Adjust spans into the for loops context before creating the new desugaring spans.

compiler/rustc_ast_lowering/src/expr.rs

@@ -1771,8 +1771,14 @@ impl<'hir> LoweringContext<'_, 'hir> {
         let pat = self.lower_pat(pat);
         let for_span =
             self.mark_span_with_reason(DesugaringKind::ForLoop, self.lower_span(e.span), None);
-        let head_span = self.mark_span_with_reason(DesugaringKind::ForLoop, head.span, None);
-        let pat_span = self.mark_span_with_reason(DesugaringKind::ForLoop, pat.span, None);
+        let for_ctxt = for_span.ctxt();
+
+        // Try to point both the head and pat spans to their position in the for loop
+        // rather than inside a macro.
+        let head_span =
+            head.span.find_ancestor_in_same_ctxt(e.span).unwrap_or(head.span).with_ctxt(for_ctxt);
+        let pat_span =
+            pat.span.find_ancestor_in_same_ctxt(e.span).unwrap_or(pat.span).with_ctxt(for_ctxt);
 
         let loop_hir_id = self.lower_node_id(e.id);
         let label = self.lower_label(opt_label, e.id, loop_hir_id);

tests/ui/for/iter_from_mac_call.rs

@@ -0,0 +1,26 @@
+macro_rules! deref {
+    ($e:expr) => { *$e };
+}
+
+fn f1<'a>(mut iter: Box<dyn Iterator<Item=&'a mut u8>>) {
+    for item in deref!(iter) { *item = 0 }
+    //~^ ERROR `dyn Iterator<Item = &'a mut u8>` is not an iterator
+}
+
+fn f2(x: &mut i32) {
+    for _item in deref!(x) {}
+    //~^ ERROR `i32` is not an iterator
+}
+
+struct Wrapped(i32);
+
+macro_rules! borrow_deref {
+    ($e:expr) => { &mut *$e };
+}
+
+fn f3<'a>(mut iter: Box<dyn Iterator<Item=&'a mut i32>>) {
+    for Wrapped(item) in borrow_deref!(iter) { *item = 0 }
+    //~^ ERROR mismatched types
+}
+
+fn main() {}

tests/ui/for/iter_from_mac_call.stderr

@@ -0,0 +1,35 @@
+error[E0277]: `dyn Iterator<Item = &'a mut u8>` is not an iterator
+  --> $DIR/iter_from_mac_call.rs:6:17
+   |
+LL |     for item in deref!(iter) { *item = 0 }
+   |                 ^^^^^^^^^^^^ the trait `IntoIterator` 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 for `dyn Iterator<Item = &'a mut u8>` to implement `IntoIterator`
+help: consider mutably borrowing here
+   |
+LL |     for item in &mut deref!(iter) { *item = 0 }
+   |                 ++++
+
+error[E0277]: `i32` is not an iterator
+  --> $DIR/iter_from_mac_call.rs:11:18
+   |
+LL |     for _item in deref!(x) {}
+   |                  ^^^^^^^^^ `i32` is not an iterator
+   |
+   = help: the trait `Iterator` is not implemented for `i32`
+   = note: if you want to iterate between `start` until a value `end`, use the exclusive range syntax `start..end` or the inclusive range syntax `start..=end`
+   = note: required for `i32` to implement `IntoIterator`
+
+error[E0308]: mismatched types
+  --> $DIR/iter_from_mac_call.rs:22:9
+   |
+LL |     for Wrapped(item) in borrow_deref!(iter) { *item = 0 }
+   |         ^^^^^^^^^^^^^    ------------------- this is an iterator with items of type `&mut i32`
+   |         |
+   |         expected `i32`, found `Wrapped`
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0277, E0308.
+For more information about an error, try `rustc --explain E0277`.

tests/ui/traits/suggest-dereferences/invalid-suggest-deref-issue-127590.stderr

@@ -60,6 +60,30 @@ help: the trait `Iterator` is implemented for `std::slice::Iter<'_, T>`
    = note: required for `Zip<std::slice::Iter<'_, {integer}>, &std::slice::Iter<'_, {integer}>>` to implement `IntoIterator`
    = note: this error originates in the macro `iterator` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: aborting due to 4 previous errors
+error[E0277]: `&std::slice::Iter<'_, {integer}>` is not an iterator
+  --> $DIR/invalid-suggest-deref-issue-127590.rs:6:24
+   |
+LL |     for (src, dest) in std::iter::zip(fields.iter(), &variant.iter()) {
+   |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `&std::slice::Iter<'_, {integer}>` is not an iterator
+   |
+   = help: the trait `Iterator` is not implemented for `&std::slice::Iter<'_, {integer}>`
+help: the trait `Iterator` is implemented for `std::slice::Iter<'_, T>`
+  --> $SRC_DIR/core/src/slice/iter.rs:LL:COL
+   = note: required for `&std::slice::Iter<'_, {integer}>` to implement `IntoIterator`
+   = note: this error originates in the macro `iterator` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0277]: `&std::slice::Iter<'_, {integer}>` is not an iterator
+  --> $DIR/invalid-suggest-deref-issue-127590.rs:13:24
+   |
+LL |     for (src, dest) in std::iter::zip(fields.iter(), &variant.iter().clone()) {
+   |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `&std::slice::Iter<'_, {integer}>` is not an iterator
+   |
+   = help: the trait `Iterator` is not implemented for `&std::slice::Iter<'_, {integer}>`
+help: the trait `Iterator` is implemented for `std::slice::Iter<'_, T>`
+  --> $SRC_DIR/core/src/slice/iter.rs:LL:COL
+   = note: required for `&std::slice::Iter<'_, {integer}>` to implement `IntoIterator`
+   = note: this error originates in the macro `iterator` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 6 previous errors
 
 For more information about this error, try `rustc --explain E0277`.