Auto merge of #89933 - est31:let_else, r=michaelwoerister

Adopt let_else across the compiler

This performs a substitution of code following the pattern:

```
let <id> = if let <pat> = ... { identity } else { ... : ! };
```

To simplify it to:

```
let <pat> = ... { identity } else { ... : ! };
```

By adopting the `let_else` feature (cc #87335).

The PR also updates the syn crate because the currently used version of the crate doesn't support `let_else` syntax yet.

Note: Generally I'm the person who *removes* usages of unstable features from the compiler, not adds more usages of them, but in this instance I think it hopefully helps the feature get stabilized sooner and in a better state. I have written a [comment](#87335 (comment)) on the tracking issue about my experience and what I feel could be improved before stabilization of `let_else`.

Cargo.lock

@@ -2728,9 +2728,9 @@ checksum = "bc881b2c22681370c6a780e47af9840ef841837bc98118431d4e1868bd0c1086"
 
 [[package]]
 name = "proc-macro2"
-version = "1.0.24"
+version = "1.0.30"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "1e0704ee1a7e00d7bb417d0770ea303c1bccbabf0ef1667dae92b5967f5f8a71"
+checksum = "edc3358ebc67bc8b7fa0c007f945b0b18226f78437d61bec735a9eb96b61ee70"
 dependencies = [
  "unicode-xid",
 ]
@@ -5093,9 +5093,9 @@ dependencies = [
 
 [[package]]
 name = "syn"
-version = "1.0.65"
+version = "1.0.80"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f3a1d708c221c5a612956ef9f75b37e454e88d1f7b899fbd3a18d4252012d663"
+checksum = "d010a1623fbd906d51d650a9916aaefc05ffa0e4053ff7fe601167f3e715d194"
 dependencies = [
  "proc-macro2",
  "quote",

compiler/rustc_borrowck/src/borrow_set.rs

@@ -315,9 +315,7 @@ impl<'a, 'tcx> GatherBorrows<'a, 'tcx> {
         //    TEMP = &foo
         //
         // so extract `temp`.
-        let temp = if let Some(temp) = assigned_place.as_local() {
-            temp
-        } else {
+        let Some(temp) = assigned_place.as_local() else {
             span_bug!(
                 self.body.source_info(start_location).span,
                 "expected 2-phase borrow to assign to a local, not `{:?}`",

compiler/rustc_borrowck/src/diagnostics/outlives_suggestion.rs

@@ -90,9 +90,7 @@ impl OutlivesSuggestionBuilder {
         let mut unified_already = FxHashSet::default();
 
         for (fr, outlived) in &self.constraints_to_add {
-            let fr_name = if let Some(fr_name) = self.region_vid_to_name(mbcx, *fr) {
-                fr_name
-            } else {
+            let Some(fr_name) = self.region_vid_to_name(mbcx, *fr) else {
                 continue;
             };
 

compiler/rustc_borrowck/src/lib.rs

@@ -7,6 +7,7 @@
 #![feature(format_args_capture)]
 #![feature(in_band_lifetimes)]
 #![feature(iter_zip)]
+#![feature(let_else)]
 #![feature(min_specialization)]
 #![feature(stmt_expr_attributes)]
 #![feature(trusted_step)]

compiler/rustc_codegen_ssa/src/back/link.rs

@@ -174,9 +174,8 @@ pub fn each_linked_rlib(
             _ => {}
         }
     }
-    let fmts = match fmts {
-        Some(f) => f,
-        None => return Err("could not find formats for rlibs".to_string()),
+    let Some(fmts) = fmts else {
+        return Err("could not find formats for rlibs".to_string());
     };
     for &cnum in crates {
         match fmts.get(cnum.as_usize() - 1) {

compiler/rustc_codegen_ssa/src/lib.rs

@@ -3,6 +3,7 @@
 #![feature(box_patterns)]
 #![feature(try_blocks)]
 #![feature(in_band_lifetimes)]
+#![feature(let_else)]
 #![feature(once_cell)]
 #![feature(nll)]
 #![feature(associated_type_bounds)]

compiler/rustc_codegen_ssa/src/mir/operand.rs

@@ -343,9 +343,7 @@ impl<'a, 'tcx, V: CodegenObject> OperandValue<V> {
             .unwrap_or_else(|| bug!("indirect_dest has non-pointer type: {:?}", indirect_dest))
             .ty;
 
-        let (llptr, llextra) = if let OperandValue::Ref(llptr, Some(llextra), _) = self {
-            (llptr, llextra)
-        } else {
+        let OperandValue::Ref(llptr, Some(llextra), _) = self else {
             bug!("store_unsized called with a sized value")
         };
 

compiler/rustc_const_eval/src/lib.rs

@@ -13,6 +13,7 @@ Rust MIR: a lowered representation of Rust.
 #![feature(exact_size_is_empty)]
 #![feature(in_band_lifetimes)]
 #![feature(iter_zip)]
+#![feature(let_else)]
 #![feature(map_try_insert)]
 #![feature(min_specialization)]
 #![feature(slice_ptr_get)]

compiler/rustc_const_eval/src/transform/check_consts/qualifs.rs

@@ -145,9 +145,7 @@ impl Qualif for NeedsNonConstDrop {
             Ok([..]) => {}
         }
 
-        let drop_trait = if let Some(did) = cx.tcx.lang_items().drop_trait() {
-            did
-        } else {
+        let Some(drop_trait) = cx.tcx.lang_items().drop_trait() else {
             // there is no way to define a type that needs non-const drop
             // without having the lang item present.
             return false;

compiler/rustc_errors/src/emitter.rs

@@ -449,11 +449,7 @@ pub trait Emitter {
         span: &mut MultiSpan,
         children: &mut Vec<SubDiagnostic>,
     ) {
-        let source_map = if let Some(ref sm) = source_map {
-            sm
-        } else {
-            return;
-        };
+        let Some(source_map) = source_map else { return };
         debug!("fix_multispans_in_extern_macros: before: span={:?} children={:?}", span, children);
         self.fix_multispan_in_extern_macros(source_map, span);
         for child in children.iter_mut() {

compiler/rustc_errors/src/lib.rs

@@ -8,6 +8,7 @@
 #![feature(if_let_guard)]
 #![feature(format_args_capture)]
 #![feature(iter_zip)]
+#![feature(let_else)]
 #![feature(nll)]
 
 #[macro_use]

compiler/rustc_expand/src/expand.rs

@@ -447,9 +447,7 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
         let mut undetermined_invocations = Vec::new();
         let (mut progress, mut force) = (false, !self.monotonic);
         loop {
-            let (invoc, ext) = if let Some(invoc) = invocations.pop() {
-                invoc
-            } else {
+            let Some((invoc, ext)) = invocations.pop() else {
                 self.resolve_imports();
                 if undetermined_invocations.is_empty() {
                     break;

compiler/rustc_expand/src/lib.rs

@@ -4,6 +4,7 @@
 #![feature(format_args_capture)]
 #![feature(if_let_guard)]
 #![feature(iter_zip)]
+#![feature(let_else)]
 #![feature(proc_macro_diagnostic)]
 #![feature(proc_macro_internals)]
 #![feature(proc_macro_span)]

compiler/rustc_expand/src/mbe/transcribe.rs

@@ -116,10 +116,8 @@ pub(super) fn transcribe<'a>(
 
     loop {
         // Look at the last frame on the stack.
-        let tree = if let Some(tree) = stack.last_mut().unwrap().next() {
-            // If it still has a TokenTree we have not looked at yet, use that tree.
-            tree
-        } else {
+        // If it still has a TokenTree we have not looked at yet, use that tree.
+        let Some(tree) = stack.last_mut().unwrap().next() else {
             // This else-case never produces a value for `tree` (it `continue`s or `return`s).
 
             // Otherwise, if we have just reached the end of a sequence and we can keep repeating,
@@ -190,9 +188,7 @@ pub(super) fn transcribe<'a>(
                     LockstepIterSize::Constraint(len, _) => {
                         // We do this to avoid an extra clone above. We know that this is a
                         // sequence already.
-                        let (sp, seq) = if let mbe::TokenTree::Sequence(sp, seq) = seq {
-                            (sp, seq)
-                        } else {
+                        let mbe::TokenTree::Sequence(sp, seq) = seq else {
                             unreachable!()
                         };
 

compiler/rustc_incremental/src/lib.rs

@@ -2,6 +2,7 @@
 
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![feature(in_band_lifetimes)]
+#![feature(let_else)]
 #![feature(nll)]
 #![recursion_limit = "256"]
 

compiler/rustc_incremental/src/persist/fs.rs

@@ -241,9 +241,7 @@ pub fn prepare_session_directory(
         // have already tried before.
         let source_directory = find_source_directory(&crate_dir, &source_directories_already_tried);
 
-        let source_directory = if let Some(dir) = source_directory {
-            dir
-        } else {
+        let Some(source_directory) = source_directory else {
             // There's nowhere to copy from, we're done
             debug!(
                 "no source directory found. Continuing with empty session \
@@ -397,15 +395,14 @@ fn copy_files(sess: &Session, target_dir: &Path, source_dir: &Path) -> Result<bo
     // We acquire a shared lock on the lock file of the directory, so that
     // nobody deletes it out from under us while we are reading from it.
     let lock_file_path = lock_file_path(source_dir);
-    let _lock = if let Ok(lock) = flock::Lock::new(
+
+    // not exclusive
+    let Ok(_lock) = flock::Lock::new(
         &lock_file_path,
         false, // don't wait,
         false, // don't create
         false,
-    ) {
-        // not exclusive
-        lock
-    } else {
+    ) else {
         // Could not acquire the lock, don't try to copy from here
         return Err(());
     };

compiler/rustc_infer/src/infer/error_reporting/nice_region_error/static_impl_trait.rs

@@ -263,9 +263,7 @@ pub fn suggest_new_region_bound(
         match fn_return.kind {
             TyKind::OpaqueDef(item_id, _) => {
                 let item = tcx.hir().item(item_id);
-                let opaque = if let ItemKind::OpaqueTy(opaque) = &item.kind {
-                    opaque
-                } else {
+                let ItemKind::OpaqueTy(opaque) = &item.kind else {
                     return;
                 };
 

compiler/rustc_infer/src/lib.rs

@@ -17,6 +17,7 @@
 #![feature(box_patterns)]
 #![feature(extend_one)]
 #![feature(iter_zip)]
+#![feature(let_else)]
 #![feature(never_type)]
 #![feature(in_band_lifetimes)]
 #![feature(control_flow_enum)]

compiler/rustc_metadata/src/lib.rs

@@ -2,6 +2,7 @@
 #![feature(crate_visibility_modifier)]
 #![feature(drain_filter)]
 #![feature(in_band_lifetimes)]
+#![feature(let_else)]
 #![feature(nll)]
 #![feature(once_cell)]
 #![feature(proc_macro_internals)]

compiler/rustc_metadata/src/rmeta/decoder.rs

@@ -472,9 +472,7 @@ impl<'a, 'tcx> Decodable<DecodeContext<'a, 'tcx>> for Span {
         let len = BytePos::decode(decoder)?;
         let hi = lo + len;
 
-        let sess = if let Some(sess) = decoder.sess {
-            sess
-        } else {
+        let Some(sess) = decoder.sess else {
             bug!("Cannot decode Span without Session.")
         };
 

compiler/rustc_middle/src/lib.rs

@@ -39,6 +39,7 @@
 #![feature(new_uninit)]
 #![feature(nll)]
 #![feature(once_cell)]
+#![feature(let_else)]
 #![feature(min_specialization)]
 #![feature(trusted_len)]
 #![feature(in_band_lifetimes)]

compiler/rustc_middle/src/ty/diagnostics.rs

@@ -221,9 +221,7 @@ pub fn suggest_constraining_type_param(
 ) -> bool {
     let param = generics.params.iter().find(|p| p.name.ident().as_str() == param_name);
 
-    let param = if let Some(param) = param {
-        param
-    } else {
+    let Some(param) = param else {
         return false;
     };
 

compiler/rustc_middle/src/ty/layout.rs

@@ -755,17 +755,14 @@ impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
                     }
 
                     // Extract the number of elements from the layout of the array field:
-                    let len = if let Ok(TyAndLayout {
+                    let Ok(TyAndLayout {
                         layout: Layout { fields: FieldsShape::Array { count, .. }, .. },
                         ..
-                    }) = self.layout_of(f0_ty)
-                    {
-                        count
-                    } else {
+                    }) = self.layout_of(f0_ty) else {
                         return Err(LayoutError::Unknown(ty));
                     };
 
-                    (*e_ty, *len, true)
+                    (*e_ty, *count, true)
                 } else {
                     // First ADT field is not an array:
                     (f0_ty, def.non_enum_variant().fields.len() as _, false)
@@ -787,9 +784,7 @@ impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
 
                 // Compute the ABI of the element type:
                 let e_ly = self.layout_of(e_ty)?;
-                let e_abi = if let Abi::Scalar(scalar) = e_ly.abi {
-                    scalar
-                } else {
+                let Abi::Scalar(e_abi) = e_ly.abi else {
                     // This error isn't caught in typeck, e.g., if
                     // the element type of the vector is generic.
                     tcx.sess.fatal(&format!(

compiler/rustc_mir_build/src/build/expr/as_place.rs

@@ -221,15 +221,13 @@ fn to_upvars_resolved_place_builder<'a, 'tcx>(
             let closure_hir_id = tcx.hir().local_def_id_to_hir_id(closure_def_id.expect_local());
             let closure_span = tcx.hir().span(closure_hir_id);
 
-            let (capture_index, capture) = if let Some(capture_details) =
+            let Some((capture_index, capture)) =
                 find_capture_matching_projections(
                     typeck_results,
                     var_hir_id,
                     closure_def_id,
                     &from_builder.projection,
-                ) {
-                capture_details
-            } else {
+                ) else {
                 if !enable_precise_capture(tcx, closure_span) {
                     bug!(
                         "No associated capture found for {:?}[{:#?}] even though \

compiler/rustc_mir_build/src/build/scope.rs

@@ -362,11 +362,7 @@ impl DropTree {
         blocks: &IndexVec<DropIdx, Option<BasicBlock>>,
     ) {
         for (drop_idx, drop_data) in self.drops.iter_enumerated().rev() {
-            let block = if let Some(block) = blocks[drop_idx] {
-                block
-            } else {
-                continue;
-            };
+            let Some(block) = blocks[drop_idx] else { continue };
             match drop_data.0.kind {
                 DropKind::Value => {
                     let terminator = TerminatorKind::Drop {

compiler/rustc_mir_build/src/lib.rs

@@ -6,6 +6,7 @@
 #![feature(crate_visibility_modifier)]
 #![feature(bool_to_option)]
 #![feature(iter_zip)]
+#![feature(let_else)]
 #![feature(once_cell)]
 #![feature(min_specialization)]
 #![recursion_limit = "256"]

compiler/rustc_mir_dataflow/src/lib.rs

@@ -6,6 +6,7 @@
 #![feature(exact_size_is_empty)]
 #![feature(in_band_lifetimes)]
 #![feature(iter_zip)]
+#![feature(let_else)]
 #![feature(min_specialization)]
 #![feature(once_cell)]
 #![feature(stmt_expr_attributes)]

compiler/rustc_mir_dataflow/src/rustc_peek.rs

@@ -290,9 +290,7 @@ impl<'tcx> RustcPeekAt<'tcx> for MaybeMutBorrowedLocals<'_, 'tcx> {
         call: PeekCall,
     ) {
         info!(?place, "peek_at");
-        let local = if let Some(l) = place.as_local() {
-            l
-        } else {
+        let Some(local) = place.as_local() else {
             tcx.sess.span_err(call.span, "rustc_peek: argument was not a local");
             return;
         };
@@ -312,9 +310,7 @@ impl<'tcx> RustcPeekAt<'tcx> for MaybeLiveLocals {
         call: PeekCall,
     ) {
         info!(?place, "peek_at");
-        let local = if let Some(l) = place.as_local() {
-            l
-        } else {
+        let Some(local) = place.as_local() else {
             tcx.sess.span_err(call.span, "rustc_peek: argument was not a local");
             return;
         };

compiler/rustc_mir_transform/src/inline.rs

@@ -673,9 +673,7 @@ impl Inliner<'tcx> {
             assert!(args.next().is_none());
 
             let tuple = Place::from(tuple);
-            let tuple_tys = if let ty::Tuple(s) = tuple.ty(caller_body, tcx).ty.kind() {
-                s
-            } else {
+            let ty::Tuple(tuple_tys) = tuple.ty(caller_body, tcx).ty.kind() else {
                 bug!("Closure arguments are not passed as a tuple");
             };
 

compiler/rustc_mir_transform/src/lib.rs

@@ -4,6 +4,7 @@
 #![cfg_attr(bootstrap, feature(const_panic))]
 #![feature(in_band_lifetimes)]
 #![feature(iter_zip)]
+#![feature(let_else)]
 #![feature(map_try_insert)]
 #![feature(min_specialization)]
 #![feature(option_get_or_insert_default)]

compiler/rustc_mir_transform/src/lower_slice_len.rs

@@ -17,9 +17,7 @@ impl<'tcx> MirPass<'tcx> for LowerSliceLenCalls {
 
 pub fn lower_slice_len_calls<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
     let language_items = tcx.lang_items();
-    let slice_len_fn_item_def_id = if let Some(slice_len_fn_item) = language_items.slice_len_fn() {
-        slice_len_fn_item
-    } else {
+    let Some(slice_len_fn_item_def_id) = language_items.slice_len_fn() else {
         // there is no language item to compare to :)
         return;
     };