32>, cell_b: Cell<&'b u32>) {
    |           --  -- lifetime `'b` defined here

src/test/ui/nll/closure-requirements/propagate-approximated-val.rs

@@ -34,10 +34,9 @@ fn demand_y<'x, 'y>(_outlives1: Cell<&&'x u32>, _outlives2: Cell<&'y &u32>, _y:
 #[rustc_regions]
 fn test<'a, 'b>(cell_a: Cell<&'a u32>, cell_b: Cell<&'b u32>) {
     establish_relationships(cell_a, cell_b, |outlives1, outlives2, x, y| {
-
         // Only works if 'x: 'y:
         demand_y(outlives1, outlives2, x.get())
-        //~^ ERROR unsatisfied lifetime constraints
+        //~^ ERROR lifetime may not live long enough
     });
 }
 

src/test/ui/nll/closure-requirements/propagate-approximated-val.stderr

@@ -3,10 +3,9 @@ note: External requirements
    |
 LL |       establish_relationships(cell_a, cell_b, |outlives1, outlives2, x, y| {
    |  _____________________________________________^
-LL | |
 LL | |         // Only works if 'x: 'y:
 LL | |         demand_y(outlives1, outlives2, x.get())
-LL | |         //~^ ERROR unsatisfied lifetime constraints
+LL | |         //~^ ERROR lifetime may not live long enough
 LL | |     });
    | |_____^
    |
@@ -24,17 +23,17 @@ note: No external requirements
    |
 LL | / fn test<'a, 'b>(cell_a: Cell<&'a u32>, cell_b: Cell<&'b u32>) {
 LL | |     establish_relationships(cell_a, cell_b, |outlives1, outlives2, x, y| {
-LL | |
 LL | |         // Only works if 'x: 'y:
-...  |
+LL | |         demand_y(outlives1, outlives2, x.get())
+LL | |         //~^ ERROR lifetime may not live long enough
 LL | |     });
 LL | | }
    | |_^
    |
    = note: defining type: DefId(0/0:6 ~ propagate_approximated_val[317d]::test[0]) with substs []
 
-error: unsatisfied lifetime constraints
-  --> $DIR/propagate-approximated-val.rs:39:9
+error: lifetime may not live long enough
+  --> $DIR/propagate-approximated-val.rs:38:9
    |
 LL | fn test<'a, 'b>(cell_a: Cell<&'a u32>, cell_b: Cell<&'b u32>) {
    |         --  -- lifetime `'b` defined here

src/test/ui/nll/closure-requirements/propagate-fail-to-approximate-longer-no-bounds.stderr

@@ -16,7 +16,7 @@ LL | |     });
    = note: late-bound region is '_#2r
    = note: late-bound region is '_#3r
 
-error: unsatisfied lifetime constraints
+error: lifetime may not live long enough
   --> $DIR/propagate-fail-to-approximate-longer-no-bounds.rs:37:9
    |
 LL |     establish_relationships(&cell_a, &cell_b, |_outlives, x, y| {

src/test/ui/nll/closure-requirements/propagate-fail-to-approximate-longer-wrong-bounds.stderr

@@ -16,7 +16,7 @@ LL | |     });
    = note: late-bound region is '_#3r
    = note: late-bound region is '_#4r
 
-error: unsatisfied lifetime constraints
+error: lifetime may not live long enough
   --> $DIR/propagate-fail-to-approximate-longer-wrong-bounds.rs:41:9
    |
 LL |     establish_relationships(&cell_a, &cell_b, |_outlives1, _outlives2, x, y| {

src/test/ui/nll/closure-requirements/region-lbr-named-does-not-outlive-static.stderr

@@ -1,4 +1,4 @@
-error: unsatisfied lifetime constraints
+error: lifetime may not live long enough
   --> $DIR/region-lbr-named-does-not-outlive-static.rs:9:5
    |
 LL | fn foo<'a>(x: &'a u32) -> &'static u32 {

src/test/ui/nll/closure-requirements/region-lbr1-does-not-outlive-ebr2.rs

@@ -7,7 +7,7 @@
 
 fn foo<'a, 'b>(x: &'a u32, y: &'b u32) -> &'b u32 {
     &*x
-        //~^ ERROR unsatisfied lifetime constraints
+    //~^ ERROR lifetime may not live long enough
 }
 
-fn main() { }
+fn main() {}

src/test/ui/nll/closure-requirements/region-lbr1-does-not-outlive-ebr2.stderr

@@ -1,4 +1,4 @@
-error: unsatisfied lifetime constraints
+error: lifetime may not live long enough
   --> $DIR/region-lbr1-does-not-outlive-ebr2.rs:9:5
    |
 LL | fn foo<'a, 'b>(x: &'a u32, y: &'b u32) -> &'b u32 {

src/test/ui/nll/closure-requirements/return-wrong-bound-region.stderr

@@ -9,7 +9,7 @@ LL |     expect_sig(|a, b| b); // ought to return `a`
                for<'r, 's> extern "rust-call" fn((&ReLateBound(DebruijnIndex(0), BrNamed(crate0:DefIndex(0:0), 'r)) i32, &ReLateBound(DebruijnIndex(0), BrNamed(crate0:DefIndex(0:0), 's)) i32)) -> &ReLateBound(DebruijnIndex(0), BrNamed(crate0:DefIndex(0:0), 'r)) i32
            ]
 
-error: unsatisfied lifetime constraints
+error: lifetime may not live long enough
   --> $DIR/return-wrong-bound-region.rs:11:23
    |
 LL |     expect_sig(|a, b| b); // ought to return `a`

src/test/ui/nll/issue-48238.stderr

@@ -1,4 +1,4 @@
-error: unsatisfied lifetime constraints
+error: lifetime may not live long enough
   --> $DIR/issue-48238.rs:11:13
    |
 LL |     move || use_val(&orig); //~ ERROR

src/test/ui/nll/issue-50716.stderr

@@ -1,4 +1,4 @@
-error: unsatisfied lifetime constraints
+error: lifetime may not live long enough
   --> $DIR/issue-50716.rs:16:14
    |
 LL | fn foo<'a, T: 'static>(s: Box<<&'a T as A>::X>)

src/test/ui/nll/issue-52113.rs

@@ -3,8 +3,8 @@
 #![allow(warnings)]
 #![feature(nll)]
 
-trait Bazinga { }
-impl<F> Bazinga for F { }
+trait Bazinga {}
+impl<F> Bazinga for F {}
 
 fn produce1<'a>(data: &'a u32) -> impl Bazinga + 'a {
     let x = move || {
@@ -21,7 +21,6 @@ fn produce2<'a>(data: &'a mut Vec<&'a u32>, value: &'a u32) -> impl Bazinga + 'a
     x
 }
 
-
 fn produce3<'a, 'b: 'a>(data: &'a mut Vec<&'a u32>, value: &'b u32) -> impl Bazinga + 'a {
     let x = move || {
         let value: &'a u32 = value;
@@ -35,7 +34,7 @@ fn produce_err<'a, 'b: 'a>(data: &'b mut Vec<&'b u32>, value: &'a u32) -> impl B
         let value: &'a u32 = value;
         data.push(value);
     };
-    x   //~ ERROR unsatisfied lifetime constraints
+    x //~ ERROR lifetime may not live long enough
 }
 
-fn main() { }
+fn main() {}

src/test/ui/nll/issue-52113.stderr

@@ -1,12 +1,12 @@
-error: unsatisfied lifetime constraints
-  --> $DIR/issue-52113.rs:38:5
+error: lifetime may not live long enough
+  --> $DIR/issue-52113.rs:37:5
    |
 LL | fn produce_err<'a, 'b: 'a>(data: &'b mut Vec<&'b u32>, value: &'a u32) -> impl Bazinga + 'b {
    |                --  -- lifetime `'b` defined here
    |                |
    |                lifetime `'a` defined here
 ...
-LL |     x   //~ ERROR unsatisfied lifetime constraints
+LL |     x //~ ERROR lifetime may not live long enough
    |     ^ returning this value requires that `'a` must outlive `'b`
 
 error: aborting due to previous error

src/test/ui/nll/issue-52742.rs

@@ -7,14 +7,14 @@ struct Foo<'a, 'b> {
 }
 
 struct Bar<'b> {
-    z: &'b u32
+    z: &'b u32,
 }
 
 impl Foo<'_, '_> {
     fn take_bar(&mut self, b: Bar<'_>) {
         self.y = b.z
-        //~^ ERROR unsatisfied lifetime constraints
+        //~^ ERROR lifetime may not live long enough
     }
 }
 
-fn main() { }
+fn main() {}

src/test/ui/nll/issue-52742.stderr

@@ -1,4 +1,4 @@
-error: unsatisfied lifetime constraints
+error: lifetime may not live long enough
   --> $DIR/issue-52742.rs:15:9
    |
 LL |     fn take_bar(&mut self, b: Bar<'_>) {

src/test/ui/nll/issue-55394.rs

@@ -8,7 +8,7 @@ struct Foo<'s> {
 
 impl Foo<'_> {
     fn new(bar: &mut Bar) -> Self {
-        Foo { bar } //~ ERROR unsatisfied lifetime constraints
+        Foo { bar } //~ERROR lifetime may not live long enough
     }
 }
 

src/test/ui/nll/issue-55394.stderr

@@ -1,11 +1,11 @@
-error: unsatisfied lifetime constraints
+error: lifetime may not live long enough
   --> $DIR/issue-55394.rs:11:9
    |
 LL |     fn new(bar: &mut Bar) -> Self {
    |                 -            ---- return type is Foo<'2>
    |                 |
    |                 let's call the lifetime of this reference `'1`
-LL |         Foo { bar } //~ ERROR unsatisfied lifetime constraints
+LL |         Foo { bar } //~ERROR lifetime may not live long enough
    |         ^^^^^^^^^^^ returning this value requires that `'1` must outlive `'2`
 
 error: aborting due to previous error

src/test/ui/nll/mir_check_cast_closure.rs

@@ -5,7 +5,7 @@
 fn bar<'a, 'b>() -> fn(&'a u32, &'b u32) -> &'a u32 {
     let g: fn(_, _) -> _ = |_x, y| y;
     g
-    //~^ ERROR unsatisfied lifetime constraints
+    //~^ ERROR lifetime may not live long enough
 }
 
 fn main() {}

src/test/ui/nll/mir_check_cast_closure.stderr

@@ -1,4 +1,4 @@
-error: unsatisfied lifetime constraints
+error: lifetime may not live long enough
   --> $DIR/mir_check_cast_closure.rs:7:5
    |
 LL | fn bar<'a, 'b>() -> fn(&'a u32, &'b u32) -> &'a u32 {

src/test/ui/nll/mir_check_cast_reify.rs

@@ -35,7 +35,7 @@ fn bar<'a>(x: &'a u32) -> &'static u32 {
     // as part of checking the `ReifyFnPointer`.
     let f: fn(_) -> _ = foo;
     f(x)
-    //~^ ERROR unsatisfied lifetime constraints
+    //~^ ERROR lifetime may not live long enough
 }
 
 fn main() {}

src/test/ui/nll/mir_check_cast_reify.stderr

@@ -1,4 +1,4 @@
-error: unsatisfied lifetime constraints
+error: lifetime may not live long enough
   --> $DIR/mir_check_cast_reify.rs:37:5
    |
 LL | fn bar<'a>(x: &'a u32) -> &'static u32 {

src/test/ui/nll/mir_check_cast_unsafe_fn.rs

@@ -7,7 +7,7 @@ fn bar<'a>(input: &'a u32, f: fn(&'a u32) -> &'a u32) -> &'static u32 {
     // in `g`. These are related via the `UnsafeFnPointer` cast.
     let g: unsafe fn(_) -> _ = f;
     unsafe { g(input) }
-    //~^ ERROR unsatisfied lifetime constraints
+    //~^ ERROR lifetime may not live long enough
 }
 
 fn main() {}

src/test/ui/nll/mir_check_cast_unsafe_fn.stderr

@@ -1,4 +1,4 @@
-error: unsatisfied lifetime constraints
+error: lifetime may not live long enough
   --> $DIR/mir_check_cast_unsafe_fn.rs:9:14
    |
 LL | fn bar<'a>(input: &'a u32, f: fn(&'a u32) -> &'a u32) -> &'static u32 {

src/test/ui/nll/mir_check_cast_unsize.rs

@@ -6,7 +6,7 @@ use std::fmt::Debug;
 
 fn bar<'a>(x: &'a u32) -> &'static dyn Debug {
     x
-    //~^ ERROR unsatisfied lifetime constraints
+    //~^ ERROR lifetime may not live long enough
 }
 
 fn main() {}

src/test/ui/nll/mir_check_cast_unsize.stderr

@@ -1,4 +1,4 @@
-error: unsatisfied lifetime constraints
+error: lifetime may not live long enough
   --> $DIR/mir_check_cast_unsize.rs:8:5
    |
 LL | fn bar<'a>(x: &'a u32) -> &'static dyn Debug {

src/test/ui/nll/ty-outlives/projection-one-region-closure.stderr

@@ -40,7 +40,7 @@ LL |     with_signature(cell, t, |cell, t| require(cell, t));
    |
    = help: consider adding an explicit lifetime bound `T: ReFree(DefId(0/0:8 ~ projection_one_region_closure[317d]::no_relationships_late[0]), BrNamed(crate0:DefIndex(1:16), 'a))`...
 
-error: unsatisfied lifetime constraints
+error: lifetime may not live long enough
   --> $DIR/projection-one-region-closure.rs:45:39
    |
 LL | fn no_relationships_late<'a, 'b, T>(cell: Cell<&'a ()>, t: T)
@@ -94,7 +94,7 @@ LL |     with_signature(cell, t, |cell, t| require(cell, t));
    |
    = help: consider adding an explicit lifetime bound `T: ReEarlyBound(0, 'a)`...
 
-error: unsatisfied lifetime constraints
+error: lifetime may not live long enough
   --> $DIR/projection-one-region-closure.rs:56:39
    |
 LL | fn no_relationships_early<'a, 'b, T>(cell: Cell<&'a ()>, t: T)

src/test/ui/nll/ty-outlives/projection-one-region-trait-bound-closure.stderr

@@ -31,7 +31,7 @@ LL | | }
                T
            ]
 
-error: unsatisfied lifetime constraints
+error: lifetime may not live long enough
   --> $DIR/projection-one-region-trait-bound-closure.rs:37:39
    |
 LL | fn no_relationships_late<'a, 'b, T>(cell: Cell<&'a ()>, t: T)
@@ -76,7 +76,7 @@ LL | | }
                T
            ]
 
-error: unsatisfied lifetime constraints
+error: lifetime may not live long enough
   --> $DIR/projection-one-region-trait-bound-closure.rs:47:39
    |
 LL | fn no_relationships_early<'a, 'b, T>(cell: Cell<&'a ()>, t: T)

src/test/ui/nll/ty-outlives/projection-two-region-trait-bound-closure.rs

@@ -85,7 +85,7 @@ where
     T: Anything<'b, 'b>,
 {
     with_signature(cell, t, |cell, t| require(cell, t));
-    //~^ ERROR unsatisfied lifetime constraints
+    //~^ ERROR lifetime may not live long enough
 }
 
 #[rustc_regions]

src/test/ui/nll/ty-outlives/projection-two-region-trait-bound-closure.stderr

@@ -217,7 +217,7 @@ LL | | where
 LL | |     T: Anything<'b, 'b>,
 LL | | {
 LL | |     with_signature(cell, t, |cell, t| require(cell, t));
-LL | |     //~^ ERROR unsatisfied lifetime constraints
+LL | |     //~^ ERROR lifetime may not live long enough
 LL | | }
    | |_^
    |
@@ -226,7 +226,7 @@ LL | | }
                T
            ]
 
-error: unsatisfied lifetime constraints
+error: lifetime may not live long enough
   --> $DIR/projection-two-region-trait-bound-closure.rs:87:29
    |
 LL | fn two_regions<'a, 'b, T>(cell: Cell<&'a ()>, t: T)