Don't create drop scopes after item statements

These scopes would not exist in MIR and can cause ICEs with invalid uses
of let expressions.

Author: matthewjasper

Diff for: compiler/rustc_hir_analysis/src/check/region.rs

@@ -149,7 +149,7 @@ fn resolve_block<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, blk: &'tcx h
                     // From now on, we continue normally.
                     visitor.cx = prev_cx;
                 }
-                hir::StmtKind::Local(..) | hir::StmtKind::Item(..) => {
+                hir::StmtKind::Local(..)=> {
                     // Each declaration introduces a subscope for bindings
                     // introduced by the declaration; this subscope covers a
                     // suffix of the block. Each subscope in a block has the
@@ -163,6 +163,10 @@ fn resolve_block<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, blk: &'tcx h
                     visitor.cx.var_parent = visitor.cx.parent;
                     visitor.visit_stmt(statement)
                 }
+                hir::StmtKind::Item(..) => {
+                    // Don't create scopes for items, since they won't be
+                    // lowered to THIR and MIR.
+                }
                 hir::StmtKind::Expr(..) | hir::StmtKind::Semi(..) => visitor.visit_stmt(statement),
             }
         }

Diff for: tests/ui/rfcs/rfc-2497-if-let-chains/avoid-invalid-mir.rs

@@ -0,0 +1,16 @@
+// Regression test for #104172
+
+const N: usize = {
+    struct U;
+    !let y = 42;
+    //~^ ERROR expected expression, found `let` statement
+    //~| ERROR `let` expressions are not supported here
+    //~| ERROR `let` expressions in this position are unstable [E0658]
+    3
+};
+
+struct S {
+    x: [(); N]
+}
+
+fn main() {}

Diff for: tests/ui/rfcs/rfc-2497-if-let-chains/avoid-invalid-mir.stderr

@@ -0,0 +1,26 @@
+error: expected expression, found `let` statement
+  --> $DIR/avoid-invalid-mir.rs:5:7
+   |
+LL |     ! let y = 42;
+   |       ^^^
+
+error: `let` expressions are not supported here
+  --> $DIR/avoid-invalid-mir.rs:5:7
+   |
+LL |     ! let y = 42;
+   |       ^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if` and `while` expressions
+
+error[E0658]: `let` expressions in this position are unstable
+  --> $DIR/avoid-invalid-mir.rs:5:7
+   |
+LL |     ! let y = 42;
+   |       ^^^^^^^^^^
+   |
+   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
+   = help: add `#![feature(let_chains)]` to the crate attributes to enable
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0658`.