Auto merge of #90352 - camsteffen:for-loop-desugar, r=oli-obk

Simplify `for` loop desugar

Basically two intermediate bindings are inlined. I could have left one intermediate binding in place as this would simplify some diagnostic logic, but I think the difference in that regard would be negligible, so it is better to have a minimal HIR.

For checking that the pattern is irrefutable, I added a special case when the `match` is found to be non-exhaustive.

The reordering of the arms is purely stylistic. I don't *think* there are any perf implications.

```diff
  match IntoIterator::into_iter($head) {
      mut iter => {
          $label: loop {
-             let mut __next;
              match Iterator::next(&mut iter) {
-                 Some(val) => __next = val,
                  None => break,
+                 Some($pat) => $block,
              }
-             let $pat = __next;
-             $block
          }
      }
  }
```

3 files changed, 3 insertions, 11 deletions

diff --git a/compiler/rustc_span/src/hygiene.rs b/compiler/rustc_span/src/hygiene.rs
index 724d1904dc3..d590776676b 100644
--- a/compiler/rustc_span/src/hygiene.rs
+++ b/compiler/rustc_span/src/hygiene.rs
@@ -1099,18 +1099,11 @@ pub enum DesugaringKind {
     OpaqueTy,
     Async,
     Await,
-    ForLoop(ForLoopLoc),
+    ForLoop,
     LetElse,
     WhileLoop,
 }
 
-/// A location in the desugaring of a `for` loop
-#[derive(Clone, Copy, PartialEq, Debug, Encodable, Decodable, HashStable_Generic)]
-pub enum ForLoopLoc {
-    Head,
-    IntoIter,
-}
-
 impl DesugaringKind {
     /// The description wording should combine well with "desugaring of {}".
     pub fn descr(self) -> &'static str {
@@ -1121,7 +1114,7 @@ impl DesugaringKind {
             DesugaringKind::QuestionMark => "operator `?`",
             DesugaringKind::TryBlock => "`try` block",
             DesugaringKind::OpaqueTy => "`impl Trait`",
-            DesugaringKind::ForLoop(_) => "`for` loop",
+            DesugaringKind::ForLoop => "`for` loop",
             DesugaringKind::LetElse => "`let...else`",
             DesugaringKind::WhileLoop => "`while` loop",
         }
diff --git a/compiler/rustc_span/src/lib.rs b/compiler/rustc_span/src/lib.rs
index 1445c59710c..66c01140abc 100644
--- a/compiler/rustc_span/src/lib.rs
+++ b/compiler/rustc_span/src/lib.rs
@@ -41,7 +41,7 @@ pub mod edition;
 use edition::Edition;
 pub mod hygiene;
 use hygiene::Transparency;
-pub use hygiene::{DesugaringKind, ExpnKind, ForLoopLoc, MacroKind};
+pub use hygiene::{DesugaringKind, ExpnKind, MacroKind};
 pub use hygiene::{ExpnData, ExpnHash, ExpnId, LocalExpnId, SyntaxContext};
 pub mod def_id;
 use def_id::{CrateNum, DefId, DefPathHash, LocalDefId, LOCAL_CRATE};
diff --git a/compiler/rustc_span/src/symbol.rs b/compiler/rustc_span/src/symbol.rs
index 9992b1f31fe..0d556b5eda6 100644
--- a/compiler/rustc_span/src/symbol.rs
+++ b/compiler/rustc_span/src/symbol.rs
@@ -269,7 +269,6 @@ symbols! {
         __D,
         __H,
         __S,
-        __next,
         __try_var,
         _args,
         _d,