Note specific regions involved in 'borrowed data escapes' error

Fixes #67007

Currently, a 'borrowed data escapes' error does not mention
the specific lifetime involved (except indirectly through a suggestion
about adding a lifetime bound). We now explain the specific lifetime
relationship that failed to hold, which improves otherwise vague
error messages.

Author: Aaron1011

compiler/rustc_borrowck/src/diagnostics/region_errors.rs

@@ -498,6 +498,27 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
             diag.span_label(*span, format!("`{}` escapes the {} body here", fr_name, escapes_from));
         }
 
+        // Only show an extra note if we can find an 'error region' for both of the region
+        // variables. This avoids showing a noisy note that just mentions 'synthetic' regions
+        // that don't help the user understand the error.
+        if self.to_error_region(errci.fr).is_some()
+            && self.to_error_region(errci.outlived_fr).is_some()
+        {
+            let fr_region_name = self.give_region_a_name(errci.fr).unwrap();
+            fr_region_name.highlight_region_name(&mut diag);
+            let outlived_fr_region_name = self.give_region_a_name(errci.outlived_fr).unwrap();
+            outlived_fr_region_name.highlight_region_name(&mut diag);
+
+            diag.span_label(
+                *span,
+                format!(
+                    "{}requires that `{}` must outlive `{}`",
+                    category.description(),
+                    fr_region_name,
+                    outlived_fr_region_name,
+                ),
+            );
+        }
         diag
     }
 

src/test/ui/async-await/issues/issue-62097.nll.stderr

@@ -20,9 +20,15 @@ error[E0521]: borrowed data escapes outside of associated function
   --> $DIR/issue-62097.rs:13:9
    |
 LL |     pub async fn run_dummy_fn(&self) {
-   |                               ----- `self` is a reference that is only valid in the associated function body
+   |                               -----
+   |                               |
+   |                               `self` is a reference that is only valid in the associated function body
+   |                               let's call the lifetime of this reference `'1`
 LL |         foo(|| self.bar()).await;
-   |         ^^^^^^^^^^^^^^^^^^ `self` escapes the associated function body here
+   |         ^^^^^^^^^^^^^^^^^^
+   |         |
+   |         `self` escapes the associated function body here
+   |         argument requires that `'1` must outlive `'static`
 
 error: aborting due to 2 previous errors
 

src/test/ui/impl-header-lifetime-elision/dyn-trait.nll.stderr

@@ -2,9 +2,14 @@ error[E0521]: borrowed data escapes outside of function
   --> $DIR/dyn-trait.rs:20:5
    |
 LL | fn with_dyn_debug_static<'a>(x: Box<dyn Debug + 'a>) {
-   |                              - `x` is a reference that is only valid in the function body
+   |                          --  - `x` is a reference that is only valid in the function body
+   |                          |
+   |                          lifetime `'a` defined here
 LL |     static_val(x);
-   |     ^^^^^^^^^^^^^ `x` escapes the function body here
+   |     ^^^^^^^^^^^^^
+   |     |
+   |     `x` escapes the function body here
+   |     argument requires that `'a` must outlive `'static`
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-16683.nll.stderr

@@ -1,10 +1,19 @@
 error[E0521]: borrowed data escapes outside of associated function
   --> $DIR/issue-16683.rs:4:9
    |
+LL | trait T<'a> {
+   |         -- lifetime `'a` defined here
+LL |     fn a(&'a self) -> &'a bool;
 LL |     fn b(&self) {
-   |          ----- `self` is a reference that is only valid in the associated function body
+   |          -----
+   |          |
+   |          `self` is a reference that is only valid in the associated function body
+   |          let's call the lifetime of this reference `'1`
 LL |         self.a();
-   |         ^^^^^^^^ `self` escapes the associated function body here
+   |         ^^^^^^^^
+   |         |
+   |         `self` escapes the associated function body here
+   |         argument requires that `'1` must outlive `'a`
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-17758.nll.stderr

@@ -1,10 +1,19 @@
 error[E0521]: borrowed data escapes outside of associated function
   --> $DIR/issue-17758.rs:7:9
    |
+LL | trait Foo<'a> {
+   |           -- lifetime `'a` defined here
+LL |     fn foo(&'a self);
 LL |     fn bar(&self) {
-   |            ----- `self` is a reference that is only valid in the associated function body
+   |            -----
+   |            |
+   |            `self` is a reference that is only valid in the associated function body
+   |            let's call the lifetime of this reference `'1`
 LL |         self.foo();
-   |         ^^^^^^^^^^ `self` escapes the associated function body here
+   |         ^^^^^^^^^^
+   |         |
+   |         `self` escapes the associated function body here
+   |         argument requires that `'1` must outlive `'a`
 
 error: aborting due to previous error
 

src/test/ui/lifetimes/lifetime-bound-will-change-warning.nll.stderr

@@ -2,19 +2,29 @@ error[E0521]: borrowed data escapes outside of function
   --> $DIR/lifetime-bound-will-change-warning.rs:34:5
    |
 LL | fn test2<'a>(x: &'a Box<dyn Fn() + 'a>) {
-   |              - `x` is a reference that is only valid in the function body
+   |          --  - `x` is a reference that is only valid in the function body
+   |          |
+   |          lifetime `'a` defined here
 LL |     // but ref_obj will not, so warn.
 LL |     ref_obj(x)
-   |     ^^^^^^^^^^ `x` escapes the function body here
+   |     ^^^^^^^^^^
+   |     |
+   |     `x` escapes the function body here
+   |     argument requires that `'a` must outlive `'static`
 
 error[E0521]: borrowed data escapes outside of function
   --> $DIR/lifetime-bound-will-change-warning.rs:39:5
    |
 LL | fn test2cc<'a>(x: &'a Box<dyn Fn() + 'a>) {
-   |                - `x` is a reference that is only valid in the function body
+   |            --  - `x` is a reference that is only valid in the function body
+   |            |
+   |            lifetime `'a` defined here
 LL |     // same as test2, but cross crate
 LL |     lib::ref_obj(x)
-   |     ^^^^^^^^^^^^^^^ `x` escapes the function body here
+   |     ^^^^^^^^^^^^^^^
+   |     |
+   |     `x` escapes the function body here
+   |     argument requires that `'a` must outlive `'static`
 
 error: aborting due to 2 previous errors
 

src/test/ui/nll/closure-requirements/propagate-approximated-shorter-to-static-no-bound.stderr

@@ -38,14 +38,19 @@ error[E0521]: borrowed data escapes outside of function
   --> $DIR/propagate-approximated-shorter-to-static-no-bound.rs:32:5
    |
 LL |   fn supply<'a, 'b>(cell_a: Cell<&'a u32>, cell_b: Cell<&'b u32>) {
-   |                     ------ `cell_a` is a reference that is only valid in the function body
+   |             --      ------ `cell_a` is a reference that is only valid in the function body
+   |             |
+   |             lifetime `'a` defined here
 LL | /     establish_relationships(&cell_a, &cell_b, |_outlives, x, y| {
 LL | |
 LL | |
 LL | |         // Only works if 'x: 'y:
 LL | |         demand_y(x, y, x.get())
 LL | |     });
-   | |______^ `cell_a` escapes the function body here
+   | |      ^
+   | |      |
+   | |______`cell_a` escapes the function body here
+   |        argument requires that `'a` must outlive `'static`
 
 error: aborting due to previous error
 

src/test/ui/nll/closure-requirements/propagate-approximated-shorter-to-static-wrong-bound.stderr

@@ -38,14 +38,19 @@ error[E0521]: borrowed data escapes outside of function
   --> $DIR/propagate-approximated-shorter-to-static-wrong-bound.rs:35:5
    |
 LL |   fn supply<'a, 'b>(cell_a: Cell<&'a u32>, cell_b: Cell<&'b u32>) {
-   |                     ------ `cell_a` is a reference that is only valid in the function body
+   |             --      ------ `cell_a` is a reference that is only valid in the function body
+   |             |
+   |             lifetime `'a` defined here
 LL | /     establish_relationships(&cell_a, &cell_b, |_outlives1, _outlives2, x, y| {
 LL | |
 LL | |
 LL | |         // Only works if 'x: 'y:
 LL | |         demand_y(x, y, x.get())
 LL | |     });
-   | |______^ `cell_a` escapes the function body here
+   | |      ^
+   | |      |
+   | |______`cell_a` escapes the function body here
+   |        argument requires that `'a` must outlive `'static`
 
 error: aborting due to previous error
 

src/test/ui/nll/issue-67007-escaping-data.rs

@@ -0,0 +1,26 @@
+// Regression test for issue #67007
+// Ensures that we show information about the specific regions involved
+
+#![feature(nll)]
+
+// Covariant over 'a, invariant over 'tcx
+struct FnCtxt<'a, 'tcx: 'a>(&'a (), *mut &'tcx ());
+
+impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
+    fn use_it(&self, _: &'tcx ()) {}
+}
+
+struct Consumer<'tcx>(&'tcx ());
+
+impl<'tcx> Consumer<'tcx> {
+    fn bad_method<'a>(&self, fcx: &FnCtxt<'a, 'tcx>) {
+        let other = self.use_fcx(fcx); //~ ERROR borrowed data
+        fcx.use_it(other);
+    }
+
+    fn use_fcx<'a>(&self, _: &FnCtxt<'a, 'tcx>) -> &'a () {
+        &()
+    }
+}
+
+fn main() {}

src/test/ui/nll/issue-67007-escaping-data.stderr

@@ -0,0 +1,21 @@
+error[E0521]: borrowed data escapes outside of associated function
+  --> $DIR/issue-67007-escaping-data.rs:17:21
+   |
+LL | impl<'tcx> Consumer<'tcx> {
+   |      ---- lifetime `'tcx` defined here
+LL |     fn bad_method<'a>(&self, fcx: &FnCtxt<'a, 'tcx>) {
+   |                   --  -----  --- `fcx` is a reference that is only valid in the associated function body
+   |                   |   |
+   |                   |   `self` declared here, outside of the associated function body
+   |                   lifetime `'a` defined here
+LL |         let other = self.use_fcx(fcx);
+   |                     ^^^^^^^^^^^^^^^^^
+   |                     |
+   |                     `fcx` escapes the associated function body here
+   |                     argument requires that `'a` must outlive `'tcx`
+   |
+   = help: consider adding the following bound: `'a: 'tcx`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0521`.