Add ConstraintCategory::Usage for handling aggregate construction

In some cases, we emit borrowcheck diagnostics pointing
at a particular field expression in a struct expression
(e.g. `MyStruct { field: my_expr }`). However, this
behavior currently relies on us choosing the
`ConstraintCategory::Boring` with the 'correct' span.
When adding additional variants to `ConstraintCategory`,
(or changing existing usages away from `ConstraintCategory::Boring`),
the current behavior can easily get broken, since a non-boring
constraint will get chosen over a boring one.

To make the diagnostic output less fragile, this commit
adds a `ConstraintCategory::Usage` variant. We use this variant
for the temporary assignments created for each field of
an aggregate we are constructing.

Using this new variant, we can emit a message mentioning
"this usage", emphasizing the fact that the error message
is related to the specific use site (in the struct expression).

This is preparation for additional work on improving NLL error messages
(see rust-lang#57374)

Author: Aaron1011

Diff for: compiler/rustc_borrowck/src/diagnostics/region_errors.rs

@@ -40,6 +40,7 @@ impl ConstraintDescription for ConstraintCategory {
             ConstraintCategory::CopyBound => "copying this value ",
             ConstraintCategory::OpaqueType => "opaque type ",
             ConstraintCategory::ClosureUpvar(_) => "closure capture ",
+            ConstraintCategory::Usage => "this usage ",
             ConstraintCategory::Boring
             | ConstraintCategory::BoringNoLocation
             | ConstraintCategory::Internal => "",

Diff for: compiler/rustc_borrowck/src/type_check/mod.rs

@@ -1388,11 +1388,24 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                             ConstraintCategory::Return(ReturnConstraint::Normal)
                         }
                     }
+                    Some(l)
+                        if matches!(
+                            body.local_decls[l].local_info,
+                            Some(box LocalInfo::AggregateTemp)
+                        ) =>
+                    {
+                        ConstraintCategory::Usage
+                    }
                     Some(l) if !body.local_decls[l].is_user_variable() => {
                         ConstraintCategory::Boring
                     }
                     _ => ConstraintCategory::Assignment,
                 };
+                debug!(
+                    "assignment category: {:?} {:?}",
+                    category,
+                    place.as_local().map(|l| &body.local_decls[l])
+                );
 
                 let place_ty = place.ty(body, tcx).ty;
                 let place_ty = self.normalize(place_ty, location);

Diff for: compiler/rustc_middle/src/mir/mod.rs

@@ -992,6 +992,9 @@ pub enum LocalInfo<'tcx> {
     StaticRef { def_id: DefId, is_thread_local: bool },
     /// A temporary created that references the const with the given `DefId`
     ConstRef { def_id: DefId },
+    /// A temporary created during the creation of an aggregate
+    /// (e.g. a temporary for `foo` in `MyStruct { my_field: foo }`)
+    AggregateTemp,
 }
 
 impl<'tcx> LocalDecl<'tcx> {

Diff for: compiler/rustc_middle/src/mir/query.rs

@@ -332,17 +332,15 @@ pub enum ConstraintCategory {
     CopyBound,
     SizedBound,
     Assignment,
+    /// A constraint that came from a usage of a variable (e.g. in an ADT expression
+    /// like `Foo { field: my_val }`)
+    Usage,
     OpaqueType,
     ClosureUpvar(hir::HirId),
 
     /// A "boring" constraint (caused by the given location) is one that
     /// the user probably doesn't want to see described in diagnostics,
     /// because it is kind of an artifact of the type system setup.
-    /// Example: `x = Foo { field: y }` technically creates
-    /// intermediate regions representing the "type of `Foo { field: y
-    /// }`", and data flows from `y` into those variables, but they
-    /// are not very interesting. The assignment into `x` on the other
-    /// hand might be.
     Boring,
     // Boring and applicable everywhere.
     BoringNoLocation,

Diff for: compiler/rustc_mir_build/src/build/expr/as_operand.rs

@@ -20,7 +20,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         expr: &Expr<'tcx>,
     ) -> BlockAnd<Operand<'tcx>> {
         let local_scope = self.local_scope();
-        self.as_operand(block, Some(local_scope), expr)
+        self.as_operand(block, Some(local_scope), expr, None)
     }
 
     /// Returns an operand suitable for use until the end of the current scope expression and
@@ -85,6 +85,11 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
     /// temporary `tmp = x`, so that we capture the value of `x` at
     /// this time.
     ///
+    /// If we end up needing to create a temporary, then we will use
+    /// `local_info` as its `LocalInfo`, unless `as_temporary`
+    /// has already assigned it a non-`None` `LocalInfo`.
+    /// Normally, you should use `None` for `local_info`
+    ///
     /// The operand is known to be live until the end of `scope`.
     ///
     /// Like `as_local_call_operand`, except that the argument will
@@ -94,15 +99,16 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         mut block: BasicBlock,
         scope: Option<region::Scope>,
         expr: &Expr<'tcx>,
+        local_info: Option<Box<LocalInfo<'tcx>>>,
     ) -> BlockAnd<Operand<'tcx>> {
-        debug!("as_operand(block={:?}, expr={:?})", block, expr);
+        debug!("as_operand(block={:?}, expr={:?} local_info={:?})", block, expr, local_info);
         let this = self;
 
         if let ExprKind::Scope { region_scope, lint_level, value } = expr.kind {
             let source_info = this.source_info(expr.span);
             let region_scope = (region_scope, source_info);
             return this.in_scope(region_scope, lint_level, |this| {
-                this.as_operand(block, scope, &this.thir[value])
+                this.as_operand(block, scope, &this.thir[value], local_info)
             });
         }
 
@@ -115,6 +121,9 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             }
             Category::Place | Category::Rvalue(..) => {
                 let operand = unpack!(block = this.as_temp(block, scope, expr, Mutability::Mut));
+                if this.local_decls[operand].local_info.is_none() {
+                    this.local_decls[operand].local_info = local_info;
+                }
                 block.and(Operand::Move(Place::from(operand)))
             }
         }
@@ -167,6 +176,6 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             }
         }
 
-        this.as_operand(block, scope, expr)
+        this.as_operand(block, scope, expr, None)
     }
 }

Diff for: compiler/rustc_mir_build/src/build/expr/as_rvalue.rs

@@ -52,16 +52,16 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             }
             ExprKind::Repeat { value, count } => {
                 let value_operand =
-                    unpack!(block = this.as_operand(block, scope, &this.thir[value]));
+                    unpack!(block = this.as_operand(block, scope, &this.thir[value], None));
                 block.and(Rvalue::Repeat(value_operand, count))
             }
             ExprKind::Binary { op, lhs, rhs } => {
-                let lhs = unpack!(block = this.as_operand(block, scope, &this.thir[lhs]));
-                let rhs = unpack!(block = this.as_operand(block, scope, &this.thir[rhs]));
+                let lhs = unpack!(block = this.as_operand(block, scope, &this.thir[lhs], None));
+                let rhs = unpack!(block = this.as_operand(block, scope, &this.thir[rhs], None));
                 this.build_binary_op(block, op, expr_span, expr.ty, lhs, rhs)
             }
             ExprKind::Unary { op, arg } => {
-                let arg = unpack!(block = this.as_operand(block, scope, &this.thir[arg]));
+                let arg = unpack!(block = this.as_operand(block, scope, &this.thir[arg], None));
                 // Check for -MIN on signed integers
                 if this.check_overflow && op == UnOp::Neg && expr.ty.is_signed() {
                     let bool_ty = this.tcx.types.bool;
@@ -116,11 +116,13 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 block.and(Rvalue::Use(Operand::Move(Place::from(result))))
             }
             ExprKind::Cast { source } => {
-                let source = unpack!(block = this.as_operand(block, scope, &this.thir[source]));
+                let source =
+                    unpack!(block = this.as_operand(block, scope, &this.thir[source], None));
                 block.and(Rvalue::Cast(CastKind::Misc, source, expr.ty))
             }
             ExprKind::Pointer { cast, source } => {
-                let source = unpack!(block = this.as_operand(block, scope, &this.thir[source]));
+                let source =
+                    unpack!(block = this.as_operand(block, scope, &this.thir[source], None));
                 block.and(Rvalue::Cast(CastKind::Pointer(cast), source, expr.ty))
             }
             ExprKind::Array { ref fields } => {
@@ -155,7 +157,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 let fields: Vec<_> = fields
                     .into_iter()
                     .copied()
-                    .map(|f| unpack!(block = this.as_operand(block, scope, &this.thir[f])))
+                    .map(|f| unpack!(block = this.as_operand(block, scope, &this.thir[f], None)))
                     .collect();
 
                 block.and(Rvalue::Aggregate(Box::new(AggregateKind::Array(el_ty)), fields))
@@ -166,7 +168,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 let fields: Vec<_> = fields
                     .into_iter()
                     .copied()
-                    .map(|f| unpack!(block = this.as_operand(block, scope, &this.thir[f])))
+                    .map(|f| unpack!(block = this.as_operand(block, scope, &this.thir[f], None)))
                     .collect();
 
                 block.and(Rvalue::Aggregate(Box::new(AggregateKind::Tuple), fields))
@@ -242,7 +244,9 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                                             &this.thir[arg],
                                         )
                                     ),
-                                    _ => unpack!(block = this.as_operand(block, scope, upvar)),
+                                    _ => {
+                                        unpack!(block = this.as_operand(block, scope, upvar, None))
+                                    }
                                 }
                             }
                         }
@@ -304,7 +308,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                     Category::of(&expr.kind),
                     Some(Category::Rvalue(RvalueFunc::AsRvalue))
                 ));
-                let operand = unpack!(block = this.as_operand(block, scope, expr));
+                let operand = unpack!(block = this.as_operand(block, scope, expr, None));
                 block.and(Rvalue::Use(operand))
             }
         }

Diff for: compiler/rustc_mir_build/src/build/expr/into.rs

@@ -326,10 +326,16 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 let fields_map: FxHashMap<_, _> = fields
                     .into_iter()
                     .map(|f| {
+                        let local_info = Box::new(LocalInfo::AggregateTemp);
                         (
                             f.name,
                             unpack!(
-                                block = this.as_operand(block, Some(scope), &this.thir[f.expr])
+                                block = this.as_operand(
+                                    block,
+                                    Some(scope),
+                                    &this.thir[f.expr],
+                                    Some(local_info)
+                                )
                             ),
                         )
                     })
@@ -508,7 +514,8 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
 
             ExprKind::Yield { value } => {
                 let scope = this.local_scope();
-                let value = unpack!(block = this.as_operand(block, Some(scope), &this.thir[value]));
+                let value =
+                    unpack!(block = this.as_operand(block, Some(scope), &this.thir[value], None));
                 let resume = this.cfg.start_new_block();
                 this.cfg.terminate(
                     block,

Diff for: src/test/ui/async-await/issue-76547.nll.stderr

@@ -1,20 +1,22 @@
 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 `'1`
-   |              assignment requires that `'1` must outlive `'2`
+   |                    -     - let's call the lifetime of this reference `'2`
+   |                    |
+   |                    let's call the lifetime of this reference `'1`
+LL |     ListFut(bufs).await
+   |             ^^^^ this usage requires that `'1` must outlive `'2`
 
 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 `'1`
-   |               assignment requires that `'1` must outlive `'2`
+   |                     -     - let's call the lifetime of this reference `'2`
+   |                     |
+   |                     let's call the lifetime of this reference `'1`
+LL |     ListFut2(bufs).await
+   |              ^^^^ this usage requires that `'1` must outlive `'2`
 
 error: aborting due to 2 previous errors
 

Diff for: src/test/ui/issues/issue-61882-2.stderr

@@ -5,7 +5,7 @@ LL |         Self(&x);
    |              ^^
    |              |
    |              borrowed value does not live long enough
-   |              requires that `x` is borrowed for `'static`
+   |              this usage requires that `x` is borrowed for `'static`
 LL |
 LL |     }
    |     - `x` dropped here while still borrowed

Diff for: src/test/ui/nll/issue-46036.stderr

@@ -5,7 +5,7 @@ LL |     let foo = Foo { x: &a };
    |                        ^^
    |                        |
    |                        borrowed value does not live long enough
-   |                        requires that `a` is borrowed for `'static`
+   |                        this usage requires that `a` is borrowed for `'static`
 LL |     loop { }
 LL | }
    | - `a` dropped here while still borrowed

Diff for: src/test/ui/nll/user-annotations/adt-brace-enums.stderr

@@ -5,7 +5,7 @@ LL |     SomeEnum::SomeVariant::<&'static u32> { t: &c };
    |                                                ^^
    |                                                |
    |                                                borrowed value does not live long enough
-   |                                                requires that `c` is borrowed for `'static`
+   |                                                this usage requires that `c` is borrowed for `'static`
 LL | }
    | - `c` dropped here while still borrowed
 
@@ -19,7 +19,7 @@ LL |     SomeEnum::SomeVariant::<&'a u32> { t: &c };
    |                                           ^^
    |                                           |
    |                                           borrowed value does not live long enough
-   |                                           requires that `c` is borrowed for `'a`
+   |                                           this usage requires that `c` is borrowed for `'a`
 LL | }
    | - `c` dropped here while still borrowed
 
@@ -33,7 +33,7 @@ LL |         SomeEnum::SomeVariant::<&'a u32> { t: &c };
    |                                               ^^
    |                                               |
    |                                               borrowed value does not live long enough
-   |                                               requires that `c` is borrowed for `'a`
+   |                                               this usage requires that `c` is borrowed for `'a`
 LL |     };
    |     - `c` dropped here while still borrowed
 

Diff for: src/test/ui/nll/user-annotations/adt-brace-structs.stderr

@@ -5,7 +5,7 @@ LL |     SomeStruct::<&'static u32> { t: &c };
    |                                     ^^
    |                                     |
    |                                     borrowed value does not live long enough
-   |                                     requires that `c` is borrowed for `'static`
+   |                                     this usage requires that `c` is borrowed for `'static`
 LL | }
    | - `c` dropped here while still borrowed
 
@@ -19,7 +19,7 @@ LL |     SomeStruct::<&'a u32> { t: &c };
    |                                ^^
    |                                |
    |                                borrowed value does not live long enough
-   |                                requires that `c` is borrowed for `'a`
+   |                                this usage requires that `c` is borrowed for `'a`
 LL | }
    | - `c` dropped here while still borrowed
 
@@ -33,7 +33,7 @@ LL |         SomeStruct::<&'a u32> { t: &c };
    |                                    ^^
    |                                    |
    |                                    borrowed value does not live long enough
-   |                                    requires that `c` is borrowed for `'a`
+   |                                    this usage requires that `c` is borrowed for `'a`
 LL |     };
    |     - `c` dropped here while still borrowed
 

Diff for: src/test/ui/nll/user-annotations/adt-tuple-enums.stderr

@@ -5,7 +5,7 @@ LL |     SomeEnum::SomeVariant::<&'static u32>(&c);
    |                                           ^^
    |                                           |
    |                                           borrowed value does not live long enough
-   |                                           requires that `c` is borrowed for `'static`
+   |                                           this usage requires that `c` is borrowed for `'static`
 LL | }
    | - `c` dropped here while still borrowed
 
@@ -19,7 +19,7 @@ LL |     SomeEnum::SomeVariant::<&'a u32>(&c);
    |                                      ^^
    |                                      |
    |                                      borrowed value does not live long enough
-   |                                      requires that `c` is borrowed for `'a`
+   |                                      this usage requires that `c` is borrowed for `'a`
 LL | }
    | - `c` dropped here while still borrowed
 
@@ -33,7 +33,7 @@ LL |         SomeEnum::SomeVariant::<&'a u32>(&c);
    |                                          ^^
    |                                          |
    |                                          borrowed value does not live long enough
-   |                                          requires that `c` is borrowed for `'a`
+   |                                          this usage requires that `c` is borrowed for `'a`
 LL |     };
    |     - `c` dropped here while still borrowed
 

Diff for: src/test/ui/nll/user-annotations/adt-tuple-struct.stderr

@@ -5,7 +5,7 @@ LL |     SomeStruct::<&'static u32>(&c);
    |                                ^^
    |                                |
    |                                borrowed value does not live long enough
-   |                                requires that `c` is borrowed for `'static`
+   |                                this usage requires that `c` is borrowed for `'static`
 LL | }
    | - `c` dropped here while still borrowed
 
@@ -19,7 +19,7 @@ LL |     SomeStruct::<&'a u32>(&c);
    |                           ^^
    |                           |
    |                           borrowed value does not live long enough
-   |                           requires that `c` is borrowed for `'a`
+   |                           this usage requires that `c` is borrowed for `'a`
 LL | }
    | - `c` dropped here while still borrowed
 
@@ -33,7 +33,7 @@ LL |         SomeStruct::<&'a u32>(&c);
    |                               ^^
    |                               |
    |                               borrowed value does not live long enough
-   |                               requires that `c` is borrowed for `'a`
+   |                               this usage requires that `c` is borrowed for `'a`
 LL |     };
    |     - `c` dropped here while still borrowed
 

Diff for: src/test/ui/nll/where_clauses_in_structs.stderr

@@ -6,7 +6,7 @@ LL | fn bar<'a, 'b>(x: Cell<&'a u32>, y: Cell<&'b u32>) {
    |        |
    |        lifetime `'a` defined here
 LL |     Foo { x, y };
-   |           ^ requires that `'a` must outlive `'b`
+   |           ^ this usage requires that `'a` must outlive `'b`
    |
    = help: consider adding the following bound: `'a: 'b`