diff --git a/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp b/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp
index 5d2df6ddfc912..eb99d560e57b5 100644
--- a/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp
+++ b/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp
@@ -784,8 +784,14 @@ extern "C" LLVMRustResult LLVMRustOptimize(
   // the PassBuilder does not create a pipeline.
   std::vector<std::function<void(ModulePassManager &, OptimizationLevel)>>
       PipelineStartEPCallbacks;
+#if LLVM_VERSION_GE(20, 0)
+  std::vector<std::function<void(ModulePassManager &, OptimizationLevel,
+                                 ThinOrFullLTOPhase)>>
+      OptimizerLastEPCallbacks;
+#else
   std::vector<std::function<void(ModulePassManager &, OptimizationLevel)>>
       OptimizerLastEPCallbacks;
+#endif
 
   if (!IsLinkerPluginLTO && SanitizerOptions && SanitizerOptions->SanitizeCFI &&
       !NoPrepopulatePasses) {
@@ -832,7 +838,12 @@ extern "C" LLVMRustResult LLVMRustOptimize(
           SanitizerOptions->SanitizeDataFlowABIList +
               SanitizerOptions->SanitizeDataFlowABIListLen);
       OptimizerLastEPCallbacks.push_back(
+#if LLVM_VERSION_GE(20, 0)
+          [ABIListFiles](ModulePassManager &MPM, OptimizationLevel Level,
+                         ThinOrFullLTOPhase phase) {
+#else
           [ABIListFiles](ModulePassManager &MPM, OptimizationLevel Level) {
+#endif
             MPM.addPass(DataFlowSanitizerPass(ABIListFiles));
           });
     }
@@ -844,23 +855,39 @@ extern "C" LLVMRustResult LLVMRustOptimize(
           /*CompileKernel=*/false,
           /*EagerChecks=*/true);
       OptimizerLastEPCallbacks.push_back(
+#if LLVM_VERSION_GE(20, 0)
+          [Options](ModulePassManager &MPM, OptimizationLevel Level,
+                    ThinOrFullLTOPhase phase) {
+#else
           [Options](ModulePassManager &MPM, OptimizationLevel Level) {
+#endif
             MPM.addPass(MemorySanitizerPass(Options));
           });
     }
 
     if (SanitizerOptions->SanitizeThread) {
-      OptimizerLastEPCallbacks.push_back([](ModulePassManager &MPM,
-                                            OptimizationLevel Level) {
-        MPM.addPass(ModuleThreadSanitizerPass());
-        MPM.addPass(createModuleToFunctionPassAdaptor(ThreadSanitizerPass()));
-      });
+      OptimizerLastEPCallbacks.push_back(
+#if LLVM_VERSION_GE(20, 0)
+          [](ModulePassManager &MPM, OptimizationLevel Level,
+             ThinOrFullLTOPhase phase) {
+#else
+          [](ModulePassManager &MPM, OptimizationLevel Level) {
+#endif
+            MPM.addPass(ModuleThreadSanitizerPass());
+            MPM.addPass(
+                createModuleToFunctionPassAdaptor(ThreadSanitizerPass()));
+          });
     }
 
     if (SanitizerOptions->SanitizeAddress ||
         SanitizerOptions->SanitizeKernelAddress) {
       OptimizerLastEPCallbacks.push_back(
+#if LLVM_VERSION_GE(20, 0)
+          [SanitizerOptions](ModulePassManager &MPM, OptimizationLevel Level,
+                             ThinOrFullLTOPhase phase) {
+#else
           [SanitizerOptions](ModulePassManager &MPM, OptimizationLevel Level) {
+#endif
             auto CompileKernel = SanitizerOptions->SanitizeKernelAddress;
             AddressSanitizerOptions opts = AddressSanitizerOptions{
                 CompileKernel,
@@ -874,7 +901,12 @@ extern "C" LLVMRustResult LLVMRustOptimize(
     }
     if (SanitizerOptions->SanitizeHWAddress) {
       OptimizerLastEPCallbacks.push_back(
+#if LLVM_VERSION_GE(20, 0)
+          [SanitizerOptions](ModulePassManager &MPM, OptimizationLevel Level,
+                             ThinOrFullLTOPhase phase) {
+#else
           [SanitizerOptions](ModulePassManager &MPM, OptimizationLevel Level) {
+#endif
             HWAddressSanitizerOptions opts(
                 /*CompileKernel=*/false,
                 SanitizerOptions->SanitizeHWAddressRecover,
@@ -935,7 +967,11 @@ extern "C" LLVMRustResult LLVMRustOptimize(
     for (const auto &C : PipelineStartEPCallbacks)
       C(MPM, OptLevel);
     for (const auto &C : OptimizerLastEPCallbacks)
+#if LLVM_VERSION_GE(20, 0)
+      C(MPM, OptLevel, ThinOrFullLTOPhase::None);
+#else
       C(MPM, OptLevel);
+#endif
   }
 
   if (ExtraPassesLen) {
diff --git a/library/core/src/char/methods.rs b/library/core/src/char/methods.rs
index 701e34b135e23..39f421df22747 100644
--- a/library/core/src/char/methods.rs
+++ b/library/core/src/char/methods.rs
@@ -711,7 +711,7 @@ impl char {
     /// '𝕊'.encode_utf16(&mut b);
     /// ```
     #[stable(feature = "unicode_encode_char", since = "1.15.0")]
-    #[rustc_const_unstable(feature = "const_char_encode_utf16", issue = "130660")]
+    #[rustc_const_stable(feature = "const_char_encode_utf16", since = "CURRENT_RUSTC_VERSION")]
     #[inline]
     pub const fn encode_utf16(self, dst: &mut [u16]) -> &mut [u16] {
         encode_utf16_raw(self as u32, dst)
@@ -1819,7 +1819,10 @@ pub const fn encode_utf8_raw(code: u32, dst: &mut [u8]) -> &mut [u8] {
 /// Panics if the buffer is not large enough.
 /// A buffer of length 2 is large enough to encode any `char`.
 #[unstable(feature = "char_internals", reason = "exposed only for libstd", issue = "none")]
-#[rustc_const_unstable(feature = "const_char_encode_utf16", issue = "130660")]
+#[cfg_attr(
+    bootstrap,
+    rustc_const_stable(feature = "const_char_encode_utf16", since = "CURRENT_RUSTC_VERSION")
+)]
 #[doc(hidden)]
 #[inline]
 pub const fn encode_utf16_raw(mut code: u32, dst: &mut [u16]) -> &mut [u16] {
diff --git a/library/core/src/fmt/num.rs b/library/core/src/fmt/num.rs
index f1540803f978d..d43f25d9fd129 100644
--- a/library/core/src/fmt/num.rs
+++ b/library/core/src/fmt/num.rs
@@ -88,7 +88,10 @@ unsafe trait GenericRadix: Sized {
                 };
             }
         }
-        let buf = &buf[curr..];
+        // SAFETY: `curr` is initialized to `buf.len()` and is only decremented, so it can't overflow. It is
+        // decremented exactly once for each digit. Since u128 is the widest fixed width integer format supported,
+        // the maximum number of digits (bits) is 128 for base-2, so `curr` won't underflow as well.
+        let buf = unsafe { buf.get_unchecked(curr..) };
         // SAFETY: The only chars in `buf` are created by `Self::digit` which are assumed to be
         // valid UTF-8
         let buf = unsafe {
diff --git a/library/core/src/lib.rs b/library/core/src/lib.rs
index 444f733adff00..87c6bb39ce6cf 100644
--- a/library/core/src/lib.rs
+++ b/library/core/src/lib.rs
@@ -115,7 +115,6 @@
 #![feature(const_align_of_val_raw)]
 #![feature(const_alloc_layout)]
 #![feature(const_black_box)]
-#![feature(const_char_encode_utf16)]
 #![feature(const_eval_select)]
 #![feature(const_float_methods)]
 #![feature(const_heap)]
diff --git a/triagebot.toml b/triagebot.toml
index 7f4def6a11ba4..45b580787d1d9 100644
--- a/triagebot.toml
+++ b/triagebot.toml
@@ -611,6 +611,10 @@ cc = ["@bjorn3"]
 [mentions."compiler/rustc_codegen_gcc"]
 cc = ["@antoyo", "@GuillaumeGomez"]
 
+[mentions."compiler/rustc_const_eval/src/"]
+message = "Some changes occurred to the CTFE machinery"
+cc = ["@rust-lang/wg-const-eval"]
+
 [mentions."compiler/rustc_const_eval/src/interpret"]
 message = "Some changes occurred to the CTFE / Miri interpreter"
 cc = ["@rust-lang/miri"]
@@ -633,7 +637,7 @@ cc = ["@compiler-errors", "@lcnr"]
 
 [mentions."compiler/rustc_middle/src/mir/interpret"]
 message = "Some changes occurred to the CTFE / Miri interpreter"
-cc = ["@rust-lang/miri"]
+cc = ["@rust-lang/miri", "@rust-lang/wg-const-eval"]
 
 [mentions."compiler/rustc_mir_transform/src/"]
 message = "Some changes occurred to MIR optimizations"
@@ -706,7 +710,7 @@ message = """
 Some changes occurred to the intrinsics. Make sure the CTFE / Miri interpreter
 gets adapted for the changes, if necessary.
 """
-cc = ["@rust-lang/miri"]
+cc = ["@rust-lang/miri", "@rust-lang/wg-const-eval"]
 
 [mentions."library/portable-simd"]
 message = """