Suggest returning a reference for unsized place from a closure

compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs

@@ -3307,6 +3307,19 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
             }
             ObligationCauseCode::SizedReturnType | ObligationCauseCode::SizedCallReturnType => {
                 err.note("the return type of a function must have a statically known size");
+                if let Some(node_body_id) = tcx.hir_node_by_def_id(body_id).body_id()
+                    && let body = tcx.hir_body(node_body_id)
+                    && let hir::Node::Expr(closure_expr) = tcx.hir_node_by_def_id(body_id)
+                    && let hir::ExprKind::Closure(closure) = closure_expr.kind
+                    && let hir::FnRetTy::DefaultReturn(_) = closure.fn_decl.output
+                {
+                    err.span_suggestion_verbose(
+                        body.value.span.shrink_to_lo(),
+                        "consider borrowing the value",
+                        "&",
+                        Applicability::MaybeIncorrect,
+                    );
+                }
             }
             ObligationCauseCode::SizedYieldType => {
                 err.note("the yield type of a coroutine must have a statically known size");

src/tools/clippy/tests/ui/crashes/ice-6251.stderr

@@ -19,6 +19,10 @@ LL | fn bug<T>() -> impl Iterator<Item = [(); { |x: [u8]| x }]> {
    |
    = help: the trait `std::marker::Sized` is not implemented for `[u8]`
    = note: the return type of a function must have a statically known size
+help: consider borrowing the value
+   |
+LL | fn bug<T>() -> impl Iterator<Item = [(); { |x: [u8]| &x }]> {
+   |                                                      +
 
 error[E0308]: mismatched types
   --> tests/ui/crashes/ice-6251.rs:4:44

tests/ui/closures/unsized-return-suggest-ref-issue-152064.fixed

@@ -0,0 +1,20 @@
+#![allow(unused_variables, for_loops_over_fallibles)]
+//@ run-rustfix
+
+fn main() {
+    // Basic case from the issue: str slice
+    let o = Some("Hello, world!");
+    for s in o.map(|s| &s[3..8]) {}
+    //~^ ERROR the size for values of type `str` cannot be known at compilation time
+    //~| ERROR the size for values of type `str` cannot be known at compilation time
+    //~| ERROR the size for values of type `str` cannot be known at compilation time
+    //~| ERROR `Option<str>` is not an iterator
+
+    // Byte slice case
+    let arr = Some(b"Hello, world!");
+    for s in arr.map(|s| &s[3..8]) {}
+    //~^ ERROR the size for values of type `[u8]` cannot be known at compilation time
+    //~| ERROR the size for values of type `[u8]` cannot be known at compilation time
+    //~| ERROR the size for values of type `[u8]` cannot be known at compilation time
+    //~| ERROR `Option<[u8]>` is not an iterator
+}

tests/ui/closures/unsized-return-suggest-ref-issue-152064.rs

@@ -0,0 +1,20 @@
+#![allow(unused_variables, for_loops_over_fallibles)]
+//@ run-rustfix
+
+fn main() {
+    // Basic case from the issue: str slice
+    let o = Some("Hello, world!");
+    for s in o.map(|s| s[3..8]) {}
+    //~^ ERROR the size for values of type `str` cannot be known at compilation time
+    //~| ERROR the size for values of type `str` cannot be known at compilation time
+    //~| ERROR the size for values of type `str` cannot be known at compilation time
+    //~| ERROR `Option<str>` is not an iterator
+
+    // Byte slice case
+    let arr = Some(b"Hello, world!");
+    for s in arr.map(|s| s[3..8]) {}
+    //~^ ERROR the size for values of type `[u8]` cannot be known at compilation time
+    //~| ERROR the size for values of type `[u8]` cannot be known at compilation time
+    //~| ERROR the size for values of type `[u8]` cannot be known at compilation time
+    //~| ERROR `Option<[u8]>` is not an iterator
+}

tests/ui/closures/unsized-return-suggest-ref-issue-152064.stderr

@@ -0,0 +1,105 @@
+error[E0277]: the size for values of type `str` cannot be known at compilation time
+  --> $DIR/unsized-return-suggest-ref-issue-152064.rs:7:16
+   |
+LL |     for s in o.map(|s| s[3..8]) {}
+   |                ^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `str`
+note: required by an implicit `Sized` bound in `Option::<T>::map`
+  --> $SRC_DIR/core/src/option.rs:LL:COL
+
+error[E0277]: the size for values of type `str` cannot be known at compilation time
+  --> $DIR/unsized-return-suggest-ref-issue-152064.rs:7:24
+   |
+LL |     for s in o.map(|s| s[3..8]) {}
+   |                        ^^^^^^^ 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
+help: consider borrowing the value
+   |
+LL |     for s in o.map(|s| &s[3..8]) {}
+   |                        +
+
+error[E0277]: the size for values of type `str` cannot be known at compilation time
+  --> $DIR/unsized-return-suggest-ref-issue-152064.rs:7:14
+   |
+LL |     for s in o.map(|s| s[3..8]) {}
+   |              ^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `str`
+note: required by an implicit `Sized` bound in `Option`
+  --> $SRC_DIR/core/src/option.rs:LL:COL
+
+error[E0277]: `Option<str>` is not an iterator
+  --> $DIR/unsized-return-suggest-ref-issue-152064.rs:7:14
+   |
+LL |     for s in o.map(|s| s[3..8]) {}
+   |              ^^^^^^^^^^^^^^^^^^ `Option<str>` is not an iterator
+   |
+   = help: the trait `IntoIterator` is not implemented for `Option<str>`
+help: the following other types implement trait `IntoIterator`
+  --> $SRC_DIR/core/src/option.rs:LL:COL
+   |
+   = note: `Option<T>`
+  ::: $SRC_DIR/core/src/option.rs:LL:COL
+   |
+   = note: `&Option<T>`
+  ::: $SRC_DIR/core/src/option.rs:LL:COL
+   |
+   = note: `&mut Option<T>`
+
+error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
+  --> $DIR/unsized-return-suggest-ref-issue-152064.rs:15:18
+   |
+LL |     for s in arr.map(|s| s[3..8]) {}
+   |                  ^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `[u8]`
+note: required by an implicit `Sized` bound in `Option::<T>::map`
+  --> $SRC_DIR/core/src/option.rs:LL:COL
+
+error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
+  --> $DIR/unsized-return-suggest-ref-issue-152064.rs:15:26
+   |
+LL |     for s in arr.map(|s| s[3..8]) {}
+   |                          ^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `[u8]`
+   = note: the return type of a function must have a statically known size
+help: consider borrowing the value
+   |
+LL |     for s in arr.map(|s| &s[3..8]) {}
+   |                          +
+
+error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
+  --> $DIR/unsized-return-suggest-ref-issue-152064.rs:15:14
+   |
+LL |     for s in arr.map(|s| s[3..8]) {}
+   |              ^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `[u8]`
+note: required by an implicit `Sized` bound in `Option`
+  --> $SRC_DIR/core/src/option.rs:LL:COL
+
+error[E0277]: `Option<[u8]>` is not an iterator
+  --> $DIR/unsized-return-suggest-ref-issue-152064.rs:15:14
+   |
+LL |     for s in arr.map(|s| s[3..8]) {}
+   |              ^^^^^^^^^^^^^^^^^^^^ `Option<[u8]>` is not an iterator
+   |
+   = help: the trait `IntoIterator` is not implemented for `Option<[u8]>`
+help: the following other types implement trait `IntoIterator`
+  --> $SRC_DIR/core/src/option.rs:LL:COL
+   |
+   = note: `Option<T>`
+  ::: $SRC_DIR/core/src/option.rs:LL:COL
+   |
+   = note: `&Option<T>`
+  ::: $SRC_DIR/core/src/option.rs:LL:COL
+   |
+   = note: `&mut Option<T>`
+
+error: aborting due to 8 previous errors
+
+For more information about this error, try `rustc --explain E0277`.