Rollup merge of #108254 - Nathan-Fenner:nathanf/error-span-ref-trait-refine, r=WaffleLapkin

Refine error span for trait error into borrowed expression

Extends the error span refinement in #106477 to drill into borrowed expressions just like tuples/struct/enum literals. For example,

```rs
trait Fancy {}
trait Good {}
impl <'a, T> Fancy for &'a T where T: Good {}
impl <S> Good for Option<S> where S: Iterator {}

fn want_fancy<F>(f: F) where F: Fancy {}

fn example() {
    want_fancy(&Some(5));
//  (BEFORE)   ^^^^^^^^ `{integer}` is not an iterator
//  (AFTER)          ^  `{integer}` is not an iterator
}
```

Existing heuristics try to find the right part of the expression to "point at"; current heuristics look at e.g. struct constructors and tuples. This PR adds a new check for borrowed expressions when looking into a borrowed type.

compiler/rustc_hir_typeck/src/fn_ctxt/adjust_fulfillment_errors.rs

@@ -549,6 +549,19 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             return Err(expr);
         };
 
+        if let (
+            hir::ExprKind::AddrOf(_borrow_kind, _borrow_mutability, borrowed_expr),
+            ty::Ref(_ty_region, ty_ref_type, _ty_mutability),
+        ) = (&expr.kind, in_ty.kind())
+        {
+            // We can "drill into" the borrowed expression.
+            return self.blame_specific_part_of_expr_corresponding_to_generic_param(
+                param,
+                borrowed_expr,
+                (*ty_ref_type).into(),
+            );
+        }
+
         if let (hir::ExprKind::Tup(expr_elements), ty::Tuple(in_ty_elements)) =
             (&expr.kind, in_ty.kind())
         {

tests/ui/errors/traits/blame-trait-error-spans-on-exprs.rs

@@ -71,6 +71,8 @@ struct DoubleWrapper<T> {
 
 impl<T: T1> T1 for DoubleWrapper<T> {}
 
+impl<'a, T: T2> T1 for &'a T {}
+
 fn example<Q>(q: Q) {
     // In each of the following examples, we expect the error span to point at the 'q' variable,
     // since the missing constraint is `Q: T3`.
@@ -126,6 +128,10 @@ fn example<Q>(q: Q) {
         Two { a: Two { a: (), b: Two { a: Two { a: (), b: q }, b: () } }, b: () },
         //~^ ERROR the trait bound `Q: T1` is not satisfied [E0277]
     );
+
+    // Verifies for reference:
+    want(&Burrito { spicy: false, filling: q });
+    //~^ ERROR the trait bound `Q: T3` is not satisfied [E0277]
 }
 
 fn main() {}

tests/ui/errors/traits/blame-trait-error-spans-on-exprs.stderr

@@ -1,5 +1,5 @@
 error[E0277]: the trait bound `Q: T3` is not satisfied
-  --> $DIR/blame-trait-error-spans-on-exprs.rs:79:60
+  --> $DIR/blame-trait-error-spans-on-exprs.rs:81:60
    |
 LL |     want(Wrapper { value: Burrito { spicy: false, filling: q } });
    |     ---- required by a bound introduced by this call       ^ the trait `T3` is not implemented for `Q`
@@ -29,7 +29,7 @@ LL | fn example<Q: T3>(q: Q) {
    |             ++++
 
 error[E0277]: the trait bound `Q: T3` is not satisfied
-  --> $DIR/blame-trait-error-spans-on-exprs.rs:83:84
+  --> $DIR/blame-trait-error-spans-on-exprs.rs:85:84
    |
 LL |     want(Wrapper { value: BurritoKinds::SmallBurrito { spicy: true, small_filling: q } });
    |     ---- required by a bound introduced by this call                               ^ the trait `T3` is not implemented for `Q`
@@ -59,7 +59,7 @@ LL | fn example<Q: T3>(q: Q) {
    |             ++++
 
 error[E0277]: the trait bound `Q: T3` is not satisfied
-  --> $DIR/blame-trait-error-spans-on-exprs.rs:87:39
+  --> $DIR/blame-trait-error-spans-on-exprs.rs:89:39
    |
 LL |     want(Wrapper { value: Taco(false, q) });
    |     ----                              ^ the trait `T3` is not implemented for `Q`
@@ -91,7 +91,7 @@ LL | fn example<Q: T3>(q: Q) {
    |             ++++
 
 error[E0277]: the trait bound `Q: T3` is not satisfied
-  --> $DIR/blame-trait-error-spans-on-exprs.rs:91:27
+  --> $DIR/blame-trait-error-spans-on-exprs.rs:93:27
    |
 LL |     want(Wrapper { value: TacoKinds::OneTaco(false, q) });
    |     ----                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `T3` is not implemented for `Q`
@@ -123,7 +123,7 @@ LL | fn example<Q: T3>(q: Q) {
    |             ++++
 
 error[E0277]: the trait bound `Q: T3` is not satisfied
-  --> $DIR/blame-trait-error-spans-on-exprs.rs:95:74
+  --> $DIR/blame-trait-error-spans-on-exprs.rs:97:74
    |
 LL |     want(Wrapper { value: GenericBurrito { spiciness: NotSpicy, filling: q } });
    |     ---- required by a bound introduced by this call                     ^ the trait `T3` is not implemented for `Q`
@@ -153,7 +153,7 @@ LL | fn example<Q: T3>(q: Q) {
    |             ++++
 
 error[E0277]: the trait bound `Q: T2` is not satisfied
-  --> $DIR/blame-trait-error-spans-on-exprs.rs:99:14
+  --> $DIR/blame-trait-error-spans-on-exprs.rs:101:14
    |
 LL |     want((3, q));
    |     ----     ^ the trait `T2` is not implemented for `Q`
@@ -178,7 +178,7 @@ LL | fn example<Q: T2>(q: Q) {
    |             ++++
 
 error[E0277]: the trait bound `Q: T3` is not satisfied
-  --> $DIR/blame-trait-error-spans-on-exprs.rs:103:31
+  --> $DIR/blame-trait-error-spans-on-exprs.rs:105:31
    |
 LL |     want(Wrapper { value: (3, q) });
    |     ----                      ^ the trait `T3` is not implemented for `Q`
@@ -210,7 +210,7 @@ LL | fn example<Q: T3>(q: Q) {
    |             ++++
 
 error[E0277]: the trait bound `Q: T3` is not satisfied
-  --> $DIR/blame-trait-error-spans-on-exprs.rs:107:15
+  --> $DIR/blame-trait-error-spans-on-exprs.rs:109:15
    |
 LL |     want(((3, q), 5));
    |     ----      ^ the trait `T3` is not implemented for `Q`
@@ -242,7 +242,7 @@ LL | fn example<Q: T3>(q: Q) {
    |             ++++
 
 error[E0277]: the trait bound `Q: T1` is not satisfied
-  --> $DIR/blame-trait-error-spans-on-exprs.rs:110:49
+  --> $DIR/blame-trait-error-spans-on-exprs.rs:112:49
    |
 LL |     want(DoubleWrapper { item: Wrapper { value: q } });
    |     ----                                        ^ the trait `T1` is not implemented for `Q`
@@ -267,7 +267,7 @@ LL | fn example<Q: T1>(q: Q) {
    |             ++++
 
 error[E0277]: the trait bound `Q: T1` is not satisfied
-  --> $DIR/blame-trait-error-spans-on-exprs.rs:113:88
+  --> $DIR/blame-trait-error-spans-on-exprs.rs:115:88
    |
 LL |     want(DoubleWrapper { item: Wrapper { value: DoubleWrapper { item: Wrapper { value: q } } } });
    |     ---- required by a bound introduced by this call                                   ^ the trait `T1` is not implemented for `Q`
@@ -292,7 +292,7 @@ LL | fn example<Q: T1>(q: Q) {
    |             ++++
 
 error[E0277]: the trait bound `Q: T3` is not satisfied
-  --> $DIR/blame-trait-error-spans-on-exprs.rs:117:27
+  --> $DIR/blame-trait-error-spans-on-exprs.rs:119:27
    |
 LL |     want(Wrapper { value: AliasBurrito { spiciness: q, filling: q } });
    |     ----                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `T3` is not implemented for `Q`
@@ -324,7 +324,7 @@ LL | fn example<Q: T3>(q: Q) {
    |             ++++
 
 error[E0277]: the trait bound `Q: T1` is not satisfied
-  --> $DIR/blame-trait-error-spans-on-exprs.rs:120:35
+  --> $DIR/blame-trait-error-spans-on-exprs.rs:122:35
    |
 LL |     want(Two { a: Two { a: (), b: q }, b: () });
    |     ----                          ^ the trait `T1` is not implemented for `Q`
@@ -349,7 +349,7 @@ LL | fn example<Q: T1>(q: Q) {
    |             ++++
 
 error[E0277]: the trait bound `Q: T1` is not satisfied
-  --> $DIR/blame-trait-error-spans-on-exprs.rs:126:59
+  --> $DIR/blame-trait-error-spans-on-exprs.rs:128:59
    |
 LL |     want(
    |     ---- required by a bound introduced by this call
@@ -375,6 +375,38 @@ help: consider restricting type parameter `Q`
 LL | fn example<Q: T1>(q: Q) {
    |             ++++
 
-error: aborting due to 13 previous errors
+error[E0277]: the trait bound `Q: T3` is not satisfied
+  --> $DIR/blame-trait-error-spans-on-exprs.rs:133:44
+   |
+LL |     want(&Burrito { spicy: false, filling: q });
+   |     ----                                   ^ the trait `T3` is not implemented for `Q`
+   |     |
+   |     required by a bound introduced by this call
+   |
+note: required for `Burrito<Q>` to implement `T2`
+  --> $DIR/blame-trait-error-spans-on-exprs.rs:22:13
+   |
+LL | impl<A: T3> T2 for Burrito<A> {}
+   |         --  ^^     ^^^^^^^^^^
+   |         |
+   |         unsatisfied trait bound introduced here
+note: required for `&Burrito<Q>` to implement `T1`
+  --> $DIR/blame-trait-error-spans-on-exprs.rs:74:17
+   |
+LL | impl<'a, T: T2> T1 for &'a T {}
+   |             --  ^^     ^^^^^
+   |             |
+   |             unsatisfied trait bound introduced here
+note: required by a bound in `want`
+  --> $DIR/blame-trait-error-spans-on-exprs.rs:53:12
+   |
+LL | fn want<V: T1>(_x: V) {}
+   |            ^^ required by this bound in `want`
+help: consider restricting type parameter `Q`
+   |
+LL | fn example<Q: T3>(q: Q) {
+   |             ++++
+
+error: aborting due to 14 previous errors
 
 For more information about this error, try `rustc --explain E0277`.

tests/ui/traits/suggest-deferences/issue-39029.stderr

@@ -1,8 +1,8 @@
 error[E0277]: the trait bound `NoToSocketAddrs: ToSocketAddrs` is not satisfied
-  --> $DIR/issue-39029.rs:16:37
+  --> $DIR/issue-39029.rs:16:38
    |
 LL |     let _errors = TcpListener::bind(&bad);
-   |                   ----------------- ^^^^ the trait `ToSocketAddrs` is not implemented for `NoToSocketAddrs`
+   |                   -----------------  ^^^ the trait `ToSocketAddrs` is not implemented for `NoToSocketAddrs`
    |                   |
    |                   required by a bound introduced by this call
    |