Auto merge of #122338 - workingjubilee:rollup-xzpt4v4, r=workingjubilee

Rollup of 15 pull requests

Successful merges:

 - #116791 (Allow codegen backends to opt-out of parallel codegen)
 - #116793 (Allow targets to override default codegen backend)
 - #117458 (LLVM Bitcode Linker: A self contained linker for nvptx and other targets)
 - #119385 (Fix type resolution of associated const equality bounds (take 2))
 - #121438 (std support for wasm32 panic=unwind)
 - #121893 (Add tests (and a bit of cleanup) for interior mut handling in promotion and const-checking)
 - #122080 (Clarity improvements to `DropTree`)
 - #122152 (Improve diagnostics for parenthesized type arguments)
 - #122166 (Remove the unused `field_remapping` field from `TypeLowering`)
 - #122249 (interpret: do not call machine read hooks during validation)
 - #122299 (Store backtrace for `must_produce_diag`)
 - #122318 (Revision-related tweaks for next-solver tests)
 - #122320 (Use ptradd for vtable indexing)
 - #122328 (unix_sigpipe: Replace `inherit` with `sig_dfl` in syntax tests)
 - #122330 (bootstrap readme: fix, improve, update)

r? `@ghost`
`@rustbot` modify labels: rollup

32 files changed, 577 insertions, 239 deletions

diff --git a/compiler/rustc_codegen_llvm/src/context.rs b/compiler/rustc_codegen_llvm/src/context.rs
index 37c690a0128..f89c8c9f836 100644
--- a/compiler/rustc_codegen_llvm/src/context.rs
+++ b/compiler/rustc_codegen_llvm/src/context.rs
@@ -77,8 +77,8 @@ pub struct CodegenCx<'ll, 'tcx> {
     /// See <https://llvm.org/docs/LangRef.html#the-llvm-compiler-used-global-variable> for details
     pub compiler_used_statics: RefCell<Vec<&'ll Value>>,
 
-    /// Mapping of non-scalar types to llvm types and field remapping if needed.
-    pub type_lowering: RefCell<FxHashMap<(Ty<'tcx>, Option<VariantIdx>), TypeLowering<'ll>>>,
+    /// Mapping of non-scalar types to llvm types.
+    pub type_lowering: RefCell<FxHashMap<(Ty<'tcx>, Option<VariantIdx>), &'ll Type>>,
 
     /// Mapping of scalar types to llvm types.
     pub scalar_lltypes: RefCell<FxHashMap<Ty<'tcx>, &'ll Type>>,
@@ -105,15 +105,6 @@ pub struct CodegenCx<'ll, 'tcx> {
     pub renamed_statics: RefCell<FxHashMap<DefId, &'ll Value>>,
 }
 
-pub struct TypeLowering<'ll> {
-    /// Associated LLVM type
-    pub lltype: &'ll Type,
-
-    /// If padding is used the slice maps fields from source order
-    /// to llvm order.
-    pub field_remapping: Option<SmallVec<[u32; 4]>>,
-}
-
 fn to_llvm_tls_model(tls_model: TlsModel) -> llvm::ThreadLocalMode {
     match tls_model {
         TlsModel::GeneralDynamic => llvm::ThreadLocalMode::GeneralDynamic,
diff --git a/compiler/rustc_codegen_llvm/src/llvm_util.rs b/compiler/rustc_codegen_llvm/src/llvm_util.rs
index 78d47f36f91..e32c38644aa 100644
--- a/compiler/rustc_codegen_llvm/src/llvm_util.rs
+++ b/compiler/rustc_codegen_llvm/src/llvm_util.rs
@@ -5,6 +5,7 @@ use crate::errors::{
 };
 use crate::llvm;
 use libc::c_int;
+use rustc_codegen_ssa::base::wants_wasm_eh;
 use rustc_codegen_ssa::traits::PrintBackendInfo;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_data_structures::small_c_str::SmallCStr;
@@ -98,6 +99,10 @@ unsafe fn configure_llvm(sess: &Session) {
             }
         }
 
+        if wants_wasm_eh(sess) {
+            add("-wasm-enable-eh", false);
+        }
+
         if sess.target.os == "emscripten" && sess.panic_strategy() == PanicStrategy::Unwind {
             add("-enable-emscripten-cxx-exceptions", false);
         }
@@ -523,6 +528,10 @@ pub(crate) fn global_llvm_features(sess: &Session, diagnostics: bool) -> Vec<Str
             .map(String::from),
     );
 
+    if wants_wasm_eh(sess) && sess.panic_strategy() == PanicStrategy::Unwind {
+        features.push("+exception-handling".into());
+    }
+
     // -Ctarget-features
     let supported_features = sess.target.supported_target_features();
     let mut featsmap = FxHashMap::default();
diff --git a/compiler/rustc_codegen_llvm/src/type_of.rs b/compiler/rustc_codegen_llvm/src/type_of.rs
index 587adefe1d2..d10a083765b 100644
--- a/compiler/rustc_codegen_llvm/src/type_of.rs
+++ b/compiler/rustc_codegen_llvm/src/type_of.rs
@@ -1,5 +1,4 @@
 use crate::common::*;
-use crate::context::TypeLowering;
 use crate::type_::Type;
 use rustc_codegen_ssa::traits::*;
 use rustc_middle::bug;
@@ -10,7 +9,6 @@ use rustc_target::abi::HasDataLayout;
 use rustc_target::abi::{Abi, Align, FieldsShape};
 use rustc_target::abi::{Int, Pointer, F128, F16, F32, F64};
 use rustc_target::abi::{Scalar, Size, Variants};
-use smallvec::{smallvec, SmallVec};
 
 use std::fmt::Write;
 
@@ -18,7 +16,6 @@ fn uncached_llvm_type<'a, 'tcx>(
     cx: &CodegenCx<'a, 'tcx>,
     layout: TyAndLayout<'tcx>,
     defer: &mut Option<(&'a Type, TyAndLayout<'tcx>)>,
-    field_remapping: &mut Option<SmallVec<[u32; 4]>>,
 ) -> &'a Type {
     match layout.abi {
         Abi::Scalar(_) => bug!("handled elsewhere"),
@@ -71,8 +68,7 @@ fn uncached_llvm_type<'a, 'tcx>(
         FieldsShape::Array { count, .. } => cx.type_array(layout.field(cx, 0).llvm_type(cx), count),
         FieldsShape::Arbitrary { .. } => match name {
             None => {
-                let (llfields, packed, new_field_remapping) = struct_llfields(cx, layout);
-                *field_remapping = new_field_remapping;
+                let (llfields, packed) = struct_llfields(cx, layout);
                 cx.type_struct(&llfields, packed)
             }
             Some(ref name) => {
@@ -87,7 +83,7 @@ fn uncached_llvm_type<'a, 'tcx>(
 fn struct_llfields<'a, 'tcx>(
     cx: &CodegenCx<'a, 'tcx>,
     layout: TyAndLayout<'tcx>,
-) -> (Vec<&'a Type>, bool, Option<SmallVec<[u32; 4]>>) {
+) -> (Vec<&'a Type>, bool) {
     debug!("struct_llfields: {:#?}", layout);
     let field_count = layout.fields.count();
 
@@ -95,7 +91,6 @@ fn struct_llfields<'a, 'tcx>(
     let mut offset = Size::ZERO;
     let mut prev_effective_align = layout.align.abi;
     let mut result: Vec<_> = Vec::with_capacity(1 + field_count * 2);
-    let mut field_remapping = smallvec![0; field_count];
     for i in layout.fields.index_by_increasing_offset() {
         let target_offset = layout.fields.offset(i as usize);
         let field = layout.field(cx, i);
@@ -120,12 +115,10 @@ fn struct_llfields<'a, 'tcx>(
             result.push(cx.type_padding_filler(padding, padding_align));
             debug!("    padding before: {:?}", padding);
         }
-        field_remapping[i] = result.len() as u32;
         result.push(field.llvm_type(cx));
         offset = target_offset + field.size;
         prev_effective_align = effective_field_align;
     }
-    let padding_used = result.len() > field_count;
     if layout.is_sized() && field_count > 0 {
         if offset > layout.size {
             bug!("layout: {:#?} stride: {:?} offset: {:?}", layout, layout.size, offset);
@@ -143,8 +136,7 @@ fn struct_llfields<'a, 'tcx>(
     } else {
         debug!("struct_llfields: offset: {:?} stride: {:?}", offset, layout.size);
     }
-    let field_remapping = padding_used.then_some(field_remapping);
-    (result, packed, field_remapping)
+    (result, packed)
 }
 
 impl<'a, 'tcx> CodegenCx<'a, 'tcx> {
@@ -224,7 +216,7 @@ impl<'tcx> LayoutLlvmExt<'tcx> for TyAndLayout<'tcx> {
             _ => None,
         };
         if let Some(llty) = cx.type_lowering.borrow().get(&(self.ty, variant_index)) {
-            return llty.lltype;
+            return llty;
         }
 
         debug!("llvm_type({:#?})", self);
@@ -236,7 +228,6 @@ impl<'tcx> LayoutLlvmExt<'tcx> for TyAndLayout<'tcx> {
         let normal_ty = cx.tcx.erase_regions(self.ty);
 
         let mut defer = None;
-        let mut field_remapping = None;
         let llty = if self.ty != normal_ty {
             let mut layout = cx.layout_of(normal_ty);
             if let Some(v) = variant_index {
@@ -244,22 +235,15 @@ impl<'tcx> LayoutLlvmExt<'tcx> for TyAndLayout<'tcx> {
             }
             layout.llvm_type(cx)
         } else {
-            uncached_llvm_type(cx, *self, &mut defer, &mut field_remapping)
+            uncached_llvm_type(cx, *self, &mut defer)
         };
         debug!("--> mapped {:#?} to llty={:?}", self, llty);
 
-        cx.type_lowering
-            .borrow_mut()
-            .insert((self.ty, variant_index), TypeLowering { lltype: llty, field_remapping });
+        cx.type_lowering.borrow_mut().insert((self.ty, variant_index), llty);
 
         if let Some((llty, layout)) = defer {
-            let (llfields, packed, new_field_remapping) = struct_llfields(cx, layout);
+            let (llfields, packed) = struct_llfields(cx, layout);
             cx.set_struct_body(llty, &llfields, packed);
-            cx.type_lowering
-                .borrow_mut()
-                .get_mut(&(self.ty, variant_index))
-                .unwrap()
-                .field_remapping = new_field_remapping;
         }
         llty
     }
diff --git a/compiler/rustc_codegen_ssa/src/back/link.rs b/compiler/rustc_codegen_ssa/src/back/link.rs
index fcb3602b734..e70cc9b6216 100644
--- a/compiler/rustc_codegen_ssa/src/back/link.rs
+++ b/compiler/rustc_codegen_ssa/src/back/link.rs
@@ -24,6 +24,7 @@ use rustc_span::symbol::Symbol;
 use rustc_target::spec::crt_objects::CrtObjects;
 use rustc_target::spec::LinkSelfContainedComponents;
 use rustc_target::spec::LinkSelfContainedDefault;
+use rustc_target::spec::LinkerFlavorCli;
 use rustc_target::spec::{Cc, LinkOutputKind, LinkerFlavor, Lld, PanicStrategy};
 use rustc_target::spec::{RelocModel, RelroLevel, SanitizerSet, SplitDebuginfo};
 
@@ -1350,6 +1351,7 @@ pub fn linker_and_flavor(sess: &Session) -> (PathBuf, LinkerFlavor) {
                         }
                     }
                     LinkerFlavor::Bpf => "bpf-linker",
+                    LinkerFlavor::Llbc => "llvm-bitcode-linker",
                     LinkerFlavor::Ptx => "rust-ptx-linker",
                 }),
                 flavor,
@@ -1367,8 +1369,17 @@ pub fn linker_and_flavor(sess: &Session) -> (PathBuf, LinkerFlavor) {
 
     // linker and linker flavor specified via command line have precedence over what the target
     // specification specifies
-    let linker_flavor =
-        sess.opts.cg.linker_flavor.map(|flavor| sess.target.linker_flavor.with_cli_hints(flavor));
+    let linker_flavor = match sess.opts.cg.linker_flavor {
+        // The linker flavors that are non-target specific can be directly translated to LinkerFlavor
+        Some(LinkerFlavorCli::Llbc) => Some(LinkerFlavor::Llbc),
+        Some(LinkerFlavorCli::Ptx) => Some(LinkerFlavor::Ptx),
+        // The linker flavors that corresponds to targets needs logic that keeps the base LinkerFlavor
+        _ => sess
+            .opts
+            .cg
+            .linker_flavor
+            .map(|flavor| sess.target.linker_flavor.with_cli_hints(flavor)),
+    };
     if let Some(ret) = infer_from(sess, sess.opts.cg.linker.clone(), linker_flavor) {
         return ret;
     }
@@ -2338,8 +2349,12 @@ fn add_order_independent_options(
         });
     }
 
-    if flavor == LinkerFlavor::Ptx {
-        // Provide the linker with fallback to internal `target-cpu`.
+    if flavor == LinkerFlavor::Llbc {
+        cmd.arg("--target");
+        cmd.arg(sess.target.llvm_target.as_ref());
+        cmd.arg("--target-cpu");
+        cmd.arg(&codegen_results.crate_info.target_cpu);
+    } else if flavor == LinkerFlavor::Ptx {
         cmd.arg("--fallback-arch");
         cmd.arg(&codegen_results.crate_info.target_cpu);
     } else if flavor == LinkerFlavor::Bpf {
diff --git a/compiler/rustc_codegen_ssa/src/back/linker.rs b/compiler/rustc_codegen_ssa/src/back/linker.rs
index e52efd86955..b4e054417f3 100644
--- a/compiler/rustc_codegen_ssa/src/back/linker.rs
+++ b/compiler/rustc_codegen_ssa/src/back/linker.rs
@@ -153,6 +153,7 @@ pub fn get_linker<'a>(
         LinkerFlavor::Msvc(..) => Box::new(MsvcLinker { cmd, sess }) as Box<dyn Linker>,
         LinkerFlavor::EmCc => Box::new(EmLinker { cmd, sess }) as Box<dyn Linker>,
         LinkerFlavor::Bpf => Box::new(BpfLinker { cmd, sess }) as Box<dyn Linker>,
+        LinkerFlavor::Llbc => Box::new(LlbcLinker { cmd, sess }) as Box<dyn Linker>,
         LinkerFlavor::Ptx => Box::new(PtxLinker { cmd, sess }) as Box<dyn Linker>,
     }
 }
@@ -1824,7 +1825,7 @@ impl<'a> Linker for PtxLinker<'a> {
             }
 
             Lto::No => {}
-        };
+        }
     }
 
     fn output_filename(&mut self, path: &Path) {
@@ -1862,6 +1863,104 @@ impl<'a> Linker for PtxLinker<'a> {
     fn linker_plugin_lto(&mut self) {}
 }
 
+/// The `self-contained` LLVM bitcode linker
+pub struct LlbcLinker<'a> {
+    cmd: Command,
+    sess: &'a Session,
+}
+
+impl<'a> Linker for LlbcLinker<'a> {
+    fn cmd(&mut self) -> &mut Command {
+        &mut self.cmd
+    }
+
+    fn set_output_kind(&mut self, _output_kind: LinkOutputKind, _out_filename: &Path) {}
+
+    fn link_dylib_by_name(&mut self, _name: &str, _verbatim: bool, _as_needed: bool) {
+        panic!("external dylibs not supported")
+    }
+
+    fn link_staticlib_by_name(
+        &mut self,
+        _name: &str,
+        _verbatim: bool,
+        _whole_archive: bool,
+        _search_paths: &SearchPaths,
+    ) {
+        panic!("staticlibs not supported")
+    }
+
+    fn link_staticlib_by_path(&mut self, path: &Path, _whole_archive: bool) {
+        self.cmd.arg(path);
+    }
+
+    fn include_path(&mut self, path: &Path) {
+        self.cmd.arg("-L").arg(path);
+    }
+
+    fn debuginfo(&mut self, _strip: Strip, _: &[PathBuf]) {
+        self.cmd.arg("--debug");
+    }
+
+    fn add_object(&mut self, path: &Path) {
+        self.cmd.arg(path);
+    }
+
+    fn optimize(&mut self) {
+        match self.sess.opts.optimize {
+            OptLevel::No => "-O0",
+            OptLevel::Less => "-O1",
+            OptLevel::Default => "-O2",
+            OptLevel::Aggressive => "-O3",
+            OptLevel::Size => "-Os",
+            OptLevel::SizeMin => "-Oz",
+        };
+    }
+
+    fn output_filename(&mut self, path: &Path) {
+        self.cmd.arg("-o").arg(path);
+    }
+
+    fn framework_path(&mut self, _path: &Path) {
+        panic!("frameworks not supported")
+    }
+
+    fn full_relro(&mut self) {}
+
+    fn partial_relro(&mut self) {}
+
+    fn no_relro(&mut self) {}
+
+    fn gc_sections(&mut self, _keep_metadata: bool) {}
+
+    fn no_gc_sections(&mut self) {}
+
+    fn pgo_gen(&mut self) {}
+
+    fn no_crt_objects(&mut self) {}
+
+    fn no_default_libraries(&mut self) {}
+
+    fn control_flow_guard(&mut self) {}
+
+    fn ehcont_guard(&mut self) {}
+
+    fn export_symbols(&mut self, _tmpdir: &Path, _crate_type: CrateType, symbols: &[String]) {
+        match _crate_type {
+            CrateType::Cdylib => {
+                for sym in symbols {
+                    self.cmd.arg("--export-symbol").arg(sym);
+                }
+            }
+            _ => (),
+        }
+    }
+
+    fn subsystem(&mut self, _subsystem: &str) {}
+
+    fn linker_plugin_lto(&mut self) {}
+}
+
 pub struct BpfLinker<'a> {
     cmd: Command,
     sess: &'a Session,
diff --git a/compiler/rustc_codegen_ssa/src/back/write.rs b/compiler/rustc_codegen_ssa/src/back/write.rs
index bbb04652119..4eda4c2f08c 100644
--- a/compiler/rustc_codegen_ssa/src/back/write.rs
+++ b/compiler/rustc_codegen_ssa/src/back/write.rs
@@ -374,6 +374,10 @@ pub struct CodegenContext<B: WriteBackendMethods> {
     pub incr_comp_session_dir: Option<PathBuf>,
     /// Channel back to the main control thread to send messages to
     pub coordinator_send: Sender<Box<dyn Any + Send>>,
+    /// `true` if the codegen should be run in parallel.
+    ///
+    /// Depends on [`CodegenBackend::supports_parallel()`] and `-Zno_parallel_backend`.
+    pub parallel: bool,
 }
 
 impl<B: WriteBackendMethods> CodegenContext<B> {
@@ -1152,6 +1156,7 @@ fn start_executing_work<B: ExtraBackendMethods>(
         target_arch: tcx.sess.target.arch.to_string(),
         split_debuginfo: tcx.sess.split_debuginfo(),
         split_dwarf_kind: tcx.sess.opts.unstable_opts.split_dwarf_kind,
+        parallel: backend.supports_parallel() && !sess.opts.unstable_opts.no_parallel_backend,
     };
 
     // This is the "main loop" of parallel work happening for parallel codegen.
@@ -1422,7 +1427,7 @@ fn start_executing_work<B: ExtraBackendMethods>(
                             .binary_search_by_key(&cost, |&(_, cost)| cost)
                             .unwrap_or_else(|e| e);
                         work_items.insert(insertion_index, (work, cost));
-                        if !cgcx.opts.unstable_opts.no_parallel_llvm {
+                        if cgcx.parallel {
                             helper.request_token();
                         }
                     }
@@ -1545,7 +1550,7 @@ fn start_executing_work<B: ExtraBackendMethods>(
                     };
                     work_items.insert(insertion_index, (llvm_work_item, cost));
 
-                    if !cgcx.opts.unstable_opts.no_parallel_llvm {
+                    if cgcx.parallel {
                         helper.request_token();
                     }
                     assert_eq!(main_thread_state, MainThreadState::Codegenning);
diff --git a/compiler/rustc_codegen_ssa/src/base.rs b/compiler/rustc_codegen_ssa/src/base.rs
index 5cba14a5dda..c316d19e041 100644
--- a/compiler/rustc_codegen_ssa/src/base.rs
+++ b/compiler/rustc_codegen_ssa/src/base.rs
@@ -165,14 +165,11 @@ pub fn unsized_info<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
                 cx.tcx().vtable_trait_upcasting_coercion_new_vptr_slot((source, target));
 
             if let Some(entry_idx) = vptr_entry_idx {
-                let ptr_ty = cx.type_ptr();
-                let ptr_align = cx.tcx().data_layout.pointer_align.abi;
-                let gep = bx.inbounds_gep(
-                    ptr_ty,
-                    old_info,
-                    &[bx.const_usize(u64::try_from(entry_idx).unwrap())],
-                );
-                let new_vptr = bx.load(ptr_ty, gep, ptr_align);
+                let ptr_size = bx.data_layout().pointer_size;
+                let ptr_align = bx.data_layout().pointer_align.abi;
+                let vtable_byte_offset = u64::try_from(entry_idx).unwrap() * ptr_size.bytes();
+                let gep = bx.inbounds_ptradd(old_info, bx.const_usize(vtable_byte_offset));
+                let new_vptr = bx.load(bx.type_ptr(), gep, ptr_align);
                 bx.nonnull_metadata(new_vptr);
                 // VTable loads are invariant.
                 bx.set_invariant_load(new_vptr);
diff --git a/compiler/rustc_codegen_ssa/src/meth.rs b/compiler/rustc_codegen_ssa/src/meth.rs
index 12146a54d3b..4f7dc9968a1 100644
--- a/compiler/rustc_codegen_ssa/src/meth.rs
+++ b/compiler/rustc_codegen_ssa/src/meth.rs
@@ -20,9 +20,13 @@ impl<'a, 'tcx> VirtualIndex {
         ty: Ty<'tcx>,
         fn_abi: &FnAbi<'tcx, Ty<'tcx>>,
     ) -> Bx::Value {
-        // Load the data pointer from the object.
+        // Load the function pointer from the object.
         debug!("get_fn({llvtable:?}, {ty:?}, {self:?})");
+
         let llty = bx.fn_ptr_backend_type(fn_abi);
+        let ptr_size = bx.data_layout().pointer_size;
+        let ptr_align = bx.data_layout().pointer_align.abi;
+        let vtable_byte_offset = self.0 * ptr_size.bytes();
 
         if bx.cx().sess().opts.unstable_opts.virtual_function_elimination
             && bx.cx().sess().lto() == Lto::Fat
@@ -30,12 +34,10 @@ impl<'a, 'tcx> VirtualIndex {
             let typeid = bx
                 .typeid_metadata(typeid_for_trait_ref(bx.tcx(), expect_dyn_trait_in_self(ty)))
                 .unwrap();
-            let vtable_byte_offset = self.0 * bx.data_layout().pointer_size.bytes();
             let func = bx.type_checked_load(llvtable, vtable_byte_offset, typeid);
             func
         } else {
-            let ptr_align = bx.tcx().data_layout.pointer_align.abi;
-            let gep = bx.inbounds_gep(llty, llvtable, &[bx.const_usize(self.0)]);
+            let gep = bx.inbounds_ptradd(llvtable, bx.const_usize(vtable_byte_offset));
             let ptr = bx.load(llty, gep, ptr_align);
             bx.nonnull_metadata(ptr);
             // VTable loads are invariant.
@@ -53,9 +55,12 @@ impl<'a, 'tcx> VirtualIndex {
         debug!("get_int({:?}, {:?})", llvtable, self);
 
         let llty = bx.type_isize();
-        let usize_align = bx.tcx().data_layout.pointer_align.abi;
-        let gep = bx.inbounds_gep(llty, llvtable, &[bx.const_usize(self.0)]);
-        let ptr = bx.load(llty, gep, usize_align);
+        let ptr_size = bx.data_layout().pointer_size;
+        let ptr_align = bx.data_layout().pointer_align.abi;
+        let vtable_byte_offset = self.0 * ptr_size.bytes();
+
+        let gep = bx.inbounds_ptradd(llvtable, bx.const_usize(vtable_byte_offset));
+        let ptr = bx.load(llty, gep, ptr_align);
         // VTable loads are invariant.
         bx.set_invariant_load(ptr);
         ptr
diff --git a/compiler/rustc_codegen_ssa/src/mir/block.rs b/compiler/rustc_codegen_ssa/src/mir/block.rs
index 89a44d2897a..d3f5de25d9a 100644
--- a/compiler/rustc_codegen_ssa/src/mir/block.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/block.rs
@@ -1597,7 +1597,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         let funclet;
         let llbb;
         let mut bx;
-        if base::wants_msvc_seh(self.cx.sess()) {
+        if base::wants_new_eh_instructions(self.cx.sess()) {
             // This is a basic block that we're aborting the program for,
             // notably in an `extern` function. These basic blocks are inserted
             // so that we assert that `extern` functions do indeed not panic,
diff --git a/compiler/rustc_codegen_ssa/src/traits/backend.rs b/compiler/rustc_codegen_ssa/src/traits/backend.rs
index cb1feff1336..e6d42c596d2 100644
--- a/compiler/rustc_codegen_ssa/src/traits/backend.rs
+++ b/compiler/rustc_codegen_ssa/src/traits/backend.rs
@@ -111,6 +111,13 @@ pub trait CodegenBackend {
         codegen_results: CodegenResults,
         outputs: &OutputFilenames,
     ) -> Result<(), ErrorGuaranteed>;
+
+    /// Returns `true` if this backend can be safely called from multiple threads.
+    ///
+    /// Defaults to `true`.
+    fn supports_parallel(&self) -> bool {
+        true
+    }
 }
 
 pub trait ExtraBackendMethods:
diff --git a/compiler/rustc_const_eval/src/const_eval/eval_queries.rs b/compiler/rustc_const_eval/src/const_eval/eval_queries.rs
index 8ee3a0cc967..90884adb28c 100644
--- a/compiler/rustc_const_eval/src/const_eval/eval_queries.rs
+++ b/compiler/rustc_const_eval/src/const_eval/eval_queries.rs
@@ -380,16 +380,12 @@ pub fn eval_in_interpreter<'mir, 'tcx>(
         }
         Ok(mplace) => {
             // Since evaluation had no errors, validate the resulting constant.
-
-            // Temporarily allow access to the static_root_alloc_id for the purpose of validation.
-            let static_root_alloc_id = ecx.machine.static_root_alloc_id.take();
-            let validation = const_validate_mplace(&ecx, &mplace, cid);
-            ecx.machine.static_root_alloc_id = static_root_alloc_id;
+            let res = const_validate_mplace(&ecx, &mplace, cid);
 
             let alloc_id = mplace.ptr().provenance.unwrap().alloc_id();
 
             // Validation failed, report an error.
-            if let Err(error) = validation {
+            if let Err(error) = res {
                 Err(const_report_error(&ecx, error, alloc_id))
             } else {
                 // Convert to raw constant
@@ -412,10 +408,10 @@ pub fn const_validate_mplace<'mir, 'tcx>(
             _ if cid.promoted.is_some() => CtfeValidationMode::Promoted,
             Some(mutbl) => CtfeValidationMode::Static { mutbl }, // a `static`
             None => {
-                // In normal `const` (not promoted), the outermost allocation is always only copied,
-                // so having `UnsafeCell` in there is okay despite them being in immutable memory.
-                let allow_immutable_unsafe_cell = cid.promoted.is_none() && !inner;
-                CtfeValidationMode::Const { allow_immutable_unsafe_cell }
+                // This is a normal `const` (not promoted).
+                // The outermost allocation is always only copied, so having `UnsafeCell` in there
+                // is okay despite them being in immutable memory.
+                CtfeValidationMode::Const { allow_immutable_unsafe_cell: !inner }
             }
         };
         ecx.const_validate_operand(&mplace.into(), path, &mut ref_tracking, mode)?;
diff --git a/compiler/rustc_const_eval/src/interpret/machine.rs b/compiler/rustc_const_eval/src/interpret/machine.rs
index 90a654a1229..305f7ade101 100644
--- a/compiler/rustc_const_eval/src/interpret/machine.rs
+++ b/compiler/rustc_const_eval/src/interpret/machine.rs
@@ -391,6 +391,8 @@ pub trait Machine<'mir, 'tcx: 'mir>: Sized {
 
     /// Hook for performing extra checks on a memory read access.
     ///
+    /// This will *not* be called during validation!
+    ///
     /// Takes read-only access to the allocation so we can keep all the memory read
     /// operations take `&self`. Use a `RefCell` in `AllocExtra` if you
     /// need to mutate.
@@ -410,6 +412,8 @@ pub trait Machine<'mir, 'tcx: 'mir>: Sized {
     /// Hook for performing extra checks on any memory read access,
     /// that involves an allocation, even ZST reads.
     ///
+    /// This will *not* be called during validation!
+    ///
     /// Used to prevent statics from self-initializing by reading from their own memory
     /// as it is being initialized.
     fn before_alloc_read(
diff --git a/compiler/rustc_const_eval/src/interpret/memory.rs b/compiler/rustc_const_eval/src/interpret/memory.rs
index 3b2208b8caa..cf7f165b87c 100644
--- a/compiler/rustc_const_eval/src/interpret/memory.rs
+++ b/compiler/rustc_const_eval/src/interpret/memory.rs
@@ -8,6 +8,7 @@
 
 use std::assert_matches::assert_matches;
 use std::borrow::Cow;
+use std::cell::Cell;
 use std::collections::VecDeque;
 use std::fmt;
 use std::ptr;
@@ -111,6 +112,11 @@ pub struct Memory<'mir, 'tcx, M: Machine<'mir, 'tcx>> {
     /// that do not exist any more.
     // FIXME: this should not be public, but interning currently needs access to it
     pub(super) dead_alloc_map: FxIndexMap<AllocId, (Size, Align)>,
+
+    /// This stores whether we are currently doing reads purely for the purpose of validation.
+    /// Those reads do not trigger the machine's hooks for memory reads.
+    /// Needless to say, this must only be set with great care!
+    validation_in_progress: Cell<bool>,
 }
 
 /// A reference to some allocation that was already bounds-checked for the given region
@@ -137,6 +143,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> Memory<'mir, 'tcx, M> {
             alloc_map: M::MemoryMap::default(),
             extra_fn_ptr_map: FxIndexMap::default(),
             dead_alloc_map: FxIndexMap::default(),
+            validation_in_progress: Cell::new(false),
         }
     }
 
@@ -624,10 +631,12 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             size,
             CheckInAllocMsg::MemoryAccessTest,
             |alloc_id, offset, prov| {
-                // We want to call the hook on *all* accesses that involve an AllocId,
-                // including zero-sized accesses. That means we have to do it here
-                // rather than below in the `Some` branch.
-                M::before_alloc_read(self, alloc_id)?;
+                if !self.memory.validation_in_progress.get() {
+                    // We want to call the hook on *all* accesses that involve an AllocId,
+                    // including zero-sized accesses. That means we have to do it here
+                    // rather than below in the `Some` branch.
+                    M::before_alloc_read(self, alloc_id)?;
+                }
                 let alloc = self.get_alloc_raw(alloc_id)?;
                 Ok((alloc.size(), alloc.align, (alloc_id, offset, prov, alloc)))
             },
@@ -635,7 +644,15 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
 
         if let Some((alloc_id, offset, prov, alloc)) = ptr_and_alloc {
             let range = alloc_range(offset, size);
-            M::before_memory_read(self.tcx, &self.machine, &alloc.extra, (alloc_id, prov), range)?;
+            if !self.memory.validation_in_progress.get() {
+                M::before_memory_read(
+                    self.tcx,
+                    &self.machine,
+                    &alloc.extra,
+                    (alloc_id, prov),
+                    range,
+                )?;
+            }
             Ok(Some(AllocRef { alloc, range, tcx: *self.tcx, alloc_id }))
         } else {
             Ok(None)
@@ -909,6 +926,21 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             }
         })
     }
+
+    /// Runs the close in "validation" mode, which means the machine's memory read hooks will be
+    /// suppressed. Needless to say, this must only be set with great care! Cannot be nested.
+    pub(super) fn run_for_validation<R>(&self, f: impl FnOnce() -> R) -> R {
+        assert!(
+            self.memory.validation_in_progress.replace(true) == false,
+            "`validation_in_progress` was already set"
+        );
+        let res = f();
+        assert!(
+            self.memory.validation_in_progress.replace(false) == true,
+            "`validation_in_progress` was unset by someone else"
+        );
+        res
+    }
 }
 
 #[doc(hidden)]
@@ -1154,6 +1186,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         };
         let src_alloc = self.get_alloc_raw(src_alloc_id)?;
         let src_range = alloc_range(src_offset, size);
+        assert!(!self.memory.validation_in_progress.get(), "we can't be copying during validation");
         M::before_memory_read(
             tcx,
             &self.machine,
diff --git a/compiler/rustc_const_eval/src/interpret/validity.rs b/compiler/rustc_const_eval/src/interpret/validity.rs
index c568f9acfd3..972424bccfa 100644
--- a/compiler/rustc_const_eval/src/interpret/validity.rs
+++ b/compiler/rustc_const_eval/src/interpret/validity.rs
@@ -971,7 +971,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         let mut visitor = ValidityVisitor { path, ref_tracking, ctfe_mode, ecx: self };
 
         // Run it.
-        match visitor.visit_value(op) {
+        match self.run_for_validation(|| visitor.visit_value(op)) {
             Ok(()) => Ok(()),
             // Pass through validation failures and "invalid program" issues.
             Err(err)
diff --git a/compiler/rustc_const_eval/src/transform/check_consts/check.rs b/compiler/rustc_const_eval/src/transform/check_consts/check.rs
index 53308cd7f90..a93e8138aa4 100644
--- a/compiler/rustc_const_eval/src/transform/check_consts/check.rs
+++ b/compiler/rustc_const_eval/src/transform/check_consts/check.rs
@@ -30,7 +30,7 @@ type QualifResults<'mir, 'tcx, Q> =
     rustc_mir_dataflow::ResultsCursor<'mir, 'tcx, FlowSensitiveAnalysis<'mir, 'mir, 'tcx, Q>>;
 
 #[derive(Default)]
-pub struct Qualifs<'mir, 'tcx> {
+pub(crate) struct Qualifs<'mir, 'tcx> {
     has_mut_interior: Option<QualifResults<'mir, 'tcx, HasMutInterior>>,
     needs_drop: Option<QualifResults<'mir, 'tcx, NeedsDrop>>,
     needs_non_const_drop: Option<QualifResults<'mir, 'tcx, NeedsNonConstDrop>>,
diff --git a/compiler/rustc_const_eval/src/transform/check_consts/qualifs.rs b/compiler/rustc_const_eval/src/transform/check_consts/qualifs.rs
index 7eb3c181d69..1847847d9d2 100644
--- a/compiler/rustc_const_eval/src/transform/check_consts/qualifs.rs
+++ b/compiler/rustc_const_eval/src/transform/check_consts/qualifs.rs
@@ -284,6 +284,7 @@ where
                 if Q::in_adt_inherently(cx, def, args) {
                     return true;
                 }
+                // Don't do any value-based reasoning for unions.
                 if def.is_union() && Q::in_any_value_of_ty(cx, rvalue.ty(cx.body, cx.tcx)) {
                     return true;
                 }
diff --git a/compiler/rustc_driver_impl/src/lib.rs b/compiler/rustc_driver_impl/src/lib.rs
index 2802f44cda7..7b1070f309b 100644
--- a/compiler/rustc_driver_impl/src/lib.rs
+++ b/compiler/rustc_driver_impl/src/lib.rs
@@ -37,7 +37,7 @@ use rustc_session::config::{nightly_options, CG_OPTIONS, Z_OPTIONS};
 use rustc_session::config::{ErrorOutputType, Input, OutFileName, OutputType};
 use rustc_session::getopts::{self, Matches};
 use rustc_session::lint::{Lint, LintId};
-use rustc_session::{config, EarlyDiagCtxt, Session};
+use rustc_session::{config, filesearch, EarlyDiagCtxt, Session};
 use rustc_span::def_id::LOCAL_CRATE;
 use rustc_span::source_map::FileLoader;
 use rustc_span::symbol::sym;
@@ -887,7 +887,11 @@ pub fn version_at_macro_invocation(
 
         let debug_flags = matches.opt_strs("Z");
         let backend_name = debug_flags.iter().find_map(|x| x.strip_prefix("codegen-backend="));
-        get_codegen_backend(early_dcx, &None, backend_name).print_version();
+        let opts = config::Options::default();
+        let sysroot = filesearch::materialize_sysroot(opts.maybe_sysroot.clone());
+        let target = config::build_target_config(early_dcx, &opts, None, &sysroot);
+
+        get_codegen_backend(early_dcx, &sysroot, backend_name, &target).print_version();
     }
 }
 
@@ -1092,7 +1096,12 @@ pub fn describe_flag_categories(early_dcx: &EarlyDiagCtxt, matches: &Matches) ->
 
     if cg_flags.iter().any(|x| *x == "passes=list") {
         let backend_name = debug_flags.iter().find_map(|x| x.strip_prefix("codegen-backend="));
-        get_codegen_backend(early_dcx, &None, backend_name).print_passes();
+
+        let opts = config::Options::default();
+        let sysroot = filesearch::materialize_sysroot(opts.maybe_sysroot.clone());
+        let target = config::build_target_config(early_dcx, &opts, None, &sysroot);
+
+        get_codegen_backend(early_dcx, &sysroot, backend_name, &target).print_passes();
         return true;
     }
 
diff --git a/compiler/rustc_errors/src/lib.rs b/compiler/rustc_errors/src/lib.rs
index bedb677c1e9..d4f884d49ea 100644
--- a/compiler/rustc_errors/src/lib.rs
+++ b/compiler/rustc_errors/src/lib.rs
@@ -442,8 +442,8 @@ struct DiagCtxtInner {
     emitter: Box<DynEmitter>,
 
     /// Must we produce a diagnostic to justify the use of the expensive
-    /// `trimmed_def_paths` function?
-    must_produce_diag: bool,
+    /// `trimmed_def_paths` function? Backtrace is the location of the call.
+    must_produce_diag: Option<Backtrace>,
 
     /// Has this diagnostic context printed any diagnostics? (I.e. has
     /// `self.emitter.emit_diagnostic()` been called?
@@ -572,10 +572,11 @@ impl Drop for DiagCtxtInner {
         }
 
         if !self.has_printed && !self.suppressed_expected_diag && !std::thread::panicking() {
-            if self.must_produce_diag {
+            if let Some(backtrace) = &self.must_produce_diag {
                 panic!(
-                    "must_produce_diag: trimmed_def_paths called but no diagnostics emitted; \
-                       use `DelayDm` for lints or `with_no_trimmed_paths` for debugging"
+                    "must_produce_diag: `trimmed_def_paths` called but no diagnostics emitted; \
+                       use `DelayDm` for lints or `with_no_trimmed_paths` for debugging. \
+                       called at: {backtrace}"
                 );
             }
         }
@@ -721,7 +722,7 @@ impl DiagCtxt {
         *delayed_bugs = Default::default();
         *deduplicated_err_count = 0;
         *deduplicated_warn_count = 0;
-        *must_produce_diag = false;
+        *must_produce_diag = None;
         *has_printed = false;
         *suppressed_expected_diag = false;
         *taught_diagnostics = Default::default();
@@ -1091,8 +1092,13 @@ impl DiagCtxt {
 
     /// Used when trimmed_def_paths is called and we must produce a diagnostic
     /// to justify its cost.
+    #[track_caller]
     pub fn set_must_produce_diag(&self) {
-        self.inner.borrow_mut().must_produce_diag = true;
+        assert!(
+            self.inner.borrow().must_produce_diag.is_none(),
+            "should only need to collect a backtrace once"
+        );
+        self.inner.borrow_mut().must_produce_diag = Some(Backtrace::capture());
     }
 }
 
@@ -1384,7 +1390,7 @@ impl DiagCtxtInner {
             deduplicated_err_count: 0,
             deduplicated_warn_count: 0,
             emitter,
-            must_produce_diag: false,
+            must_produce_diag: None,
             has_printed: false,
             suppressed_expected_diag: false,
             taught_diagnostics: Default::default(),
diff --git a/compiler/rustc_hir_analysis/src/astconv/bounds.rs b/compiler/rustc_hir_analysis/src/astconv/bounds.rs
index 6d8a5bc0e90..17e0aeb044c 100644
--- a/compiler/rustc_hir_analysis/src/astconv/bounds.rs
+++ b/compiler/rustc_hir_analysis/src/astconv/bounds.rs
@@ -408,7 +408,7 @@ impl<'tcx> dyn AstConv<'tcx> + '_ {
             // Create the generic arguments for the associated type or constant by joining the
             // parent arguments (the arguments of the trait) and the own arguments (the ones of
             // the associated item itself) and construct an alias type using them.
-            candidate.map_bound(|trait_ref| {
+            let alias_ty = candidate.map_bound(|trait_ref| {
                 let ident = Ident::new(assoc_item.name, binding.ident.span);
                 let item_segment = hir::PathSegment {
                     ident,
@@ -430,7 +430,25 @@ impl<'tcx> dyn AstConv<'tcx> + '_ {
                 // *constants* to represent *const projections*. Alias *term* would be a more
                 // appropriate name but alas.
                 ty::AliasTy::new(tcx, assoc_item.def_id, alias_args)
-            })
+            });
+
+            // Provide the resolved type of the associated constant to `type_of(AnonConst)`.
+            if !speculative
+                && let hir::TypeBindingKind::Equality { term: hir::Term::Const(anon_const) } =
+                    binding.kind
+            {
+                let ty = alias_ty.map_bound(|ty| tcx.type_of(ty.def_id).instantiate(tcx, ty.args));
+                // Since the arguments passed to the alias type above may contain early-bound
+                // generic parameters, the instantiated type may contain some as well.
+                // Therefore wrap it in `EarlyBinder`.
+                // FIXME(fmease): Reject escaping late-bound vars.
+                tcx.feed_anon_const_type(
+                    anon_const.def_id,
+                    ty::EarlyBinder::bind(ty.skip_binder()),
+                );
+            }
+
+            alias_ty
         };
 
         match binding.kind {
diff --git a/compiler/rustc_hir_analysis/src/collect/type_of.rs b/compiler/rustc_hir_analysis/src/collect/type_of.rs
index a1345c2d81b..598dba922d0 100644
--- a/compiler/rustc_hir_analysis/src/collect/type_of.rs
+++ b/compiler/rustc_hir_analysis/src/collect/type_of.rs
@@ -79,37 +79,6 @@ fn anon_const_type_of<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> Ty<'tcx> {
                 .expect("const parameter types cannot be generic");
         }
 
-        Node::TypeBinding(binding @ &TypeBinding { hir_id: binding_id, .. })
-            if let Node::TraitRef(trait_ref) = tcx.parent_hir_node(binding_id) =>
-        {
-            let Some(trait_def_id) = trait_ref.trait_def_id() else {
-                return Ty::new_error_with_message(
-                    tcx,
-                    tcx.def_span(def_id),
-                    "Could not find trait",
-                );
-            };
-            let assoc_items = tcx.associated_items(trait_def_id);
-            let assoc_item = assoc_items.find_by_name_and_kind(
-                tcx,
-                binding.ident,
-                ty::AssocKind::Const,
-                def_id.to_def_id(),
-            );
-            return if let Some(assoc_item) = assoc_item {
-                tcx.type_of(assoc_item.def_id)
-                    .no_bound_vars()
-                    .expect("const parameter types cannot be generic")
-            } else {
-                // FIXME(associated_const_equality): add a useful error message here.
-                Ty::new_error_with_message(
-                    tcx,
-                    tcx.def_span(def_id),
-                    "Could not find associated const on trait",
-                )
-            };
-        }
-
         // This match arm is for when the def_id appears in a GAT whose
         // path can't be resolved without typechecking e.g.
         //
diff --git a/compiler/rustc_interface/src/interface.rs b/compiler/rustc_interface/src/interface.rs
index 284d965979e..1a82e6c6910 100644
--- a/compiler/rustc_interface/src/interface.rs
+++ b/compiler/rustc_interface/src/interface.rs
@@ -16,7 +16,7 @@ use rustc_parse::maybe_new_parser_from_source_str;
 use rustc_query_impl::QueryCtxt;
 use rustc_query_system::query::print_query_stack;
 use rustc_session::config::{self, Cfg, CheckCfg, ExpectedValues, Input, OutFileName};
-use rustc_session::filesearch::sysroot_candidates;
+use rustc_session::filesearch::{self, sysroot_candidates};
 use rustc_session::parse::ParseSess;
 use rustc_session::{lint, CompilerIO, EarlyDiagCtxt, Session};
 use rustc_span::source_map::FileLoader;
@@ -339,16 +339,53 @@ pub fn run_compiler<R: Send>(config: Config, f: impl FnOnce(&Compiler) -> R + Se
 
             let early_dcx = EarlyDiagCtxt::new(config.opts.error_format);
 
-            let codegen_backend = if let Some(make_codegen_backend) = config.make_codegen_backend {
-                make_codegen_backend(&config.opts)
-            } else {
-                util::get_codegen_backend(
-                    &early_dcx,
-                    &config.opts.maybe_sysroot,
-                    config.opts.unstable_opts.codegen_backend.as_deref(),
-                )
+            let sysroot = filesearch::materialize_sysroot(config.opts.maybe_sysroot.clone());
+
+            let (codegen_backend, target_override) = match config.make_codegen_backend {
+                None => {
+                    // Build a target without override, so that it can override the backend if needed
+                    let target =
+                        config::build_target_config(&early_dcx, &config.opts, None, &sysroot);
+
+                    let backend = util::get_codegen_backend(
+                        &early_dcx,
+                        &sysroot,
+                        config.opts.unstable_opts.codegen_backend.as_deref(),
+                        &target,
+                    );
+
+                    // target_override is documented to be called before init(), so this is okay
+                    let target_override = backend.target_override(&config.opts);
+
+                    // Assert that we don't use target's override of the backend and
+                    // backend's override of the target at the same time
+                    if config.opts.unstable_opts.codegen_backend.is_none()
+                        && target.default_codegen_backend.is_some()
+                        && target_override.is_some()
+                    {
+                        rustc_middle::bug!(
+                            "Codegen backend requested target override even though the target requested the backend"
+                        );
+                    }
+
+                    (backend, target_override)
+                }
+                Some(make_codegen_backend) => {
+                    // N.B. `make_codegen_backend` takes precedence over `target.default_codegen_backend`,
+                    //      which is ignored in this case.
+                    let backend = make_codegen_backend(&config.opts);
+
+                    // target_override is documented to be called before init(), so this is okay
+                    let target_override = backend.target_override(&config.opts);
+
+                    (backend, target_override)
+                }
             };
 
+            // Re-build target with the (potential) override
+            let target_cfg =
+                config::build_target_config(&early_dcx, &config.opts, target_override, &sysroot);
+
             let temps_dir = config.opts.unstable_opts.temps_dir.as_deref().map(PathBuf::from);
 
             let bundle = match rustc_errors::fluent_bundle(
@@ -367,9 +404,6 @@ pub fn run_compiler<R: Send>(config: Config, f: impl FnOnce(&Compiler) -> R + Se
             let mut locale_resources = Vec::from(config.locale_resources);
             locale_resources.push(codegen_backend.locale_resource());
 
-            // target_override is documented to be called before init(), so this is okay
-            let target_override = codegen_backend.target_override(&config.opts);
-
             let mut sess = rustc_session::build_session(
                 early_dcx,
                 config.opts,
@@ -384,7 +418,8 @@ pub fn run_compiler<R: Send>(config: Config, f: impl FnOnce(&Compiler) -> R + Se
                 locale_resources,
                 config.lint_caps,
                 config.file_loader,
-                target_override,
+                target_cfg,
+                sysroot,
                 util::rustc_version_str().unwrap_or("unknown"),
                 config.ice_file,
                 config.using_internal_features,
diff --git a/compiler/rustc_interface/src/tests.rs b/compiler/rustc_interface/src/tests.rs
index 4f15d1c1d3a..42fff01c11c 100644
--- a/compiler/rustc_interface/src/tests.rs
+++ b/compiler/rustc_interface/src/tests.rs
@@ -13,7 +13,7 @@ use rustc_session::config::{
 use rustc_session::lint::Level;
 use rustc_session::search_paths::SearchPath;
 use rustc_session::utils::{CanonicalizedPath, NativeLib, NativeLibKind};
-use rustc_session::{build_session, getopts, CompilerIO, EarlyDiagCtxt, Session};
+use rustc_session::{build_session, filesearch, getopts, CompilerIO, EarlyDiagCtxt, Session};
 use rustc_span::edition::{Edition, DEFAULT_EDITION};
 use rustc_span::symbol::sym;
 use rustc_span::{FileName, SourceFileHashAlgorithm};
@@ -37,6 +37,12 @@ fn mk_session(matches: getopts::Matches) -> (Session, Cfg) {
         output_file: None,
         temps_dir,
     };
+
+    let sysroot = filesearch::materialize_sysroot(sessopts.maybe_sysroot.clone());
+
+    let target_cfg =
+        rustc_session::config::build_target_config(&early_dcx, &sessopts, None, &sysroot);
+
     let sess = build_session(
         early_dcx,
         sessopts,
@@ -46,7 +52,8 @@ fn mk_session(matches: getopts::Matches) -> (Session, Cfg) {
         vec![],
         Default::default(),
         None,
-        None,
+        target_cfg,
+        sysroot,
         "",
         None,
         Arc::default(),
@@ -685,7 +692,7 @@ fn test_unstable_options_tracking_hash() {
     untracked!(nll_facts, true);
     untracked!(no_analysis, true);
     untracked!(no_leak_check, true);
-    untracked!(no_parallel_llvm, true);
+    untracked!(no_parallel_backend, true);
     untracked!(parse_only, true);
     // `pre_link_arg` is omitted because it just forwards to `pre_link_args`.
     untracked!(pre_link_args, vec![String::from("abc"), String::from("def")]);
diff --git a/compiler/rustc_interface/src/util.rs b/compiler/rustc_interface/src/util.rs
index 23bd2dac57e..829b00aabc1 100644
--- a/compiler/rustc_interface/src/util.rs
+++ b/compiler/rustc_interface/src/util.rs
@@ -14,13 +14,14 @@ use rustc_session::{filesearch, output, Session};
 use rustc_span::edit_distance::find_best_match_for_name;
 use rustc_span::edition::Edition;
 use rustc_span::symbol::{sym, Symbol};
+use rustc_target::spec::Target;
 use session::EarlyDiagCtxt;
-use std::env;
 use std::env::consts::{DLL_PREFIX, DLL_SUFFIX};
 use std::path::{Path, PathBuf};
 use std::sync::atomic::{AtomicBool, Ordering};
 use std::sync::OnceLock;
 use std::thread;
+use std::{env, iter};
 
 /// Function pointer type that constructs a new CodegenBackend.
 pub type MakeBackendFn = fn() -> Box<dyn CodegenBackend>;
@@ -195,21 +196,25 @@ fn load_backend_from_dylib(early_dcx: &EarlyDiagCtxt, path: &Path) -> MakeBacken
 /// A name of `None` indicates that the default backend should be used.
 pub fn get_codegen_backend(
     early_dcx: &EarlyDiagCtxt,
-    maybe_sysroot: &Option<PathBuf>,
+    sysroot: &Path,
     backend_name: Option<&str>,
+    target: &Target,
 ) -> Box<dyn CodegenBackend> {
     static LOAD: OnceLock<unsafe fn() -> Box<dyn CodegenBackend>> = OnceLock::new();
 
     let load = LOAD.get_or_init(|| {
-        let default_codegen_backend = option_env!("CFG_DEFAULT_CODEGEN_BACKEND").unwrap_or("llvm");
+        let backend = backend_name
+            .or(target.default_codegen_backend.as_deref())
+            .or(option_env!("CFG_DEFAULT_CODEGEN_BACKEND"))
+            .unwrap_or("llvm");
 
-        match backend_name.unwrap_or(default_codegen_backend) {
+        match backend {
             filename if filename.contains('.') => {
                 load_backend_from_dylib(early_dcx, filename.as_ref())
             }
             #[cfg(feature = "llvm")]
             "llvm" => rustc_codegen_llvm::LlvmCodegenBackend::new,
-            backend_name => get_codegen_sysroot(early_dcx, maybe_sysroot, backend_name),
+            backend_name => get_codegen_sysroot(early_dcx, sysroot, backend_name),
         }
     });
 
@@ -244,7 +249,7 @@ fn get_rustc_path_inner(bin_path: &str) -> Option<PathBuf> {
 #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
 fn get_codegen_sysroot(
     early_dcx: &EarlyDiagCtxt,
-    maybe_sysroot: &Option<PathBuf>,
+    sysroot: &Path,
     backend_name: &str,
 ) -> MakeBackendFn {
     // For now we only allow this function to be called once as it'll dlopen a
@@ -261,28 +266,28 @@ fn get_codegen_sysroot(
     let target = session::config::host_triple();
     let sysroot_candidates = sysroot_candidates();
 
-    let sysroot = maybe_sysroot
-        .iter()
-        .chain(sysroot_candidates.iter())
+    let sysroot = iter::once(sysroot)
+        .chain(sysroot_candidates.iter().map(<_>::as_ref))
         .map(|sysroot| {
             filesearch::make_target_lib_path(sysroot, target).with_file_name("codegen-backends")
         })
         .find(|f| {
             info!("codegen backend candidate: {}", f.display());
             f.exists()
-        });
-    let sysroot = sysroot.unwrap_or_else(|| {
-        let candidates = sysroot_candidates
-            .iter()
-            .map(|p| p.display().to_string())
-            .collect::<Vec<_>>()
-            .join("\n* ");
-        let err = format!(
-            "failed to find a `codegen-backends` folder \
+        })
+        .unwrap_or_else(|| {
+            let candidates = sysroot_candidates
+                .iter()
+                .map(|p| p.display().to_string())
+                .collect::<Vec<_>>()
+                .join("\n* ");
+            let err = format!(
+                "failed to find a `codegen-backends` folder \
                            in the sysroot candidates:\n* {candidates}"
-        );
-        early_dcx.early_fatal(err);
-    });
+            );
+            early_dcx.early_fatal(err);
+        });
+
     info!("probing {} for a codegen backend", sysroot.display());
 
     let d = sysroot.read_dir().unwrap_or_else(|e| {
diff --git a/compiler/rustc_mir_build/src/build/scope.rs b/compiler/rustc_mir_build/src/build/scope.rs
index 961502566ba..88ac05cabb6 100644
--- a/compiler/rustc_mir_build/src/build/scope.rs
+++ b/compiler/rustc_mir_build/src/build/scope.rs
@@ -203,16 +203,31 @@ const ROOT_NODE: DropIdx = DropIdx::from_u32(0);
 /// in `build_mir`.
 #[derive(Debug)]
 struct DropTree {
-    /// Drops in the tree.
-    drops: IndexVec<DropIdx, (DropData, DropIdx)>,
-    /// Map for finding the inverse of the `next_drop` relation:
-    ///
-    /// `previous_drops[(drops[i].1, drops[i].0.local, drops[i].0.kind)] == i`
-    previous_drops: FxHashMap<(DropIdx, Local, DropKind), DropIdx>,
+    /// Nodes in the drop tree, containing drop data and a link to the next node.
+    drops: IndexVec<DropIdx, DropNode>,
+    /// Map for finding the index of an existing node, given its contents.
+    existing_drops_map: FxHashMap<DropNodeKey, DropIdx>,
     /// Edges into the `DropTree` that need to be added once it's lowered.
     entry_points: Vec<(DropIdx, BasicBlock)>,
 }
 
+/// A single node in the drop tree.
+#[derive(Debug)]
+struct DropNode {
+    /// Info about the drop to be performed at this node in the drop tree.
+    data: DropData,
+    /// Index of the "next" drop to perform (in drop order, not declaration order).
+    next: DropIdx,
+}
+
+/// Subset of [`DropNode`] used for reverse lookup in a hash table.
+#[derive(Debug, PartialEq, Eq, Hash)]
+struct DropNodeKey {
+    next: DropIdx,
+    local: Local,
+    kind: DropKind,
+}
+
 impl Scope {
     /// Whether there's anything to do for the cleanup path, that is,
     /// when unwinding through this scope. This includes destructors,
@@ -247,7 +262,7 @@ trait DropTreeBuilder<'tcx> {
 
     /// Links a block outside the drop tree, `from`, to the block `to` inside
     /// the drop tree.
-    fn add_entry(cfg: &mut CFG<'tcx>, from: BasicBlock, to: BasicBlock);
+    fn link_entry_point(cfg: &mut CFG<'tcx>, from: BasicBlock, to: BasicBlock);
 }
 
 impl DropTree {
@@ -258,20 +273,29 @@ impl DropTree {
         let fake_source_info = SourceInfo::outermost(DUMMY_SP);
         let fake_data =
             DropData { source_info: fake_source_info, local: Local::MAX, kind: DropKind::Storage };
-        let drop_idx = DropIdx::MAX;
-        let drops = IndexVec::from_elem_n((fake_data, drop_idx), 1);
-        Self { drops, entry_points: Vec::new(), previous_drops: FxHashMap::default() }
+        let drops = IndexVec::from_raw(vec![DropNode { data: fake_data, next: DropIdx::MAX }]);
+        Self { drops, entry_points: Vec::new(), existing_drops_map: FxHashMap::default() }
     }
 
-    fn add_drop(&mut self, drop: DropData, next: DropIdx) -> DropIdx {
+    /// Adds a node to the drop tree, consisting of drop data and the index of
+    /// the "next" drop (in drop order), which could be the sentinel [`ROOT_NODE`].
+    ///
+    /// If there is already an equivalent node in the tree, nothing is added, and
+    /// that node's index is returned. Otherwise, the new node's index is returned.
+    fn add_drop(&mut self, data: DropData, next: DropIdx) -> DropIdx {
         let drops = &mut self.drops;
         *self
-            .previous_drops
-            .entry((next, drop.local, drop.kind))
-            .or_insert_with(|| drops.push((drop, next)))
+            .existing_drops_map
+            .entry(DropNodeKey { next, local: data.local, kind: data.kind })
+            // Create a new node, and also add its index to the map.
+            .or_insert_with(|| drops.push(DropNode { data, next }))
     }
 
-    fn add_entry(&mut self, from: BasicBlock, to: DropIdx) {
+    /// Registers `from` as an entry point to this drop tree, at `to`.
+    ///
+    /// During [`Self::build_mir`], `from` will be linked to the corresponding
+    /// block within the drop tree.
+    fn add_entry_point(&mut self, from: BasicBlock, to: DropIdx) {
         debug_assert!(to < self.drops.next_index());
         self.entry_points.push((to, from));
     }
@@ -326,13 +350,13 @@ impl DropTree {
         let entry_points = &mut self.entry_points;
         entry_points.sort();
 
-        for (drop_idx, drop_data) in self.drops.iter_enumerated().rev() {
+        for (drop_idx, drop_node) in self.drops.iter_enumerated().rev() {
             if entry_points.last().is_some_and(|entry_point| entry_point.0 == drop_idx) {
                 let block = *blocks[drop_idx].get_or_insert_with(|| T::make_block(cfg));
                 needs_block[drop_idx] = Block::Own;
                 while entry_points.last().is_some_and(|entry_point| entry_point.0 == drop_idx) {
                     let entry_block = entry_points.pop().unwrap().1;
-                    T::add_entry(cfg, entry_block, block);
+                    T::link_entry_point(cfg, entry_block, block);
                 }
             }
             match needs_block[drop_idx] {
@@ -344,10 +368,10 @@ impl DropTree {
                     blocks[drop_idx] = blocks[pred];
                 }
             }
-            if let DropKind::Value = drop_data.0.kind {
-                needs_block[drop_data.1] = Block::Own;
+            if let DropKind::Value = drop_node.data.kind {
+                needs_block[drop_node.next] = Block::Own;
             } else if drop_idx != ROOT_NODE {
-                match &mut needs_block[drop_data.1] {
+                match &mut needs_block[drop_node.next] {
                     pred @ Block::None => *pred = Block::Shares(drop_idx),
                     pred @ Block::Shares(_) => *pred = Block::Own,
                     Block::Own => (),
@@ -364,34 +388,35 @@ impl DropTree {
         cfg: &mut CFG<'tcx>,
         blocks: &IndexSlice<DropIdx, Option<BasicBlock>>,
     ) {
-        for (drop_idx, drop_data) in self.drops.iter_enumerated().rev() {
+        for (drop_idx, drop_node) in self.drops.iter_enumerated().rev() {
             let Some(block) = blocks[drop_idx] else { continue };
-            match drop_data.0.kind {
+            match drop_node.data.kind {
                 DropKind::Value => {
                     let terminator = TerminatorKind::Drop {
-                        target: blocks[drop_data.1].unwrap(),
+                        target: blocks[drop_node.next].unwrap(),
                         // The caller will handle this if needed.
                         unwind: UnwindAction::Terminate(UnwindTerminateReason::InCleanup),
-                        place: drop_data.0.local.into(),
+                        place: drop_node.data.local.into(),
                         replace: false,
                     };
-                    cfg.terminate(block, drop_data.0.source_info, terminator);
+                    cfg.terminate(block, drop_node.data.source_info, terminator);
                 }
                 // Root nodes don't correspond to a drop.
                 DropKind::Storage if drop_idx == ROOT_NODE => {}
                 DropKind::Storage => {
                     let stmt = Statement {
-                        source_info: drop_data.0.source_info,
-                        kind: StatementKind::StorageDead(drop_data.0.local),
+                        source_info: drop_node.data.source_info,
+                        kind: StatementKind::StorageDead(drop_node.data.local),
                     };
                     cfg.push(block, stmt);
-                    let target = blocks[drop_data.1].unwrap();
+                    let target = blocks[drop_node.next].unwrap();
                     if target != block {
                         // Diagnostics don't use this `Span` but debuginfo
                         // might. Since we don't want breakpoints to be placed
                         // here, especially when this is on an unwind path, we
                         // use `DUMMY_SP`.
-                        let source_info = SourceInfo { span: DUMMY_SP, ..drop_data.0.source_info };
+                        let source_info =
+                            SourceInfo { span: DUMMY_SP, ..drop_node.data.source_info };
                         let terminator = TerminatorKind::Goto { target };
                         cfg.terminate(block, source_info, terminator);
                     }
@@ -673,11 +698,12 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             .flat_map(|scope| &scope.drops)
             .fold(ROOT_NODE, |drop_idx, &drop| drops.add_drop(drop, drop_idx));
 
-        drops.add_entry(block, drop_idx);
+        drops.add_entry_point(block, drop_idx);
 
         // `build_drop_trees` doesn't have access to our source_info, so we
         // create a dummy terminator now. `TerminatorKind::UnwindResume` is used
         // because MIR type checking will panic if it hasn't been overwritten.
+        // (See `<ExitScopes as DropTreeBuilder>::link_entry_point`.)
         self.cfg.terminate(block, source_info, TerminatorKind::UnwindResume);
 
         self.cfg.start_new_block().unit()
@@ -708,11 +734,12 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 drop_idx = drops.add_drop(*drop, drop_idx);
             }
         }
-        drops.add_entry(block, drop_idx);
+        drops.add_entry_point(block, drop_idx);
 
         // `build_drop_trees` doesn't have access to our source_info, so we
         // create a dummy terminator now. `TerminatorKind::UnwindResume` is used
         // because MIR type checking will panic if it hasn't been overwritten.
+        // (See `<ExitScopes as DropTreeBuilder>::link_entry_point`.)
         self.cfg.terminate(block, source_info, TerminatorKind::UnwindResume);
     }
 
@@ -1119,7 +1146,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         );
 
         let next_drop = self.diverge_cleanup();
-        self.scopes.unwind_drops.add_entry(start, next_drop);
+        self.scopes.unwind_drops.add_entry_point(start, next_drop);
     }
 
     /// Sets up a path that performs all required cleanup for dropping a
@@ -1153,7 +1180,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             scope.cached_coroutine_drop_block = Some(cached_drop);
         }
 
-        self.scopes.coroutine_drops.add_entry(yield_block, cached_drop);
+        self.scopes.coroutine_drops.add_entry_point(yield_block, cached_drop);
     }
 
     /// Utility function for *non*-scope code to build their own drops
@@ -1274,9 +1301,9 @@ fn build_scope_drops<'tcx>(
                 // `unwind_to` should drop the value that we're about to
                 // schedule. If dropping this value panics, then we continue
                 // with the *next* value on the unwind path.
-                debug_assert_eq!(unwind_drops.drops[unwind_to].0.local, drop_data.local);
-                debug_assert_eq!(unwind_drops.drops[unwind_to].0.kind, drop_data.kind);
-                unwind_to = unwind_drops.drops[unwind_to].1;
+                debug_assert_eq!(unwind_drops.drops[unwind_to].data.local, drop_data.local);
+                debug_assert_eq!(unwind_drops.drops[unwind_to].data.kind, drop_data.kind);
+                unwind_to = unwind_drops.drops[unwind_to].next;
 
                 // If the operand has been moved, and we are not on an unwind
                 // path, then don't generate the drop. (We only take this into
@@ -1286,7 +1313,7 @@ fn build_scope_drops<'tcx>(
                     continue;
                 }
 
-                unwind_drops.add_entry(block, unwind_to);
+                unwind_drops.add_entry_point(block, unwind_to);
 
                 let next = cfg.start_new_block();
                 cfg.terminate(
@@ -1303,9 +1330,9 @@ fn build_scope_drops<'tcx>(
             }
             DropKind::Storage => {
                 if storage_dead_on_unwind {
-                    debug_assert_eq!(unwind_drops.drops[unwind_to].0.local, drop_data.local);
-                    debug_assert_eq!(unwind_drops.drops[unwind_to].0.kind, drop_data.kind);
-                    unwind_to = unwind_drops.drops[unwind_to].1;
+                    debug_assert_eq!(unwind_drops.drops[unwind_to].data.local, drop_data.local);
+                    debug_assert_eq!(unwind_drops.drops[unwind_to].data.kind, drop_data.kind);
+                    unwind_to = unwind_drops.drops[unwind_to].next;
                 }
                 // Only temps and vars need their storage dead.
                 assert!(local.index() > arg_count);
@@ -1335,30 +1362,31 @@ impl<'a, 'tcx: 'a> Builder<'a, 'tcx> {
         let is_coroutine = self.coroutine.is_some();
 
         // Link the exit drop tree to unwind drop tree.
-        if drops.drops.iter().any(|(drop, _)| drop.kind == DropKind::Value) {
+        if drops.drops.iter().any(|drop_node| drop_node.data.kind == DropKind::Value) {
             let unwind_target = self.diverge_cleanup_target(else_scope, span);
             let mut unwind_indices = IndexVec::from_elem_n(unwind_target, 1);
-            for (drop_idx, drop_data) in drops.drops.iter_enumerated().skip(1) {
-                match drop_data.0.kind {
+            for (drop_idx, drop_node) in drops.drops.iter_enumerated().skip(1) {
+                match drop_node.data.kind {
                     DropKind::Storage => {
                         if is_coroutine {
                             let unwind_drop = self
                                 .scopes
                                 .unwind_drops
-                                .add_drop(drop_data.0, unwind_indices[drop_data.1]);
+                                .add_drop(drop_node.data, unwind_indices[drop_node.next]);
                             unwind_indices.push(unwind_drop);
                         } else {
-                            unwind_indices.push(unwind_indices[drop_data.1]);
+                            unwind_indices.push(unwind_indices[drop_node.next]);
                         }
                     }
                     DropKind::Value => {
                         let unwind_drop = self
                             .scopes
                             .unwind_drops
-                            .add_drop(drop_data.0, unwind_indices[drop_data.1]);
-                        self.scopes
-                            .unwind_drops
-                            .add_entry(blocks[drop_idx].unwrap(), unwind_indices[drop_data.1]);
+                            .add_drop(drop_node.data, unwind_indices[drop_node.next]);
+                        self.scopes.unwind_drops.add_entry_point(
+                            blocks[drop_idx].unwrap(),
+                            unwind_indices[drop_node.next],
+                        );
                         unwind_indices.push(unwind_drop);
                     }
                 }
@@ -1408,10 +1436,10 @@ impl<'a, 'tcx: 'a> Builder<'a, 'tcx> {
         // prevent drop elaboration from creating drop flags that would have
         // to be captured by the coroutine. I'm not sure how important this
         // optimization is, but it is here.
-        for (drop_idx, drop_data) in drops.drops.iter_enumerated() {
-            if let DropKind::Value = drop_data.0.kind {
-                debug_assert!(drop_data.1 < drops.drops.next_index());
-                drops.entry_points.push((drop_data.1, blocks[drop_idx].unwrap()));
+        for (drop_idx, drop_node) in drops.drops.iter_enumerated() {
+            if let DropKind::Value = drop_node.data.kind {
+                debug_assert!(drop_node.next < drops.drops.next_index());
+                drops.entry_points.push((drop_node.next, blocks[drop_idx].unwrap()));
             }
         }
         Self::build_unwind_tree(cfg, drops, fn_span, resume_block);
@@ -1442,8 +1470,16 @@ impl<'tcx> DropTreeBuilder<'tcx> for ExitScopes {
     fn make_block(cfg: &mut CFG<'tcx>) -> BasicBlock {
         cfg.start_new_block()
     }
-    fn add_entry(cfg: &mut CFG<'tcx>, from: BasicBlock, to: BasicBlock) {
-        cfg.block_data_mut(from).terminator_mut().kind = TerminatorKind::Goto { target: to };
+    fn link_entry_point(cfg: &mut CFG<'tcx>, from: BasicBlock, to: BasicBlock) {
+        // There should be an existing terminator with real source info and a
+        // dummy TerminatorKind. Replace it with a proper goto.
+        // (The dummy is added by `break_scope` and `break_for_else`.)
+        let term = cfg.block_data_mut(from).terminator_mut();
+        if let TerminatorKind::UnwindResume = term.kind {
+            term.kind = TerminatorKind::Goto { target: to };
+        } else {
+            span_bug!(term.source_info.span, "unexpected dummy terminator kind: {:?}", term.kind);
+        }
     }
 }
 
@@ -1453,7 +1489,7 @@ impl<'tcx> DropTreeBuilder<'tcx> for CoroutineDrop {
     fn make_block(cfg: &mut CFG<'tcx>) -> BasicBlock {
         cfg.start_new_block()
     }
-    fn add_entry(cfg: &mut CFG<'tcx>, from: BasicBlock, to: BasicBlock) {
+    fn link_entry_point(cfg: &mut CFG<'tcx>, from: BasicBlock, to: BasicBlock) {
         let term = cfg.block_data_mut(from).terminator_mut();
         if let TerminatorKind::Yield { ref mut drop, .. } = term.kind {
             *drop = Some(to);
@@ -1473,7 +1509,7 @@ impl<'tcx> DropTreeBuilder<'tcx> for Unwind {
     fn make_block(cfg: &mut CFG<'tcx>) -> BasicBlock {
         cfg.start_new_cleanup_block()
     }
-    fn add_entry(cfg: &mut CFG<'tcx>, from: BasicBlock, to: BasicBlock) {
+    fn link_entry_point(cfg: &mut CFG<'tcx>, from: BasicBlock, to: BasicBlock) {
         let term = &mut cfg.block_data_mut(from).terminator_mut();
         match &mut term.kind {
             TerminatorKind::Drop { unwind, .. } => {
diff --git a/compiler/rustc_parse/src/parser/path.rs b/compiler/rustc_parse/src/parser/path.rs
index 545db5138a3..2edf2111de7 100644
--- a/compiler/rustc_parse/src/parser/path.rs
+++ b/compiler/rustc_parse/src/parser/path.rs
@@ -1,6 +1,7 @@
 use super::ty::{AllowPlus, RecoverQPath, RecoverReturnSign};
 use super::{Parser, Restrictions, TokenType};
 use crate::errors::PathSingleColon;
+use crate::parser::{CommaRecoveryMode, RecoverColon, RecoverComma};
 use crate::{errors, maybe_whole};
 use ast::token::IdentIsRaw;
 use rustc_ast::ptr::P;
@@ -10,7 +11,7 @@ use rustc_ast::{
     AssocConstraintKind, BlockCheckMode, GenericArg, GenericArgs, Generics, ParenthesizedArgs,
     Path, PathSegment, QSelf,
 };
-use rustc_errors::{Applicability, PResult};
+use rustc_errors::{Applicability, Diag, PResult};
 use rustc_span::symbol::{kw, sym, Ident};
 use rustc_span::{BytePos, Span};
 use std::mem;
@@ -373,7 +374,38 @@ impl<'a> Parser<'a> {
                     .into()
                 } else {
                     // `(T, U) -> R`
-                    let (inputs, _) = self.parse_paren_comma_seq(|p| p.parse_ty())?;
+
+                    let prev_token_before_parsing = self.prev_token.clone();
+                    let token_before_parsing = self.token.clone();
+                    let mut snapshot = None;
+                    if self.may_recover()
+                        && prev_token_before_parsing.kind == token::ModSep
+                        && (style == PathStyle::Expr && self.token.can_begin_expr()
+                            || style == PathStyle::Pat && self.token.can_begin_pattern())
+                    {
+                        snapshot = Some(self.create_snapshot_for_diagnostic());
+                    }
+
+                    let (inputs, _) = match self.parse_paren_comma_seq(|p| p.parse_ty()) {
+                        Ok(output) => output,
+                        Err(mut error) if prev_token_before_parsing.kind == token::ModSep => {
+                            error.span_label(
+                                prev_token_before_parsing.span.to(token_before_parsing.span),
+                                "while parsing this parenthesized list of type arguments starting here",
+                            );
+
+                            if let Some(mut snapshot) = snapshot {
+                                snapshot.recover_fn_call_leading_path_sep(
+                                    style,
+                                    prev_token_before_parsing,
+                                    &mut error,
+                                )
+                            }
+
+                            return Err(error);
+                        }
+                        Err(error) => return Err(error),
+                    };
                     let inputs_span = lo.to(self.prev_token.span);
                     let output =
                         self.parse_ret_ty(AllowPlus::No, RecoverQPath::No, RecoverReturnSign::No)?;
@@ -399,6 +431,56 @@ impl<'a> Parser<'a> {
         }
     }
 
+    /// Recover `$path::(...)` as `$path(...)`.
+    ///
+    /// ```ignore (diagnostics)
+    /// foo::(420, "bar")
+    ///    ^^ remove extra separator to make the function call
+    /// // or
+    /// match x {
+    ///    Foo::(420, "bar") => { ... },
+    ///       ^^ remove extra separator to turn this into tuple struct pattern
+    ///    _ => { ... },
+    /// }
+    /// ```
+    fn recover_fn_call_leading_path_sep(
+        &mut self,
+        style: PathStyle,
+        prev_token_before_parsing: Token,
+        error: &mut Diag<'_>,
+    ) {
+        if ((style == PathStyle::Expr && self.parse_paren_comma_seq(|p| p.parse_expr()).is_ok())
+            || (style == PathStyle::Pat
+                && self
+                    .parse_paren_comma_seq(|p| {
+                        p.parse_pat_allow_top_alt(
+                            None,
+                            RecoverComma::No,
+                            RecoverColon::No,
+                            CommaRecoveryMode::LikelyTuple,
+                        )
+                    })
+                    .is_ok()))
+            && !matches!(self.token.kind, token::ModSep | token::RArrow)
+        {
+            error.span_suggestion_verbose(
+                prev_token_before_parsing.span,
+                format!(
+                    "consider removing the `::` here to {}",
+                    match style {
+                        PathStyle::Expr => "call the expression",
+                        PathStyle::Pat => "turn this into a tuple struct pattern",
+                        _ => {
+                            return;
+                        }
+                    }
+                ),
+                "",
+                Applicability::MaybeIncorrect,
+            );
+        }
+    }
+
     /// Parses generic args (within a path segment) with recovery for extra leading angle brackets.
     /// For the purposes of understanding the parsing logic of generic arguments, this function
     /// can be thought of being the same as just calling `self.parse_angle_args()` if the source
diff --git a/compiler/rustc_session/src/config.rs b/compiler/rustc_session/src/config.rs
index 1573920c8c0..b947ac81805 100644
--- a/compiler/rustc_session/src/config.rs
+++ b/compiler/rustc_session/src/config.rs
@@ -8,7 +8,7 @@ pub use crate::options::*;
 use crate::errors::FileWriteFail;
 use crate::search_paths::SearchPath;
 use crate::utils::{CanonicalizedPath, NativeLib, NativeLibKind};
-use crate::{lint, HashStableContext};
+use crate::{filesearch, lint, HashStableContext};
 use crate::{EarlyDiagCtxt, Session};
 use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexMap, FxIndexSet};
 use rustc_data_structures::stable_hasher::{StableOrd, ToStableHashKey};
@@ -1564,7 +1564,7 @@ pub fn build_configuration(sess: &Session, mut user_cfg: Cfg) -> Cfg {
     user_cfg
 }
 
-pub(super) fn build_target_config(
+pub fn build_target_config(
     early_dcx: &EarlyDiagCtxt,
     opts: &Options,
     target_override: Option<Target>,
@@ -2863,16 +2863,8 @@ pub fn build_session_options(early_dcx: &mut EarlyDiagCtxt, matches: &getopts::M
 
     let logical_env = parse_logical_env(early_dcx, matches);
 
-    // Try to find a directory containing the Rust `src`, for more details see
-    // the doc comment on the `real_rust_source_base_dir` field.
-    let tmp_buf;
-    let sysroot = match &sysroot_opt {
-        Some(s) => s,
-        None => {
-            tmp_buf = crate::filesearch::get_or_default_sysroot().expect("Failed finding sysroot");
-            &tmp_buf
-        }
-    };
+    let sysroot = filesearch::materialize_sysroot(sysroot_opt);
+
     let real_rust_source_base_dir = {
         // This is the location used by the `rust-src` `rustup` component.
         let mut candidate = sysroot.join("lib/rustlib/src/rust");
@@ -2916,7 +2908,7 @@ pub fn build_session_options(early_dcx: &mut EarlyDiagCtxt, matches: &getopts::M
         describe_lints,
         output_types,
         search_paths,
-        maybe_sysroot: sysroot_opt,
+        maybe_sysroot: Some(sysroot),
         target_triple,
         test,
         incremental,
diff --git a/compiler/rustc_session/src/filesearch.rs b/compiler/rustc_session/src/filesearch.rs
index 2456b4b3a15..4f0e3354680 100644
--- a/compiler/rustc_session/src/filesearch.rs
+++ b/compiler/rustc_session/src/filesearch.rs
@@ -193,6 +193,12 @@ pub fn sysroot_candidates() -> SmallVec<[PathBuf; 2]> {
     return sysroot_candidates;
 }
 
+/// Returns the provided sysroot or calls [`get_or_default_sysroot`] if it's none.
+/// Panics if [`get_or_default_sysroot`]  returns an error.
+pub fn materialize_sysroot(maybe_sysroot: Option<PathBuf>) -> PathBuf {
+    maybe_sysroot.unwrap_or_else(|| get_or_default_sysroot().expect("Failed finding sysroot"))
+}
+
 /// This function checks if sysroot is found using env::args().next(), and if it
 /// is not found, finds sysroot from current rustc_driver dll.
 pub fn get_or_default_sysroot() -> Result<PathBuf, String> {
diff --git a/compiler/rustc_session/src/options.rs b/compiler/rustc_session/src/options.rs
index 93bef82e4ba..ea4b8f2463e 100644
--- a/compiler/rustc_session/src/options.rs
+++ b/compiler/rustc_session/src/options.rs
@@ -1772,7 +1772,7 @@ options! {
         "disable the 'leak check' for subtyping; unsound, but useful for tests"),
     no_link: bool = (false, parse_no_flag, [TRACKED],
         "compile without linking"),
-    no_parallel_llvm: bool = (false, parse_no_flag, [UNTRACKED],
+    no_parallel_backend: bool = (false, parse_no_flag, [UNTRACKED],
         "run LLVM in non-parallel mode (while keeping codegen-units and ThinLTO)"),
     no_profiler_runtime: bool = (false, parse_no_flag, [TRACKED],
         "prevent automatic injection of the profiler_builtins crate"),
diff --git a/compiler/rustc_session/src/session.rs b/compiler/rustc_session/src/session.rs
index e9b8a300939..ab636b14d19 100644
--- a/compiler/rustc_session/src/session.rs
+++ b/compiler/rustc_session/src/session.rs
@@ -1016,7 +1016,8 @@ pub fn build_session(
     fluent_resources: Vec<&'static str>,
     driver_lint_caps: FxHashMap<lint::LintId, lint::Level>,
     file_loader: Option<Box<dyn FileLoader + Send + Sync + 'static>>,
-    target_override: Option<Target>,
+    target_cfg: Target,
+    sysroot: PathBuf,
     cfg_version: &'static str,
     ice_file: Option<PathBuf>,
     using_internal_features: Arc<AtomicBool>,
@@ -1033,12 +1034,6 @@ pub fn build_session(
     let cap_lints_allow = sopts.lint_cap.is_some_and(|cap| cap == lint::Allow);
     let can_emit_warnings = !(warnings_allow || cap_lints_allow);
 
-    let sysroot = match &sopts.maybe_sysroot {
-        Some(sysroot) => sysroot.clone(),
-        None => filesearch::get_or_default_sysroot().expect("Failed finding sysroot"),
-    };
-
-    let target_cfg = config::build_target_config(&early_dcx, &sopts, target_override, &sysroot);
     let host_triple = TargetTriple::from_triple(config::host_triple());
     let (host, target_warnings) = Target::search(&host_triple, &sysroot).unwrap_or_else(|e| {
         early_dcx.early_fatal(format!("Error loading host specification: {e}"))
diff --git a/compiler/rustc_target/src/spec/mod.rs b/compiler/rustc_target/src/spec/mod.rs
index 6552810d113..6bca86af30c 100644
--- a/compiler/rustc_target/src/spec/mod.rs
+++ b/compiler/rustc_target/src/spec/mod.rs
@@ -123,6 +123,8 @@ pub enum LinkerFlavor {
     Bpf,
     /// Linker tool for Nvidia PTX.
     Ptx,
+    /// LLVM bitcode linker that can be used as a `self-contained` linker
+    Llbc,
 }
 
 /// Linker flavors available externally through command line (`-Clinker-flavor`)
@@ -141,6 +143,7 @@ pub enum LinkerFlavorCli {
     EmCc,
     Bpf,
     Ptx,
+    Llbc,
 
     // Legacy stable values
     Gcc,
@@ -160,6 +163,7 @@ impl LinkerFlavorCli {
             | LinkerFlavorCli::Msvc(Lld::Yes)
             | LinkerFlavorCli::EmCc
             | LinkerFlavorCli::Bpf
+            | LinkerFlavorCli::Llbc
             | LinkerFlavorCli::Ptx => true,
             LinkerFlavorCli::Gcc
             | LinkerFlavorCli::Ld
@@ -219,6 +223,7 @@ impl LinkerFlavor {
             LinkerFlavorCli::Msvc(lld) => LinkerFlavor::Msvc(lld),
             LinkerFlavorCli::EmCc => LinkerFlavor::EmCc,
             LinkerFlavorCli::Bpf => LinkerFlavor::Bpf,
+            LinkerFlavorCli::Llbc => LinkerFlavor::Llbc,
             LinkerFlavorCli::Ptx => LinkerFlavor::Ptx,
 
             // Below: legacy stable values
@@ -258,6 +263,7 @@ impl LinkerFlavor {
             LinkerFlavor::Msvc(..) => LinkerFlavorCli::Msvc(Lld::No),
             LinkerFlavor::EmCc => LinkerFlavorCli::Em,
             LinkerFlavor::Bpf => LinkerFlavorCli::Bpf,
+            LinkerFlavor::Llbc => LinkerFlavorCli::Llbc,
             LinkerFlavor::Ptx => LinkerFlavorCli::Ptx,
         }
     }
@@ -272,6 +278,7 @@ impl LinkerFlavor {
             LinkerFlavor::Msvc(lld) => LinkerFlavorCli::Msvc(lld),
             LinkerFlavor::EmCc => LinkerFlavorCli::EmCc,
             LinkerFlavor::Bpf => LinkerFlavorCli::Bpf,
+            LinkerFlavor::Llbc => LinkerFlavorCli::Llbc,
             LinkerFlavor::Ptx => LinkerFlavorCli::Ptx,
         }
     }
@@ -286,6 +293,7 @@ impl LinkerFlavor {
             LinkerFlavorCli::Msvc(lld) => (Some(Cc::No), Some(lld)),
             LinkerFlavorCli::EmCc => (Some(Cc::Yes), Some(Lld::Yes)),
             LinkerFlavorCli::Bpf | LinkerFlavorCli::Ptx => (None, None),
+            LinkerFlavorCli::Llbc => (None, None),
 
             // Below: legacy stable values
             LinkerFlavorCli::Gcc => (Some(Cc::Yes), None),
@@ -340,7 +348,7 @@ impl LinkerFlavor {
             LinkerFlavor::WasmLld(cc) => LinkerFlavor::WasmLld(cc_hint.unwrap_or(cc)),
             LinkerFlavor::Unix(cc) => LinkerFlavor::Unix(cc_hint.unwrap_or(cc)),
             LinkerFlavor::Msvc(lld) => LinkerFlavor::Msvc(lld_hint.unwrap_or(lld)),
-            LinkerFlavor::EmCc | LinkerFlavor::Bpf | LinkerFlavor::Ptx => self,
+            LinkerFlavor::EmCc | LinkerFlavor::Bpf | LinkerFlavor::Llbc | LinkerFlavor::Ptx => self,
         }
     }
 
@@ -355,8 +363,8 @@ impl LinkerFlavor {
     pub fn check_compatibility(self, cli: LinkerFlavorCli) -> Option<String> {
         let compatible = |cli| {
             // The CLI flavor should be compatible with the target if:
-            // 1. they are counterparts: they have the same principal flavor.
             match (self, cli) {
+                // 1. they are counterparts: they have the same principal flavor.
                 (LinkerFlavor::Gnu(..), LinkerFlavorCli::Gnu(..))
                 | (LinkerFlavor::Darwin(..), LinkerFlavorCli::Darwin(..))
                 | (LinkerFlavor::WasmLld(..), LinkerFlavorCli::WasmLld(..))
@@ -364,11 +372,14 @@ impl LinkerFlavor {
                 | (LinkerFlavor::Msvc(..), LinkerFlavorCli::Msvc(..))
                 | (LinkerFlavor::EmCc, LinkerFlavorCli::EmCc)
                 | (LinkerFlavor::Bpf, LinkerFlavorCli::Bpf)
+                | (LinkerFlavor::Llbc, LinkerFlavorCli::Llbc)
                 | (LinkerFlavor::Ptx, LinkerFlavorCli::Ptx) => return true,
+                // 2. The linker flavor is independent of target and compatible
+                (LinkerFlavor::Ptx, LinkerFlavorCli::Llbc) => return true,
                 _ => {}
             }
 
-            // 2. or, the flavor is legacy and survives this roundtrip.
+            // 3. or, the flavor is legacy and survives this roundtrip.
             cli == self.with_cli_hints(cli).to_cli()
         };
         (!compatible(cli)).then(|| {
@@ -387,6 +398,7 @@ impl LinkerFlavor {
             | LinkerFlavor::Unix(..)
             | LinkerFlavor::EmCc
             | LinkerFlavor::Bpf
+            | LinkerFlavor::Llbc
             | LinkerFlavor::Ptx => LldFlavor::Ld,
             LinkerFlavor::Darwin(..) => LldFlavor::Ld64,
             LinkerFlavor::WasmLld(..) => LldFlavor::Wasm,
@@ -412,6 +424,7 @@ impl LinkerFlavor {
             | LinkerFlavor::Msvc(_)
             | LinkerFlavor::Unix(_)
             | LinkerFlavor::Bpf
+            | LinkerFlavor::Llbc
             | LinkerFlavor::Ptx => false,
         }
     }
@@ -431,6 +444,7 @@ impl LinkerFlavor {
             | LinkerFlavor::Msvc(_)
             | LinkerFlavor::Unix(_)
             | LinkerFlavor::Bpf
+            | LinkerFlavor::Llbc
             | LinkerFlavor::Ptx => false,
         }
     }
@@ -480,6 +494,7 @@ linker_flavor_cli_impls! {
     (LinkerFlavorCli::Msvc(Lld::No)) "msvc"
     (LinkerFlavorCli::EmCc) "em-cc"
     (LinkerFlavorCli::Bpf) "bpf"
+    (LinkerFlavorCli::Llbc) "llbc"
     (LinkerFlavorCli::Ptx) "ptx"
 
     // Legacy stable flavors
@@ -2070,6 +2085,14 @@ pub struct TargetOptions {
     /// Default number of codegen units to use in debug mode
     pub default_codegen_units: Option<u64>,
 
+    /// Default codegen backend used for this target. Defaults to `None`.
+    ///
+    /// If `None`, then `CFG_DEFAULT_CODEGEN_BACKEND` environmental variable captured when
+    /// compiling `rustc` will be used instead (or llvm if it is not set).
+    ///
+    /// N.B. when *using* the compiler, backend can always be overriden with `-Zcodegen-backend`.
+    pub default_codegen_backend: Option<StaticCow<str>>,
+
     /// Whether to generate trap instructions in places where optimization would
     /// otherwise produce control flow that falls through into unrelated memory.
     pub trap_unreachable: bool,
@@ -2220,6 +2243,7 @@ fn add_link_args_iter(
         | LinkerFlavor::Unix(..)
         | LinkerFlavor::EmCc
         | LinkerFlavor::Bpf
+        | LinkerFlavor::Llbc
         | LinkerFlavor::Ptx => {}
     }
 }
@@ -2376,6 +2400,7 @@ impl Default for TargetOptions {
             stack_probes: StackProbeType::None,
             min_global_align: None,
             default_codegen_units: None,
+            default_codegen_backend: None,
             trap_unreachable: true,
             requires_lto: false,
             singlethread: false,
diff --git a/compiler/rustc_target/src/spec/targets/nvptx64_nvidia_cuda.rs b/compiler/rustc_target/src/spec/targets/nvptx64_nvidia_cuda.rs
index e54661b29d7..0b0b31b503b 100644
--- a/compiler/rustc_target/src/spec/targets/nvptx64_nvidia_cuda.rs
+++ b/compiler/rustc_target/src/spec/targets/nvptx64_nvidia_cuda.rs
@@ -1,3 +1,4 @@
+use crate::spec::LinkSelfContainedDefault;
 use crate::spec::{LinkerFlavor, MergeFunctions, PanicStrategy, Target, TargetOptions};
 
 pub fn target() -> Target {
@@ -52,6 +53,9 @@ pub fn target() -> Target {
             // The LLVM backend does not support stack canaries for this target
             supports_stack_protector: false,
 
+            // Support using `self-contained` linkers like the llvm-bitcode-linker
+            link_self_contained: LinkSelfContainedDefault::True,
+
             ..Default::default()
         },
     }
diff --git a/compiler/rustc_target/src/spec/tests/tests_impl.rs b/compiler/rustc_target/src/spec/tests/tests_impl.rs
index 3fefd60f7cd..3be18ef3127 100644
--- a/compiler/rustc_target/src/spec/tests/tests_impl.rs
+++ b/compiler/rustc_target/src/spec/tests/tests_impl.rs
@@ -56,7 +56,10 @@ impl Target {
                     LinkerFlavor::Msvc(..) => {
                         assert_matches!(flavor, LinkerFlavor::Msvc(..))
                     }
-                    LinkerFlavor::EmCc | LinkerFlavor::Bpf | LinkerFlavor::Ptx => {
+                    LinkerFlavor::EmCc
+                    | LinkerFlavor::Bpf
+                    | LinkerFlavor::Ptx
+                    | LinkerFlavor::Llbc => {
                         assert_eq!(flavor, self.linker_flavor)
                     }
                 }