Merge from rustc

75 files changed, 673 insertions, 374 deletions

diff --git a/compiler/rustc/src/main.rs b/compiler/rustc/src/main.rs
index 434b978ae31..7ba58406ef1 100644
--- a/compiler/rustc/src/main.rs
+++ b/compiler/rustc/src/main.rs
@@ -1,5 +1,3 @@
-#![feature(unix_sigpipe)]
-
 // A note about jemalloc: rustc uses jemalloc when built for CI and
 // distribution. The obvious way to do this is with the `#[global_allocator]`
 // mechanism. However, for complicated reasons (see
@@ -34,7 +32,6 @@
 // https://github.com/rust-lang/rust/commit/b90cfc887c31c3e7a9e6d462e2464db1fe506175#diff-43914724af6e464c1da2171e4a9b6c7e607d5bc1203fa95c0ab85be4122605ef
 // for an example of how to do so.
 
-#[unix_sigpipe = "sig_dfl"]
 fn main() {
     // See the comment at the top of this file for an explanation of this.
     #[cfg(feature = "jemalloc-sys")]
diff --git a/compiler/rustc_abi/src/lib.rs b/compiler/rustc_abi/src/lib.rs
index 6a2943da4a3..5e3f64540e4 100644
--- a/compiler/rustc_abi/src/lib.rs
+++ b/compiler/rustc_abi/src/lib.rs
@@ -21,6 +21,8 @@ use rustc_macros::{Decodable_Generic, Encodable_Generic};
 use std::iter::Step;
 
 mod layout;
+#[cfg(test)]
+mod tests;
 
 pub use layout::LayoutCalculator;
 
diff --git a/compiler/rustc_abi/src/tests.rs b/compiler/rustc_abi/src/tests.rs
new file mode 100644
index 00000000000..d993012378c
--- /dev/null
+++ b/compiler/rustc_abi/src/tests.rs
@@ -0,0 +1,7 @@
+use super::*;
+
+#[test]
+fn align_constants() {
+    assert_eq!(Align::ONE, Align::from_bytes(1).unwrap());
+    assert_eq!(Align::EIGHT, Align::from_bytes(8).unwrap());
+}
diff --git a/compiler/rustc_ast_lowering/src/index.rs b/compiler/rustc_ast_lowering/src/index.rs
index be6c7da4108..7254be2b2f4 100644
--- a/compiler/rustc_ast_lowering/src/index.rs
+++ b/compiler/rustc_ast_lowering/src/index.rs
@@ -62,7 +62,7 @@ pub(super) fn index_hir<'hir>(
         if let Node::Err(span) = node.node {
             let hir_id = HirId { owner: item.def_id(), local_id };
             let msg = format!("ID {hir_id} not encountered when visiting item HIR");
-            tcx.dcx().span_delayed_bug(*span, msg);
+            tcx.dcx().span_delayed_bug(span, msg);
         }
     }
 
@@ -376,7 +376,7 @@ impl<'a, 'hir> Visitor<'hir> for NodeCollector<'a, 'hir> {
         }
     }
 
-    fn visit_array_length(&mut self, len: &'hir ArrayLen) {
+    fn visit_array_length(&mut self, len: &'hir ArrayLen<'hir>) {
         match len {
             ArrayLen::Infer(inf) => self.insert(inf.span, inf.hir_id, Node::ArrayLenInfer(inf)),
             ArrayLen::Body(..) => intravisit::walk_array_len(self, len),
diff --git a/compiler/rustc_ast_lowering/src/item.rs b/compiler/rustc_ast_lowering/src/item.rs
index dcd8f5d5f15..156e369c458 100644
--- a/compiler/rustc_ast_lowering/src/item.rs
+++ b/compiler/rustc_ast_lowering/src/item.rs
@@ -1589,11 +1589,12 @@ impl<'hir> LoweringContext<'_, 'hir> {
                             }),
                         )),
                     )),
-                    default: Some(hir::AnonConst {
+                    default: Some(self.arena.alloc(hir::AnonConst {
                         def_id: anon_const,
                         hir_id: const_id,
                         body: const_body,
-                    }),
+                        span,
+                    })),
                     is_host_effect: true,
                 },
                 colon_span: None,
diff --git a/compiler/rustc_ast_lowering/src/lib.rs b/compiler/rustc_ast_lowering/src/lib.rs
index dd8c09ce485..6c54363e306 100644
--- a/compiler/rustc_ast_lowering/src/lib.rs
+++ b/compiler/rustc_ast_lowering/src/lib.rs
@@ -1178,14 +1178,17 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                                     tokens: None,
                                 };
 
-                                let ct = self.with_new_scopes(span, |this| hir::AnonConst {
-                                    def_id,
-                                    hir_id: this.lower_node_id(node_id),
-                                    body: this.lower_const_body(path_expr.span, Some(&path_expr)),
+                                let ct = self.with_new_scopes(span, |this| {
+                                    self.arena.alloc(hir::AnonConst {
+                                        def_id,
+                                        hir_id: this.lower_node_id(node_id),
+                                        body: this
+                                            .lower_const_body(path_expr.span, Some(&path_expr)),
+                                        span,
+                                    })
                                 });
                                 return GenericArg::Const(ConstArg {
                                     value: ct,
-                                    span,
                                     is_desugared_from_effects: false,
                                 });
                             }
@@ -1197,7 +1200,6 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             }
             ast::GenericArg::Const(ct) => GenericArg::Const(ConstArg {
                 value: self.lower_anon_const(ct),
-                span: self.lower_span(ct.value.span),
                 is_desugared_from_effects: false,
             }),
         }
@@ -2315,7 +2317,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     }
 
     #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
-    fn lower_array_length(&mut self, c: &AnonConst) -> hir::ArrayLen {
+    fn lower_array_length(&mut self, c: &AnonConst) -> hir::ArrayLen<'hir> {
         match c.value.kind {
             ExprKind::Underscore => {
                 if self.tcx.features().generic_arg_infer {
@@ -2338,12 +2340,13 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         }
     }
 
-    fn lower_anon_const(&mut self, c: &AnonConst) -> hir::AnonConst {
-        self.with_new_scopes(c.value.span, |this| hir::AnonConst {
+    fn lower_anon_const(&mut self, c: &AnonConst) -> &'hir hir::AnonConst {
+        self.arena.alloc(self.with_new_scopes(c.value.span, |this| hir::AnonConst {
             def_id: this.local_def_id(c.id),
             hir_id: this.lower_node_id(c.id),
             body: this.lower_const_body(c.value.span, Some(&c.value)),
-        })
+            span: this.lower_span(c.value.span),
+        }))
     }
 
     fn lower_unsafe_source(&mut self, u: UnsafeSource) -> hir::UnsafeSource {
@@ -2650,8 +2653,7 @@ impl<'hir> GenericArgsCtor<'hir> {
 
         lcx.children.push((def_id, hir::MaybeOwner::NonOwner(hir_id)));
         self.args.push(hir::GenericArg::Const(hir::ConstArg {
-            value: hir::AnonConst { def_id, hir_id, body },
-            span,
+            value: lcx.arena.alloc(hir::AnonConst { def_id, hir_id, body, span }),
             is_desugared_from_effects: true,
         }))
     }
diff --git a/compiler/rustc_builtin_macros/src/format.rs b/compiler/rustc_builtin_macros/src/format.rs
index 2c717661a1c..2450ac8f4b3 100644
--- a/compiler/rustc_builtin_macros/src/format.rs
+++ b/compiler/rustc_builtin_macros/src/format.rs
@@ -307,7 +307,7 @@ fn make_format_args(
         return ExpandResult::Ready(Err(guar));
     }
 
-    let to_span = |inner_span: rustc_parse_format::InnerSpan| {
+    let to_span = |inner_span: parse::InnerSpan| {
         is_source_literal.then(|| {
             fmt_span.from_inner(InnerSpan { start: inner_span.start, end: inner_span.end })
         })
@@ -577,7 +577,7 @@ fn make_format_args(
 fn invalid_placeholder_type_error(
     ecx: &ExtCtxt<'_>,
     ty: &str,
-    ty_span: Option<rustc_parse_format::InnerSpan>,
+    ty_span: Option<parse::InnerSpan>,
     fmt_span: Span,
 ) {
     let sp = ty_span.map(|sp| fmt_span.from_inner(InnerSpan::new(sp.start, sp.end)));
diff --git a/compiler/rustc_codegen_gcc/src/context.rs b/compiler/rustc_codegen_gcc/src/context.rs
index 16a85b4e8fa..4a1f5188a80 100644
--- a/compiler/rustc_codegen_gcc/src/context.rs
+++ b/compiler/rustc_codegen_gcc/src/context.rs
@@ -6,7 +6,8 @@ use gccjit::{
 use rustc_codegen_ssa::base::wants_msvc_seh;
 use rustc_codegen_ssa::errors as ssa_errors;
 use rustc_codegen_ssa::traits::{BackendTypes, BaseTypeMethods, MiscMethods};
-use rustc_data_structures::base_n;
+use rustc_data_structures::base_n::ToBaseN;
+use rustc_data_structures::base_n::ALPHANUMERIC_ONLY;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_middle::mir::mono::CodegenUnit;
 use rustc_middle::span_bug;
@@ -621,7 +622,7 @@ impl<'b, 'tcx> CodegenCx<'b, 'tcx> {
         let mut name = String::with_capacity(prefix.len() + 6);
         name.push_str(prefix);
         name.push_str(".");
-        base_n::push_str(idx as u128, base_n::ALPHANUMERIC_ONLY, &mut name);
+        name.push_str(&(idx as u64).to_base(ALPHANUMERIC_ONLY));
         name
     }
 }
diff --git a/compiler/rustc_codegen_llvm/src/builder.rs b/compiler/rustc_codegen_llvm/src/builder.rs
index 5c8f358d03a..1a1b4ae3831 100644
--- a/compiler/rustc_codegen_llvm/src/builder.rs
+++ b/compiler/rustc_codegen_llvm/src/builder.rs
@@ -17,7 +17,7 @@ use rustc_data_structures::small_c_str::SmallCStr;
 use rustc_hir::def_id::DefId;
 use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrs;
 use rustc_middle::ty::layout::{
-    FnAbiError, FnAbiOfHelpers, FnAbiRequest, HasTyCtxt, LayoutError, LayoutOfHelpers, TyAndLayout,
+    FnAbiError, FnAbiOfHelpers, FnAbiRequest, LayoutError, LayoutOfHelpers, TyAndLayout,
 };
 use rustc_middle::ty::{self, Instance, Ty, TyCtxt};
 use rustc_sanitizers::{cfi, kcfi};
@@ -27,7 +27,6 @@ use rustc_target::abi::{self, call::FnAbi, Align, Size, WrappingRange};
 use rustc_target::spec::{HasTargetSpec, SanitizerSet, Target};
 use smallvec::SmallVec;
 use std::borrow::Cow;
-use std::ffi::CString;
 use std::iter;
 use std::ops::Deref;
 use std::ptr;
@@ -1705,13 +1704,21 @@ impl<'a, 'll, 'tcx> Builder<'a, 'll, 'tcx> {
         kcfi_bundle
     }
 
+    /// Emits a call to `llvm.instrprof.mcdc.parameters`.
+    ///
+    /// This doesn't produce any code directly, but is used as input by
+    /// the LLVM pass that handles coverage instrumentation.
+    ///
+    /// (See clang's [`CodeGenPGO::emitMCDCParameters`] for comparison.)
+    ///
+    /// [`CodeGenPGO::emitMCDCParameters`]:
+    ///     https://github.com/rust-lang/llvm-project/blob/5399a24/clang/lib/CodeGen/CodeGenPGO.cpp#L1124
     pub(crate) fn mcdc_parameters(
         &mut self,
         fn_name: &'ll Value,
         hash: &'ll Value,
         bitmap_bytes: &'ll Value,
-        max_decision_depth: u32,
-    ) -> Vec<&'ll Value> {
+    ) {
         debug!("mcdc_parameters() with args ({:?}, {:?}, {:?})", fn_name, hash, bitmap_bytes);
 
         assert!(llvm_util::get_version() >= (18, 0, 0), "MCDC intrinsics require LLVM 18 or later");
@@ -1724,8 +1731,6 @@ impl<'a, 'll, 'tcx> Builder<'a, 'll, 'tcx> {
         let args = &[fn_name, hash, bitmap_bytes];
         let args = self.check_call("call", llty, llfn, args);
 
-        let mut cond_bitmaps = vec![];
-
         unsafe {
             let _ = llvm::LLVMRustBuildCall(
                 self.llbuilder,
@@ -1736,23 +1741,7 @@ impl<'a, 'll, 'tcx> Builder<'a, 'll, 'tcx> {
                 [].as_ptr(),
                 0 as c_uint,
             );
-            // Create condition bitmap named `mcdc.addr`.
-            for i in 0..=max_decision_depth {
-                let mut bx = Builder::with_cx(self.cx);
-                bx.position_at_start(llvm::LLVMGetFirstBasicBlock(self.llfn()));
-
-                let name = CString::new(format!("mcdc.addr.{i}")).unwrap();
-                let cond_bitmap = {
-                    let alloca =
-                        llvm::LLVMBuildAlloca(bx.llbuilder, bx.cx.type_i32(), name.as_ptr());
-                    llvm::LLVMSetAlignment(alloca, 4);
-                    alloca
-                };
-                bx.store(self.const_i32(0), cond_bitmap, self.tcx().data_layout.i32_align.abi);
-                cond_bitmaps.push(cond_bitmap);
-            }
         }
-        cond_bitmaps
     }
 
     pub(crate) fn mcdc_tvbitmap_update(
@@ -1794,8 +1783,7 @@ impl<'a, 'll, 'tcx> Builder<'a, 'll, 'tcx> {
                 0 as c_uint,
             );
         }
-        let i32_align = self.tcx().data_layout.i32_align.abi;
-        self.store(self.const_i32(0), mcdc_temp, i32_align);
+        self.store(self.const_i32(0), mcdc_temp, self.tcx.data_layout.i32_align.abi);
     }
 
     pub(crate) fn mcdc_condbitmap_update(
diff --git a/compiler/rustc_codegen_llvm/src/context.rs b/compiler/rustc_codegen_llvm/src/context.rs
index d32baa6dc02..653abb4e41b 100644
--- a/compiler/rustc_codegen_llvm/src/context.rs
+++ b/compiler/rustc_codegen_llvm/src/context.rs
@@ -11,7 +11,8 @@ use crate::value::Value;
 use rustc_codegen_ssa::base::{wants_msvc_seh, wants_wasm_eh};
 use rustc_codegen_ssa::errors as ssa_errors;
 use rustc_codegen_ssa::traits::*;
-use rustc_data_structures::base_n;
+use rustc_data_structures::base_n::ToBaseN;
+use rustc_data_structures::base_n::ALPHANUMERIC_ONLY;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::small_c_str::SmallCStr;
 use rustc_hir::def_id::DefId;
@@ -1015,7 +1016,7 @@ impl CodegenCx<'_, '_> {
         let mut name = String::with_capacity(prefix.len() + 6);
         name.push_str(prefix);
         name.push('.');
-        base_n::push_str(idx as u128, base_n::ALPHANUMERIC_ONLY, &mut name);
+        name.push_str(&(idx as u64).to_base(ALPHANUMERIC_ONLY));
         name
     }
 }
diff --git a/compiler/rustc_codegen_llvm/src/coverageinfo/mod.rs b/compiler/rustc_codegen_llvm/src/coverageinfo/mod.rs
index 679c6e1a2ff..c51a7744a30 100644
--- a/compiler/rustc_codegen_llvm/src/coverageinfo/mod.rs
+++ b/compiler/rustc_codegen_llvm/src/coverageinfo/mod.rs
@@ -13,10 +13,10 @@ use rustc_codegen_ssa::traits::{
 use rustc_data_structures::fx::{FxHashMap, FxIndexMap};
 use rustc_llvm::RustString;
 use rustc_middle::bug;
-use rustc_middle::mir::coverage::{CoverageKind, FunctionCoverageInfo};
+use rustc_middle::mir::coverage::CoverageKind;
 use rustc_middle::ty::layout::HasTyCtxt;
 use rustc_middle::ty::Instance;
-use rustc_target::abi::Align;
+use rustc_target::abi::{Align, Size};
 
 use std::cell::RefCell;
 
@@ -91,6 +91,42 @@ impl<'ll, 'tcx> CodegenCx<'ll, 'tcx> {
 }
 
 impl<'tcx> CoverageInfoBuilderMethods<'tcx> for Builder<'_, '_, 'tcx> {
+    fn init_coverage(&mut self, instance: Instance<'tcx>) {
+        let Some(function_coverage_info) =
+            self.tcx.instance_mir(instance.def).function_coverage_info.as_deref()
+        else {
+            return;
+        };
+
+        // If there are no MC/DC bitmaps to set up, return immediately.
+        if function_coverage_info.mcdc_bitmap_bytes == 0 {
+            return;
+        }
+
+        let fn_name = self.get_pgo_func_name_var(instance);
+        let hash = self.const_u64(function_coverage_info.function_source_hash);
+        let bitmap_bytes = self.const_u32(function_coverage_info.mcdc_bitmap_bytes);
+        self.mcdc_parameters(fn_name, hash, bitmap_bytes);
+
+        // Create pointers named `mcdc.addr.{i}` to stack-allocated condition bitmaps.
+        let mut cond_bitmaps = vec![];
+        for i in 0..function_coverage_info.mcdc_num_condition_bitmaps {
+            // MC/DC intrinsics will perform loads/stores that use the ABI default
+            // alignment for i32, so our variable declaration should match.
+            let align = self.tcx.data_layout.i32_align.abi;
+            let cond_bitmap = self.alloca(Size::from_bytes(4), align);
+            llvm::set_value_name(cond_bitmap, format!("mcdc.addr.{i}").as_bytes());
+            self.store(self.const_i32(0), cond_bitmap, align);
+            cond_bitmaps.push(cond_bitmap);
+        }
+
+        self.coverage_context()
+            .expect("always present when coverage is enabled")
+            .mcdc_condition_bitmap_map
+            .borrow_mut()
+            .insert(instance, cond_bitmaps);
+    }
+
     #[instrument(level = "debug", skip(self))]
     fn add_coverage(&mut self, instance: Instance<'tcx>, kind: &CoverageKind) {
         // Our caller should have already taken care of inlining subtleties,
@@ -109,10 +145,6 @@ impl<'tcx> CoverageInfoBuilderMethods<'tcx> for Builder<'_, '_, 'tcx> {
             return;
         };
 
-        if function_coverage_info.mcdc_bitmap_bytes > 0 {
-            ensure_mcdc_parameters(bx, instance, function_coverage_info);
-        }
-
         let Some(coverage_context) = bx.coverage_context() else { return };
         let mut coverage_map = coverage_context.function_coverage_map.borrow_mut();
         let func_coverage = coverage_map
@@ -193,28 +225,6 @@ impl<'tcx> CoverageInfoBuilderMethods<'tcx> for Builder<'_, '_, 'tcx> {
     }
 }
 
-fn ensure_mcdc_parameters<'ll, 'tcx>(
-    bx: &mut Builder<'_, 'll, 'tcx>,
-    instance: Instance<'tcx>,
-    function_coverage_info: &FunctionCoverageInfo,
-) {
-    let Some(cx) = bx.coverage_context() else { return };
-    if cx.mcdc_condition_bitmap_map.borrow().contains_key(&instance) {
-        return;
-    }
-
-    let fn_name = bx.get_pgo_func_name_var(instance);
-    let hash = bx.const_u64(function_coverage_info.function_source_hash);
-    let bitmap_bytes = bx.const_u32(function_coverage_info.mcdc_bitmap_bytes);
-    let max_decision_depth = function_coverage_info.mcdc_max_decision_depth;
-    let cond_bitmap = bx.mcdc_parameters(fn_name, hash, bitmap_bytes, max_decision_depth as u32);
-    bx.coverage_context()
-        .expect("already checked above")
-        .mcdc_condition_bitmap_map
-        .borrow_mut()
-        .insert(instance, cond_bitmap);
-}
-
 /// Calls llvm::createPGOFuncNameVar() with the given function instance's
 /// mangled function name. The LLVM API returns an llvm::GlobalVariable
 /// containing the function name, with the specific variable name and linkage
diff --git a/compiler/rustc_codegen_ssa/src/mir/mod.rs b/compiler/rustc_codegen_ssa/src/mir/mod.rs
index 0064c16f5d9..cf6e2e8d14c 100644
--- a/compiler/rustc_codegen_ssa/src/mir/mod.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/mod.rs
@@ -259,6 +259,10 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     // Apply debuginfo to the newly allocated locals.
     fx.debug_introduce_locals(&mut start_bx);
 
+    // If the backend supports coverage, and coverage is enabled for this function,
+    // do any necessary start-of-function codegen (e.g. locals for MC/DC bitmaps).
+    start_bx.init_coverage(instance);
+
     // The builders will be created separately for each basic block at `codegen_block`.
     // So drop the builder of `start_llbb` to avoid having two at the same time.
     drop(start_bx);
diff --git a/compiler/rustc_codegen_ssa/src/traits/coverageinfo.rs b/compiler/rustc_codegen_ssa/src/traits/coverageinfo.rs
index d1d813bd389..906d8b87d3b 100644
--- a/compiler/rustc_codegen_ssa/src/traits/coverageinfo.rs
+++ b/compiler/rustc_codegen_ssa/src/traits/coverageinfo.rs
@@ -3,6 +3,11 @@ use rustc_middle::mir::coverage::CoverageKind;
 use rustc_middle::ty::Instance;
 
 pub trait CoverageInfoBuilderMethods<'tcx>: BackendTypes {
+    /// Performs any start-of-function codegen needed for coverage instrumentation.
+    ///
+    /// Can be a no-op in backends that don't support coverage instrumentation.
+    fn init_coverage(&mut self, _instance: Instance<'tcx>) {}
+
     /// Handle the MIR coverage info in a backend-specific way.
     ///
     /// This can potentially be a no-op in backends that don't support
diff --git a/compiler/rustc_const_eval/src/const_eval/dummy_machine.rs b/compiler/rustc_const_eval/src/const_eval/dummy_machine.rs
index 7b6828c6e18..afc60d33647 100644
--- a/compiler/rustc_const_eval/src/const_eval/dummy_machine.rs
+++ b/compiler/rustc_const_eval/src/const_eval/dummy_machine.rs
@@ -105,7 +105,7 @@ impl<'mir, 'tcx: 'mir> interpret::Machine<'mir, 'tcx> for DummyMachine {
         _destination: &interpret::MPlaceTy<'tcx, Self::Provenance>,
         _target: Option<BasicBlock>,
         _unwind: UnwindAction,
-    ) -> interpret::InterpResult<'tcx> {
+    ) -> interpret::InterpResult<'tcx, Option<ty::Instance<'tcx>>> {
         unimplemented!()
     }
 
diff --git a/compiler/rustc_const_eval/src/const_eval/machine.rs b/compiler/rustc_const_eval/src/const_eval/machine.rs
index dd835279df3..e9cf5a3d769 100644
--- a/compiler/rustc_const_eval/src/const_eval/machine.rs
+++ b/compiler/rustc_const_eval/src/const_eval/machine.rs
@@ -459,16 +459,26 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
         dest: &MPlaceTy<'tcx, Self::Provenance>,
         target: Option<mir::BasicBlock>,
         _unwind: mir::UnwindAction,
-    ) -> InterpResult<'tcx> {
+    ) -> InterpResult<'tcx, Option<ty::Instance<'tcx>>> {
         // Shared intrinsics.
         if ecx.emulate_intrinsic(instance, args, dest, target)? {
-            return Ok(());
+            return Ok(None);
         }
         let intrinsic_name = ecx.tcx.item_name(instance.def_id());
 
         // CTFE-specific intrinsics.
         let Some(ret) = target else {
-            throw_unsup_format!("intrinsic `{intrinsic_name}` is not supported at compile-time");
+            // Handle diverging intrinsics. We can't handle any of them (that are not already
+            // handled above), but check if there is a fallback body.
+            if ecx.tcx.intrinsic(instance.def_id()).unwrap().must_be_overridden {
+                throw_unsup_format!(
+                    "intrinsic `{intrinsic_name}` is not supported at compile-time"
+                );
+            }
+            return Ok(Some(ty::Instance {
+                def: ty::InstanceDef::Item(instance.def_id()),
+                args: instance.args,
+            }));
         };
         match intrinsic_name {
             sym::ptr_guaranteed_cmp => {
@@ -536,14 +546,21 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
             // not the optimization stage.)
             sym::is_val_statically_known => ecx.write_scalar(Scalar::from_bool(false), dest)?,
             _ => {
-                throw_unsup_format!(
-                    "intrinsic `{intrinsic_name}` is not supported at compile-time"
-                );
+                // We haven't handled the intrinsic, let's see if we can use a fallback body.
+                if ecx.tcx.intrinsic(instance.def_id()).unwrap().must_be_overridden {
+                    throw_unsup_format!(
+                        "intrinsic `{intrinsic_name}` is not supported at compile-time"
+                    );
+                }
+                return Ok(Some(ty::Instance {
+                    def: ty::InstanceDef::Item(instance.def_id()),
+                    args: instance.args,
+                }));
             }
         }
 
         ecx.go_to_block(ret);
-        Ok(())
+        Ok(None)
     }
 
     fn assert_panic(
diff --git a/compiler/rustc_const_eval/src/interpret/intrinsics.rs b/compiler/rustc_const_eval/src/interpret/intrinsics.rs
index 4d37c3c22cd..f73293856c7 100644
--- a/compiler/rustc_const_eval/src/interpret/intrinsics.rs
+++ b/compiler/rustc_const_eval/src/interpret/intrinsics.rs
@@ -414,7 +414,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 }
                 self.copy_op(&self.project_index(&input, index)?, dest)?;
             }
-            sym::likely | sym::unlikely | sym::black_box => {
+            sym::black_box => {
                 // These just return their argument
                 self.copy_op(&args[0], dest)?;
             }
diff --git a/compiler/rustc_const_eval/src/interpret/machine.rs b/compiler/rustc_const_eval/src/interpret/machine.rs
index 8bc569bed54..8405d0746df 100644
--- a/compiler/rustc_const_eval/src/interpret/machine.rs
+++ b/compiler/rustc_const_eval/src/interpret/machine.rs
@@ -216,6 +216,9 @@ pub trait Machine<'mir, 'tcx: 'mir>: Sized {
 
     /// Directly process an intrinsic without pushing a stack frame. It is the hook's
     /// responsibility to advance the instruction pointer as appropriate.
+    ///
+    /// Returns `None` if the intrinsic was fully handled.
+    /// Otherwise, returns an `Instance` of the function that implements the intrinsic.
     fn call_intrinsic(
         ecx: &mut InterpCx<'mir, 'tcx, Self>,
         instance: ty::Instance<'tcx>,
@@ -223,7 +226,7 @@ pub trait Machine<'mir, 'tcx: 'mir>: Sized {
         destination: &MPlaceTy<'tcx, Self::Provenance>,
         target: Option<mir::BasicBlock>,
         unwind: mir::UnwindAction,
-    ) -> InterpResult<'tcx>;
+    ) -> InterpResult<'tcx, Option<ty::Instance<'tcx>>>;
 
     /// Called to evaluate `Assert` MIR terminators that trigger a panic.
     fn assert_panic(
diff --git a/compiler/rustc_const_eval/src/interpret/terminator.rs b/compiler/rustc_const_eval/src/interpret/terminator.rs
index 9c31532a9ce..07425f9a686 100644
--- a/compiler/rustc_const_eval/src/interpret/terminator.rs
+++ b/compiler/rustc_const_eval/src/interpret/terminator.rs
@@ -539,14 +539,28 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             ty::InstanceDef::Intrinsic(def_id) => {
                 assert!(self.tcx.intrinsic(def_id).is_some());
                 // FIXME: Should `InPlace` arguments be reset to uninit?
-                M::call_intrinsic(
+                if let Some(fallback) = M::call_intrinsic(
                     self,
                     instance,
                     &self.copy_fn_args(args),
                     destination,
                     target,
                     unwind,
-                )
+                )? {
+                    assert!(!self.tcx.intrinsic(fallback.def_id()).unwrap().must_be_overridden);
+                    assert!(matches!(fallback.def, ty::InstanceDef::Item(_)));
+                    return self.eval_fn_call(
+                        FnVal::Instance(fallback),
+                        (caller_abi, caller_fn_abi),
+                        args,
+                        with_caller_location,
+                        destination,
+                        target,
+                        unwind,
+                    );
+                } else {
+                    Ok(())
+                }
             }
             ty::InstanceDef::VTableShim(..)
             | ty::InstanceDef::ReifyShim(..)
diff --git a/compiler/rustc_data_structures/src/base_n.rs b/compiler/rustc_data_structures/src/base_n.rs
index a3eb2b9c416..aed89fadc4c 100644
--- a/compiler/rustc_data_structures/src/base_n.rs
+++ b/compiler/rustc_data_structures/src/base_n.rs
@@ -1,6 +1,7 @@
 /// Converts unsigned integers into a string representation with some base.
 /// Bases up to and including 36 can be used for case-insensitive things.
-use std::str;
+use std::ascii;
+use std::fmt;
 
 #[cfg(test)]
 mod tests;
@@ -9,36 +10,101 @@ pub const MAX_BASE: usize = 64;
 pub const ALPHANUMERIC_ONLY: usize = 62;
 pub const CASE_INSENSITIVE: usize = 36;
 
-const BASE_64: &[u8; MAX_BASE] =
-    b"0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ@$";
+const BASE_64: [ascii::Char; MAX_BASE] = {
+    let bytes = b"0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ@$";
+    let Some(ascii) = bytes.as_ascii() else { panic!() };
+    *ascii
+};
 
-#[inline]
-pub fn push_str(mut n: u128, base: usize, output: &mut String) {
-    debug_assert!(base >= 2 && base <= MAX_BASE);
-    let mut s = [0u8; 128];
-    let mut index = s.len();
+pub struct BaseNString {
+    start: usize,
+    buf: [ascii::Char; 128],
+}
+
+impl std::ops::Deref for BaseNString {
+    type Target = str;
 
-    let base = base as u128;
+    fn deref(&self) -> &str {
+        self.buf[self.start..].as_str()
+    }
+}
+
+impl AsRef<str> for BaseNString {
+    fn as_ref(&self) -> &str {
+        self.buf[self.start..].as_str()
+    }
+}
+
+impl fmt::Display for BaseNString {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(self)
+    }
+}
+
+// This trait just lets us reserve the exact right amount of space when doing fixed-length
+// case-insensitve encoding. Add any impls you need.
+pub trait ToBaseN: Into<u128> {
+    fn encoded_len(base: usize) -> usize;
+
+    fn to_base_fixed_len(self, base: usize) -> BaseNString {
+        let mut encoded = self.to_base(base);
+        encoded.start = encoded.buf.len() - Self::encoded_len(base);
+        encoded
+    }
 
-    loop {
-        index -= 1;
-        s[index] = BASE_64[(n % base) as usize];
-        n /= base;
+    fn to_base(self, base: usize) -> BaseNString {
+        let mut output = [ascii::Char::Digit0; 128];
 
-        if n == 0 {
-            break;
+        let mut n: u128 = self.into();
+
+        let mut index = output.len();
+        loop {
+            index -= 1;
+            output[index] = BASE_64[(n % base as u128) as usize];
+            n /= base as u128;
+
+            if n == 0 {
+                break;
+            }
+        }
+        assert_eq!(n, 0);
+
+        BaseNString { start: index, buf: output }
+    }
+}
+
+impl ToBaseN for u128 {
+    fn encoded_len(base: usize) -> usize {
+        let mut max = u128::MAX;
+        let mut len = 0;
+        while max > 0 {
+            len += 1;
+            max /= base as u128;
         }
+        len
     }
+}
 
-    output.push_str(unsafe {
-        // SAFETY: `s` is populated using only valid utf8 characters from `BASE_64`
-        str::from_utf8_unchecked(&s[index..])
-    });
+impl ToBaseN for u64 {
+    fn encoded_len(base: usize) -> usize {
+        let mut max = u64::MAX;
+        let mut len = 0;
+        while max > 0 {
+            len += 1;
+            max /= base as u64;
+        }
+        len
+    }
 }
 
-#[inline]
-pub fn encode(n: u128, base: usize) -> String {
-    let mut s = String::new();
-    push_str(n, base, &mut s);
-    s
+impl ToBaseN for u32 {
+    fn encoded_len(base: usize) -> usize {
+        let mut max = u32::MAX;
+        let mut len = 0;
+        while max > 0 {
+            len += 1;
+            max /= base as u32;
+        }
+        len
+    }
 }
diff --git a/compiler/rustc_data_structures/src/base_n/tests.rs b/compiler/rustc_data_structures/src/base_n/tests.rs
index 2be2f0532c9..148d8dde02a 100644
--- a/compiler/rustc_data_structures/src/base_n/tests.rs
+++ b/compiler/rustc_data_structures/src/base_n/tests.rs
@@ -1,9 +1,17 @@
 use super::*;
 
 #[test]
-fn test_encode() {
+fn limits() {
+    assert_eq!(Ok(u128::MAX), u128::from_str_radix(&u128::MAX.to_base(36), 36));
+    assert_eq!(Ok(u64::MAX), u64::from_str_radix(&u64::MAX.to_base(36), 36));
+    assert_eq!(Ok(u32::MAX), u32::from_str_radix(&u32::MAX.to_base(36), 36));
+}
+
+#[test]
+fn test_to_base() {
     fn test(n: u128, base: usize) {
-        assert_eq!(Ok(n), u128::from_str_radix(&encode(n, base), base as u32));
+        assert_eq!(Ok(n), u128::from_str_radix(&n.to_base(base), base as u32));
+        assert_eq!(Ok(n), u128::from_str_radix(&n.to_base_fixed_len(base), base as u32));
     }
 
     for base in 2..37 {
diff --git a/compiler/rustc_data_structures/src/lib.rs b/compiler/rustc_data_structures/src/lib.rs
index cf54e700e2b..8dd85b25e0e 100644
--- a/compiler/rustc_data_structures/src/lib.rs
+++ b/compiler/rustc_data_structures/src/lib.rs
@@ -16,6 +16,8 @@
 #![doc(rust_logo)]
 #![feature(allocator_api)]
 #![feature(array_windows)]
+#![feature(ascii_char)]
+#![feature(ascii_char_variants)]
 #![feature(auto_traits)]
 #![feature(cfg_match)]
 #![feature(core_intrinsics)]
diff --git a/compiler/rustc_driver_impl/Cargo.toml b/compiler/rustc_driver_impl/Cargo.toml
index a8bba3afb7e..cb37644d570 100644
--- a/compiler/rustc_driver_impl/Cargo.toml
+++ b/compiler/rustc_driver_impl/Cargo.toml
@@ -42,9 +42,8 @@ rustc_privacy = { path = "../rustc_privacy" }
 rustc_query_system = { path = "../rustc_query_system" }
 rustc_resolve = { path = "../rustc_resolve" }
 rustc_session = { path = "../rustc_session" }
-rustc_smir ={ path = "../rustc_smir" }
+rustc_smir = { path = "../rustc_smir" }
 rustc_span = { path = "../rustc_span" }
-rustc_symbol_mangling = { path = "../rustc_symbol_mangling" }
 rustc_target = { path = "../rustc_target" }
 rustc_trait_selection = { path = "../rustc_trait_selection" }
 rustc_ty_utils = { path = "../rustc_ty_utils" }
diff --git a/compiler/rustc_errors/src/diagnostic_impls.rs b/compiler/rustc_errors/src/diagnostic_impls.rs
index b3a1e29f8e2..4696917554f 100644
--- a/compiler/rustc_errors/src/diagnostic_impls.rs
+++ b/compiler/rustc_errors/src/diagnostic_impls.rs
@@ -13,7 +13,7 @@ use rustc_span::symbol::{Ident, MacroRulesNormalizedIdent, Symbol};
 use rustc_span::Span;
 use rustc_target::abi::TargetDataLayoutErrors;
 use rustc_target::spec::{PanicStrategy, SplitDebuginfo, StackProtector, TargetTriple};
-use rustc_type_ir as type_ir;
+use rustc_type_ir::{ClosureKind, FloatTy};
 use std::backtrace::Backtrace;
 use std::borrow::Cow;
 use std::fmt;
@@ -196,7 +196,7 @@ impl IntoDiagArg for ast::token::TokenKind {
     }
 }
 
-impl IntoDiagArg for type_ir::FloatTy {
+impl IntoDiagArg for FloatTy {
     fn into_diag_arg(self) -> DiagArgValue {
         DiagArgValue::Str(Cow::Borrowed(self.name_str()))
     }
@@ -252,7 +252,7 @@ impl IntoDiagArg for Level {
     }
 }
 
-impl IntoDiagArg for type_ir::ClosureKind {
+impl IntoDiagArg for ClosureKind {
     fn into_diag_arg(self) -> DiagArgValue {
         DiagArgValue::Str(self.as_str().into())
     }
diff --git a/compiler/rustc_feature/src/builtin_attrs.rs b/compiler/rustc_feature/src/builtin_attrs.rs
index 9d3aac66c41..9c33cc8ed0b 100644
--- a/compiler/rustc_feature/src/builtin_attrs.rs
+++ b/compiler/rustc_feature/src/builtin_attrs.rs
@@ -396,10 +396,6 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     ),
 
     // Entry point:
-    gated!(
-        unix_sigpipe, Normal, template!(NameValueStr: "inherit|sig_ign|sig_dfl"), ErrorFollowing,
-        EncodeCrossCrate::Yes, experimental!(unix_sigpipe)
-    ),
     ungated!(start, Normal, template!(Word), WarnFollowing, EncodeCrossCrate::No),
     ungated!(no_start, CrateLevel, template!(Word), WarnFollowing, EncodeCrossCrate::No),
     ungated!(no_main, CrateLevel, template!(Word), WarnFollowing, EncodeCrossCrate::No),
diff --git a/compiler/rustc_feature/src/unstable.rs b/compiler/rustc_feature/src/unstable.rs
index e7d7a9f380b..fe50499db76 100644
--- a/compiler/rustc_feature/src/unstable.rs
+++ b/compiler/rustc_feature/src/unstable.rs
@@ -619,8 +619,6 @@ declare_features! (
     /// Allows creation of instances of a struct by moving fields that have
     /// not changed from prior instances of the same struct (RFC #2528)
     (unstable, type_changing_struct_update, "1.58.0", Some(86555)),
-    /// Enables rustc to generate code that instructs libstd to NOT ignore SIGPIPE.
-    (unstable, unix_sigpipe, "1.65.0", Some(97889)),
     /// Allows unnamed fields of struct and union type
     (incomplete, unnamed_fields, "1.74.0", Some(49804)),
     /// Allows unsized fn parameters.
diff --git a/compiler/rustc_hir/src/hir.rs b/compiler/rustc_hir/src/hir.rs
index fc8f7466694..244c479120d 100644
--- a/compiler/rustc_hir/src/hir.rs
+++ b/compiler/rustc_hir/src/hir.rs
@@ -229,9 +229,8 @@ impl<'hir> PathSegment<'hir> {
 }
 
 #[derive(Clone, Copy, Debug, HashStable_Generic)]
-pub struct ConstArg {
-    pub value: AnonConst,
-    pub span: Span,
+pub struct ConstArg<'hir> {
+    pub value: &'hir AnonConst,
     /// Indicates whether this comes from a `~const` desugaring.
     pub is_desugared_from_effects: bool,
 }
@@ -252,7 +251,7 @@ impl InferArg {
 pub enum GenericArg<'hir> {
     Lifetime(&'hir Lifetime),
     Type(&'hir Ty<'hir>),
-    Const(ConstArg),
+    Const(ConstArg<'hir>),
     Infer(InferArg),
 }
 
@@ -261,7 +260,7 @@ impl GenericArg<'_> {
         match self {
             GenericArg::Lifetime(l) => l.ident.span,
             GenericArg::Type(t) => t.span,
-            GenericArg::Const(c) => c.span,
+            GenericArg::Const(c) => c.value.span,
             GenericArg::Infer(i) => i.span,
         }
     }
@@ -490,7 +489,7 @@ pub enum GenericParamKind<'hir> {
     Const {
         ty: &'hir Ty<'hir>,
         /// Optional default value for the const generic param
-        default: Option<AnonConst>,
+        default: Option<&'hir AnonConst>,
         is_host_effect: bool,
     },
 }
@@ -1562,12 +1561,12 @@ impl fmt::Display for ConstContext {
 pub type Lit = Spanned<LitKind>;
 
 #[derive(Copy, Clone, Debug, HashStable_Generic)]
-pub enum ArrayLen {
+pub enum ArrayLen<'hir> {
     Infer(InferArg),
-    Body(AnonConst),
+    Body(&'hir AnonConst),
 }
 
-impl ArrayLen {
+impl ArrayLen<'_> {
     pub fn hir_id(&self) -> HirId {
         match self {
             ArrayLen::Infer(InferArg { hir_id, .. }) | ArrayLen::Body(AnonConst { hir_id, .. }) => {
@@ -1590,6 +1589,7 @@ pub struct AnonConst {
     pub hir_id: HirId,
     pub def_id: LocalDefId,
     pub body: BodyId,
+    pub span: Span,
 }
 
 /// An inline constant expression `const { something }`.
@@ -2002,7 +2002,7 @@ pub enum ExprKind<'hir> {
     ///
     /// E.g., `[1; 5]`. The first expression is the element
     /// to be repeated; the second is the number of times to repeat it.
-    Repeat(&'hir Expr<'hir>, ArrayLen),
+    Repeat(&'hir Expr<'hir>, ArrayLen<'hir>),
 
     /// A suspension point for coroutines (i.e., `yield <expr>`).
     Yield(&'hir Expr<'hir>, YieldSource),
@@ -2382,7 +2382,7 @@ pub struct TypeBinding<'hir> {
 #[derive(Debug, Clone, Copy, HashStable_Generic)]
 pub enum Term<'hir> {
     Ty(&'hir Ty<'hir>),
-    Const(AnonConst),
+    Const(&'hir AnonConst),
 }
 
 impl<'hir> From<&'hir Ty<'hir>> for Term<'hir> {
@@ -2391,8 +2391,8 @@ impl<'hir> From<&'hir Ty<'hir>> for Term<'hir> {
     }
 }
 
-impl<'hir> From<AnonConst> for Term<'hir> {
-    fn from(c: AnonConst) -> Self {
+impl<'hir> From<&'hir AnonConst> for Term<'hir> {
+    fn from(c: &'hir AnonConst) -> Self {
         Term::Const(c)
     }
 }
@@ -2683,7 +2683,7 @@ pub enum TyKind<'hir> {
     /// A variable length slice (i.e., `[T]`).
     Slice(&'hir Ty<'hir>),
     /// A fixed length array (i.e., `[T; n]`).
-    Array(&'hir Ty<'hir>, ArrayLen),
+    Array(&'hir Ty<'hir>, ArrayLen<'hir>),
     /// A raw pointer (i.e., `*const T` or `*mut T`).
     Ptr(MutTy<'hir>),
     /// A reference (i.e., `&'a T` or `&'a mut T`).
@@ -2712,7 +2712,7 @@ pub enum TyKind<'hir> {
     /// where `Bound` is a trait or a lifetime.
     TraitObject(&'hir [PolyTraitRef<'hir>], &'hir Lifetime, TraitObjectSyntax),
     /// Unused for now.
-    Typeof(AnonConst),
+    Typeof(&'hir AnonConst),
     /// `TyKind::Infer` means the type should be inferred instead of it having been
     /// specified. This can appear anywhere in a type.
     Infer,
@@ -2745,10 +2745,10 @@ pub enum InlineAsmOperand<'hir> {
         out_expr: Option<&'hir Expr<'hir>>,
     },
     Const {
-        anon_const: AnonConst,
+        anon_const: &'hir AnonConst,
     },
     SymFn {
-        anon_const: AnonConst,
+        anon_const: &'hir AnonConst,
     },
     SymStatic {
         path: QPath<'hir>,
@@ -2950,7 +2950,7 @@ pub struct Variant<'hir> {
     /// Fields and constructor id of the variant.
     pub data: VariantData<'hir>,
     /// Explicit discriminant (e.g., `Foo = 1`).
-    pub disr_expr: Option<AnonConst>,
+    pub disr_expr: Option<&'hir AnonConst>,
     /// Span
     pub span: Span,
 }
@@ -3479,15 +3479,13 @@ impl<'hir> OwnerNode<'hir> {
         }
     }
 
-    // Span by reference to pass to `Node::Err`.
-    #[allow(rustc::pass_by_value)]
-    pub fn span(&self) -> &'hir Span {
+    pub fn span(&self) -> Span {
         match self {
             OwnerNode::Item(Item { span, .. })
             | OwnerNode::ForeignItem(ForeignItem { span, .. })
             | OwnerNode::ImplItem(ImplItem { span, .. })
-            | OwnerNode::TraitItem(TraitItem { span, .. }) => span,
-            OwnerNode::Crate(Mod { spans: ModSpans { inner_span, .. }, .. }) => inner_span,
+            | OwnerNode::TraitItem(TraitItem { span, .. }) => *span,
+            OwnerNode::Crate(Mod { spans: ModSpans { inner_span, .. }, .. }) => *inner_span,
             OwnerNode::Synthetic => unreachable!(),
         }
     }
@@ -3632,9 +3630,7 @@ pub enum Node<'hir> {
     PreciseCapturingNonLifetimeArg(&'hir PreciseCapturingNonLifetimeArg),
     // Created by query feeding
     Synthetic,
-    // Span by reference to minimize `Node`'s size
-    #[allow(rustc::pass_by_value)]
-    Err(&'hir Span),
+    Err(Span),
 }
 
 impl<'hir> Node<'hir> {
@@ -3871,7 +3867,7 @@ mod size_asserts {
     static_assert_size!(FnDecl<'_>, 40);
     static_assert_size!(ForeignItem<'_>, 72);
     static_assert_size!(ForeignItemKind<'_>, 40);
-    static_assert_size!(GenericArg<'_>, 32);
+    static_assert_size!(GenericArg<'_>, 24);
     static_assert_size!(GenericBound<'_>, 48);
     static_assert_size!(Generics<'_>, 56);
     static_assert_size!(Impl<'_>, 80);
diff --git a/compiler/rustc_hir/src/intravisit.rs b/compiler/rustc_hir/src/intravisit.rs
index cd9f9ff9109..fa89a4a44ef 100644
--- a/compiler/rustc_hir/src/intravisit.rs
+++ b/compiler/rustc_hir/src/intravisit.rs
@@ -338,7 +338,7 @@ pub trait Visitor<'v>: Sized {
     fn visit_pat_field(&mut self, f: &'v PatField<'v>) -> Self::Result {
         walk_pat_field(self, f)
     }
-    fn visit_array_length(&mut self, len: &'v ArrayLen) -> Self::Result {
+    fn visit_array_length(&mut self, len: &'v ArrayLen<'v>) -> Self::Result {
         walk_array_len(self, len)
     }
     fn visit_anon_const(&mut self, c: &'v AnonConst) -> Self::Result {
@@ -703,7 +703,7 @@ pub fn walk_pat_field<'v, V: Visitor<'v>>(visitor: &mut V, field: &'v PatField<'
     visitor.visit_pat(field.pat)
 }
 
-pub fn walk_array_len<'v, V: Visitor<'v>>(visitor: &mut V, len: &'v ArrayLen) -> V::Result {
+pub fn walk_array_len<'v, V: Visitor<'v>>(visitor: &mut V, len: &'v ArrayLen<'v>) -> V::Result {
     match len {
         // FIXME: Use `visit_infer` here.
         ArrayLen::Infer(InferArg { hir_id, span: _ }) => visitor.visit_id(*hir_id),
diff --git a/compiler/rustc_hir_analysis/src/bounds.rs b/compiler/rustc_hir_analysis/src/bounds.rs
index aafb5c1c0b4..5562b81871f 100644
--- a/compiler/rustc_hir_analysis/src/bounds.rs
+++ b/compiler/rustc_hir_analysis/src/bounds.rs
@@ -23,7 +23,7 @@ use rustc_span::Span;
 /// include the self type (e.g., `trait_bounds`) but in others we do not
 #[derive(Default, PartialEq, Eq, Clone, Debug)]
 pub struct Bounds<'tcx> {
-    pub clauses: Vec<(ty::Clause<'tcx>, Span)>,
+    clauses: Vec<(ty::Clause<'tcx>, Span)>,
 }
 
 impl<'tcx> Bounds<'tcx> {
diff --git a/compiler/rustc_hir_analysis/src/collect.rs b/compiler/rustc_hir_analysis/src/collect.rs
index 0f0736f8756..9198ceb8f59 100644
--- a/compiler/rustc_hir_analysis/src/collect.rs
+++ b/compiler/rustc_hir_analysis/src/collect.rs
@@ -143,7 +143,7 @@ impl<'v> Visitor<'v> for HirPlaceholderCollector {
             _ => {}
         }
     }
-    fn visit_array_length(&mut self, length: &'v hir::ArrayLen) {
+    fn visit_array_length(&mut self, length: &'v hir::ArrayLen<'v>) {
         if let hir::ArrayLen::Infer(inf) = length {
             self.0.push(inf.span);
         }
diff --git a/compiler/rustc_hir_analysis/src/collect/type_of.rs b/compiler/rustc_hir_analysis/src/collect/type_of.rs
index 9d7deebac48..5ccfd06f258 100644
--- a/compiler/rustc_hir_analysis/src/collect/type_of.rs
+++ b/compiler/rustc_hir_analysis/src/collect/type_of.rs
@@ -24,7 +24,7 @@ fn anon_const_type_of<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> Ty<'tcx> {
     let hir_id = tcx.local_def_id_to_hir_id(def_id);
 
     let node = tcx.hir_node(hir_id);
-    let Node::AnonConst(_) = node else {
+    let Node::AnonConst(&AnonConst { span, .. }) = node else {
         span_bug!(
             tcx.def_span(def_id),
             "expected anon const in `anon_const_type_of`, got {node:?}"
@@ -134,7 +134,7 @@ fn anon_const_type_of<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> Ty<'tcx> {
                 // I dont think it's possible to reach this but I'm not 100% sure - BoxyUwU
                 return Ty::new_error_with_message(
                     tcx,
-                    tcx.def_span(def_id),
+                    span,
                     "unexpected non-GAT usage of an anon const",
                 );
             }
@@ -152,7 +152,7 @@ fn anon_const_type_of<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> Ty<'tcx> {
             let Some(type_dependent_def) = tables.type_dependent_def_id(parent_node_id) else {
                 return Ty::new_error_with_message(
                     tcx,
-                    tcx.def_span(def_id),
+                    span,
                     format!("unable to find type-dependent def for {parent_node_id:?}"),
                 );
             };
@@ -194,7 +194,7 @@ fn anon_const_type_of<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> Ty<'tcx> {
                     } else {
                         return Ty::new_error_with_message(
                             tcx,
-                            tcx.def_span(def_id),
+                            span,
                             format!("unable to find const parent for {hir_id} in pat {pat:?}"),
                         );
                     }
@@ -202,7 +202,7 @@ fn anon_const_type_of<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> Ty<'tcx> {
                 _ => {
                     return Ty::new_error_with_message(
                         tcx,
-                        tcx.def_span(def_id),
+                        span,
                         format!("unexpected const parent path {parent_node:?}"),
                     );
                 }
@@ -226,11 +226,7 @@ fn anon_const_type_of<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> Ty<'tcx> {
                             .map(|idx| (idx, seg))
                     })
             }) else {
-                return Ty::new_error_with_message(
-                    tcx,
-                    tcx.def_span(def_id),
-                    "no arg matching AnonConst in path",
-                );
+                return Ty::new_error_with_message(tcx, span, "no arg matching AnonConst in path");
             };
 
             let generics = match tcx.res_generics_def_id(segment.res) {
@@ -238,7 +234,7 @@ fn anon_const_type_of<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> Ty<'tcx> {
                 None => {
                     return Ty::new_error_with_message(
                         tcx,
-                        tcx.def_span(def_id),
+                        span,
                         format!("unexpected anon const res {:?} in path: {:?}", segment.res, path),
                     );
                 }
@@ -250,7 +246,7 @@ fn anon_const_type_of<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> Ty<'tcx> {
         _ => {
             return Ty::new_error_with_message(
                 tcx,
-                tcx.def_span(def_id),
+                span,
                 format!("unexpected const parent in type_of(): {parent_node:?}"),
             );
         }
@@ -278,7 +274,7 @@ fn anon_const_type_of<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> Ty<'tcx> {
     } else {
         return Ty::new_error_with_message(
             tcx,
-            tcx.def_span(def_id),
+            span,
             format!("const generic parameter not found in {generics:?} at position {arg_idx:?}"),
         );
     }
diff --git a/compiler/rustc_hir_pretty/src/lib.rs b/compiler/rustc_hir_pretty/src/lib.rs
index 285b99c2c69..4f5fbd024a9 100644
--- a/compiler/rustc_hir_pretty/src/lib.rs
+++ b/compiler/rustc_hir_pretty/src/lib.rs
@@ -968,7 +968,7 @@ impl<'a> State<'a> {
         self.print_else(elseopt)
     }
 
-    fn print_array_length(&mut self, len: &hir::ArrayLen) {
+    fn print_array_length(&mut self, len: &hir::ArrayLen<'_>) {
         match len {
             hir::ArrayLen::Infer(..) => self.word("_"),
             hir::ArrayLen::Body(ct) => self.print_anon_const(ct),
@@ -1052,7 +1052,7 @@ impl<'a> State<'a> {
         self.end()
     }
 
-    fn print_expr_repeat(&mut self, element: &hir::Expr<'_>, count: &hir::ArrayLen) {
+    fn print_expr_repeat(&mut self, element: &hir::Expr<'_>, count: &hir::ArrayLen<'_>) {
         self.ibox(INDENT_UNIT);
         self.word("[");
         self.print_expr(element);
diff --git a/compiler/rustc_hir_typeck/Cargo.toml b/compiler/rustc_hir_typeck/Cargo.toml
index 9e7f0776b60..73a775690d6 100644
--- a/compiler/rustc_hir_typeck/Cargo.toml
+++ b/compiler/rustc_hir_typeck/Cargo.toml
@@ -12,7 +12,6 @@ rustc_attr = { path = "../rustc_attr" }
 rustc_data_structures = { path = "../rustc_data_structures" }
 rustc_errors = { path = "../rustc_errors" }
 rustc_fluent_macro = { path = "../rustc_fluent_macro" }
-rustc_graphviz = { path = "../rustc_graphviz" }
 rustc_hir = { path = "../rustc_hir" }
 rustc_hir_analysis = { path = "../rustc_hir_analysis" }
 rustc_hir_pretty = { path = "../rustc_hir_pretty" }
diff --git a/compiler/rustc_hir_typeck/src/expr.rs b/compiler/rustc_hir_typeck/src/expr.rs
index 8b1ea7c952c..7b552bb7077 100644
--- a/compiler/rustc_hir_typeck/src/expr.rs
+++ b/compiler/rustc_hir_typeck/src/expr.rs
@@ -1444,7 +1444,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             return;
         };
         if let hir::TyKind::Array(_, length) = ty.peel_refs().kind
-            && let hir::ArrayLen::Body(hir::AnonConst { hir_id, .. }) = length
+            && let hir::ArrayLen::Body(&hir::AnonConst { hir_id, .. }) = length
         {
             let span = self.tcx.hir().span(hir_id);
             self.dcx().try_steal_modify_and_emit_err(
@@ -1483,7 +1483,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     fn check_expr_repeat(
         &self,
         element: &'tcx hir::Expr<'tcx>,
-        count: &'tcx hir::ArrayLen,
+        count: &'tcx hir::ArrayLen<'tcx>,
         expected: Expectation<'tcx>,
         expr: &'tcx hir::Expr<'tcx>,
     ) -> Ty<'tcx> {
diff --git a/compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs b/compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs
index 2060e08aacf..552747bdc52 100644
--- a/compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs
+++ b/compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs
@@ -438,7 +438,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         }
     }
 
-    pub fn lower_array_length(&self, length: &hir::ArrayLen) -> ty::Const<'tcx> {
+    pub fn lower_array_length(&self, length: &hir::ArrayLen<'tcx>) -> ty::Const<'tcx> {
         match length {
             hir::ArrayLen::Infer(inf) => self.ct_infer(self.tcx.types.usize, None, inf.span),
             hir::ArrayLen::Body(anon_const) => {
diff --git a/compiler/rustc_hir_typeck/src/pat.rs b/compiler/rustc_hir_typeck/src/pat.rs
index 382dba5f95f..8c7ae7f8e98 100644
--- a/compiler/rustc_hir_typeck/src/pat.rs
+++ b/compiler/rustc_hir_typeck/src/pat.rs
@@ -10,7 +10,6 @@ use rustc_hir::pat_util::EnumerateAndAdjustIterator;
 use rustc_hir::{self as hir, BindingMode, ByRef, HirId, Mutability, Pat, PatKind};
 use rustc_infer::infer;
 use rustc_infer::infer::type_variable::TypeVariableOrigin;
-use rustc_lint as lint;
 use rustc_middle::mir::interpret::ErrorHandled;
 use rustc_middle::ty::{self, Ty, TypeVisitableExt};
 use rustc_session::lint::builtin::NON_EXHAUSTIVE_OMITTED_PATTERNS;
@@ -684,7 +683,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             {
                 // `mut x` resets the binding mode in edition <= 2021.
                 self.tcx.emit_node_span_lint(
-                    lint::builtin::DEREFERENCING_MUT_BINDING,
+                    rustc_lint::builtin::DEREFERENCING_MUT_BINDING,
                     pat.hir_id,
                     pat.span,
                     errors::DereferencingMutBinding { span: pat.span },
diff --git a/compiler/rustc_incremental/src/persist/fs.rs b/compiler/rustc_incremental/src/persist/fs.rs
index 3d7c0cfc30a..193042b8cdf 100644
--- a/compiler/rustc_incremental/src/persist/fs.rs
+++ b/compiler/rustc_incremental/src/persist/fs.rs
@@ -104,10 +104,14 @@
 //! implemented.
 
 use crate::errors;
+use rustc_data_structures::base_n;
+use rustc_data_structures::base_n::BaseNString;
+use rustc_data_structures::base_n::ToBaseN;
+use rustc_data_structures::base_n::CASE_INSENSITIVE;
+use rustc_data_structures::flock;
 use rustc_data_structures::fx::{FxHashSet, FxIndexSet};
 use rustc_data_structures::svh::Svh;
 use rustc_data_structures::unord::{UnordMap, UnordSet};
-use rustc_data_structures::{base_n, flock};
 use rustc_errors::ErrorGuaranteed;
 use rustc_fs_util::{link_or_copy, try_canonicalize, LinkOrCopy};
 use rustc_middle::bug;
@@ -333,31 +337,24 @@ pub fn finalize_session_directory(sess: &Session, svh: Option<Svh>) {
 
     debug!("finalize_session_directory() - session directory: {}", incr_comp_session_dir.display());
 
-    let old_sub_dir_name = incr_comp_session_dir
+    let mut sub_dir_name = incr_comp_session_dir
         .file_name()
         .unwrap()
         .to_str()
-        .expect("malformed session dir name: contains non-Unicode characters");
+        .expect("malformed session dir name: contains non-Unicode characters")
+        .to_string();
 
-    // Keep the 's-{timestamp}-{random-number}' prefix, but replace the
-    // '-working' part with the SVH of the crate
-    let dash_indices: Vec<_> = old_sub_dir_name.match_indices('-').map(|(idx, _)| idx).collect();
-    if dash_indices.len() != 3 {
-        bug!(
-            "Encountered incremental compilation session directory with \
-              malformed name: {}",
-            incr_comp_session_dir.display()
-        )
-    }
-
-    // State: "s-{timestamp}-{random-number}-"
-    let mut new_sub_dir_name = String::from(&old_sub_dir_name[..=dash_indices[2]]);
+    // Keep the 's-{timestamp}-{random-number}' prefix, but replace "working" with the SVH of the crate
+    sub_dir_name.truncate(sub_dir_name.len() - "working".len());
+    // Double-check that we kept this: "s-{timestamp}-{random-number}-"
+    assert!(sub_dir_name.ends_with('-'), "{:?}", sub_dir_name);
+    assert!(sub_dir_name.as_bytes().iter().filter(|b| **b == b'-').count() == 3);
 
-    // Append the svh
-    base_n::push_str(svh.as_u128(), INT_ENCODE_BASE, &mut new_sub_dir_name);
+    // Append the SVH
+    sub_dir_name.push_str(&svh.as_u128().to_base_fixed_len(CASE_INSENSITIVE));
 
     // Create the full path
-    let new_path = incr_comp_session_dir.parent().unwrap().join(new_sub_dir_name);
+    let new_path = incr_comp_session_dir.parent().unwrap().join(&*sub_dir_name);
     debug!("finalize_session_directory() - new path: {}", new_path.display());
 
     match rename_path_with_retry(&*incr_comp_session_dir, &new_path, 3) {
@@ -453,11 +450,11 @@ fn generate_session_dir_path(crate_dir: &Path) -> PathBuf {
     let random_number = thread_rng().next_u32();
     debug!("generate_session_dir_path: random_number = {}", random_number);
 
-    let directory_name = format!(
-        "s-{}-{}-working",
-        timestamp,
-        base_n::encode(random_number as u128, INT_ENCODE_BASE)
-    );
+    // Chop the first 3 characters off the timestamp. Those 3 bytes will be zero for a while.
+    let (zeroes, timestamp) = timestamp.split_at(3);
+    assert_eq!(zeroes, "000");
+    let directory_name =
+        format!("s-{}-{}-working", timestamp, random_number.to_base_fixed_len(CASE_INSENSITIVE));
     debug!("generate_session_dir_path: directory_name = {}", directory_name);
     let directory_path = crate_dir.join(directory_name);
     debug!("generate_session_dir_path: directory_path = {}", directory_path.display());
@@ -588,10 +585,10 @@ fn extract_timestamp_from_session_dir(directory_name: &str) -> Result<SystemTime
     string_to_timestamp(&directory_name[dash_indices[0] + 1..dash_indices[1]])
 }
 
-fn timestamp_to_string(timestamp: SystemTime) -> String {
+fn timestamp_to_string(timestamp: SystemTime) -> BaseNString {
     let duration = timestamp.duration_since(UNIX_EPOCH).unwrap();
     let micros = duration.as_secs() * 1_000_000 + (duration.subsec_nanos() as u64) / 1000;
-    base_n::encode(micros as u128, INT_ENCODE_BASE)
+    micros.to_base_fixed_len(CASE_INSENSITIVE)
 }
 
 fn string_to_timestamp(s: &str) -> Result<SystemTime, &'static str> {
@@ -622,9 +619,8 @@ fn crate_path(sess: &Session) -> PathBuf {
         sess.cfg_version,
     );
 
-    let stable_crate_id = base_n::encode(stable_crate_id.as_u64() as u128, INT_ENCODE_BASE);
-
-    let crate_name = format!("{crate_name}-{stable_crate_id}");
+    let crate_name =
+        format!("{crate_name}-{}", stable_crate_id.as_u64().to_base_fixed_len(CASE_INSENSITIVE));
     incr_dir.join(crate_name)
 }
 
diff --git a/compiler/rustc_interface/src/passes.rs b/compiler/rustc_interface/src/passes.rs
index b593c41a8ea..76d5d7a3ac2 100644
--- a/compiler/rustc_interface/src/passes.rs
+++ b/compiler/rustc_interface/src/passes.rs
@@ -4,7 +4,6 @@ use crate::proc_macro_decls;
 use crate::util;
 
 use rustc_ast::{self as ast, visit};
-use rustc_borrowck as mir_borrowck;
 use rustc_codegen_ssa::traits::CodegenBackend;
 use rustc_data_structures::parallel;
 use rustc_data_structures::steal::Steal;
@@ -20,7 +19,6 @@ use rustc_middle::arena::Arena;
 use rustc_middle::dep_graph::DepGraph;
 use rustc_middle::ty::{self, GlobalCtxt, RegisteredTools, TyCtxt};
 use rustc_middle::util::Providers;
-use rustc_mir_build as mir_build;
 use rustc_parse::{parse_crate_from_file, parse_crate_from_source_str, validate_attr};
 use rustc_passes::{abi_test, hir_stats, layout_test};
 use rustc_resolve::Resolver;
@@ -616,8 +614,8 @@ pub static DEFAULT_QUERY_PROVIDERS: LazyLock<Providers> = LazyLock::new(|| {
     proc_macro_decls::provide(providers);
     rustc_const_eval::provide(providers);
     rustc_middle::hir::provide(providers);
-    mir_borrowck::provide(providers);
-    mir_build::provide(providers);
+    rustc_borrowck::provide(providers);
+    rustc_mir_build::provide(providers);
     rustc_mir_transform::provide(providers);
     rustc_monomorphize::provide(providers);
     rustc_privacy::provide(providers);
diff --git a/compiler/rustc_interface/src/tests.rs b/compiler/rustc_interface/src/tests.rs
index db150cc1f87..36f9dda7250 100644
--- a/compiler/rustc_interface/src/tests.rs
+++ b/compiler/rustc_interface/src/tests.rs
@@ -20,7 +20,7 @@ use rustc_span::source_map::{RealFileLoader, SourceMapInputs};
 use rustc_span::symbol::sym;
 use rustc_span::{FileName, SourceFileHashAlgorithm};
 use rustc_target::spec::{
-    CodeModel, LinkerFlavorCli, MergeFunctions, PanicStrategy, RelocModel, WasmCAbi,
+    CodeModel, LinkerFlavorCli, MergeFunctions, OnBrokenPipe, PanicStrategy, RelocModel, WasmCAbi,
 };
 use rustc_target::spec::{RelroLevel, SanitizerSet, SplitDebuginfo, StackProtector, TlsModel};
 use std::collections::{BTreeMap, BTreeSet};
@@ -809,6 +809,7 @@ fn test_unstable_options_tracking_hash() {
     tracked!(no_profiler_runtime, true);
     tracked!(no_trait_vptr, true);
     tracked!(no_unique_section_names, true);
+    tracked!(on_broken_pipe, OnBrokenPipe::Kill);
     tracked!(oom, OomStrategy::Panic);
     tracked!(osx_rpath_install_name, true);
     tracked!(packed_bundled_libs, true);
diff --git a/compiler/rustc_interface/src/util.rs b/compiler/rustc_interface/src/util.rs
index 02dcfe9c8df..ce4d3825015 100644
--- a/compiler/rustc_interface/src/util.rs
+++ b/compiler/rustc_interface/src/util.rs
@@ -7,18 +7,16 @@ use rustc_data_structures::sync;
 use rustc_metadata::{load_symbol_from_dylib, DylibError};
 use rustc_middle::ty::CurrentGcx;
 use rustc_parse::validate_attr;
-use rustc_session as session;
-use rustc_session::config::{Cfg, OutFileName, OutputFilenames, OutputTypes};
+use rustc_session::config::{host_triple, Cfg, OutFileName, OutputFilenames, OutputTypes};
 use rustc_session::filesearch::sysroot_candidates;
 use rustc_session::lint::{self, BuiltinLintDiag, LintBuffer};
-use rustc_session::{filesearch, Session};
+use rustc_session::output::{categorize_crate_type, CRATE_TYPES};
+use rustc_session::{filesearch, EarlyDiagCtxt, Session};
 use rustc_span::edit_distance::find_best_match_for_name;
 use rustc_span::edition::Edition;
 use rustc_span::source_map::SourceMapInputs;
 use rustc_span::symbol::sym;
 use rustc_target::spec::Target;
-use session::output::{categorize_crate_type, CRATE_TYPES};
-use session::EarlyDiagCtxt;
 use std::env::consts::{DLL_PREFIX, DLL_SUFFIX};
 use std::path::{Path, PathBuf};
 use std::sync::atomic::{AtomicBool, Ordering};
@@ -286,7 +284,7 @@ fn get_codegen_sysroot(
         "cannot load the default codegen backend twice"
     );
 
-    let target = session::config::host_triple();
+    let target = host_triple();
     let sysroot_candidates = sysroot_candidates();
 
     let sysroot = iter::once(sysroot)
diff --git a/compiler/rustc_lint/src/pass_by_value.rs b/compiler/rustc_lint/src/pass_by_value.rs
index 1f0a8a0b567..c1f5cd45dc8 100644
--- a/compiler/rustc_lint/src/pass_by_value.rs
+++ b/compiler/rustc_lint/src/pass_by_value.rs
@@ -74,9 +74,12 @@ fn gen_args(cx: &LateContext<'_>, segment: &PathSegment<'_>) -> String {
                 GenericArg::Type(ty) => {
                     cx.tcx.sess.source_map().span_to_snippet(ty.span).unwrap_or_else(|_| "_".into())
                 }
-                GenericArg::Const(c) => {
-                    cx.tcx.sess.source_map().span_to_snippet(c.span).unwrap_or_else(|_| "_".into())
-                }
+                GenericArg::Const(c) => cx
+                    .tcx
+                    .sess
+                    .source_map()
+                    .span_to_snippet(c.value.span)
+                    .unwrap_or_else(|_| "_".into()),
                 GenericArg::Infer(_) => String::from("_"),
             })
             .collect::<Vec<_>>();
diff --git a/compiler/rustc_middle/src/hir/map/mod.rs b/compiler/rustc_middle/src/hir/map/mod.rs
index c0c773c6285..c7aea137b68 100644
--- a/compiler/rustc_middle/src/hir/map/mod.rs
+++ b/compiler/rustc_middle/src/hir/map/mod.rs
@@ -885,7 +885,7 @@ impl<'hir> Map<'hir> {
             Node::ImplItem(impl_item) => impl_item.span,
             Node::Variant(variant) => variant.span,
             Node::Field(field) => field.span,
-            Node::AnonConst(constant) => self.body(constant.body).value.span,
+            Node::AnonConst(constant) => constant.span,
             Node::ConstBlock(constant) => self.body(constant.body).value.span,
             Node::Expr(expr) => expr.span,
             Node::ExprField(field) => field.span,
@@ -912,7 +912,7 @@ impl<'hir> Map<'hir> {
             Node::ArrayLenInfer(inf) => inf.span,
             Node::PreciseCapturingNonLifetimeArg(param) => param.ident.span,
             Node::Synthetic => unreachable!(),
-            Node::Err(span) => *span,
+            Node::Err(span) => span,
         }
     }
 
diff --git a/compiler/rustc_middle/src/mir/coverage.rs b/compiler/rustc_middle/src/mir/coverage.rs
index 9d9ca22247a..477303e2434 100644
--- a/compiler/rustc_middle/src/mir/coverage.rs
+++ b/compiler/rustc_middle/src/mir/coverage.rs
@@ -277,7 +277,7 @@ pub struct FunctionCoverageInfo {
     pub mappings: Vec<Mapping>,
     /// The depth of the deepest decision is used to know how many
     /// temp condbitmaps should be allocated for the function.
-    pub mcdc_max_decision_depth: u16,
+    pub mcdc_num_condition_bitmaps: usize,
 }
 
 /// Branch information recorded during THIR-to-MIR lowering, and stored in MIR.
diff --git a/compiler/rustc_middle/src/mir/mono.rs b/compiler/rustc_middle/src/mir/mono.rs
index 6d4585cfbc6..daab6c85581 100644
--- a/compiler/rustc_middle/src/mir/mono.rs
+++ b/compiler/rustc_middle/src/mir/mono.rs
@@ -1,7 +1,9 @@
 use crate::dep_graph::{DepNode, WorkProduct, WorkProductId};
 use crate::ty::{GenericArgs, Instance, InstanceDef, SymbolName, TyCtxt};
 use rustc_attr::InlineAttr;
-use rustc_data_structures::base_n;
+use rustc_data_structures::base_n::BaseNString;
+use rustc_data_structures::base_n::ToBaseN;
+use rustc_data_structures::base_n::CASE_INSENSITIVE;
 use rustc_data_structures::fingerprint::Fingerprint;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::fx::FxIndexMap;
@@ -337,14 +339,11 @@ impl<'tcx> CodegenUnit<'tcx> {
         self.is_code_coverage_dead_code_cgu = true;
     }
 
-    pub fn mangle_name(human_readable_name: &str) -> String {
-        // We generate a 80 bit hash from the name. This should be enough to
-        // avoid collisions and is still reasonably short for filenames.
+    pub fn mangle_name(human_readable_name: &str) -> BaseNString {
         let mut hasher = StableHasher::new();
         human_readable_name.hash(&mut hasher);
         let hash: Hash128 = hasher.finish();
-        let hash = hash.as_u128() & ((1u128 << 80) - 1);
-        base_n::encode(hash, base_n::CASE_INSENSITIVE)
+        hash.as_u128().to_base_fixed_len(CASE_INSENSITIVE)
     }
 
     pub fn compute_size_estimate(&mut self) {
diff --git a/compiler/rustc_middle/src/traits/solve.rs b/compiler/rustc_middle/src/traits/solve.rs
index 4c34bf88c7f..3ad6b68d129 100644
--- a/compiler/rustc_middle/src/traits/solve.rs
+++ b/compiler/rustc_middle/src/traits/solve.rs
@@ -273,6 +273,8 @@ pub enum GoalSource {
     /// they are from an impl where-clause. This is necessary due to
     /// backwards compatability, cc trait-system-refactor-initiatitive#70.
     ImplWhereBound,
+    /// Instantiating a higher-ranked goal and re-proving it.
+    InstantiateHigherRanked,
 }
 
 /// Possible ways the given goal can be proven.
diff --git a/compiler/rustc_middle/src/traits/solve/inspect/format.rs b/compiler/rustc_middle/src/traits/solve/inspect/format.rs
index 2d73be387fd..11aa0e10931 100644
--- a/compiler/rustc_middle/src/traits/solve/inspect/format.rs
+++ b/compiler/rustc_middle/src/traits/solve/inspect/format.rs
@@ -127,6 +127,7 @@ impl<'a, 'b> ProofTreeFormatter<'a, 'b> {
                         let source = match source {
                             GoalSource::Misc => "misc",
                             GoalSource::ImplWhereBound => "impl where-bound",
+                            GoalSource::InstantiateHigherRanked => "higher-ranked goal",
                         };
                         writeln!(this.f, "ADDED GOAL ({source}): {goal:?}")?
                     }
diff --git a/compiler/rustc_mir_build/src/build/mod.rs b/compiler/rustc_mir_build/src/build/mod.rs
index d6e6cc957b4..794e7ebb7b4 100644
--- a/compiler/rustc_mir_build/src/build/mod.rs
+++ b/compiler/rustc_mir_build/src/build/mod.rs
@@ -566,7 +566,8 @@ fn construct_const<'a, 'tcx>(
             span,
             ..
         }) => (*span, ty.span),
-        Node::AnonConst(_) | Node::ConstBlock(_) => {
+        Node::AnonConst(ct) => (ct.span, ct.span),
+        Node::ConstBlock(_) => {
             let span = tcx.def_span(def);
             (span, span)
         }
diff --git a/compiler/rustc_mir_transform/src/copy_prop.rs b/compiler/rustc_mir_transform/src/copy_prop.rs
index 0119b95cced..c1f9313a377 100644
--- a/compiler/rustc_mir_transform/src/copy_prop.rs
+++ b/compiler/rustc_mir_transform/src/copy_prop.rs
@@ -3,7 +3,6 @@ use rustc_index::IndexSlice;
 use rustc_middle::mir::visit::*;
 use rustc_middle::mir::*;
 use rustc_middle::ty::TyCtxt;
-use rustc_mir_dataflow::impls::borrowed_locals;
 
 use crate::ssa::SsaLocals;
 
@@ -32,8 +31,8 @@ impl<'tcx> MirPass<'tcx> for CopyProp {
 }
 
 fn propagate_ssa<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
-    let borrowed_locals = borrowed_locals(body);
-    let ssa = SsaLocals::new(body);
+    let param_env = tcx.param_env_reveal_all_normalized(body.source.def_id());
+    let ssa = SsaLocals::new(tcx, body, param_env);
 
     let fully_moved = fully_moved_locals(&ssa, body);
     debug!(?fully_moved);
@@ -51,7 +50,7 @@ fn propagate_ssa<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
         tcx,
         copy_classes: ssa.copy_classes(),
         fully_moved,
-        borrowed_locals,
+        borrowed_locals: ssa.borrowed_locals(),
         storage_to_remove,
     }
     .visit_body_preserves_cfg(body);
@@ -101,7 +100,7 @@ struct Replacer<'a, 'tcx> {
     tcx: TyCtxt<'tcx>,
     fully_moved: BitSet<Local>,
     storage_to_remove: BitSet<Local>,
-    borrowed_locals: BitSet<Local>,
+    borrowed_locals: &'a BitSet<Local>,
     copy_classes: &'a IndexSlice<Local, Local>,
 }
 
@@ -112,6 +111,12 @@ impl<'tcx> MutVisitor<'tcx> for Replacer<'_, 'tcx> {
 
     fn visit_local(&mut self, local: &mut Local, ctxt: PlaceContext, _: Location) {
         let new_local = self.copy_classes[*local];
+        // We must not unify two locals that are borrowed. But this is fine if one is borrowed and
+        // the other is not. We chose to check the original local, and not the target. That way, if
+        // the original local is borrowed and the target is not, we do not pessimize the whole class.
+        if self.borrowed_locals.contains(*local) {
+            return;
+        }
         match ctxt {
             // Do not modify the local in storage statements.
             PlaceContext::NonUse(NonUseContext::StorageLive | NonUseContext::StorageDead) => {}
@@ -122,32 +127,14 @@ impl<'tcx> MutVisitor<'tcx> for Replacer<'_, 'tcx> {
         }
     }
 
-    fn visit_place(&mut self, place: &mut Place<'tcx>, ctxt: PlaceContext, loc: Location) {
+    fn visit_place(&mut self, place: &mut Place<'tcx>, _: PlaceContext, loc: Location) {
         if let Some(new_projection) = self.process_projection(place.projection, loc) {
             place.projection = self.tcx().mk_place_elems(&new_projection);
         }
 
-        let observes_address = match ctxt {
-            PlaceContext::NonMutatingUse(
-                NonMutatingUseContext::SharedBorrow
-                | NonMutatingUseContext::FakeBorrow
-                | NonMutatingUseContext::AddressOf,
-            ) => true,
-            // For debuginfo, merging locals is ok.
-            PlaceContext::NonUse(NonUseContext::VarDebugInfo) => {
-                self.borrowed_locals.contains(place.local)
-            }
-            _ => false,
-        };
-        if observes_address && !place.is_indirect() {
-            // We observe the address of `place.local`. Do not replace it.
-        } else {
-            self.visit_local(
-                &mut place.local,
-                PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy),
-                loc,
-            )
-        }
+        // Any non-mutating use context is ok.
+        let ctxt = PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy);
+        self.visit_local(&mut place.local, ctxt, loc)
     }
 
     fn visit_operand(&mut self, operand: &mut Operand<'tcx>, loc: Location) {
diff --git a/compiler/rustc_mir_transform/src/coverage/mod.rs b/compiler/rustc_mir_transform/src/coverage/mod.rs
index 9edde666246..ffe61e761c5 100644
--- a/compiler/rustc_mir_transform/src/coverage/mod.rs
+++ b/compiler/rustc_mir_transform/src/coverage/mod.rs
@@ -102,7 +102,7 @@ fn instrument_function_for_coverage<'tcx>(tcx: TyCtxt<'tcx>, mir_body: &mut mir:
 
     inject_mcdc_statements(mir_body, &basic_coverage_blocks, &coverage_spans);
 
-    let mcdc_max_decision_depth = coverage_spans
+    let mcdc_num_condition_bitmaps = coverage_spans
         .mappings
         .iter()
         .filter_map(|bcb_mapping| match bcb_mapping.kind {
@@ -110,7 +110,7 @@ fn instrument_function_for_coverage<'tcx>(tcx: TyCtxt<'tcx>, mir_body: &mut mir:
             _ => None,
         })
         .max()
-        .unwrap_or(0);
+        .map_or(0, |max| usize::from(max) + 1);
 
     mir_body.function_coverage_info = Some(Box::new(FunctionCoverageInfo {
         function_source_hash: hir_info.function_source_hash,
@@ -118,7 +118,7 @@ fn instrument_function_for_coverage<'tcx>(tcx: TyCtxt<'tcx>, mir_body: &mut mir:
         mcdc_bitmap_bytes: coverage_spans.test_vector_bitmap_bytes(),
         expressions: coverage_counters.into_expressions(),
         mappings,
-        mcdc_max_decision_depth,
+        mcdc_num_condition_bitmaps,
     }));
 }
 
diff --git a/compiler/rustc_mir_transform/src/gvn.rs b/compiler/rustc_mir_transform/src/gvn.rs
index 24832086b16..4dd595ce1e1 100644
--- a/compiler/rustc_mir_transform/src/gvn.rs
+++ b/compiler/rustc_mir_transform/src/gvn.rs
@@ -121,7 +121,7 @@ impl<'tcx> MirPass<'tcx> for GVN {
 
 fn propagate_ssa<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
     let param_env = tcx.param_env_reveal_all_normalized(body.source.def_id());
-    let ssa = SsaLocals::new(body);
+    let ssa = SsaLocals::new(tcx, body, param_env);
     // Clone dominators as we need them while mutating the body.
     let dominators = body.basic_blocks.dominators().clone();
 
@@ -724,6 +724,14 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
         // Invariant: `value` holds the value up-to the `index`th projection excluded.
         let mut value = self.locals[place.local]?;
         for (index, proj) in place.projection.iter().enumerate() {
+            if let Value::Projection(pointer, ProjectionElem::Deref) = *self.get(value)
+                && let Value::Address { place: mut pointee, kind, .. } = *self.get(pointer)
+                && let AddressKind::Ref(BorrowKind::Shared) = kind
+                && let Some(v) = self.simplify_place_value(&mut pointee, location)
+            {
+                value = v;
+                place_ref = pointee.project_deeper(&place.projection[index..], self.tcx).as_ref();
+            }
             if let Some(local) = self.try_as_local(value, location) {
                 // Both `local` and `Place { local: place.local, projection: projection[..index] }`
                 // hold the same value. Therefore, following place holds the value in the original
@@ -735,6 +743,14 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
             value = self.project(base, value, proj)?;
         }
 
+        if let Value::Projection(pointer, ProjectionElem::Deref) = *self.get(value)
+            && let Value::Address { place: mut pointee, kind, .. } = *self.get(pointer)
+            && let AddressKind::Ref(BorrowKind::Shared) = kind
+            && let Some(v) = self.simplify_place_value(&mut pointee, location)
+        {
+            value = v;
+            place_ref = pointee.project_deeper(&[], self.tcx).as_ref();
+        }
         if let Some(new_local) = self.try_as_local(value, location) {
             place_ref = PlaceRef { local: new_local, projection: &[] };
         }
diff --git a/compiler/rustc_mir_transform/src/normalize_array_len.rs b/compiler/rustc_mir_transform/src/normalize_array_len.rs
index 128634bd7f2..c26a5461633 100644
--- a/compiler/rustc_mir_transform/src/normalize_array_len.rs
+++ b/compiler/rustc_mir_transform/src/normalize_array_len.rs
@@ -22,7 +22,8 @@ impl<'tcx> MirPass<'tcx> for NormalizeArrayLen {
 }
 
 fn normalize_array_len_calls<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
-    let ssa = SsaLocals::new(body);
+    let param_env = tcx.param_env_reveal_all_normalized(body.source.def_id());
+    let ssa = SsaLocals::new(tcx, body, param_env);
 
     let slice_lengths = compute_slice_length(tcx, &ssa, body);
     debug!(?slice_lengths);
diff --git a/compiler/rustc_mir_transform/src/ref_prop.rs b/compiler/rustc_mir_transform/src/ref_prop.rs
index d5642be5513..044ae32c1d4 100644
--- a/compiler/rustc_mir_transform/src/ref_prop.rs
+++ b/compiler/rustc_mir_transform/src/ref_prop.rs
@@ -82,7 +82,8 @@ impl<'tcx> MirPass<'tcx> for ReferencePropagation {
 }
 
 fn propagate_ssa<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) -> bool {
-    let ssa = SsaLocals::new(body);
+    let param_env = tcx.param_env_reveal_all_normalized(body.source.def_id());
+    let ssa = SsaLocals::new(tcx, body, param_env);
 
     let mut replacer = compute_replacement(tcx, body, &ssa);
     debug!(?replacer.targets);
diff --git a/compiler/rustc_mir_transform/src/ssa.rs b/compiler/rustc_mir_transform/src/ssa.rs
index fddc62e6652..55fed7d9da2 100644
--- a/compiler/rustc_mir_transform/src/ssa.rs
+++ b/compiler/rustc_mir_transform/src/ssa.rs
@@ -2,8 +2,9 @@
 //! 1/ They are only assigned-to once, either as a function parameter, or in an assign statement;
 //! 2/ This single assignment dominates all uses;
 //!
-//! As a consequence of rule 2, we consider that borrowed locals are not SSA, even if they are
-//! `Freeze`, as we do not track that the assignment dominates all uses of the borrow.
+//! As we do not track indirect assignments, a local that has its address taken (either by
+//! AddressOf or by borrowing) is considered non-SSA. However, it is UB to modify through an
+//! immutable borrow of a `Freeze` local. Those can still be considered to be SSA.
 
 use rustc_data_structures::graph::dominators::Dominators;
 use rustc_index::bit_set::BitSet;
@@ -11,6 +12,7 @@ use rustc_index::{IndexSlice, IndexVec};
 use rustc_middle::middle::resolve_bound_vars::Set1;
 use rustc_middle::mir::visit::*;
 use rustc_middle::mir::*;
+use rustc_middle::ty::{ParamEnv, TyCtxt};
 
 pub struct SsaLocals {
     /// Assignments to each local. This defines whether the local is SSA.
@@ -24,6 +26,8 @@ pub struct SsaLocals {
     /// Number of "direct" uses of each local, ie. uses that are not dereferences.
     /// We ignore non-uses (Storage statements, debuginfo).
     direct_uses: IndexVec<Local, u32>,
+    /// Set of SSA locals that are immutably borrowed.
+    borrowed_locals: BitSet<Local>,
 }
 
 pub enum AssignedValue<'a, 'tcx> {
@@ -33,15 +37,22 @@ pub enum AssignedValue<'a, 'tcx> {
 }
 
 impl SsaLocals {
-    pub fn new<'tcx>(body: &Body<'tcx>) -> SsaLocals {
+    pub fn new<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>, param_env: ParamEnv<'tcx>) -> SsaLocals {
         let assignment_order = Vec::with_capacity(body.local_decls.len());
 
         let assignments = IndexVec::from_elem(Set1::Empty, &body.local_decls);
         let dominators = body.basic_blocks.dominators();
 
         let direct_uses = IndexVec::from_elem(0, &body.local_decls);
-        let mut visitor =
-            SsaVisitor { body, assignments, assignment_order, dominators, direct_uses };
+        let borrowed_locals = BitSet::new_empty(body.local_decls.len());
+        let mut visitor = SsaVisitor {
+            body,
+            assignments,
+            assignment_order,
+            dominators,
+            direct_uses,
+            borrowed_locals,
+        };
 
         for local in body.args_iter() {
             visitor.assignments[local] = Set1::One(DefLocation::Argument);
@@ -58,6 +69,16 @@ impl SsaLocals {
             visitor.visit_var_debug_info(var_debug_info);
         }
 
+        // The immutability of shared borrows only works on `Freeze` locals. If the visitor found
+        // borrows, we need to check the types. For raw pointers and mutable borrows, the locals
+        // have already been marked as non-SSA.
+        debug!(?visitor.borrowed_locals);
+        for local in visitor.borrowed_locals.iter() {
+            if !body.local_decls[local].ty.is_freeze(tcx, param_env) {
+                visitor.assignments[local] = Set1::Many;
+            }
+        }
+
         debug!(?visitor.assignments);
         debug!(?visitor.direct_uses);
 
@@ -70,6 +91,7 @@ impl SsaLocals {
             assignments: visitor.assignments,
             assignment_order: visitor.assignment_order,
             direct_uses: visitor.direct_uses,
+            borrowed_locals: visitor.borrowed_locals,
             // This is filled by `compute_copy_classes`.
             copy_classes: IndexVec::default(),
         };
@@ -174,6 +196,11 @@ impl SsaLocals {
         &self.copy_classes
     }
 
+    /// Set of SSA locals that are immutably borrowed.
+    pub fn borrowed_locals(&self) -> &BitSet<Local> {
+        &self.borrowed_locals
+    }
+
     /// Make a property uniform on a copy equivalence class by removing elements.
     pub fn meet_copy_equivalence(&self, property: &mut BitSet<Local>) {
         // Consolidate to have a local iff all its copies are.
@@ -208,6 +235,8 @@ struct SsaVisitor<'tcx, 'a> {
     assignments: IndexVec<Local, Set1<DefLocation>>,
     assignment_order: Vec<Local>,
     direct_uses: IndexVec<Local, u32>,
+    // Track locals that are immutably borrowed, so we can check their type is `Freeze` later.
+    borrowed_locals: BitSet<Local>,
 }
 
 impl SsaVisitor<'_, '_> {
@@ -232,16 +261,18 @@ impl<'tcx> Visitor<'tcx> for SsaVisitor<'tcx, '_> {
             PlaceContext::MutatingUse(MutatingUseContext::Projection)
             | PlaceContext::NonMutatingUse(NonMutatingUseContext::Projection) => bug!(),
             // Anything can happen with raw pointers, so remove them.
-            // We do not verify that all uses of the borrow dominate the assignment to `local`,
-            // so we have to remove them too.
-            PlaceContext::NonMutatingUse(
-                NonMutatingUseContext::SharedBorrow
-                | NonMutatingUseContext::FakeBorrow
-                | NonMutatingUseContext::AddressOf,
-            )
+            PlaceContext::NonMutatingUse(NonMutatingUseContext::AddressOf)
             | PlaceContext::MutatingUse(_) => {
                 self.assignments[local] = Set1::Many;
             }
+            // Immutable borrows are ok, but we need to delay a check that the type is `Freeze`.
+            PlaceContext::NonMutatingUse(
+                NonMutatingUseContext::SharedBorrow | NonMutatingUseContext::FakeBorrow,
+            ) => {
+                self.borrowed_locals.insert(local);
+                self.check_dominates(local, loc);
+                self.direct_uses[local] += 1;
+            }
             PlaceContext::NonMutatingUse(_) => {
                 self.check_dominates(local, loc);
                 self.direct_uses[local] += 1;
diff --git a/compiler/rustc_passes/messages.ftl b/compiler/rustc_passes/messages.ftl
index a545c170297..8878310d6e9 100644
--- a/compiler/rustc_passes/messages.ftl
+++ b/compiler/rustc_passes/messages.ftl
@@ -695,9 +695,6 @@ passes_transparent_incompatible =
 passes_undefined_naked_function_abi =
     Rust ABI is unsupported in naked functions
 
-passes_unix_sigpipe_values =
-    valid values for `#[unix_sigpipe = "..."]` are `inherit`, `sig_ign`, or `sig_dfl`
-
 passes_unknown_external_lang_item =
     unknown external lang item: `{$lang_item}`
 
diff --git a/compiler/rustc_passes/src/check_attr.rs b/compiler/rustc_passes/src/check_attr.rs
index c403e9196fa..e60aa27dba2 100644
--- a/compiler/rustc_passes/src/check_attr.rs
+++ b/compiler/rustc_passes/src/check_attr.rs
@@ -2523,7 +2523,6 @@ fn check_invalid_crate_level_attr(tcx: TyCtxt<'_>, attrs: &[Attribute]) {
         sym::automatically_derived,
         sym::start,
         sym::rustc_main,
-        sym::unix_sigpipe,
         sym::derive,
         sym::test,
         sym::test_case,
diff --git a/compiler/rustc_passes/src/entry.rs b/compiler/rustc_passes/src/entry.rs
index 438c583db49..d52092f2aa9 100644
--- a/compiler/rustc_passes/src/entry.rs
+++ b/compiler/rustc_passes/src/entry.rs
@@ -12,8 +12,7 @@ use rustc_span::symbol::sym;
 use rustc_span::{Span, Symbol};
 
 use crate::errors::{
-    AttrOnlyInFunctions, AttrOnlyOnMain, AttrOnlyOnRootMain, ExternMain, MultipleRustcMain,
-    MultipleStartFunctions, NoMainErr, UnixSigpipeValues,
+    AttrOnlyInFunctions, ExternMain, MultipleRustcMain, MultipleStartFunctions, NoMainErr,
 };
 
 struct EntryContext<'tcx> {
@@ -67,11 +66,7 @@ fn find_item(id: ItemId, ctxt: &mut EntryContext<'_>) {
         ctxt.tcx.opt_item_name(id.owner_id.to_def_id()),
     );
     match entry_point_type {
-        EntryPointType::None => {
-            if let Some(span) = attr_span_by_symbol(ctxt, id, sym::unix_sigpipe) {
-                ctxt.tcx.dcx().emit_err(AttrOnlyOnMain { span, attr: sym::unix_sigpipe });
-            }
-        }
+        EntryPointType::None => (),
         _ if !matches!(ctxt.tcx.def_kind(id.owner_id), DefKind::Fn) => {
             for attr in [sym::start, sym::rustc_main] {
                 if let Some(span) = attr_span_by_symbol(ctxt, id, attr) {
@@ -81,9 +76,6 @@ fn find_item(id: ItemId, ctxt: &mut EntryContext<'_>) {
         }
         EntryPointType::MainNamed => (),
         EntryPointType::OtherMain => {
-            if let Some(span) = attr_span_by_symbol(ctxt, id, sym::unix_sigpipe) {
-                ctxt.tcx.dcx().emit_err(AttrOnlyOnRootMain { span, attr: sym::unix_sigpipe });
-            }
             ctxt.non_main_fns.push(ctxt.tcx.def_span(id.owner_id));
         }
         EntryPointType::RustcMainAttr => {
@@ -98,9 +90,6 @@ fn find_item(id: ItemId, ctxt: &mut EntryContext<'_>) {
             }
         }
         EntryPointType::Start => {
-            if let Some(span) = attr_span_by_symbol(ctxt, id, sym::unix_sigpipe) {
-                ctxt.tcx.dcx().emit_err(AttrOnlyOnMain { span, attr: sym::unix_sigpipe });
-            }
             if ctxt.start_fn.is_none() {
                 ctxt.start_fn = Some((id.owner_id.def_id, ctxt.tcx.def_span(id.owner_id)));
             } else {
@@ -120,7 +109,7 @@ fn configure_main(tcx: TyCtxt<'_>, visitor: &EntryContext<'_>) -> Option<(DefId,
         Some((def_id.to_def_id(), EntryFnType::Start))
     } else if let Some((local_def_id, _)) = visitor.attr_main_fn {
         let def_id = local_def_id.to_def_id();
-        Some((def_id, EntryFnType::Main { sigpipe: sigpipe(tcx, def_id) }))
+        Some((def_id, EntryFnType::Main { sigpipe: sigpipe(tcx) }))
     } else {
         if let Some(main_def) = tcx.resolutions(()).main_def
             && let Some(def_id) = main_def.opt_fn_def_id()
@@ -133,31 +122,19 @@ fn configure_main(tcx: TyCtxt<'_>, visitor: &EntryContext<'_>) -> Option<(DefId,
                 return None;
             }
 
-            return Some((def_id, EntryFnType::Main { sigpipe: sigpipe(tcx, def_id) }));
+            return Some((def_id, EntryFnType::Main { sigpipe: sigpipe(tcx) }));
         }
         no_main_err(tcx, visitor);
         None
     }
 }
 
-fn sigpipe(tcx: TyCtxt<'_>, def_id: DefId) -> u8 {
-    if let Some(attr) = tcx.get_attr(def_id, sym::unix_sigpipe) {
-        match (attr.value_str(), attr.meta_item_list()) {
-            (Some(sym::inherit), None) => sigpipe::INHERIT,
-            (Some(sym::sig_ign), None) => sigpipe::SIG_IGN,
-            (Some(sym::sig_dfl), None) => sigpipe::SIG_DFL,
-            (Some(_), None) => {
-                tcx.dcx().emit_err(UnixSigpipeValues { span: attr.span });
-                sigpipe::DEFAULT
-            }
-            _ => {
-                // Keep going so that `fn emit_malformed_attribute()` can print
-                // an excellent error message
-                sigpipe::DEFAULT
-            }
-        }
-    } else {
-        sigpipe::DEFAULT
+fn sigpipe(tcx: TyCtxt<'_>) -> u8 {
+    match tcx.sess.opts.unstable_opts.on_broken_pipe {
+        rustc_target::spec::OnBrokenPipe::Default => sigpipe::DEFAULT,
+        rustc_target::spec::OnBrokenPipe::Kill => sigpipe::SIG_DFL,
+        rustc_target::spec::OnBrokenPipe::Error => sigpipe::SIG_IGN,
+        rustc_target::spec::OnBrokenPipe::Inherit => sigpipe::INHERIT,
     }
 }
 
diff --git a/compiler/rustc_passes/src/errors.rs b/compiler/rustc_passes/src/errors.rs
index 03a607348e8..743faf54560 100644
--- a/compiler/rustc_passes/src/errors.rs
+++ b/compiler/rustc_passes/src/errors.rs
@@ -1259,13 +1259,6 @@ pub struct ExternMain {
     pub span: Span,
 }
 
-#[derive(Diagnostic)]
-#[diag(passes_unix_sigpipe_values)]
-pub struct UnixSigpipeValues {
-    #[primary_span]
-    pub span: Span,
-}
-
 pub struct NoMainErr {
     pub sp: Span,
     pub crate_name: Symbol,
diff --git a/compiler/rustc_privacy/Cargo.toml b/compiler/rustc_privacy/Cargo.toml
index e7a32771f35..f998e0ff154 100644
--- a/compiler/rustc_privacy/Cargo.toml
+++ b/compiler/rustc_privacy/Cargo.toml
@@ -11,7 +11,6 @@ rustc_data_structures = { path = "../rustc_data_structures" }
 rustc_errors = { path = "../rustc_errors" }
 rustc_fluent_macro = { path = "../rustc_fluent_macro" }
 rustc_hir = { path = "../rustc_hir" }
-rustc_hir_analysis = { path = "../rustc_hir_analysis" }
 rustc_macros = { path = "../rustc_macros" }
 rustc_middle = { path = "../rustc_middle" }
 rustc_session = { path = "../rustc_session" }
diff --git a/compiler/rustc_query_system/Cargo.toml b/compiler/rustc_query_system/Cargo.toml
index 6ea87a4a633..4d845ab0d07 100644
--- a/compiler/rustc_query_system/Cargo.toml
+++ b/compiler/rustc_query_system/Cargo.toml
@@ -19,7 +19,6 @@ rustc_serialize = { path = "../rustc_serialize" }
 rustc_session = { path = "../rustc_session" }
 rustc_span = { path = "../rustc_span" }
 rustc_target = { path = "../rustc_target" }
-rustc_type_ir = { path = "../rustc_type_ir" }
 smallvec = { version = "1.8.1", features = ["union", "may_dangle"] }
 thin-vec = "0.2.12"
 tracing = "0.1"
diff --git a/compiler/rustc_resolve/src/diagnostics.rs b/compiler/rustc_resolve/src/diagnostics.rs
index 12484462f82..01e279b6d04 100644
--- a/compiler/rustc_resolve/src/diagnostics.rs
+++ b/compiler/rustc_resolve/src/diagnostics.rs
@@ -1617,7 +1617,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
             let post = format!(", consider renaming `{}` into `{snippet}`", suggestion.candidate);
             (span, snippet, post)
         } else {
-            (span, suggestion.candidate.to_string(), String::new())
+            (span, suggestion.candidate.to_ident_string(), String::new())
         };
         let msg = match suggestion.target {
             SuggestionTarget::SimilarlyNamed => format!(
diff --git a/compiler/rustc_resolve/src/macros.rs b/compiler/rustc_resolve/src/macros.rs
index 82e41b6c314..35bf3f761df 100644
--- a/compiler/rustc_resolve/src/macros.rs
+++ b/compiler/rustc_resolve/src/macros.rs
@@ -141,7 +141,7 @@ pub(crate) fn registered_tools(tcx: TyCtxt<'_>, (): ()) -> RegisteredTools {
     }
     // We implicitly add `rustfmt`, `clippy`, `diagnostic` to known tools,
     // but it's not an error to register them explicitly.
-    let predefined_tools = [sym::clippy, sym::rustfmt, sym::diagnostic];
+    let predefined_tools = [sym::clippy, sym::rustfmt, sym::diagnostic, sym::miri];
     registered_tools.extend(predefined_tools.iter().cloned().map(Ident::with_dummy_span));
     registered_tools
 }
diff --git a/compiler/rustc_sanitizers/src/cfi/typeid/itanium_cxx_abi/encode.rs b/compiler/rustc_sanitizers/src/cfi/typeid/itanium_cxx_abi/encode.rs
index 40cd0c14b05..d67132d2dd4 100644
--- a/compiler/rustc_sanitizers/src/cfi/typeid/itanium_cxx_abi/encode.rs
+++ b/compiler/rustc_sanitizers/src/cfi/typeid/itanium_cxx_abi/encode.rs
@@ -4,7 +4,9 @@
 //!
 //! For more information about LLVM CFI and cross-language LLVM CFI support for the Rust compiler,
 //! see design document in the tracking issue #89653.
-use rustc_data_structures::base_n;
+use rustc_data_structures::base_n::ToBaseN;
+use rustc_data_structures::base_n::ALPHANUMERIC_ONLY;
+use rustc_data_structures::base_n::CASE_INSENSITIVE;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_hir as hir;
 use rustc_middle::bug;
@@ -736,7 +738,7 @@ fn encode_ty_name(tcx: TyCtxt<'_>, def_id: DefId) -> String {
 /// <https://rust-lang.github.io/rfcs/2603-rust-symbol-name-mangling-v0.html>).
 fn to_disambiguator(num: u64) -> String {
     if let Some(num) = num.checked_sub(1) {
-        format!("s{}_", base_n::encode(num as u128, base_n::ALPHANUMERIC_ONLY))
+        format!("s{}_", num.to_base(ALPHANUMERIC_ONLY))
     } else {
         "s_".to_string()
     }
@@ -746,7 +748,7 @@ fn to_disambiguator(num: u64) -> String {
 /// <https://itanium-cxx-abi.github.io/cxx-abi/abi.html#mangle.seq-id>).
 fn to_seq_id(num: usize) -> String {
     if let Some(num) = num.checked_sub(1) {
-        base_n::encode(num as u128, base_n::CASE_INSENSITIVE).to_uppercase()
+        (num as u64).to_base(CASE_INSENSITIVE).to_uppercase()
     } else {
         "".to_string()
     }
diff --git a/compiler/rustc_session/src/config.rs b/compiler/rustc_session/src/config.rs
index ad66e5e1c2b..f2bdabbf394 100644
--- a/compiler/rustc_session/src/config.rs
+++ b/compiler/rustc_session/src/config.rs
@@ -2909,7 +2909,9 @@ pub(crate) mod dep_tracking {
     use rustc_feature::UnstableFeatures;
     use rustc_span::edition::Edition;
     use rustc_span::RealFileName;
-    use rustc_target::spec::{CodeModel, MergeFunctions, PanicStrategy, RelocModel, WasmCAbi};
+    use rustc_target::spec::{
+        CodeModel, MergeFunctions, OnBrokenPipe, PanicStrategy, RelocModel, WasmCAbi,
+    };
     use rustc_target::spec::{
         RelroLevel, SanitizerSet, SplitDebuginfo, StackProtector, TargetTriple, TlsModel,
     };
@@ -2973,6 +2975,7 @@ pub(crate) mod dep_tracking {
         InstrumentXRay,
         CrateType,
         MergeFunctions,
+        OnBrokenPipe,
         PanicStrategy,
         RelroLevel,
         OptLevel,
diff --git a/compiler/rustc_session/src/config/sigpipe.rs b/compiler/rustc_session/src/config/sigpipe.rs
index 1fadc75cfd0..1830ee03485 100644
--- a/compiler/rustc_session/src/config/sigpipe.rs
+++ b/compiler/rustc_session/src/config/sigpipe.rs
@@ -1,6 +1,6 @@
 //! NOTE: Keep these constants in sync with `library/std/src/sys/pal/unix/mod.rs`!
 
-/// The default value if `#[unix_sigpipe]` is not specified. This resolves
+/// The default value if `-Zon-broken-pipe=...` is not specified. This resolves
 /// to `SIG_IGN` in `library/std/src/sys/pal/unix/mod.rs`.
 ///
 /// Note that `SIG_IGN` has been the Rust default since 2014. See
diff --git a/compiler/rustc_session/src/options.rs b/compiler/rustc_session/src/options.rs
index 7355e5b6953..65660286dd7 100644
--- a/compiler/rustc_session/src/options.rs
+++ b/compiler/rustc_session/src/options.rs
@@ -12,7 +12,7 @@ use rustc_span::edition::Edition;
 use rustc_span::RealFileName;
 use rustc_span::SourceFileHashAlgorithm;
 use rustc_target::spec::{
-    CodeModel, LinkerFlavorCli, MergeFunctions, PanicStrategy, SanitizerSet, WasmCAbi,
+    CodeModel, LinkerFlavorCli, MergeFunctions, OnBrokenPipe, PanicStrategy, SanitizerSet, WasmCAbi,
 };
 use rustc_target::spec::{
     RelocModel, RelroLevel, SplitDebuginfo, StackProtector, TargetTriple, TlsModel,
@@ -378,6 +378,7 @@ mod desc {
     pub const parse_time_passes_format: &str = "`text` (default) or `json`";
     pub const parse_passes: &str = "a space-separated list of passes, or `all`";
     pub const parse_panic_strategy: &str = "either `unwind` or `abort`";
+    pub const parse_on_broken_pipe: &str = "either `kill`, `error`, or `inherit`";
     pub const parse_opt_panic_strategy: &str = parse_panic_strategy;
     pub const parse_oom_strategy: &str = "either `panic` or `abort`";
     pub const parse_relro_level: &str = "one of: `full`, `partial`, or `off`";
@@ -708,6 +709,17 @@ mod parse {
         true
     }
 
+    pub(crate) fn parse_on_broken_pipe(slot: &mut OnBrokenPipe, v: Option<&str>) -> bool {
+        match v {
+            // OnBrokenPipe::Default can't be explicitly specified
+            Some("kill") => *slot = OnBrokenPipe::Kill,
+            Some("error") => *slot = OnBrokenPipe::Error,
+            Some("inherit") => *slot = OnBrokenPipe::Inherit,
+            _ => return false,
+        }
+        true
+    }
+
     pub(crate) fn parse_oom_strategy(slot: &mut OomStrategy, v: Option<&str>) -> bool {
         match v {
             Some("panic") => *slot = OomStrategy::Panic,
@@ -1833,6 +1845,8 @@ options! {
         "do not use unique names for text and data sections when -Z function-sections is used"),
     normalize_docs: bool = (false, parse_bool, [TRACKED],
         "normalize associated items in rustdoc when generating documentation"),
+    on_broken_pipe: OnBrokenPipe = (OnBrokenPipe::Default, parse_on_broken_pipe, [TRACKED],
+        "behavior of std::io::ErrorKind::BrokenPipe (SIGPIPE)"),
     oom: OomStrategy = (OomStrategy::Abort, parse_oom_strategy, [TRACKED],
         "panic strategy for out-of-memory handling"),
     osx_rpath_install_name: bool = (false, parse_bool, [TRACKED],
diff --git a/compiler/rustc_span/src/symbol.rs b/compiler/rustc_span/src/symbol.rs
index 7fe94c2e82b..9c556f1152a 100644
--- a/compiler/rustc_span/src/symbol.rs
+++ b/compiler/rustc_span/src/symbol.rs
@@ -1936,7 +1936,6 @@ symbols! {
         unit,
         universal_impl_trait,
         unix,
-        unix_sigpipe,
         unlikely,
         unmarked_api,
         unnamed_fields,
diff --git a/compiler/rustc_symbol_mangling/src/v0.rs b/compiler/rustc_symbol_mangling/src/v0.rs
index fc2c3dcb2d9..5cdfb773b5c 100644
--- a/compiler/rustc_symbol_mangling/src/v0.rs
+++ b/compiler/rustc_symbol_mangling/src/v0.rs
@@ -1,4 +1,4 @@
-use rustc_data_structures::base_n;
+use rustc_data_structures::base_n::ToBaseN;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::intern::Interned;
 use rustc_hir as hir;
@@ -832,7 +832,7 @@ impl<'tcx> Printer<'tcx> for SymbolMangler<'tcx> {
 ///   e.g. `1` becomes `"0_"`, `62` becomes `"Z_"`, etc.
 pub(crate) fn push_integer_62(x: u64, output: &mut String) {
     if let Some(x) = x.checked_sub(1) {
-        base_n::push_str(x as u128, base_n::ALPHANUMERIC_ONLY, output);
+        output.push_str(&x.to_base(62));
     }
     output.push('_');
 }
diff --git a/compiler/rustc_target/src/spec/base/fuchsia.rs b/compiler/rustc_target/src/spec/base/fuchsia.rs
index 4c2775850d1..d1ac22f3a09 100644
--- a/compiler/rustc_target/src/spec/base/fuchsia.rs
+++ b/compiler/rustc_target/src/spec/base/fuchsia.rs
@@ -1,4 +1,6 @@
-use crate::spec::{crt_objects, cvs, Cc, LinkOutputKind, LinkerFlavor, Lld, TargetOptions};
+use crate::spec::{
+    crt_objects, cvs, Cc, FramePointer, LinkOutputKind, LinkerFlavor, Lld, TargetOptions,
+};
 
 pub fn opts() -> TargetOptions {
     // This mirrors the linker options provided by clang. We presume lld for
@@ -38,6 +40,7 @@ pub fn opts() -> TargetOptions {
         ]),
         position_independent_executables: true,
         has_thread_local: true,
+        frame_pointer: FramePointer::NonLeaf,
         ..Default::default()
     }
 }
diff --git a/compiler/rustc_target/src/spec/mod.rs b/compiler/rustc_target/src/spec/mod.rs
index bd347c1b4b3..cbb248a0fc2 100644
--- a/compiler/rustc_target/src/spec/mod.rs
+++ b/compiler/rustc_target/src/spec/mod.rs
@@ -778,6 +778,14 @@ pub enum PanicStrategy {
     Abort,
 }
 
+#[derive(Clone, Copy, Debug, PartialEq, Hash, Encodable, Decodable, HashStable_Generic)]
+pub enum OnBrokenPipe {
+    Default,
+    Kill,
+    Error,
+    Inherit,
+}
+
 impl PanicStrategy {
     pub fn desc(&self) -> &str {
         match *self {
diff --git a/compiler/rustc_trait_selection/src/solve/assembly/mod.rs b/compiler/rustc_trait_selection/src/solve/assembly/mod.rs
index 280975f63bd..f2ca42a0be9 100644
--- a/compiler/rustc_trait_selection/src/solve/assembly/mod.rs
+++ b/compiler/rustc_trait_selection/src/solve/assembly/mod.rs
@@ -58,15 +58,15 @@ pub(super) trait GoalKind<'tcx>:
     /// goal by equating it with the assumption.
     fn probe_and_consider_implied_clause(
         ecx: &mut EvalCtxt<'_, 'tcx>,
-        source: CandidateSource,
+        parent_source: CandidateSource,
         goal: Goal<'tcx, Self>,
         assumption: ty::Clause<'tcx>,
-        requirements: impl IntoIterator<Item = Goal<'tcx, ty::Predicate<'tcx>>>,
+        requirements: impl IntoIterator<Item = (GoalSource, Goal<'tcx, ty::Predicate<'tcx>>)>,
     ) -> Result<Candidate<'tcx>, NoSolution> {
-        Self::probe_and_match_goal_against_assumption(ecx, source, goal, assumption, |ecx| {
-            // FIXME(-Znext-solver=coinductive): check whether this should be
-            // `GoalSource::ImplWhereBound` for any caller.
-            ecx.add_goals(GoalSource::Misc, requirements);
+        Self::probe_and_match_goal_against_assumption(ecx, parent_source, goal, assumption, |ecx| {
+            for (nested_source, goal) in requirements {
+                ecx.add_goal(nested_source, goal);
+            }
             ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
         })
     }
@@ -85,9 +85,8 @@ pub(super) trait GoalKind<'tcx>:
             let ty::Dynamic(bounds, _, _) = *goal.predicate.self_ty().kind() else {
                 bug!("expected object type in `probe_and_consider_object_bound_candidate`");
             };
-            // FIXME(-Znext-solver=coinductive): Should this be `GoalSource::ImplWhereBound`?
             ecx.add_goals(
-                GoalSource::Misc,
+                GoalSource::ImplWhereBound,
                 structural_traits::predicates_for_object_candidate(
                     ecx,
                     goal.param_env,
diff --git a/compiler/rustc_trait_selection/src/solve/eval_ctxt/canonical.rs b/compiler/rustc_trait_selection/src/solve/eval_ctxt/canonical.rs
index 6722abd709c..d6bf2b596ef 100644
--- a/compiler/rustc_trait_selection/src/solve/eval_ctxt/canonical.rs
+++ b/compiler/rustc_trait_selection/src/solve/eval_ctxt/canonical.rs
@@ -90,6 +90,8 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
         &mut self,
         certainty: Certainty,
     ) -> QueryResult<'tcx> {
+        self.inspect.make_canonical_response(certainty);
+
         let goals_certainty = self.try_evaluate_added_goals()?;
         assert_eq!(
             self.tainted,
@@ -98,8 +100,6 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
             previous call to `try_evaluate_added_goals!`"
         );
 
-        self.inspect.make_canonical_response(certainty);
-
         // When normalizing, we've replaced the expected term with an unconstrained
         // inference variable. This means that we dropped information which could
         // have been important. We handle this by instead returning the nested goals
diff --git a/compiler/rustc_trait_selection/src/solve/eval_ctxt/mod.rs b/compiler/rustc_trait_selection/src/solve/eval_ctxt/mod.rs
index 1710746ae50..bae1c6b6011 100644
--- a/compiler/rustc_trait_selection/src/solve/eval_ctxt/mod.rs
+++ b/compiler/rustc_trait_selection/src/solve/eval_ctxt/mod.rs
@@ -454,7 +454,7 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
         } else {
             self.infcx.enter_forall(kind, |kind| {
                 let goal = goal.with(self.tcx(), ty::Binder::dummy(kind));
-                self.add_goal(GoalSource::Misc, goal);
+                self.add_goal(GoalSource::InstantiateHigherRanked, goal);
                 self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
             })
         }
diff --git a/compiler/rustc_trait_selection/src/solve/fulfill.rs b/compiler/rustc_trait_selection/src/solve/fulfill.rs
index 3fa409eefff..796222129f1 100644
--- a/compiler/rustc_trait_selection/src/solve/fulfill.rs
+++ b/compiler/rustc_trait_selection/src/solve/fulfill.rs
@@ -1,15 +1,19 @@
 use std::mem;
+use std::ops::ControlFlow;
 
 use rustc_infer::infer::InferCtxt;
-use rustc_infer::traits::solve::MaybeCause;
+use rustc_infer::traits::query::NoSolution;
+use rustc_infer::traits::solve::inspect::ProbeKind;
+use rustc_infer::traits::solve::{CandidateSource, GoalSource, MaybeCause};
 use rustc_infer::traits::{
-    query::NoSolution, FulfillmentError, FulfillmentErrorCode, MismatchedProjectionTypes,
-    PredicateObligation, SelectionError, TraitEngine,
+    self, FulfillmentError, FulfillmentErrorCode, MismatchedProjectionTypes, Obligation,
+    ObligationCause, PredicateObligation, SelectionError, TraitEngine,
 };
-use rustc_middle::ty;
 use rustc_middle::ty::error::{ExpectedFound, TypeError};
+use rustc_middle::ty::{self, TyCtxt};
 
 use super::eval_ctxt::GenerateProofTree;
+use super::inspect::{ProofTreeInferCtxtExt, ProofTreeVisitor};
 use super::{Certainty, InferCtxtEvalExt};
 
 /// A trait engine using the new trait solver.
@@ -133,9 +137,9 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentCtxt<'tcx> {
             .collect();
 
         errors.extend(self.obligations.overflowed.drain(..).map(|obligation| FulfillmentError {
-            root_obligation: obligation.clone(),
+            obligation: find_best_leaf_obligation(infcx, &obligation),
             code: FulfillmentErrorCode::Ambiguity { overflow: Some(true) },
-            obligation,
+            root_obligation: obligation,
         }));
 
         errors
@@ -192,8 +196,10 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentCtxt<'tcx> {
 
 fn fulfillment_error_for_no_solution<'tcx>(
     infcx: &InferCtxt<'tcx>,
-    obligation: PredicateObligation<'tcx>,
+    root_obligation: PredicateObligation<'tcx>,
 ) -> FulfillmentError<'tcx> {
+    let obligation = find_best_leaf_obligation(infcx, &root_obligation);
+
     let code = match obligation.predicate.kind().skip_binder() {
         ty::PredicateKind::Clause(ty::ClauseKind::Projection(_)) => {
             FulfillmentErrorCode::ProjectionError(
@@ -234,7 +240,8 @@ fn fulfillment_error_for_no_solution<'tcx>(
             bug!("unexpected goal: {obligation:?}")
         }
     };
-    FulfillmentError { root_obligation: obligation.clone(), code, obligation }
+
+    FulfillmentError { obligation, code, root_obligation }
 }
 
 fn fulfillment_error_for_stalled<'tcx>(
@@ -258,5 +265,136 @@ fn fulfillment_error_for_stalled<'tcx>(
         }
     });
 
-    FulfillmentError { obligation: obligation.clone(), code, root_obligation: obligation }
+    FulfillmentError {
+        obligation: find_best_leaf_obligation(infcx, &obligation),
+        code,
+        root_obligation: obligation,
+    }
+}
+
+fn find_best_leaf_obligation<'tcx>(
+    infcx: &InferCtxt<'tcx>,
+    obligation: &PredicateObligation<'tcx>,
+) -> PredicateObligation<'tcx> {
+    let obligation = infcx.resolve_vars_if_possible(obligation.clone());
+    infcx
+        .visit_proof_tree(
+            obligation.clone().into(),
+            &mut BestObligation { obligation: obligation.clone() },
+        )
+        .break_value()
+        .unwrap_or(obligation)
+}
+
+struct BestObligation<'tcx> {
+    obligation: PredicateObligation<'tcx>,
+}
+
+impl<'tcx> BestObligation<'tcx> {
+    fn with_derived_obligation(
+        &mut self,
+        derived_obligation: PredicateObligation<'tcx>,
+        and_then: impl FnOnce(&mut Self) -> <Self as ProofTreeVisitor<'tcx>>::Result,
+    ) -> <Self as ProofTreeVisitor<'tcx>>::Result {
+        let old_obligation = std::mem::replace(&mut self.obligation, derived_obligation);
+        let res = and_then(self);
+        self.obligation = old_obligation;
+        res
+    }
+}
+
+impl<'tcx> ProofTreeVisitor<'tcx> for BestObligation<'tcx> {
+    type Result = ControlFlow<PredicateObligation<'tcx>>;
+
+    fn span(&self) -> rustc_span::Span {
+        self.obligation.cause.span
+    }
+
+    fn visit_goal(&mut self, goal: &super::inspect::InspectGoal<'_, 'tcx>) -> Self::Result {
+        // FIXME: Throw out candidates that have no failing WC and >0 failing misc goal.
+        // This most likely means that the goal just didn't unify at all, e.g. a param
+        // candidate with an alias in it.
+        let candidates = goal.candidates();
+
+        let [candidate] = candidates.as_slice() else {
+            return ControlFlow::Break(self.obligation.clone());
+        };
+
+        // FIXME: Could we extract a trait ref from a projection here too?
+        // FIXME: Also, what about considering >1 layer up the stack? May be necessary
+        // for normalizes-to.
+        let Some(parent_trait_pred) = goal.goal().predicate.to_opt_poly_trait_pred() else {
+            return ControlFlow::Break(self.obligation.clone());
+        };
+
+        let tcx = goal.infcx().tcx;
+        let mut impl_where_bound_count = 0;
+        for nested_goal in candidate.instantiate_nested_goals(self.span()) {
+            let obligation;
+            match nested_goal.source() {
+                GoalSource::Misc => {
+                    continue;
+                }
+                GoalSource::ImplWhereBound => {
+                    obligation = Obligation {
+                        cause: derive_cause(
+                            tcx,
+                            candidate.kind(),
+                            self.obligation.cause.clone(),
+                            impl_where_bound_count,
+                            parent_trait_pred,
+                        ),
+                        param_env: nested_goal.goal().param_env,
+                        predicate: nested_goal.goal().predicate,
+                        recursion_depth: self.obligation.recursion_depth + 1,
+                    };
+                    impl_where_bound_count += 1;
+                }
+                GoalSource::InstantiateHigherRanked => {
+                    obligation = self.obligation.clone();
+                }
+            }
+
+            // Skip nested goals that hold.
+            //FIXME: We should change the max allowed certainty based on if we're
+            // visiting an ambiguity or error obligation.
+            if matches!(nested_goal.result(), Ok(Certainty::Yes)) {
+                continue;
+            }
+
+            self.with_derived_obligation(obligation, |this| nested_goal.visit_with(this))?;
+        }
+
+        ControlFlow::Break(self.obligation.clone())
+    }
+}
+
+fn derive_cause<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    candidate_kind: ProbeKind<'tcx>,
+    mut cause: ObligationCause<'tcx>,
+    idx: usize,
+    parent_trait_pred: ty::PolyTraitPredicate<'tcx>,
+) -> ObligationCause<'tcx> {
+    match candidate_kind {
+        ProbeKind::TraitCandidate { source: CandidateSource::Impl(impl_def_id), result: _ } => {
+            if let Some((_, span)) =
+                tcx.predicates_of(impl_def_id).instantiate_identity(tcx).iter().nth(idx)
+            {
+                cause = cause.derived_cause(parent_trait_pred, |derived| {
+                    traits::ImplDerivedObligation(Box::new(traits::ImplDerivedObligationCause {
+                        derived,
+                        impl_or_alias_def_id: impl_def_id,
+                        impl_def_predicate_index: Some(idx),
+                        span,
+                    }))
+                })
+            }
+        }
+        ProbeKind::TraitCandidate { source: CandidateSource::BuiltinImpl(..), result: _ } => {
+            cause = cause.derived_cause(parent_trait_pred, traits::BuiltinDerivedObligation);
+        }
+        _ => {}
+    };
+    cause
 }
diff --git a/compiler/rustc_trait_selection/src/solve/inspect/analyse.rs b/compiler/rustc_trait_selection/src/solve/inspect/analyse.rs
index 97de25295b8..4f79f1b2aaf 100644
--- a/compiler/rustc_trait_selection/src/solve/inspect/analyse.rs
+++ b/compiler/rustc_trait_selection/src/solve/inspect/analyse.rs
@@ -41,6 +41,7 @@ pub struct InspectGoal<'a, 'tcx> {
     result: Result<Certainty, NoSolution>,
     evaluation_kind: inspect::CanonicalGoalEvaluationKind<'tcx>,
     normalizes_to_term_hack: Option<NormalizesToTermHack<'tcx>>,
+    source: GoalSource,
 }
 
 /// The expected term of a `NormalizesTo` goal gets replaced
@@ -90,7 +91,7 @@ impl<'tcx> NormalizesToTermHack<'tcx> {
 pub struct InspectCandidate<'a, 'tcx> {
     goal: &'a InspectGoal<'a, 'tcx>,
     kind: inspect::ProbeKind<'tcx>,
-    nested_goals: Vec<inspect::CanonicalState<'tcx, Goal<'tcx, ty::Predicate<'tcx>>>>,
+    nested_goals: Vec<(GoalSource, inspect::CanonicalState<'tcx, Goal<'tcx, ty::Predicate<'tcx>>>)>,
     final_state: inspect::CanonicalState<'tcx, ()>,
     result: QueryResult<'tcx>,
     shallow_certainty: Certainty,
@@ -125,10 +126,8 @@ impl<'a, 'tcx> InspectCandidate<'a, 'tcx> {
     /// back their inference constraints. This function modifies
     /// the state of the `infcx`.
     pub fn visit_nested_no_probe<V: ProofTreeVisitor<'tcx>>(&self, visitor: &mut V) -> V::Result {
-        if self.goal.depth < visitor.config().max_depth {
-            for goal in self.instantiate_nested_goals(visitor.span()) {
-                try_visit!(visitor.visit_goal(&goal));
-            }
+        for goal in self.instantiate_nested_goals(visitor.span()) {
+            try_visit!(goal.visit_with(visitor));
         }
 
         V::Result::output()
@@ -143,13 +142,16 @@ impl<'a, 'tcx> InspectCandidate<'a, 'tcx> {
         let instantiated_goals: Vec<_> = self
             .nested_goals
             .iter()
-            .map(|goal| {
-                canonical::instantiate_canonical_state(
-                    infcx,
-                    span,
-                    param_env,
-                    &mut orig_values,
-                    *goal,
+            .map(|(source, goal)| {
+                (
+                    *source,
+                    canonical::instantiate_canonical_state(
+                        infcx,
+                        span,
+                        param_env,
+                        &mut orig_values,
+                        *goal,
+                    ),
                 )
             })
             .collect();
@@ -171,7 +173,7 @@ impl<'a, 'tcx> InspectCandidate<'a, 'tcx> {
 
         instantiated_goals
             .into_iter()
-            .map(|goal| match goal.predicate.kind().no_bound_vars() {
+            .map(|(source, goal)| match goal.predicate.kind().no_bound_vars() {
                 Some(ty::PredicateKind::NormalizesTo(ty::NormalizesTo { alias, term })) => {
                     let unconstrained_term = match term.unpack() {
                         ty::TermKind::Ty(_) => infcx
@@ -195,6 +197,7 @@ impl<'a, 'tcx> InspectCandidate<'a, 'tcx> {
                         self.goal.depth + 1,
                         proof_tree.unwrap(),
                         Some(NormalizesToTermHack { term, unconstrained_term }),
+                        source,
                     )
                 }
                 _ => InspectGoal::new(
@@ -202,6 +205,7 @@ impl<'a, 'tcx> InspectCandidate<'a, 'tcx> {
                     self.goal.depth + 1,
                     infcx.evaluate_root_goal(goal, GenerateProofTree::Yes).1.unwrap(),
                     None,
+                    source,
                 ),
             })
             .collect()
@@ -227,16 +231,23 @@ impl<'a, 'tcx> InspectGoal<'a, 'tcx> {
         self.result
     }
 
+    pub fn source(&self) -> GoalSource {
+        self.source
+    }
+
     fn candidates_recur(
         &'a self,
         candidates: &mut Vec<InspectCandidate<'a, 'tcx>>,
-        nested_goals: &mut Vec<inspect::CanonicalState<'tcx, Goal<'tcx, ty::Predicate<'tcx>>>>,
+        nested_goals: &mut Vec<(
+            GoalSource,
+            inspect::CanonicalState<'tcx, Goal<'tcx, ty::Predicate<'tcx>>>,
+        )>,
         probe: &inspect::Probe<'tcx>,
     ) {
         let mut shallow_certainty = None;
         for step in &probe.steps {
             match step {
-                &inspect::ProbeStep::AddGoal(_source, goal) => nested_goals.push(goal),
+                &inspect::ProbeStep::AddGoal(source, goal) => nested_goals.push((source, goal)),
                 inspect::ProbeStep::NestedProbe(ref probe) => {
                     // Nested probes have to prove goals added in their parent
                     // but do not leak them, so we truncate the added goals
@@ -319,6 +330,7 @@ impl<'a, 'tcx> InspectGoal<'a, 'tcx> {
         depth: usize,
         root: inspect::GoalEvaluation<'tcx>,
         normalizes_to_term_hack: Option<NormalizesToTermHack<'tcx>>,
+        source: GoalSource,
     ) -> Self {
         let inspect::GoalEvaluation { uncanonicalized_goal, kind, evaluation } = root;
         let inspect::GoalEvaluationKind::Root { orig_values } = kind else { unreachable!() };
@@ -341,8 +353,17 @@ impl<'a, 'tcx> InspectGoal<'a, 'tcx> {
             result,
             evaluation_kind: evaluation.kind,
             normalizes_to_term_hack,
+            source,
         }
     }
+
+    pub(crate) fn visit_with<V: ProofTreeVisitor<'tcx>>(&self, visitor: &mut V) -> V::Result {
+        if self.depth < visitor.config().max_depth {
+            try_visit!(visitor.visit_goal(self));
+        }
+
+        V::Result::output()
+    }
 }
 
 /// The public API to interact with proof trees.
@@ -367,6 +388,6 @@ impl<'tcx> InferCtxt<'tcx> {
     ) -> V::Result {
         let (_, proof_tree) = self.evaluate_root_goal(goal, GenerateProofTree::Yes);
         let proof_tree = proof_tree.unwrap();
-        visitor.visit_goal(&InspectGoal::new(self, 0, proof_tree, None))
+        visitor.visit_goal(&InspectGoal::new(self, 0, proof_tree, None, GoalSource::Misc))
     }
 }
diff --git a/compiler/rustc_trait_selection/src/solve/normalizes_to/mod.rs b/compiler/rustc_trait_selection/src/solve/normalizes_to/mod.rs
index dab87fffe46..f886c588650 100644
--- a/compiler/rustc_trait_selection/src/solve/normalizes_to/mod.rs
+++ b/compiler/rustc_trait_selection/src/solve/normalizes_to/mod.rs
@@ -389,7 +389,7 @@ impl<'tcx> assembly::GoalKind<'tcx> for NormalizesTo<'tcx> {
             CandidateSource::BuiltinImpl(BuiltinImplSource::Misc),
             goal,
             pred,
-            [goal.with(tcx, output_is_sized_pred)],
+            [(GoalSource::ImplWhereBound, goal.with(tcx, output_is_sized_pred))],
         )
     }
 
@@ -473,7 +473,8 @@ impl<'tcx> assembly::GoalKind<'tcx> for NormalizesTo<'tcx> {
             pred,
             [goal.with(tcx, output_is_sized_pred)]
                 .into_iter()
-                .chain(nested_preds.into_iter().map(|pred| goal.with(tcx, pred))),
+                .chain(nested_preds.into_iter().map(|pred| goal.with(tcx, pred)))
+                .map(|goal| (GoalSource::ImplWhereBound, goal)),
         )
     }
 
diff --git a/compiler/rustc_trait_selection/src/solve/trait_goals.rs b/compiler/rustc_trait_selection/src/solve/trait_goals.rs
index c8cb14abb55..d2b893d6383 100644
--- a/compiler/rustc_trait_selection/src/solve/trait_goals.rs
+++ b/compiler/rustc_trait_selection/src/solve/trait_goals.rs
@@ -321,7 +321,7 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
             CandidateSource::BuiltinImpl(BuiltinImplSource::Misc),
             goal,
             pred,
-            [goal.with(tcx, output_is_sized_pred)],
+            [(GoalSource::ImplWhereBound, goal.with(tcx, output_is_sized_pred))],
         )
     }
 
@@ -367,7 +367,8 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
             pred,
             [goal.with(tcx, output_is_sized_pred)]
                 .into_iter()
-                .chain(nested_preds.into_iter().map(|pred| goal.with(tcx, pred))),
+                .chain(nested_preds.into_iter().map(|pred| goal.with(tcx, pred)))
+                .map(|goal| (GoalSource::ImplWhereBound, goal)),
         )
     }