fix(rustc): suggest items be borrowed in for i in items[x..]

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

@@ -23,7 +23,7 @@ use rustc_middle::ty::{
 use rustc_middle::ty::{TypeAndMut, TypeckResults};
 use rustc_span::def_id::LOCAL_CRATE;
 use rustc_span::symbol::{kw, sym, Ident, Symbol};
-use rustc_span::{BytePos, MultiSpan, Span, DUMMY_SP};
+use rustc_span::{BytePos, DesugaringKind, ExpnKind, ForLoopLoc, MultiSpan, Span, DUMMY_SP};
 use rustc_target::spec::abi;
 use std::fmt;
 
@@ -680,7 +680,13 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         points_at_arg: bool,
         has_custom_message: bool,
     ) -> bool {
-        if !points_at_arg {
+        let span = obligation.cause.span;
+        let points_at_for_iter = matches!(
+            span.ctxt().outer_expn_data().kind,
+            ExpnKind::Desugaring(DesugaringKind::ForLoop(ForLoopLoc::IntoIter))
+        );
+
+        if !points_at_arg && !points_at_for_iter {
             return false;
         }
 
@@ -695,7 +701,6 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
 
         never_suggest_borrow.push(self.tcx.get_diagnostic_item(sym::send_trait).unwrap());
 
-        let span = obligation.cause.span;
         let param_env = obligation.param_env;
         let trait_ref = trait_ref.skip_binder();
 
@@ -754,7 +759,11 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                     );
 
                     // This if is to prevent a special edge-case
-                    if !span.from_expansion() {
+                    if matches!(
+                        span.ctxt().outer_expn_data().kind,
+                        ExpnKind::Root
+                            | ExpnKind::Desugaring(DesugaringKind::ForLoop(ForLoopLoc::IntoIter))
+                    ) {
                         // We don't want a borrowing suggestion on the fields in structs,
                         // ```
                         // struct Foo {

src/test/ui/issues/issue-20605.stderr

@@ -2,9 +2,12 @@ error[E0277]: the size for values of type `dyn Iterator<Item = &'a mut u8>` cann
   --> $DIR/issue-20605.rs:2:17
    |
 LL |     for item in *things { *item = 0 }
-   |                 ^^^^^^^ doesn't have a size known at compile-time
+   |                 ^^^^^^^
+   |                 |
+   |                 expected an implementor of trait `IntoIterator`
+   |                 help: consider mutably borrowing here: `&mut *things`
    |
-   = help: the trait `Sized` is not implemented for `dyn Iterator<Item = &'a mut u8>`
+   = note: the trait bound `dyn Iterator<Item = &'a mut u8>: IntoIterator` is not satisfied
    = note: required because of the requirements on the impl of `IntoIterator` for `dyn Iterator<Item = &'a mut u8>`
 note: required by `into_iter`
   --> $SRC_DIR/core/src/iter/traits/collect.rs:LL:COL

src/test/ui/suggestions/slice-issue-87994.rs

@@ -0,0 +1,16 @@
+fn main() {
+  let v = vec![1i32, 2, 3];
+  for _ in v[1..] {
+    //~^ ERROR [i32]` is not an iterator [E0277]
+    //~^^ ERROR known at compilation time
+  }
+  struct K {
+    n: i32,
+  }
+  let mut v2 = vec![K { n: 1 }, K { n: 1 }, K { n: 1 }];
+  for i2 in v2[1..] {
+    //~^ ERROR [K]` is not an iterator [E0277]
+    //~^^ ERROR known at compilation time
+    i2.n = 2;
+  }
+}

src/test/ui/suggestions/slice-issue-87994.stderr

@@ -0,0 +1,71 @@
+error[E0277]: the size for values of type `[i32]` cannot be known at compilation time
+  --> $DIR/slice-issue-87994.rs:3:12
+   |
+LL |   for _ in v[1..] {
+   |            ^^^^^^
+   |            |
+   |            expected an implementor of trait `IntoIterator`
+   |            help: consider borrowing here: `&v[1..]`
+   |
+   = note: the trait bound `[i32]: IntoIterator` is not satisfied
+   = note: required because of the requirements on the impl of `IntoIterator` for `[i32]`
+note: required by `into_iter`
+  --> $SRC_DIR/core/src/iter/traits/collect.rs:LL:COL
+   |
+LL |     fn into_iter(self) -> Self::IntoIter;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0277]: `[i32]` is not an iterator
+  --> $DIR/slice-issue-87994.rs:3:12
+   |
+LL |   for _ in v[1..] {
+   |            ^^^^^^
+   |            |
+   |            expected an implementor of trait `IntoIterator`
+   |            help: consider borrowing here: `&v[1..]`
+   |
+   = note: the trait bound `[i32]: IntoIterator` is not satisfied
+   = note: required because of the requirements on the impl of `IntoIterator` for `[i32]`
+note: required by `into_iter`
+  --> $SRC_DIR/core/src/iter/traits/collect.rs:LL:COL
+   |
+LL |     fn into_iter(self) -> Self::IntoIter;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0277]: the size for values of type `[K]` cannot be known at compilation time
+  --> $DIR/slice-issue-87994.rs:11:13
+   |
+LL |   for i2 in v2[1..] {
+   |             ^^^^^^^
+   |             |
+   |             expected an implementor of trait `IntoIterator`
+   |             help: consider borrowing here: `&v2[1..]`
+   |
+   = note: the trait bound `[K]: IntoIterator` is not satisfied
+   = note: required because of the requirements on the impl of `IntoIterator` for `[K]`
+note: required by `into_iter`
+  --> $SRC_DIR/core/src/iter/traits/collect.rs:LL:COL
+   |
+LL |     fn into_iter(self) -> Self::IntoIter;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0277]: `[K]` is not an iterator
+  --> $DIR/slice-issue-87994.rs:11:13
+   |
+LL |   for i2 in v2[1..] {
+   |             ^^^^^^^
+   |             |
+   |             expected an implementor of trait `IntoIterator`
+   |             help: consider borrowing here: `&v2[1..]`
+   |
+   = note: the trait bound `[K]: IntoIterator` is not satisfied
+   = note: required because of the requirements on the impl of `IntoIterator` for `[K]`
+note: required by `into_iter`
+  --> $SRC_DIR/core/src/iter/traits/collect.rs:LL:COL
+   |
+LL |     fn into_iter(self) -> Self::IntoIter;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0277`.