Rollup merge of #131035 - dingxiangfei2009:tweak-if-let-rescope-lint, r=jieyouxu

Preserve brackets around if-lets and skip while-lets

r? `@jieyouxu`

Tracked by #124085

Fresh out of #129466, we have discovered 9 crates that the lint did not successfully migrate because the span of `if let` includes the surrounding brackets `(..)` like the following, which surprised me a bit.

```rust
if (if let .. { .. } else { .. }) {
// ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
// the span somehow includes the surrounding brackets
}
```

There is one crate that failed the migration because some suggestion spans cross the macro expansion boundaries. Surely there is no way to patch them with `match` rewrite. To handle this case, we will instead require all spans to be tested for admissibility as suggestion spans.

Besides, there are 4 false negative cases discovered with desugared-`while let`. We don't need to lint them, because the `else` branch surely contains exactly one statement because the drop order is not changed whatsoever in this case.

```rust
while let Some(value) = droppy().get() {
..
}
// is desugared into
loop {
    if let Some(value) = droppy().get() {
        ..
    } else {
        break;
        // here can be nothing observable in this block
    }
}
```

I believe this is the one and only false positive that I have found. I think we have finally nailed all the corner cases this time.

1 files changed, 28 insertions, 2 deletions

diff --git a/compiler/rustc_lint/src/if_let_rescope.rs b/compiler/rustc_lint/src/if_let_rescope.rs
index c6218fe1e74..cdd0e80c458 100644
--- a/compiler/rustc_lint/src/if_let_rescope.rs
+++ b/compiler/rustc_lint/src/if_let_rescope.rs
@@ -122,7 +122,11 @@ impl IfLetRescope {
         }
         let tcx = cx.tcx;
         let source_map = tcx.sess.source_map();
-        let expr_end = expr.span.shrink_to_hi();
+        let expr_end = match expr.kind {
+            hir::ExprKind::If(_cond, conseq, None) => conseq.span.shrink_to_hi(),
+            hir::ExprKind::If(_cond, _conseq, Some(alt)) => alt.span.shrink_to_hi(),
+            _ => return,
+        };
         let mut add_bracket_to_match_head = match_head_needs_bracket(tcx, expr);
         let mut significant_droppers = vec![];
         let mut lifetime_ends = vec![];
@@ -145,7 +149,10 @@ impl IfLetRescope {
                 recovered: Recovered::No,
             }) = cond.kind
             {
-                let if_let_pat = expr.span.shrink_to_lo().between(init.span);
+                // Peel off round braces
+                let if_let_pat = source_map
+                    .span_take_while(expr.span, |&ch| ch == '(' || ch.is_whitespace())
+                    .between(init.span);
                 // The consequent fragment is always a block.
                 let before_conseq = conseq.span.shrink_to_lo();
                 let lifetime_end = source_map.end_point(conseq.span);
@@ -159,6 +166,8 @@ impl IfLetRescope {
                     if ty_ascription.is_some()
                         || !expr.span.can_be_used_for_suggestions()
                         || !pat.span.can_be_used_for_suggestions()
+                        || !if_let_pat.can_be_used_for_suggestions()
+                        || !before_conseq.can_be_used_for_suggestions()
                     {
                         // Our `match` rewrites does not support type ascription,
                         // so we just bail.
@@ -240,6 +249,23 @@ impl<'tcx> LateLintPass<'tcx> for IfLetRescope {
         if let (Level::Allow, _) = cx.tcx.lint_level_at_node(IF_LET_RESCOPE, expr.hir_id) {
             return;
         }
+        if let hir::ExprKind::Loop(block, _label, hir::LoopSource::While, _span) = expr.kind
+            && let Some(value) = block.expr
+            && let hir::ExprKind::If(cond, _conseq, _alt) = value.kind
+            && let hir::ExprKind::Let(..) = cond.kind
+        {
+            // Recall that `while let` is lowered into this:
+            // ```
+            // loop {
+            //     if let .. { body } else { break; }
+            // }
+            // ```
+            // There is no observable change in drop order on the overall `if let` expression
+            // given that the `{ break; }` block is trivial so the edition change
+            // means nothing substantial to this `while` statement.
+            self.skip.insert(value.hir_id);
+            return;
+        }
         if expr_parent_is_stmt(cx.tcx, expr.hir_id)
             && matches!(expr.kind, hir::ExprKind::If(_cond, _conseq, None))
         {