best_blame_constraint: avoid blaming constraints from MIR generated by desugaring

compiler/rustc_borrowck/src/region_infer/mod.rs

@@ -22,6 +22,7 @@ use rustc_middle::ty::fold::fold_regions;
 use rustc_middle::ty::{self, RegionVid, Ty, TyCtxt, TypeFoldable, UniverseIndex};
 use rustc_mir_dataflow::points::DenseLocationMap;
 use rustc_span::Span;
+use rustc_span::hygiene::DesugaringKind;
 use tracing::{debug, instrument, trace};
 
 use crate::BorrowckInferCtxt;
@@ -2033,7 +2034,13 @@ impl<'tcx> RegionInferenceContext<'tcx> {
                     | ConstraintCategory::BoringNoLocation
                     | ConstraintCategory::Internal
                     | ConstraintCategory::Predicate(_)
-            )
+            ) && constraint.span.desugaring_kind().is_none_or(|kind| {
+                // Try to avoid blaming constraints from desugarings, since they may not clearly
+                // clearly match what users have written. As an exception, allow blaming returns
+                // generated by `?` desugaring, since the correspondence is fairly clear.
+                kind == DesugaringKind::QuestionMark
+                    && matches!(constraint.category, ConstraintCategory::Return(_))
+            })
         };
 
         let best_choice = if blame_source {

tests/ui/async-await/issue-76547.rs

@@ -17,8 +17,8 @@ impl<'a> Future for ListFut<'a> {
 }
 
 async fn fut(bufs: &mut [&mut [u8]]) {
-    //~^ ERROR lifetime may not live long enough
     ListFut(bufs).await
+    //~^ ERROR lifetime may not live long enough
 }
 
 pub struct ListFut2<'a>(&'a mut [&'a mut [u8]]);
@@ -31,8 +31,8 @@ impl<'a> Future for ListFut2<'a> {
 }
 
 async fn fut2(bufs: &mut [&mut [u8]]) -> i32 {
-    //~^ ERROR lifetime may not live long enough
     ListFut2(bufs).await
+    //~^ ERROR lifetime may not live long enough
 }
 
 fn main() {}

tests/ui/async-await/issue-76547.stderr

@@ -1,11 +1,12 @@
 error: lifetime may not live long enough
-  --> $DIR/issue-76547.rs:19:14
+  --> $DIR/issue-76547.rs:20:13
    |
 LL | async fn fut(bufs: &mut [&mut [u8]]) {
-   |              ^^^^  -     - let's call the lifetime of this reference `'2`
+   |                    -     - let's call the lifetime of this reference `'2`
-   |              |     |
+   |                    |
-   |              |     let's call the lifetime of this reference `'1`
+   |                    let's call the lifetime of this reference `'1`
-   |              assignment requires that `'1` must outlive `'2`
+LL |     ListFut(bufs).await
+   |             ^^^^ this usage requires that `'1` must outlive `'2`
    |
 help: consider introducing a named lifetime parameter
    |
@@ -13,13 +14,14 @@ LL | async fn fut<'a>(bufs: &'a mut [&'a mut [u8]]) {
    |             ++++        ++       ++
 
 error: lifetime may not live long enough
-  --> $DIR/issue-76547.rs:33:15
+  --> $DIR/issue-76547.rs:34:14
    |
 LL | async fn fut2(bufs: &mut [&mut [u8]]) -> i32 {
-   |               ^^^^  -     - let's call the lifetime of this reference `'2`
+   |                     -     - let's call the lifetime of this reference `'2`
-   |               |     |
+   |                     |
-   |               |     let's call the lifetime of this reference `'1`
+   |                     let's call the lifetime of this reference `'1`
-   |               assignment requires that `'1` must outlive `'2`
+LL |     ListFut2(bufs).await
+   |              ^^^^ this usage requires that `'1` must outlive `'2`
    |
 help: consider introducing a named lifetime parameter
    |

tests/ui/async-await/issues/issue-63388-1.rs

@@ -11,8 +11,8 @@ impl Xyz {
         &'a self, foo: &dyn Foo
     ) -> &dyn Foo  //~ WARNING elided lifetime has a name
     {
-        //~^ ERROR explicit lifetime required in the type of `foo` [E0621]
         foo
+        //~^ ERROR explicit lifetime required in the type of `foo` [E0621]
     }
 }
 

tests/ui/async-await/issues/issue-63388-1.stderr

@@ -10,16 +10,13 @@ LL |     ) -> &dyn Foo
    = note: `#[warn(elided_named_lifetimes)]` on by default
 
 error[E0621]: explicit lifetime required in the type of `foo`
-  --> $DIR/issue-63388-1.rs:13:5
+  --> $DIR/issue-63388-1.rs:14:9
    |
-LL |           &'a self, foo: &dyn Foo
+LL |         &'a self, foo: &dyn Foo
-   |                          -------- help: add explicit lifetime `'a` to the type of `foo`: `&'a (dyn Foo + 'a)`
+   |                        -------- help: add explicit lifetime `'a` to the type of `foo`: `&'a (dyn Foo + 'a)`
-LL |       ) -> &dyn Foo
+...
-LL | /     {
+LL |         foo
-LL | |
+   |         ^^^ lifetime `'a` required
-LL | |         foo
-LL | |     }
-   | |_____^ lifetime `'a` required
 
 error: aborting due to 1 previous error; 1 warning emitted
 

tests/ui/async-await/issues/issue-63388-2.rs

@@ -11,8 +11,8 @@ impl Xyz {
         foo: &dyn Foo, bar: &'a dyn Foo
     ) -> &dyn Foo //~ ERROR missing lifetime specifier
     {
-        //~^ ERROR explicit lifetime required in the type of `foo` [E0621]
         foo
+        //~^ ERROR explicit lifetime required in the type of `foo` [E0621]
     }
 }
 

tests/ui/async-await/issues/issue-63388-2.stderr

@@ -13,16 +13,13 @@ LL |     ) -> &'a dyn Foo
    |           ++
 
 error[E0621]: explicit lifetime required in the type of `foo`
-  --> $DIR/issue-63388-2.rs:13:5
+  --> $DIR/issue-63388-2.rs:14:9
    |
-LL |           foo: &dyn Foo, bar: &'a dyn Foo
+LL |         foo: &dyn Foo, bar: &'a dyn Foo
-   |                -------- help: add explicit lifetime `'a` to the type of `foo`: `&'a (dyn Foo + 'a)`
+   |              -------- help: add explicit lifetime `'a` to the type of `foo`: `&'a (dyn Foo + 'a)`
-LL |       ) -> &dyn Foo
+...
-LL | /     {
+LL |         foo
-LL | |
+   |         ^^^ lifetime `'a` required
-LL | |         foo
-LL | |     }
-   | |_____^ lifetime `'a` required
 
 error: aborting due to 2 previous errors