Show named lifetime in closure upvar diagnostics

[arferreira]

Fixes #153545.

When a closure captures a variable whose type involves a named lifetime from the parent function (e.g., 'a), the borrow checker's region renumbering creates a separate RegionVid for each occurrence of the erased lifetime in the closure's upvar type. These new RegionVids have no external_name, so diagnostics fall back to synthetic names like '1.

This PR recovers the original lifetime name by matching the closure's upvar type against the parent function's parameter type (obtained via liberate_late_bound_regions). Both types have the same structure but different region representations, the parent has ReLateParam/ReEarlyParam with real names, the closure has ReVar from renumbering. Walking them in parallel with for_each_free_region lets us find which named lifetime corresponds to the anonymous RegionVid.

Before:

error[E0716]: temporary value dropped while borrowed
 --> test.rs:11:21
  |
6 | fn apply<'a>(
  |          -- lifetime `'1` defined here

After:

error[E0716]: temporary value dropped while borrowed
 --> test.rs:11:21
  |
6 | fn apply<'a>(
  |          -- lifetime `'a` defined here

The fix gracefully falls back to the existing '1 naming when it can't match, nested closures, local variables (not parameters), destructuring patterns, or partial captures where the type structures diverge.

[rustbot]

r? @jieyouxu

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[jieyouxu]

Thanks, I think the change looks reasonable. There might be some false-matches, but the current diff is better than previous anyway.

@bors r+ rollup

View changes since this review

[jieyouxu]

Meta suggestion: for future PRs that does diagnostic changes like these, it's particularly helpful for the reviewer to evaluate the diagnostic difference if you can structure the PR into 2 commits:

1. Commit 1 adds the test with a blessed stderr that reflects the current diagnostics (i.e. before your changes)
2. Commit 2 contains your change and the diff in diagnostic output (i.e. after your change)

[jieyouxu]

FWIW, the diff is

diff --git b/tests/ui/borrowck/closure-upvar-named-lifetime.stderr a/tests/ui/borrowck/closure-upvar-named-lifetime.stderr
index 5c2e17ef177..fc7c59fa97c 100644
--- b/tests/ui/borrowck/closure-upvar-named-lifetime.stderr
+++ a/tests/ui/borrowck/closure-upvar-named-lifetime.stderr
@@ -1,8 +1,8 @@
 error[E0597]: `map` does not live long enough
   --> $DIR/closure-upvar-named-lifetime.rs:19:21
    |
-LL |     f: Arc<dyn Fn(Entry<'a, String, String>) + 'a>,
-   |     - lifetime `'1` appears in the type of `f`
+LL | fn apply<'a>(
+   |          -- lifetime `'a` defined here
 ...
 LL |     move |map| {
    |           --- binding `map` declared here
@@ -11,18 +11,18 @@ LL |         let value = map.borrow_mut().entry("foo".to_string());
    |                     ^^^ borrowed value does not live long enough
 ...
 LL |         f(wrapped_value);
-   |         ---------------- argument requires that `map` is borrowed for `'1`
+   |         ---------------- argument requires that `map` is borrowed for `'a`
 LL |     }
    |     - `map` dropped here while still borrowed
    |
-note: requirement that the value outlives `'1` introduced here
+note: requirement that the value outlives `'a` introduced here
   --> $SRC_DIR/core/src/ops/function.rs:LL:COL
 
 error[E0716]: temporary value dropped while borrowed
   --> $DIR/closure-upvar-named-lifetime.rs:19:21
    |
-LL |     f: Arc<dyn Fn(Entry<'a, String, String>) + 'a>,
-   |     - lifetime `'1` appears in the type of `f`
+LL | fn apply<'a>(
+   |          -- lifetime `'a` defined here
 ...
 LL |         let value = map.borrow_mut().entry("foo".to_string());
    |                     ^^^^^^^^^^^^^^^^                         - temporary value is freed at the end of this statement
@@ -30,9 +30,9 @@ LL |         let value = map.borrow_mut().entry("foo".to_string());
    |                     creates a temporary value which is freed while still in use
 ...
 LL |         f(wrapped_value);
-   |         ---------------- argument requires that borrow lasts for `'1`
+   |         ---------------- argument requires that borrow lasts for `'a`
    |
-note: requirement that the value outlives `'1` introduced here
+note: requirement that the value outlives `'a` introduced here
   --> $SRC_DIR/core/src/ops/function.rs:LL:COL
 
 error[E0700]: hidden type for `impl Fn(RefCell<HashMap<String, String>>)` captures lifetime that does not appear in bounds
@@ -60,8 +60,8 @@ LL | ) -> impl Fn(RefCell<HashMap<String, String>>) + use<'a>
 error[E0597]: `map` does not live long enough
   --> $DIR/closure-upvar-named-lifetime.rs:34:21
    |
-LL |     f: Box<dyn Fn(Entry<'a, String, String>) + 'a>,
-   |     - lifetime `'1` appears in the type of `f`
+LL | fn apply_box<'a>(
+   |              -- lifetime `'a` defined here
 ...
 LL |     move |map| {
    |           --- binding `map` declared here
@@ -70,15 +70,15 @@ LL |         let value = map.borrow_mut().entry("foo".to_string());
    |                     ^^^ borrowed value does not live long enough
 ...
 LL |         f(value);
-   |         -------- argument requires that `map` is borrowed for `'1`
+   |         -------- argument requires that `map` is borrowed for `'a`
 LL |     }
    |     - `map` dropped here while still borrowed
 
 error[E0716]: temporary value dropped while borrowed
   --> $DIR/closure-upvar-named-lifetime.rs:34:21
    |
-LL |     f: Box<dyn Fn(Entry<'a, String, String>) + 'a>,
-   |     - lifetime `'1` appears in the type of `f`
+LL | fn apply_box<'a>(
+   |              -- lifetime `'a` defined here
 ...
 LL |         let value = map.borrow_mut().entry("foo".to_string());
    |                     ^^^^^^^^^^^^^^^^                         - temporary value is freed at the end of this statement
@@ -86,7 +86,7 @@ LL |         let value = map.borrow_mut().entry("foo".to_string());
    |                     creates a temporary value which is freed while still in use
 ...
 LL |         f(value);
-   |         -------- argument requires that borrow lasts for `'1`
+   |         -------- argument requires that borrow lasts for `'a`
 
 error[E0700]: hidden type for `impl Fn(RefCell<HashMap<String, String>>)` captures lifetime that does not appear in bounds
   --> $DIR/closure-upvar-named-lifetime.rs:32:5

[rust-bors]

📌 Commit 99e1940 has been approved by jieyouxu

It is now in the queue for this repository.