Do not suggest borrow that is already there in fully-qualified call

When encountering `&str::from("value")` do not suggest `&&str::from("value")`.

Fix #132041.

3 files changed, 40 insertions, 0 deletions

diff --git a/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs b/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
index 2bbf90ed3ed..a81f7574c5e 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
@@ -1195,6 +1195,15 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
             c @ ObligationCauseCode::WhereClauseInExpr(_, _, hir_id, _)
                 if self.tcx.hir_span(*hir_id).lo() == span.lo() =>
             {
+                if let hir::Node::Expr(expr) = self.tcx.parent_hir_node(*hir_id)
+                    && let hir::ExprKind::Call(base, _) = expr.kind
+                    && let hir::ExprKind::Path(hir::QPath::TypeRelative(ty, _)) = base.kind
+                    && ty.span == span
+                {
+                    // Do not suggest borrowing when we already do so. This would happen with
+                    // `let _ = &str::from("");` where the expression corresponds to the `str`.
+                    return false;
+                }
                 c
             }
             c if matches!(
diff --git a/tests/ui/suggestions/dont-suggest-borrowing-existing-borrow.rs b/tests/ui/suggestions/dont-suggest-borrowing-existing-borrow.rs
new file mode 100644
index 00000000000..c25bde5d0a2
--- /dev/null
+++ b/tests/ui/suggestions/dont-suggest-borrowing-existing-borrow.rs
@@ -0,0 +1,5 @@
+fn main() {
+    let _ = &str::from("value");
+    //~^ ERROR the trait bound `str: From<_>` is not satisfied
+    //~| ERROR the size for values of type `str` cannot be known at compilation time
+}
diff --git a/tests/ui/suggestions/dont-suggest-borrowing-existing-borrow.stderr b/tests/ui/suggestions/dont-suggest-borrowing-existing-borrow.stderr
new file mode 100644
index 00000000000..025014aa022
--- /dev/null
+++ b/tests/ui/suggestions/dont-suggest-borrowing-existing-borrow.stderr
@@ -0,0 +1,26 @@
+error[E0277]: the trait bound `str: From<_>` is not satisfied
+  --> $DIR/dont-suggest-borrowing-existing-borrow.rs:2:14
+   |
+LL |     let _ = &str::from("value");
+   |              ^^^ the trait `From<_>` is not implemented for `str`
+   |
+   = help: the following other types implement trait `From<T>`:
+             `String` implements `From<&String>`
+             `String` implements `From<&mut str>`
+             `String` implements `From<&str>`
+             `String` implements `From<Box<str>>`
+             `String` implements `From<Cow<'_, str>>`
+             `String` implements `From<char>`
+
+error[E0277]: the size for values of type `str` cannot be known at compilation time
+  --> $DIR/dont-suggest-borrowing-existing-borrow.rs:2:14
+   |
+LL |     let _ = &str::from("value");
+   |              ^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `str`
+   = note: the return type of a function must have a statically known size
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.