Auto merge of #107251 - dingxiangfei2009:let-chain-rescope, r=jieyouxu

Rescope temp lifetime in if-let into IfElse with migration lint

Tracking issue #124085

This PR shortens the temporary lifetime to cover only the pattern matching and consequent branch of a `if let`.

At the expression location, means that the lifetime is shortened from previously the deepest enclosing block or statement in Edition 2021. This warrants an Edition change.

Coming with the Edition change, this patch also implements an edition lint to warn about the change and a safe rewrite suggestion to preserve the 2021 semantics in most cases.

Related to #103108.
Related crater runs: https://github.com/rust-lang/rust/pull/129466.

3 files changed, 18 insertions, 1 deletions

diff --git a/compiler/rustc_middle/src/middle/region.rs b/compiler/rustc_middle/src/middle/region.rs
index 6ef1801717c..999743ed73b 100644
--- a/compiler/rustc_middle/src/middle/region.rs
+++ b/compiler/rustc_middle/src/middle/region.rs
@@ -96,6 +96,7 @@ impl fmt::Debug for Scope {
             ScopeData::Arguments => write!(fmt, "Arguments({:?})", self.id),
             ScopeData::Destruction => write!(fmt, "Destruction({:?})", self.id),
             ScopeData::IfThen => write!(fmt, "IfThen({:?})", self.id),
+            ScopeData::IfThenRescope => write!(fmt, "IfThen[edition2024]({:?})", self.id),
             ScopeData::Remainder(fsi) => write!(
                 fmt,
                 "Remainder {{ block: {:?}, first_statement_index: {}}}",
@@ -126,6 +127,11 @@ pub enum ScopeData {
     /// Used for variables introduced in an if-let expression.
     IfThen,
 
+    /// Scope of the condition and then block of an if expression
+    /// Used for variables introduced in an if-let expression,
+    /// whose lifetimes do not cross beyond this scope.
+    IfThenRescope,
+
     /// Scope following a `let id = expr;` binding in a block.
     Remainder(FirstStatementIndex),
 }
diff --git a/compiler/rustc_middle/src/mir/mod.rs b/compiler/rustc_middle/src/mir/mod.rs
index 26addb1e357..e312e65cc21 100644
--- a/compiler/rustc_middle/src/mir/mod.rs
+++ b/compiler/rustc_middle/src/mir/mod.rs
@@ -1084,6 +1084,9 @@ pub enum LocalInfo<'tcx> {
     /// (with no intervening statement context).
     // FIXME(matthewjasper) Don't store in this in `Body`
     BlockTailTemp(BlockTailInfo),
+    /// A temporary created during evaluating `if` predicate, possibly for pattern matching for `let`s,
+    /// and subject to Edition 2024 temporary lifetime rules
+    IfThenRescopeTemp { if_then: HirId },
     /// A temporary created during the pass `Derefer` to avoid it's retagging
     DerefTemp,
     /// A temporary created for borrow checking.
diff --git a/compiler/rustc_middle/src/ty/rvalue_scopes.rs b/compiler/rustc_middle/src/ty/rvalue_scopes.rs
index bcab54cf8ba..37dcf7c0d64 100644
--- a/compiler/rustc_middle/src/ty/rvalue_scopes.rs
+++ b/compiler/rustc_middle/src/ty/rvalue_scopes.rs
@@ -41,7 +41,15 @@ impl RvalueScopes {
                     debug!("temporary_scope({expr_id:?}) = {id:?} [enclosing]");
                     return Some(id);
                 }
-                _ => id = p,
+                ScopeData::IfThenRescope => {
+                    debug!("temporary_scope({expr_id:?}) = {p:?} [enclosing]");
+                    return Some(p);
+                }
+                ScopeData::Node
+                | ScopeData::CallSite
+                | ScopeData::Arguments
+                | ScopeData::IfThen
+                | ScopeData::Remainder(_) => id = p,
             }
         }