Rollup merge of #72389 - Aaron1011:feature/move-fn-self-msg, r=nikomatsakis

Explain move errors that occur due to method calls involving `self`

When calling a method that takes `self` (e.g. `vec.into_iter()`), the method receiver is moved out of. If the method receiver is used again, a move error will be emitted::

```rust
fn main() {
    let a = vec![true];
    a.into_iter();
    a;
}
```

emits

```
error[E0382]: use of moved value: `a`
 --> src/main.rs:4:5
  |
2 |     let a = vec![true];
  |         - move occurs because `a` has type `std::vec::Vec<bool>`, which does not implement the `Copy` trait
3 |     a.into_iter();
  |     - value moved here
4 |     a;
  |     ^ value used here after move
```

However, the error message doesn't make it clear that the move is caused by the call to `into_iter`.

This PR adds additional messages to move errors when the move is caused by using a value as the receiver of a `self` method::

```
error[E0382]: use of moved value: `a`
   --> vec.rs:4:5
    |
2   |     let a = vec![true];
    |         - move occurs because `a` has type `std::vec::Vec<bool>`, which does not implement the `Copy` trait
3   |     a.into_iter();
    |     ------------- value moved due to this method call
4   |     a;
    |     ^ value used here after move
    |
note: this function takes `self`, which moves the receiver
   --> /home/aaron/repos/rust/src/libcore/iter/traits/collect.rs:239:5
    |
239 |     fn into_iter(self) -> Self::IntoIter;
```

TODO:

- [x] Add special handling for `FnOnce/FnMut/Fn` - we probably don't want to point at the unstable trait methods
- [x] Consider adding additional context for operations (e.g. `Shr::shr`) when the call was generated using the operator syntax (e.g. `a >> b`)
- [x] Consider pointing to the method parent (impl or trait block) in addition to the method itself.

Author: RalfJung

src/librustc_ast_lowering/expr.rs

@@ -9,7 +9,7 @@ use rustc_data_structures::thin_vec::ThinVec;
 use rustc_errors::struct_span_err;
 use rustc_hir as hir;
 use rustc_hir::def::Res;
-use rustc_span::source_map::{respan, DesugaringKind, Span, Spanned};
+use rustc_span::source_map::{respan, DesugaringKind, ForLoopLoc, Span, Spanned};
 use rustc_span::symbol::{sym, Ident, Symbol};
 use rustc_target::asm;
 use std::collections::hash_map::Entry;
@@ -25,6 +25,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
     }
 
     pub(super) fn lower_expr_mut(&mut self, e: &Expr) -> hir::Expr<'hir> {
+        let mut span = e.span;
         ensure_sufficient_stack(|| {
             let kind = match e.kind {
                 ExprKind::Box(ref inner) => hir::ExprKind::Box(self.lower_expr(inner)),
@@ -53,6 +54,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     hir::ExprKind::MethodCall(hir_seg, seg.ident.span, args, span)
                 }
                 ExprKind::Binary(binop, ref lhs, ref rhs) => {
+                    span = self.mark_span_with_reason(DesugaringKind::Operator, e.span, None);
                     let binop = self.lower_binop(binop);
                     let lhs = self.lower_expr(lhs);
                     let rhs = self.lower_expr(rhs);
@@ -222,7 +224,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
             hir::Expr {
                 hir_id: self.lower_node_id(e.id),
                 kind,
-                span: e.span,
+                span,
                 attrs: e.attrs.iter().map(|a| self.lower_attr(a)).collect::<Vec<_>>().into(),
             }
         })
@@ -237,6 +239,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
     }
 
     fn lower_binop(&mut self, b: BinOp) -> hir::BinOp {
+        let span = self.mark_span_with_reason(DesugaringKind::Operator, b.span, None);
         Spanned {
             node: match b.node {
                 BinOpKind::Add => hir::BinOpKind::Add,
@@ -258,7 +261,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 BinOpKind::Ge => hir::BinOpKind::Ge,
                 BinOpKind::Gt => hir::BinOpKind::Gt,
             },
-            span: b.span,
+            span,
         }
     }
 
@@ -1360,9 +1363,14 @@ impl<'hir> LoweringContext<'_, 'hir> {
         body: &Block,
         opt_label: Option<Label>,
     ) -> hir::Expr<'hir> {
+        let orig_head_span = head.span;
         // expand <head>
         let mut head = self.lower_expr_mut(head);
-        let desugared_span = self.mark_span_with_reason(DesugaringKind::ForLoop, head.span, None);
+        let desugared_span = self.mark_span_with_reason(
+            DesugaringKind::ForLoop(ForLoopLoc::Head),
+            orig_head_span,
+            None,
+        );
         head.span = desugared_span;
 
         let iter = Ident::with_dummy_span(sym::iter);
@@ -1457,10 +1465,16 @@ impl<'hir> LoweringContext<'_, 'hir> {
         // `mut iter => { ... }`
         let iter_arm = self.arm(iter_pat, loop_expr);
 
+        let into_iter_span = self.mark_span_with_reason(
+            DesugaringKind::ForLoop(ForLoopLoc::IntoIter),
+            orig_head_span,
+            None,
+        );
+
         // `match ::std::iter::IntoIterator::into_iter(<head>) { ... }`
         let into_iter_expr = {
             let into_iter_path = &[sym::iter, sym::IntoIterator, sym::into_iter];
-            self.expr_call_std_path(desugared_span, into_iter_path, arena_vec![self; head])
+            self.expr_call_std_path(into_iter_span, into_iter_path, arena_vec![self; head])
         };
 
         let match_expr = self.arena.alloc(self.expr_match(

src/librustc_infer/infer/error_reporting/need_type_info.rs

@@ -455,7 +455,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             let msg = if let Some(simple_ident) = pattern.simple_ident() {
                 match pattern.span.desugaring_kind() {
                     None => format!("consider giving `{}` {}", simple_ident, suffix),
-                    Some(DesugaringKind::ForLoop) => {
+                    Some(DesugaringKind::ForLoop(_)) => {
                         "the element type for this iterator is not specified".to_string()
                     }
                     _ => format!("this needs {}", suffix),