Auto merge of #104246 - Manishearth:rollup-9o3txc7, r=Manishearth

Rollup of 9 pull requests

Successful merges:

 - #101939 (Add loongarch64 abi support)
 - #103863 (Use `TraitEngine` in more places, restrict visibility of `FulfillmentCtxt` constructor)
 - #104036 (Suggest `is_some` when we've found `Option` but expected `bool`)
 - #104060 (Make `Hash`, `Hasher` and `BuildHasher` `#[const_trait]` and make `Sip` const `Hasher`)
 - #104077 (Use aapcs for efiapi calling convention on arm)
 - #104186 (Tighten the 'introduce new binding' suggestion)
 - #104194 (`EarlyBinder` docs)
 - #104233 (Don't ICE when encountering `ConstKind::Error` in `RequiredConstsVisitor`)
 - #104235 (Use `const_error_with_guaranteed` more)

Failed merges:

 - #104078 (Print "Checking/Building ..." message even when --dry-run is passed)
 - #104169 (Migrate `:target` rules to use CSS variables)

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

38 files changed, 827 insertions, 104 deletions

diff --git a/compiler/rustc_hir_typeck/src/coercion.rs b/compiler/rustc_hir_typeck/src/coercion.rs
index 4d8ab2c1c7a..71949b42118 100644
--- a/compiler/rustc_hir_typeck/src/coercion.rs
+++ b/compiler/rustc_hir_typeck/src/coercion.rs
@@ -62,6 +62,7 @@ use rustc_span::{self, BytePos, DesugaringKind, Span};
 use rustc_target::spec::abi::Abi;
 use rustc_trait_selection::infer::InferCtxtExt as _;
 use rustc_trait_selection::traits::error_reporting::TypeErrCtxtExt as _;
+use rustc_trait_selection::traits::TraitEngineExt as _;
 use rustc_trait_selection::traits::{self, ObligationCause, ObligationCauseCode};
 
 use smallvec::{smallvec, SmallVec};
@@ -1038,7 +1039,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             let Ok(ok) = coerce.coerce(source, target) else {
                 return false;
             };
-            let mut fcx = traits::FulfillmentContext::new_in_snapshot();
+            let mut fcx = <dyn TraitEngine<'tcx>>::new_in_snapshot(self.tcx);
             fcx.register_predicate_obligations(self, ok.obligations);
             fcx.select_where_possible(&self).is_empty()
         })
diff --git a/compiler/rustc_hir_typeck/src/demand.rs b/compiler/rustc_hir_typeck/src/demand.rs
index 16febfc46da..7f78f5fb8a7 100644
--- a/compiler/rustc_hir_typeck/src/demand.rs
+++ b/compiler/rustc_hir_typeck/src/demand.rs
@@ -42,7 +42,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             || self.suggest_boxing_when_appropriate(err, expr, expected, expr_ty)
             || self.suggest_block_to_brackets_peeling_refs(err, expr, expr_ty, expected)
             || self.suggest_copied_or_cloned(err, expr, expr_ty, expected)
-            || self.suggest_into(err, expr, expr_ty, expected);
+            || self.suggest_into(err, expr, expr_ty, expected)
+            || self.suggest_option_to_bool(err, expr, expr_ty, expected);
 
         self.note_type_is_not_clone(err, expected, expr_ty, expr);
         self.note_need_for_fn_pointer(err, expected, expr_ty);
diff --git a/compiler/rustc_hir_typeck/src/expr.rs b/compiler/rustc_hir_typeck/src/expr.rs
index 43669489e69..e948d832e32 100644
--- a/compiler/rustc_hir_typeck/src/expr.rs
+++ b/compiler/rustc_hir_typeck/src/expr.rs
@@ -103,8 +103,16 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         }
 
         if let Some(mut err) = self.demand_suptype_diag(expr.span, expected_ty, ty) {
-            let expr = expr.peel_drop_temps();
-            self.suggest_deref_ref_or_into(&mut err, expr, expected_ty, ty, None);
+            // FIXME(compiler-errors): We probably should fold some of the
+            // `suggest_` functions from  `emit_coerce_suggestions` into here,
+            // since some of those aren't necessarily just coerce suggestions.
+            let _ = self.suggest_deref_ref_or_into(
+                &mut err,
+                expr.peel_drop_temps(),
+                expected_ty,
+                ty,
+                None,
+            ) || self.suggest_option_to_bool(&mut err, expr, ty, expected_ty);
             extend_err(&mut err);
             err.emit();
         }
diff --git a/compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs b/compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs
index e3b3fb499b1..a14759e254c 100644
--- a/compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs
+++ b/compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs
@@ -13,7 +13,7 @@ use rustc_hir_analysis::astconv::AstConv;
 use rustc_infer::infer::{self, TyCtxtInferExt};
 use rustc_infer::traits::{self, StatementAsExpression};
 use rustc_middle::lint::in_external_macro;
-use rustc_middle::ty::{self, Binder, IsSuggestable, ToPredicate, Ty};
+use rustc_middle::ty::{self, Binder, DefIdTree, IsSuggestable, ToPredicate, Ty};
 use rustc_session::errors::ExprParenthesesNeeded;
 use rustc_span::symbol::sym;
 use rustc_span::Span;
@@ -1116,6 +1116,53 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         false
     }
 
+    /// When expecting a `bool` and finding an `Option`, suggests using `let Some(..)` or `.is_some()`
+    pub(crate) fn suggest_option_to_bool(
+        &self,
+        diag: &mut Diagnostic,
+        expr: &hir::Expr<'_>,
+        expr_ty: Ty<'tcx>,
+        expected_ty: Ty<'tcx>,
+    ) -> bool {
+        if !expected_ty.is_bool() {
+            return false;
+        }
+
+        let ty::Adt(def, _) = expr_ty.peel_refs().kind() else { return false; };
+        if !self.tcx.is_diagnostic_item(sym::Option, def.did()) {
+            return false;
+        }
+
+        let hir = self.tcx.hir();
+        let cond_parent = hir.parent_iter(expr.hir_id).skip_while(|(_, node)| {
+            matches!(node, hir::Node::Expr(hir::Expr { kind: hir::ExprKind::Binary(op, _, _), .. }) if op.node == hir::BinOpKind::And)
+        }).next();
+        // Don't suggest:
+        //     `let Some(_) = a.is_some() && b`
+        //                     ++++++++++
+        // since the user probably just misunderstood how `let else`
+        // and `&&` work together.
+        if let Some((_, hir::Node::Local(local))) = cond_parent
+            && let hir::PatKind::Path(qpath) | hir::PatKind::TupleStruct(qpath, _, _) = &local.pat.kind
+            && let hir::QPath::Resolved(None, path) = qpath
+            && let Some(did) = path.res.opt_def_id()
+                .and_then(|did| self.tcx.opt_parent(did))
+                .and_then(|did| self.tcx.opt_parent(did))
+            && self.tcx.is_diagnostic_item(sym::Option, did)
+        {
+            return false;
+        }
+
+        diag.span_suggestion(
+            expr.span.shrink_to_hi(),
+            "use `Option::is_some` to test if the `Option` has a value",
+            ".is_some()",
+            Applicability::MachineApplicable,
+        );
+
+        true
+    }
+
     /// Suggest wrapping the block in square brackets instead of curly braces
     /// in case the block was mistaken array syntax, e.g. `{ 1 }` -> `[ 1 ]`.
     pub(crate) fn suggest_block_to_brackets(
diff --git a/compiler/rustc_middle/src/mir/interpret/mod.rs b/compiler/rustc_middle/src/mir/interpret/mod.rs
index 5e3dfcbcc49..32ec5855769 100644
--- a/compiler/rustc_middle/src/mir/interpret/mod.rs
+++ b/compiler/rustc_middle/src/mir/interpret/mod.rs
@@ -106,6 +106,7 @@ use rustc_ast::LitKind;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::sync::{HashMapExt, Lock};
 use rustc_data_structures::tiny_list::TinyList;
+use rustc_errors::ErrorGuaranteed;
 use rustc_hir::def_id::DefId;
 use rustc_macros::HashStable;
 use rustc_middle::ty::print::with_no_trimmed_paths;
@@ -176,7 +177,7 @@ pub enum LitToConstError {
     /// This is used for graceful error handling (`delay_span_bug`) in
     /// type checking (`Const::from_anon_const`).
     TypeError,
-    Reported,
+    Reported(ErrorGuaranteed),
 }
 
 #[derive(Copy, Clone, Eq, Hash, Ord, PartialEq, PartialOrd)]
diff --git a/compiler/rustc_middle/src/mir/mod.rs b/compiler/rustc_middle/src/mir/mod.rs
index 5290d5aae46..54f3964d28f 100644
--- a/compiler/rustc_middle/src/mir/mod.rs
+++ b/compiler/rustc_middle/src/mir/mod.rs
@@ -2251,7 +2251,9 @@ impl<'tcx> ConstantKind<'tcx> {
                 match tcx.const_eval_resolve(param_env, uneval, None) {
                     Ok(val) => Self::Val(val, ty),
                     Err(ErrorHandled::TooGeneric | ErrorHandled::Linted) => self,
-                    Err(_) => Self::Ty(tcx.const_error(ty)),
+                    Err(ErrorHandled::Reported(guar)) => {
+                        Self::Ty(tcx.const_error_with_guaranteed(ty, guar))
+                    }
                 }
             }
         }
diff --git a/compiler/rustc_middle/src/ty/consts.rs b/compiler/rustc_middle/src/ty/consts.rs
index 33fdf1a8370..e2e2761501b 100644
--- a/compiler/rustc_middle/src/ty/consts.rs
+++ b/compiler/rustc_middle/src/ty/consts.rs
@@ -2,7 +2,6 @@ use crate::mir::interpret::LitToConstInput;
 use crate::mir::ConstantKind;
 use crate::ty::{self, InternalSubsts, ParamEnv, ParamEnvAnd, Ty, TyCtxt};
 use rustc_data_structures::intern::Interned;
-use rustc_errors::ErrorGuaranteed;
 use rustc_hir as hir;
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_macros::HashStable;
@@ -225,7 +224,7 @@ impl<'tcx> Const<'tcx> {
         if let Some(val) = self.kind().try_eval_for_typeck(tcx, param_env) {
             match val {
                 Ok(val) => Const::from_value(tcx, val, self.ty()),
-                Err(ErrorGuaranteed { .. }) => tcx.const_error(self.ty()),
+                Err(guar) => tcx.const_error_with_guaranteed(self.ty(), guar),
             }
         } else {
             // Either the constant isn't evaluatable or ValTree creation failed.
@@ -240,7 +239,7 @@ impl<'tcx> Const<'tcx> {
         if let Some(val) = self.kind().try_eval_for_mir(tcx, param_env) {
             match val {
                 Ok(const_val) => ConstantKind::from_value(const_val, self.ty()),
-                Err(ErrorGuaranteed { .. }) => ConstantKind::Ty(tcx.const_error(self.ty())),
+                Err(guar) => ConstantKind::Ty(tcx.const_error_with_guaranteed(self.ty(), guar)),
             }
         } else {
             ConstantKind::Ty(self)
diff --git a/compiler/rustc_middle/src/ty/subst.rs b/compiler/rustc_middle/src/ty/subst.rs
index 0660e9b79a7..2bcb2d82484 100644
--- a/compiler/rustc_middle/src/ty/subst.rs
+++ b/compiler/rustc_middle/src/ty/subst.rs
@@ -506,6 +506,9 @@ impl<'tcx, T: TypeVisitable<'tcx>> TypeVisitable<'tcx> for &'tcx ty::List<T> {
     }
 }
 
+/// Similar to [`super::Binder`] except that it tracks early bound generics, i.e. `struct Foo<T>(T)`
+/// needs `T` substituted immediately. This type primarily exists to avoid forgetting to call
+/// `subst`.
 #[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Debug)]
 #[derive(Encodable, Decodable, HashStable)]
 pub struct EarlyBinder<T>(pub T);
diff --git a/compiler/rustc_mir_build/src/build/expr/as_constant.rs b/compiler/rustc_mir_build/src/build/expr/as_constant.rs
index 98df9c3f0e8..7d8a940bde5 100644
--- a/compiler/rustc_mir_build/src/build/expr/as_constant.rs
+++ b/compiler/rustc_mir_build/src/build/expr/as_constant.rs
@@ -9,6 +9,7 @@ use rustc_middle::mir::interpret::{
 use rustc_middle::mir::*;
 use rustc_middle::thir::*;
 use rustc_middle::ty::{self, CanonicalUserTypeAnnotation, TyCtxt};
+use rustc_span::DUMMY_SP;
 use rustc_target::abi::Size;
 
 impl<'a, 'tcx> Builder<'a, 'tcx> {
@@ -26,7 +27,9 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 let literal =
                     match lit_to_mir_constant(tcx, LitToConstInput { lit: &lit.node, ty, neg }) {
                         Ok(c) => c,
-                        Err(LitToConstError::Reported) => ConstantKind::Ty(tcx.const_error(ty)),
+                        Err(LitToConstError::Reported(guar)) => {
+                            ConstantKind::Ty(tcx.const_error_with_guaranteed(ty, guar))
+                        }
                         Err(LitToConstError::TypeError) => {
                             bug!("encountered type error in `lit_to_mir_constant")
                         }
@@ -105,7 +108,15 @@ pub(crate) fn lit_to_mir_constant<'tcx>(
     let LitToConstInput { lit, ty, neg } = lit_input;
     let trunc = |n| {
         let param_ty = ty::ParamEnv::reveal_all().and(ty);
-        let width = tcx.layout_of(param_ty).map_err(|_| LitToConstError::Reported)?.size;
+        let width = tcx
+            .layout_of(param_ty)
+            .map_err(|_| {
+                LitToConstError::Reported(tcx.sess.delay_span_bug(
+                    DUMMY_SP,
+                    format!("couldn't compute width of literal: {:?}", lit_input.lit),
+                ))
+            })?
+            .size;
         trace!("trunc {} with size {} and shift {}", n, width.bits(), 128 - width.bits());
         let result = width.truncate(n);
         trace!("trunc result: {}", result);
@@ -136,12 +147,20 @@ pub(crate) fn lit_to_mir_constant<'tcx>(
         (ast::LitKind::Int(n, _), ty::Uint(_)) | (ast::LitKind::Int(n, _), ty::Int(_)) => {
             trunc(if neg { (*n as i128).overflowing_neg().0 as u128 } else { *n })?
         }
-        (ast::LitKind::Float(n, _), ty::Float(fty)) => {
-            parse_float_into_constval(*n, *fty, neg).ok_or(LitToConstError::Reported)?
-        }
+        (ast::LitKind::Float(n, _), ty::Float(fty)) => parse_float_into_constval(*n, *fty, neg)
+            .ok_or_else(|| {
+                LitToConstError::Reported(tcx.sess.delay_span_bug(
+                    DUMMY_SP,
+                    format!("couldn't parse float literal: {:?}", lit_input.lit),
+                ))
+            })?,
         (ast::LitKind::Bool(b), ty::Bool) => ConstValue::Scalar(Scalar::from_bool(*b)),
         (ast::LitKind::Char(c), ty::Char) => ConstValue::Scalar(Scalar::from_char(*c)),
-        (ast::LitKind::Err, _) => return Err(LitToConstError::Reported),
+        (ast::LitKind::Err, _) => {
+            return Err(LitToConstError::Reported(
+                tcx.sess.delay_span_bug(DUMMY_SP, "encountered LitKind::Err during mir build"),
+            ));
+        }
         _ => return Err(LitToConstError::TypeError),
     };
 
diff --git a/compiler/rustc_mir_build/src/thir/constant.rs b/compiler/rustc_mir_build/src/thir/constant.rs
index f626571b5b2..85e8801bda3 100644
--- a/compiler/rustc_mir_build/src/thir/constant.rs
+++ b/compiler/rustc_mir_build/src/thir/constant.rs
@@ -1,6 +1,7 @@
 use rustc_ast as ast;
 use rustc_middle::mir::interpret::{LitToConstError, LitToConstInput};
 use rustc_middle::ty::{self, ParamEnv, ScalarInt, TyCtxt};
+use rustc_span::DUMMY_SP;
 
 pub(crate) fn lit_to_const<'tcx>(
     tcx: TyCtxt<'tcx>,
@@ -10,7 +11,15 @@ pub(crate) fn lit_to_const<'tcx>(
 
     let trunc = |n| {
         let param_ty = ParamEnv::reveal_all().and(ty);
-        let width = tcx.layout_of(param_ty).map_err(|_| LitToConstError::Reported)?.size;
+        let width = tcx
+            .layout_of(param_ty)
+            .map_err(|_| {
+                LitToConstError::Reported(tcx.sess.delay_span_bug(
+                    DUMMY_SP,
+                    format!("couldn't compute width of literal: {:?}", lit_input.lit),
+                ))
+            })?
+            .size;
         trace!("trunc {} with size {} and shift {}", n, width.bits(), 128 - width.bits());
         let result = width.truncate(n);
         trace!("trunc result: {}", result);
@@ -44,7 +53,11 @@ pub(crate) fn lit_to_const<'tcx>(
         }
         (ast::LitKind::Bool(b), ty::Bool) => ty::ValTree::from_scalar_int((*b).into()),
         (ast::LitKind::Char(c), ty::Char) => ty::ValTree::from_scalar_int((*c).into()),
-        (ast::LitKind::Err, _) => return Err(LitToConstError::Reported),
+        (ast::LitKind::Err, _) => {
+            return Err(LitToConstError::Reported(
+                tcx.sess.delay_span_bug(DUMMY_SP, "encountered LitKind::Err during mir build"),
+            ));
+        }
         _ => return Err(LitToConstError::TypeError),
     };
 
diff --git a/compiler/rustc_mir_build/src/thir/pattern/mod.rs b/compiler/rustc_mir_build/src/thir/pattern/mod.rs
index 2526522a25c..776c748c7e5 100644
--- a/compiler/rustc_mir_build/src/thir/pattern/mod.rs
+++ b/compiler/rustc_mir_build/src/thir/pattern/mod.rs
@@ -614,7 +614,7 @@ impl<'a, 'tcx> PatCtxt<'a, 'tcx> {
             LitToConstInput { lit: &lit.node, ty: self.typeck_results.expr_ty(expr), neg };
         match self.tcx.at(expr.span).lit_to_mir_constant(lit_input) {
             Ok(constant) => self.const_to_pat(constant, expr.hir_id, lit.span, false).kind,
-            Err(LitToConstError::Reported) => PatKind::Wild,
+            Err(LitToConstError::Reported(_)) => PatKind::Wild,
             Err(LitToConstError::TypeError) => bug!("lower_lit: had type error"),
         }
     }
diff --git a/compiler/rustc_mir_transform/src/required_consts.rs b/compiler/rustc_mir_transform/src/required_consts.rs
index cc75947d9dd..0ea8f2ba93f 100644
--- a/compiler/rustc_mir_transform/src/required_consts.rs
+++ b/compiler/rustc_mir_transform/src/required_consts.rs
@@ -17,7 +17,7 @@ impl<'tcx> Visitor<'tcx> for RequiredConstsVisitor<'_, 'tcx> {
         let literal = constant.literal;
         match literal {
             ConstantKind::Ty(c) => match c.kind() {
-                ConstKind::Param(_) => {}
+                ConstKind::Param(_) | ConstKind::Error(_) => {}
                 _ => bug!("only ConstKind::Param should be encountered here, got {:#?}", c),
             },
             ConstantKind::Unevaluated(..) => self.required_consts.push(*constant),
diff --git a/compiler/rustc_resolve/src/late.rs b/compiler/rustc_resolve/src/late.rs
index 6d2ee25df32..ede67813883 100644
--- a/compiler/rustc_resolve/src/late.rs
+++ b/compiler/rustc_resolve/src/late.rs
@@ -527,6 +527,7 @@ struct DiagnosticMetadata<'ast> {
 
     /// Used to detect possible new binding written without `let` and to provide structured suggestion.
     in_assignment: Option<&'ast Expr>,
+    is_assign_rhs: bool,
 
     /// If we are currently in a trait object definition. Used to point at the bounds when
     /// encountering a struct or enum.
@@ -3963,10 +3964,15 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
                 self.resolve_expr(elem, Some(expr));
                 self.visit_expr(idx);
             }
-            ExprKind::Assign(..) => {
-                let old = self.diagnostic_metadata.in_assignment.replace(expr);
-                visit::walk_expr(self, expr);
-                self.diagnostic_metadata.in_assignment = old;
+            ExprKind::Assign(ref lhs, ref rhs, _) => {
+                if !self.diagnostic_metadata.is_assign_rhs {
+                    self.diagnostic_metadata.in_assignment = Some(expr);
+                }
+                self.visit_expr(lhs);
+                self.diagnostic_metadata.is_assign_rhs = true;
+                self.diagnostic_metadata.in_assignment = None;
+                self.visit_expr(rhs);
+                self.diagnostic_metadata.is_assign_rhs = false;
             }
             _ => {
                 visit::walk_expr(self, expr);
diff --git a/compiler/rustc_resolve/src/late/diagnostics.rs b/compiler/rustc_resolve/src/late/diagnostics.rs
index cdc28db13f3..2587a9c4b34 100644
--- a/compiler/rustc_resolve/src/late/diagnostics.rs
+++ b/compiler/rustc_resolve/src/late/diagnostics.rs
@@ -1810,29 +1810,22 @@ impl<'a: 'ast, 'ast> LateResolutionVisitor<'a, '_, 'ast> {
         false
     }
 
-    fn let_binding_suggestion(&self, err: &mut Diagnostic, ident_span: Span) -> bool {
-        // try to give a suggestion for this pattern: `name = 1`, which is common in other languages
-        let mut added_suggestion = false;
-        if let Some(Expr { kind: ExprKind::Assign(lhs, _rhs, _), .. }) = self.diagnostic_metadata.in_assignment &&
+    // try to give a suggestion for this pattern: `name = blah`, which is common in other languages
+    // suggest `let name = blah` to introduce a new binding
+    fn let_binding_suggestion(&mut self, err: &mut Diagnostic, ident_span: Span) -> bool {
+        if let Some(Expr { kind: ExprKind::Assign(lhs, .. ), .. }) = self.diagnostic_metadata.in_assignment &&
             let ast::ExprKind::Path(None, _) = lhs.kind {
-                let sm = self.r.session.source_map();
-                let line_span = sm.span_extend_to_line(ident_span);
-                let ident_name = sm.span_to_snippet(ident_span).unwrap();
-                // HACK(chenyukang): make sure ident_name is at the starting of the line to protect against macros
-                if sm
-                    .span_to_snippet(line_span)
-                    .map_or(false, |s| s.trim().starts_with(&ident_name))
-                {
+                if !ident_span.from_expansion() {
                     err.span_suggestion_verbose(
                         ident_span.shrink_to_lo(),
                         "you might have meant to introduce a new binding",
                         "let ".to_string(),
                         Applicability::MaybeIncorrect,
                     );
-                    added_suggestion = true;
+                    return true;
                 }
             }
-        added_suggestion
+        false
     }
 
     fn find_module(&mut self, def_id: DefId) -> Option<(Module<'a>, ImportSuggestion)> {
diff --git a/compiler/rustc_target/src/abi/call/loongarch.rs b/compiler/rustc_target/src/abi/call/loongarch.rs
new file mode 100644
index 00000000000..d29b479de5d
--- /dev/null
+++ b/compiler/rustc_target/src/abi/call/loongarch.rs
@@ -0,0 +1,342 @@
+use crate::abi::call::{ArgAbi, ArgExtension, CastTarget, FnAbi, PassMode, Reg, RegKind, Uniform};
+use crate::abi::{self, Abi, FieldsShape, HasDataLayout, Size, TyAbiInterface, TyAndLayout};
+use crate::spec::HasTargetSpec;
+
+#[derive(Copy, Clone)]
+enum RegPassKind {
+    Float(Reg),
+    Integer(Reg),
+    Unknown,
+}
+
+#[derive(Copy, Clone)]
+enum FloatConv {
+    FloatPair(Reg, Reg),
+    Float(Reg),
+    MixedPair(Reg, Reg),
+}
+
+#[derive(Copy, Clone)]
+struct CannotUseFpConv;
+
+fn is_loongarch_aggregate<'a, Ty>(arg: &ArgAbi<'a, Ty>) -> bool {
+    match arg.layout.abi {
+        Abi::Vector { .. } => true,
+        _ => arg.layout.is_aggregate(),
+    }
+}
+
+fn should_use_fp_conv_helper<'a, Ty, C>(
+    cx: &C,
+    arg_layout: &TyAndLayout<'a, Ty>,
+    xlen: u64,
+    flen: u64,
+    field1_kind: &mut RegPassKind,
+    field2_kind: &mut RegPassKind,
+) -> Result<(), CannotUseFpConv>
+where
+    Ty: TyAbiInterface<'a, C> + Copy,
+{
+    match arg_layout.abi {
+        Abi::Scalar(scalar) => match scalar.primitive() {
+            abi::Int(..) | abi::Pointer => {
+                if arg_layout.size.bits() > xlen {
+                    return Err(CannotUseFpConv);
+                }
+                match (*field1_kind, *field2_kind) {
+                    (RegPassKind::Unknown, _) => {
+                        *field1_kind = RegPassKind::Integer(Reg {
+                            kind: RegKind::Integer,
+                            size: arg_layout.size,
+                        });
+                    }
+                    (RegPassKind::Float(_), RegPassKind::Unknown) => {
+                        *field2_kind = RegPassKind::Integer(Reg {
+                            kind: RegKind::Integer,
+                            size: arg_layout.size,
+                        });
+                    }
+                    _ => return Err(CannotUseFpConv),
+                }
+            }
+            abi::F32 | abi::F64 => {
+                if arg_layout.size.bits() > flen {
+                    return Err(CannotUseFpConv);
+                }
+                match (*field1_kind, *field2_kind) {
+                    (RegPassKind::Unknown, _) => {
+                        *field1_kind =
+                            RegPassKind::Float(Reg { kind: RegKind::Float, size: arg_layout.size });
+                    }
+                    (_, RegPassKind::Unknown) => {
+                        *field2_kind =
+                            RegPassKind::Float(Reg { kind: RegKind::Float, size: arg_layout.size });
+                    }
+                    _ => return Err(CannotUseFpConv),
+                }
+            }
+        },
+        Abi::Vector { .. } | Abi::Uninhabited => return Err(CannotUseFpConv),
+        Abi::ScalarPair(..) | Abi::Aggregate { .. } => match arg_layout.fields {
+            FieldsShape::Primitive => {
+                unreachable!("aggregates can't have `FieldsShape::Primitive`")
+            }
+            FieldsShape::Union(_) => {
+                if !arg_layout.is_zst() {
+                    return Err(CannotUseFpConv);
+                }
+            }
+            FieldsShape::Array { count, .. } => {
+                for _ in 0..count {
+                    let elem_layout = arg_layout.field(cx, 0);
+                    should_use_fp_conv_helper(
+                        cx,
+                        &elem_layout,
+                        xlen,
+                        flen,
+                        field1_kind,
+                        field2_kind,
+                    )?;
+                }
+            }
+            FieldsShape::Arbitrary { .. } => {
+                match arg_layout.variants {
+                    abi::Variants::Multiple { .. } => return Err(CannotUseFpConv),
+                    abi::Variants::Single { .. } => (),
+                }
+                for i in arg_layout.fields.index_by_increasing_offset() {
+                    let field = arg_layout.field(cx, i);
+                    should_use_fp_conv_helper(cx, &field, xlen, flen, field1_kind, field2_kind)?;
+                }
+            }
+        },
+    }
+    Ok(())
+}
+
+fn should_use_fp_conv<'a, Ty, C>(
+    cx: &C,
+    arg: &TyAndLayout<'a, Ty>,
+    xlen: u64,
+    flen: u64,
+) -> Option<FloatConv>
+where
+    Ty: TyAbiInterface<'a, C> + Copy,
+{
+    let mut field1_kind = RegPassKind::Unknown;
+    let mut field2_kind = RegPassKind::Unknown;
+    if should_use_fp_conv_helper(cx, arg, xlen, flen, &mut field1_kind, &mut field2_kind).is_err() {
+        return None;
+    }
+    match (field1_kind, field2_kind) {
+        (RegPassKind::Integer(l), RegPassKind::Float(r)) => Some(FloatConv::MixedPair(l, r)),
+        (RegPassKind::Float(l), RegPassKind::Integer(r)) => Some(FloatConv::MixedPair(l, r)),
+        (RegPassKind::Float(l), RegPassKind::Float(r)) => Some(FloatConv::FloatPair(l, r)),
+        (RegPassKind::Float(f), RegPassKind::Unknown) => Some(FloatConv::Float(f)),
+        _ => None,
+    }
+}
+
+fn classify_ret<'a, Ty, C>(cx: &C, arg: &mut ArgAbi<'a, Ty>, xlen: u64, flen: u64) -> bool
+where
+    Ty: TyAbiInterface<'a, C> + Copy,
+{
+    if let Some(conv) = should_use_fp_conv(cx, &arg.layout, xlen, flen) {
+        match conv {
+            FloatConv::Float(f) => {
+                arg.cast_to(f);
+            }
+            FloatConv::FloatPair(l, r) => {
+                arg.cast_to(CastTarget::pair(l, r));
+            }
+            FloatConv::MixedPair(l, r) => {
+                arg.cast_to(CastTarget::pair(l, r));
+            }
+        }
+        return false;
+    }
+
+    let total = arg.layout.size;
+
+    // "Scalars wider than 2✕XLEN are passed by reference and are replaced in
+    // the argument list with the address."
+    // "Aggregates larger than 2✕XLEN bits are passed by reference and are
+    // replaced in the argument list with the address, as are C++ aggregates
+    // with nontrivial copy constructors, destructors, or vtables."
+    if total.bits() > 2 * xlen {
+        // We rely on the LLVM backend lowering code to lower passing a scalar larger than 2*XLEN.
+        if is_loongarch_aggregate(arg) {
+            arg.make_indirect();
+        }
+        return true;
+    }
+
+    let xlen_reg = match xlen {
+        32 => Reg::i32(),
+        64 => Reg::i64(),
+        _ => unreachable!("Unsupported XLEN: {}", xlen),
+    };
+    if is_loongarch_aggregate(arg) {
+        if total.bits() <= xlen {
+            arg.cast_to(xlen_reg);
+        } else {
+            arg.cast_to(Uniform { unit: xlen_reg, total: Size::from_bits(xlen * 2) });
+        }
+        return false;
+    }
+
+    // "When passed in registers, scalars narrower than XLEN bits are widened
+    // according to the sign of their type up to 32 bits, then sign-extended to
+    // XLEN bits."
+    extend_integer_width(arg, xlen);
+    false
+}
+
+fn classify_arg<'a, Ty, C>(
+    cx: &C,
+    arg: &mut ArgAbi<'a, Ty>,
+    xlen: u64,
+    flen: u64,
+    is_vararg: bool,
+    avail_gprs: &mut u64,
+    avail_fprs: &mut u64,
+) where
+    Ty: TyAbiInterface<'a, C> + Copy,
+{
+    if !is_vararg {
+        match should_use_fp_conv(cx, &arg.layout, xlen, flen) {
+            Some(FloatConv::Float(f)) if *avail_fprs >= 1 => {
+                *avail_fprs -= 1;
+                arg.cast_to(f);
+                return;
+            }
+            Some(FloatConv::FloatPair(l, r)) if *avail_fprs >= 2 => {
+                *avail_fprs -= 2;
+                arg.cast_to(CastTarget::pair(l, r));
+                return;
+            }
+            Some(FloatConv::MixedPair(l, r)) if *avail_fprs >= 1 && *avail_gprs >= 1 => {
+                *avail_gprs -= 1;
+                *avail_fprs -= 1;
+                arg.cast_to(CastTarget::pair(l, r));
+                return;
+            }
+            _ => (),
+        }
+    }
+
+    let total = arg.layout.size;
+    let align = arg.layout.align.abi.bits();
+
+    // "Scalars wider than 2✕XLEN are passed by reference and are replaced in
+    // the argument list with the address."
+    // "Aggregates larger than 2✕XLEN bits are passed by reference and are
+    // replaced in the argument list with the address, as are C++ aggregates
+    // with nontrivial copy constructors, destructors, or vtables."
+    if total.bits() > 2 * xlen {
+        // We rely on the LLVM backend lowering code to lower passing a scalar larger than 2*XLEN.
+        if is_loongarch_aggregate(arg) {
+            arg.make_indirect();
+        }
+        if *avail_gprs >= 1 {
+            *avail_gprs -= 1;
+        }
+        return;
+    }
+
+    let double_xlen_reg = match xlen {
+        32 => Reg::i64(),
+        64 => Reg::i128(),
+        _ => unreachable!("Unsupported XLEN: {}", xlen),
+    };
+
+    let xlen_reg = match xlen {
+        32 => Reg::i32(),
+        64 => Reg::i64(),
+        _ => unreachable!("Unsupported XLEN: {}", xlen),
+    };
+
+    if total.bits() > xlen {
+        let align_regs = align > xlen;
+        if is_loongarch_aggregate(arg) {
+            arg.cast_to(Uniform {
+                unit: if align_regs { double_xlen_reg } else { xlen_reg },
+                total: Size::from_bits(xlen * 2),
+            });
+        }
+        if align_regs && is_vararg {
+            *avail_gprs -= *avail_gprs % 2;
+        }
+        if *avail_gprs >= 2 {
+            *avail_gprs -= 2;
+        } else {
+            *avail_gprs = 0;
+        }
+        return;
+    } else if is_loongarch_aggregate(arg) {
+        arg.cast_to(xlen_reg);
+        if *avail_gprs >= 1 {
+            *avail_gprs -= 1;
+        }
+        return;
+    }
+
+    // "When passed in registers, scalars narrower than XLEN bits are widened
+    // according to the sign of their type up to 32 bits, then sign-extended to
+    // XLEN bits."
+    if *avail_gprs >= 1 {
+        extend_integer_width(arg, xlen);
+        *avail_gprs -= 1;
+    }
+}
+
+fn extend_integer_width<'a, Ty>(arg: &mut ArgAbi<'a, Ty>, xlen: u64) {
+    if let Abi::Scalar(scalar) = arg.layout.abi {
+        if let abi::Int(i, _) = scalar.primitive() {
+            // 32-bit integers are always sign-extended
+            if i.size().bits() == 32 && xlen > 32 {
+                if let PassMode::Direct(ref mut attrs) = arg.mode {
+                    attrs.ext(ArgExtension::Sext);
+                    return;
+                }
+            }
+        }
+    }
+
+    arg.extend_integer_width_to(xlen);
+}
+
+pub fn compute_abi_info<'a, Ty, C>(cx: &C, fn_abi: &mut FnAbi<'a, Ty>)
+where
+    Ty: TyAbiInterface<'a, C> + Copy,
+    C: HasDataLayout + HasTargetSpec,
+{
+    let xlen = cx.data_layout().pointer_size.bits();
+    let flen = match &cx.target_spec().llvm_abiname[..] {
+        "ilp32f" | "lp64f" => 32,
+        "ilp32d" | "lp64d" => 64,
+        _ => 0,
+    };
+
+    let mut avail_gprs = 8;
+    let mut avail_fprs = 8;
+
+    if !fn_abi.ret.is_ignore() && classify_ret(cx, &mut fn_abi.ret, xlen, flen) {
+        avail_gprs -= 1;
+    }
+
+    for (i, arg) in fn_abi.args.iter_mut().enumerate() {
+        if arg.is_ignore() {
+            continue;
+        }
+        classify_arg(
+            cx,
+            arg,
+            xlen,
+            flen,
+            i >= fn_abi.fixed_count as usize,
+            &mut avail_gprs,
+            &mut avail_fprs,
+        );
+    }
+}
diff --git a/compiler/rustc_target/src/abi/call/mod.rs b/compiler/rustc_target/src/abi/call/mod.rs
index 9e5f0e4d158..c5a6f9893b6 100644
--- a/compiler/rustc_target/src/abi/call/mod.rs
+++ b/compiler/rustc_target/src/abi/call/mod.rs
@@ -10,6 +10,7 @@ mod arm;
 mod avr;
 mod bpf;
 mod hexagon;
+mod loongarch;
 mod m68k;
 mod mips;
 mod mips64;
@@ -696,6 +697,7 @@ impl<'a, Ty> FnAbi<'a, Ty> {
             "amdgpu" => amdgpu::compute_abi_info(cx, self),
             "arm" => arm::compute_abi_info(cx, self),
             "avr" => avr::compute_abi_info(self),
+            "loongarch64" => loongarch::compute_abi_info(cx, self),
             "m68k" => m68k::compute_abi_info(self),
             "mips" => mips::compute_abi_info(cx, self),
             "mips64" => mips64::compute_abi_info(cx, self),
diff --git a/compiler/rustc_target/src/spec/mod.rs b/compiler/rustc_target/src/spec/mod.rs
index 1bcb02ecb30..77d9a092003 100644
--- a/compiler/rustc_target/src/spec/mod.rs
+++ b/compiler/rustc_target/src/spec/mod.rs
@@ -1911,6 +1911,7 @@ impl Target {
                 Abi::Stdcall { unwind }
             }
             Abi::System { unwind } => Abi::C { unwind },
+            Abi::EfiApi if self.arch == "arm" => Abi::Aapcs { unwind: false },
             Abi::EfiApi if self.arch == "x86_64" => Abi::Win64 { unwind: false },
             Abi::EfiApi => Abi::C { unwind: false },
 
diff --git a/compiler/rustc_trait_selection/src/autoderef.rs b/compiler/rustc_trait_selection/src/autoderef.rs
index 46ee2f35976..54c738d8389 100644
--- a/compiler/rustc_trait_selection/src/autoderef.rs
+++ b/compiler/rustc_trait_selection/src/autoderef.rs
@@ -1,6 +1,6 @@
 use crate::errors::AutoDerefReachedRecursionLimit;
 use crate::traits::query::evaluate_obligation::InferCtxtExt;
-use crate::traits::{self, TraitEngine};
+use crate::traits::{self, TraitEngine, TraitEngineExt};
 use rustc_hir as hir;
 use rustc_infer::infer::InferCtxt;
 use rustc_middle::ty::{self, TraitRef, Ty, TyCtxt};
@@ -139,7 +139,7 @@ impl<'a, 'tcx> Autoderef<'a, 'tcx> {
             return None;
         }
 
-        let mut fulfillcx = traits::FulfillmentContext::new_in_snapshot();
+        let mut fulfillcx = <dyn TraitEngine<'tcx>>::new_in_snapshot(tcx);
         let normalized_ty = fulfillcx.normalize_projection_type(
             &self.infcx,
             self.param_env,
diff --git a/compiler/rustc_trait_selection/src/traits/chalk_fulfill.rs b/compiler/rustc_trait_selection/src/traits/chalk_fulfill.rs
index d32a990f182..8f9d5eaac9d 100644
--- a/compiler/rustc_trait_selection/src/traits/chalk_fulfill.rs
+++ b/compiler/rustc_trait_selection/src/traits/chalk_fulfill.rs
@@ -19,7 +19,7 @@ pub struct FulfillmentContext<'tcx> {
 }
 
 impl FulfillmentContext<'_> {
-    pub(crate) fn new() -> Self {
+    pub(super) fn new() -> Self {
         FulfillmentContext {
             obligations: FxIndexSet::default(),
             relationships: FxHashMap::default(),
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
index e64586407c9..98c13ffdafb 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
@@ -3,13 +3,14 @@ pub mod on_unimplemented;
 pub mod suggestions;
 
 use super::{
-    FulfillmentContext, FulfillmentError, FulfillmentErrorCode, MismatchedProjectionTypes,
-    Obligation, ObligationCause, ObligationCauseCode, OutputTypeParameterMismatch, Overflow,
-    PredicateObligation, SelectionContext, SelectionError, TraitNotObjectSafe,
+    FulfillmentError, FulfillmentErrorCode, MismatchedProjectionTypes, Obligation, ObligationCause,
+    ObligationCauseCode, OutputTypeParameterMismatch, Overflow, PredicateObligation,
+    SelectionContext, SelectionError, TraitNotObjectSafe,
 };
 use crate::infer::error_reporting::{TyCategory, TypeAnnotationNeeded as ErrorCode};
 use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use crate::infer::{self, InferCtxt, TyCtxtInferExt};
+use crate::traits::engine::TraitEngineExt as _;
 use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
 use crate::traits::query::normalize::AtExt as _;
 use crate::traits::specialize::to_pretty_impl_header;
@@ -352,7 +353,7 @@ impl<'tcx> InferCtxtExt<'tcx> for InferCtxt<'tcx> {
                     })
                     .to_predicate(self.tcx),
                 );
-                let mut fulfill_cx = FulfillmentContext::new_in_snapshot();
+                let mut fulfill_cx = <dyn TraitEngine<'tcx>>::new_in_snapshot(self.tcx);
                 fulfill_cx.register_predicate_obligation(self, obligation);
                 if fulfill_cx.select_all_or_error(self).is_empty() {
                     return Ok((
diff --git a/compiler/rustc_trait_selection/src/traits/fulfill.rs b/compiler/rustc_trait_selection/src/traits/fulfill.rs
index a417e1440b9..b486c07f354 100644
--- a/compiler/rustc_trait_selection/src/traits/fulfill.rs
+++ b/compiler/rustc_trait_selection/src/traits/fulfill.rs
@@ -85,7 +85,7 @@ static_assert_size!(PendingPredicateObligation<'_>, 72);
 
 impl<'a, 'tcx> FulfillmentContext<'tcx> {
     /// Creates a new fulfillment context.
-    pub fn new() -> FulfillmentContext<'tcx> {
+    pub(super) fn new() -> FulfillmentContext<'tcx> {
         FulfillmentContext {
             predicates: ObligationForest::new(),
             relationships: FxHashMap::default(),
@@ -93,7 +93,7 @@ impl<'a, 'tcx> FulfillmentContext<'tcx> {
         }
     }
 
-    pub fn new_in_snapshot() -> FulfillmentContext<'tcx> {
+    pub(super) fn new_in_snapshot() -> FulfillmentContext<'tcx> {
         FulfillmentContext {
             predicates: ObligationForest::new(),
             relationships: FxHashMap::default(),
diff --git a/compiler/rustc_ty_utils/src/consts.rs b/compiler/rustc_ty_utils/src/consts.rs
index 3cef47c0f8b..cb41c4f94e2 100644
--- a/compiler/rustc_ty_utils/src/consts.rs
+++ b/compiler/rustc_ty_utils/src/consts.rs
@@ -235,7 +235,9 @@ impl<'a, 'tcx> AbstractConstBuilder<'a, 'tcx> {
                     neg,
                 }) {
                     Ok(c) => c,
-                    Err(LitToConstError::Reported) => self.tcx.const_error(node.ty),
+                    Err(LitToConstError::Reported(guar)) => {
+                        self.tcx.const_error_with_guaranteed(node.ty, guar)
+                    }
                     Err(LitToConstError::TypeError) => {
                         bug!("encountered type error in lit_to_const")
                     }
diff --git a/library/core/src/hash/mod.rs b/library/core/src/hash/mod.rs
index aa13435e680..c755afa39eb 100644
--- a/library/core/src/hash/mod.rs
+++ b/library/core/src/hash/mod.rs
@@ -86,7 +86,8 @@
 #![stable(feature = "rust1", since = "1.0.0")]
 
 use crate::fmt;
-use crate::marker;
+use crate::intrinsics::const_eval_select;
+use crate::marker::{self, Destruct};
 
 #[stable(feature = "rust1", since = "1.0.0")]
 #[allow(deprecated)]
@@ -183,6 +184,7 @@ mod sip;
 /// [impl]: ../../std/primitive.str.html#impl-Hash-for-str
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_diagnostic_item = "Hash"]
+#[const_trait]
 pub trait Hash {
     /// Feeds this value into the given [`Hasher`].
     ///
@@ -234,13 +236,25 @@ pub trait Hash {
     /// [`hash`]: Hash::hash
     /// [`hash_slice`]: Hash::hash_slice
     #[stable(feature = "hash_slice", since = "1.3.0")]
-    fn hash_slice<H: Hasher>(data: &[Self], state: &mut H)
+    fn hash_slice<H: ~const Hasher>(data: &[Self], state: &mut H)
     where
         Self: Sized,
     {
-        for piece in data {
-            piece.hash(state);
+        //FIXME(const_trait_impl): revert to only a for loop
+        fn rt<T: Hash, H: Hasher>(data: &[T], state: &mut H) {
+            for piece in data {
+                piece.hash(state)
+            }
+        }
+        const fn ct<T: ~const Hash, H: ~const Hasher>(data: &[T], state: &mut H) {
+            let mut i = 0;
+            while i < data.len() {
+                data[i].hash(state);
+                i += 1;
+            }
         }
+        // SAFETY: same behavior, CT just uses while instead of for
+        unsafe { const_eval_select((data, state), ct, rt) };
     }
 }
 
@@ -313,6 +327,7 @@ pub use macros::Hash;
 /// [`write_u8`]: Hasher::write_u8
 /// [`write_u32`]: Hasher::write_u32
 #[stable(feature = "rust1", since = "1.0.0")]
+#[const_trait]
 pub trait Hasher {
     /// Returns the hash value for the values written so far.
     ///
@@ -558,7 +573,8 @@ pub trait Hasher {
 }
 
 #[stable(feature = "indirect_hasher_impl", since = "1.22.0")]
-impl<H: Hasher + ?Sized> Hasher for &mut H {
+#[rustc_const_unstable(feature = "const_hash", issue = "104061")]
+impl<H: ~const Hasher + ?Sized> const Hasher for &mut H {
     fn finish(&self) -> u64 {
         (**self).finish()
     }
@@ -638,6 +654,7 @@ impl<H: Hasher + ?Sized> Hasher for &mut H {
 /// [`build_hasher`]: BuildHasher::build_hasher
 /// [`HashMap`]: ../../std/collections/struct.HashMap.html
 #[stable(since = "1.7.0", feature = "build_hasher")]
+#[const_trait]
 pub trait BuildHasher {
     /// Type of the hasher that will be created.
     #[stable(since = "1.7.0", feature = "build_hasher")]
@@ -698,9 +715,10 @@ pub trait BuildHasher {
     /// );
     /// ```
     #[unstable(feature = "build_hasher_simple_hash_one", issue = "86161")]
-    fn hash_one<T: Hash>(&self, x: T) -> u64
+    fn hash_one<T: ~const Hash + ~const Destruct>(&self, x: T) -> u64
     where
         Self: Sized,
+        Self::Hasher: ~const Hasher + ~const Destruct,
     {
         let mut hasher = self.build_hasher();
         x.hash(&mut hasher);
@@ -764,7 +782,8 @@ impl<H> fmt::Debug for BuildHasherDefault<H> {
 }
 
 #[stable(since = "1.7.0", feature = "build_hasher")]
-impl<H: Default + Hasher> BuildHasher for BuildHasherDefault<H> {
+#[rustc_const_unstable(feature = "const_hash", issue = "104061")]
+impl<H: ~const Default + Hasher> const BuildHasher for BuildHasherDefault<H> {
     type Hasher = H;
 
     fn build_hasher(&self) -> H {
@@ -806,14 +825,15 @@ mod impls {
     macro_rules! impl_write {
         ($(($ty:ident, $meth:ident),)*) => {$(
             #[stable(feature = "rust1", since = "1.0.0")]
-            impl Hash for $ty {
+            #[rustc_const_unstable(feature = "const_hash", issue = "104061")]
+            impl const Hash for $ty {
                 #[inline]
-                fn hash<H: Hasher>(&self, state: &mut H) {
+                fn hash<H: ~const Hasher>(&self, state: &mut H) {
                     state.$meth(*self)
                 }
 
                 #[inline]
-                fn hash_slice<H: Hasher>(data: &[$ty], state: &mut H) {
+                fn hash_slice<H: ~const Hasher>(data: &[$ty], state: &mut H) {
                     let newlen = data.len() * mem::size_of::<$ty>();
                     let ptr = data.as_ptr() as *const u8;
                     // SAFETY: `ptr` is valid and aligned, as this macro is only used
@@ -842,33 +862,37 @@ mod impls {
     }
 
     #[stable(feature = "rust1", since = "1.0.0")]
-    impl Hash for bool {
+    #[rustc_const_unstable(feature = "const_hash", issue = "104061")]
+    impl const Hash for bool {
         #[inline]
-        fn hash<H: Hasher>(&self, state: &mut H) {
+        fn hash<H: ~const Hasher>(&self, state: &mut H) {
             state.write_u8(*self as u8)
         }
     }
 
     #[stable(feature = "rust1", since = "1.0.0")]
-    impl Hash for char {
+    #[rustc_const_unstable(feature = "const_hash", issue = "104061")]
+    impl const Hash for char {
         #[inline]
-        fn hash<H: Hasher>(&self, state: &mut H) {
+        fn hash<H: ~const Hasher>(&self, state: &mut H) {
             state.write_u32(*self as u32)
         }
     }
 
     #[stable(feature = "rust1", since = "1.0.0")]
-    impl Hash for str {
+    #[rustc_const_unstable(feature = "const_hash", issue = "104061")]
+    impl const Hash for str {
         #[inline]
-        fn hash<H: Hasher>(&self, state: &mut H) {
+        fn hash<H: ~const Hasher>(&self, state: &mut H) {
             state.write_str(self);
         }
     }
 
     #[stable(feature = "never_hash", since = "1.29.0")]
-    impl Hash for ! {
+    #[rustc_const_unstable(feature = "const_hash", issue = "104061")]
+    impl const Hash for ! {
         #[inline]
-        fn hash<H: Hasher>(&self, _: &mut H) {
+        fn hash<H: ~const Hasher>(&self, _: &mut H) {
             *self
         }
     }
@@ -876,9 +900,10 @@ mod impls {
     macro_rules! impl_hash_tuple {
         () => (
             #[stable(feature = "rust1", since = "1.0.0")]
-            impl Hash for () {
+            #[rustc_const_unstable(feature = "const_hash", issue = "104061")]
+            impl const Hash for () {
                 #[inline]
-                fn hash<H: Hasher>(&self, _state: &mut H) {}
+                fn hash<H: ~const Hasher>(&self, _state: &mut H) {}
             }
         );
 
@@ -886,10 +911,11 @@ mod impls {
             maybe_tuple_doc! {
                 $($name)+ @
                 #[stable(feature = "rust1", since = "1.0.0")]
-                impl<$($name: Hash),+> Hash for ($($name,)+) where last_type!($($name,)+): ?Sized {
+                #[rustc_const_unstable(feature = "const_hash", issue = "104061")]
+                impl<$($name: ~const Hash),+> const Hash for ($($name,)+) where last_type!($($name,)+): ?Sized {
                     #[allow(non_snake_case)]
                     #[inline]
-                    fn hash<S: Hasher>(&self, state: &mut S) {
+                    fn hash<S: ~const Hasher>(&self, state: &mut S) {
                         let ($(ref $name,)+) = *self;
                         $($name.hash(state);)+
                     }
@@ -932,24 +958,27 @@ mod impls {
     impl_hash_tuple! { T B C D E F G H I J K L }
 
     #[stable(feature = "rust1", since = "1.0.0")]
-    impl<T: Hash> Hash for [T] {
+    #[rustc_const_unstable(feature = "const_hash", issue = "104061")]
+    impl<T: ~const Hash> const Hash for [T] {
         #[inline]
-        fn hash<H: Hasher>(&self, state: &mut H) {
+        fn hash<H: ~const Hasher>(&self, state: &mut H) {
             state.write_length_prefix(self.len());
             Hash::hash_slice(self, state)
         }
     }
 
     #[stable(feature = "rust1", since = "1.0.0")]
-    impl<T: ?Sized + Hash> Hash for &T {
+    #[rustc_const_unstable(feature = "const_hash", issue = "104061")]
+    impl<T: ?Sized + ~const Hash> const Hash for &T {
         #[inline]
-        fn hash<H: Hasher>(&self, state: &mut H) {
+        fn hash<H: ~const Hasher>(&self, state: &mut H) {
             (**self).hash(state);
         }
     }
 
     #[stable(feature = "rust1", since = "1.0.0")]
-    impl<T: ?Sized + Hash> Hash for &mut T {
+    #[rustc_const_unstable(feature = "const_hash", issue = "104061")]
+    impl<T: ?Sized + ~const Hash> const Hash for &mut T {
         #[inline]
         fn hash<H: Hasher>(&self, state: &mut H) {
             (**self).hash(state);
diff --git a/library/core/src/hash/sip.rs b/library/core/src/hash/sip.rs
index 81bf1dfdf45..7f8287bf56f 100644
--- a/library/core/src/hash/sip.rs
+++ b/library/core/src/hash/sip.rs
@@ -118,7 +118,7 @@ macro_rules! load_int_le {
 /// Safety: this performs unchecked indexing of `buf` at `start..start+len`, so
 /// that must be in-bounds.
 #[inline]
-unsafe fn u8to64_le(buf: &[u8], start: usize, len: usize) -> u64 {
+const unsafe fn u8to64_le(buf: &[u8], start: usize, len: usize) -> u64 {
     debug_assert!(len < 8);
     let mut i = 0; // current byte index (from LSB) in the output u64
     let mut out = 0;
@@ -138,7 +138,8 @@ unsafe fn u8to64_le(buf: &[u8], start: usize, len: usize) -> u64 {
         out |= (unsafe { *buf.get_unchecked(start + i) } as u64) << (i * 8);
         i += 1;
     }
-    debug_assert_eq!(i, len);
+    //FIXME(fee1-dead): use debug_assert_eq
+    debug_assert!(i == len);
     out
 }
 
@@ -150,8 +151,9 @@ impl SipHasher {
         since = "1.13.0",
         note = "use `std::collections::hash_map::DefaultHasher` instead"
     )]
+    #[rustc_const_unstable(feature = "const_hash", issue = "104061")]
     #[must_use]
-    pub fn new() -> SipHasher {
+    pub const fn new() -> SipHasher {
         SipHasher::new_with_keys(0, 0)
     }
 
@@ -162,8 +164,9 @@ impl SipHasher {
         since = "1.13.0",
         note = "use `std::collections::hash_map::DefaultHasher` instead"
     )]
+    #[rustc_const_unstable(feature = "const_hash", issue = "104061")]
     #[must_use]
-    pub fn new_with_keys(key0: u64, key1: u64) -> SipHasher {
+    pub const fn new_with_keys(key0: u64, key1: u64) -> SipHasher {
         SipHasher(SipHasher24 { hasher: Hasher::new_with_keys(key0, key1) })
     }
 }
@@ -176,7 +179,8 @@ impl SipHasher13 {
         since = "1.13.0",
         note = "use `std::collections::hash_map::DefaultHasher` instead"
     )]
-    pub fn new() -> SipHasher13 {
+    #[rustc_const_unstable(feature = "const_hash", issue = "104061")]
+    pub const fn new() -> SipHasher13 {
         SipHasher13::new_with_keys(0, 0)
     }
 
@@ -187,14 +191,15 @@ impl SipHasher13 {
         since = "1.13.0",
         note = "use `std::collections::hash_map::DefaultHasher` instead"
     )]
-    pub fn new_with_keys(key0: u64, key1: u64) -> SipHasher13 {
+    #[rustc_const_unstable(feature = "const_hash", issue = "104061")]
+    pub const fn new_with_keys(key0: u64, key1: u64) -> SipHasher13 {
         SipHasher13 { hasher: Hasher::new_with_keys(key0, key1) }
     }
 }
 
 impl<S: Sip> Hasher<S> {
     #[inline]
-    fn new_with_keys(key0: u64, key1: u64) -> Hasher<S> {
+    const fn new_with_keys(key0: u64, key1: u64) -> Hasher<S> {
         let mut state = Hasher {
             k0: key0,
             k1: key1,
@@ -209,7 +214,7 @@ impl<S: Sip> Hasher<S> {
     }
 
     #[inline]
-    fn reset(&mut self) {
+    const fn reset(&mut self) {
         self.length = 0;
         self.state.v0 = self.k0 ^ 0x736f6d6570736575;
         self.state.v1 = self.k1 ^ 0x646f72616e646f6d;
@@ -220,7 +225,8 @@ impl<S: Sip> Hasher<S> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl super::Hasher for SipHasher {
+#[rustc_const_unstable(feature = "const_hash", issue = "104061")]
+impl const super::Hasher for SipHasher {
     #[inline]
     fn write(&mut self, msg: &[u8]) {
         self.0.hasher.write(msg)
@@ -238,7 +244,8 @@ impl super::Hasher for SipHasher {
 }
 
 #[unstable(feature = "hashmap_internals", issue = "none")]
-impl super::Hasher for SipHasher13 {
+#[rustc_const_unstable(feature = "const_hash", issue = "104061")]
+impl const super::Hasher for SipHasher13 {
     #[inline]
     fn write(&mut self, msg: &[u8]) {
         self.hasher.write(msg)
@@ -255,7 +262,7 @@ impl super::Hasher for SipHasher13 {
     }
 }
 
-impl<S: Sip> super::Hasher for Hasher<S> {
+impl<S: ~const Sip> const super::Hasher for Hasher<S> {
     // Note: no integer hashing methods (`write_u*`, `write_i*`) are defined
     // for this type. We could add them, copy the `short_write` implementation
     // in librustc_data_structures/sip128.rs, and add `write_u*`/`write_i*`
@@ -335,7 +342,7 @@ impl<S: Sip> super::Hasher for Hasher<S> {
     }
 }
 
-impl<S: Sip> Clone for Hasher<S> {
+impl<S: Sip> const Clone for Hasher<S> {
     #[inline]
     fn clone(&self) -> Hasher<S> {
         Hasher {
@@ -359,6 +366,7 @@ impl<S: Sip> Default for Hasher<S> {
 }
 
 #[doc(hidden)]
+#[const_trait]
 trait Sip {
     fn c_rounds(_: &mut State);
     fn d_rounds(_: &mut State);
@@ -367,7 +375,7 @@ trait Sip {
 #[derive(Debug, Clone, Default)]
 struct Sip13Rounds;
 
-impl Sip for Sip13Rounds {
+impl const Sip for Sip13Rounds {
     #[inline]
     fn c_rounds(state: &mut State) {
         compress!(state);
@@ -384,7 +392,7 @@ impl Sip for Sip13Rounds {
 #[derive(Debug, Clone, Default)]
 struct Sip24Rounds;
 
-impl Sip for Sip24Rounds {
+impl const Sip for Sip24Rounds {
     #[inline]
     fn c_rounds(state: &mut State) {
         compress!(state);
diff --git a/library/core/src/lib.rs b/library/core/src/lib.rs
index 659409557c9..5dc7427bee0 100644
--- a/library/core/src/lib.rs
+++ b/library/core/src/lib.rs
@@ -112,6 +112,7 @@
 #![feature(const_float_bits_conv)]
 #![feature(const_float_classify)]
 #![feature(const_fmt_arguments_new)]
+#![feature(const_hash)]
 #![feature(const_heap)]
 #![feature(const_convert)]
 #![feature(const_index_range_slice_index)]
diff --git a/library/core/tests/hash/mod.rs b/library/core/tests/hash/mod.rs
index f7934d062a3..267245f05dc 100644
--- a/library/core/tests/hash/mod.rs
+++ b/library/core/tests/hash/mod.rs
@@ -9,16 +9,19 @@ struct MyHasher {
     hash: u64,
 }
 
-impl Default for MyHasher {
+impl const Default for MyHasher {
     fn default() -> MyHasher {
         MyHasher { hash: 0 }
     }
 }
 
-impl Hasher for MyHasher {
+impl const Hasher for MyHasher {
     fn write(&mut self, buf: &[u8]) {
-        for byte in buf {
-            self.hash += *byte as u64;
+        // FIXME(const_trait_impl): change to for loop
+        let mut i = 0;
+        while i < buf.len() {
+            self.hash += buf[i] as u64;
+            i += 1;
         }
     }
     fn write_str(&mut self, s: &str) {
@@ -32,12 +35,25 @@ impl Hasher for MyHasher {
 
 #[test]
 fn test_writer_hasher() {
-    fn hash<T: Hash>(t: &T) -> u64 {
+    const fn hash<T: ~const Hash>(t: &T) -> u64 {
         let mut s = MyHasher { hash: 0 };
         t.hash(&mut s);
         s.finish()
     }
 
+    const {
+        // FIXME(fee1-dead): assert_eq
+        assert!(hash(&()) == 0);
+        assert!(hash(&5_u8) == 5);
+        assert!(hash(&5_u16) == 5);
+        assert!(hash(&5_u32) == 5);
+
+        assert!(hash(&'a') == 97);
+
+        let s: &str = "a";
+        assert!(hash(&s) == 97 + 0xFF);
+    };
+
     assert_eq!(hash(&()), 0);
 
     assert_eq!(hash(&5_u8), 5);
@@ -97,7 +113,7 @@ struct CustomHasher {
     output: u64,
 }
 
-impl Hasher for CustomHasher {
+impl const Hasher for CustomHasher {
     fn finish(&self) -> u64 {
         self.output
     }
@@ -109,27 +125,29 @@ impl Hasher for CustomHasher {
     }
 }
 
-impl Default for CustomHasher {
+impl const Default for CustomHasher {
     fn default() -> CustomHasher {
         CustomHasher { output: 0 }
     }
 }
 
-impl Hash for Custom {
-    fn hash<H: Hasher>(&self, state: &mut H) {
+impl const Hash for Custom {
+    fn hash<H: ~const Hasher>(&self, state: &mut H) {
         state.write_u64(self.hash);
     }
 }
 
 #[test]
 fn test_custom_state() {
-    fn hash<T: Hash>(t: &T) -> u64 {
+    const fn hash<T: ~const Hash>(t: &T) -> u64 {
         let mut c = CustomHasher { output: 0 };
         t.hash(&mut c);
         c.finish()
     }
 
     assert_eq!(hash(&Custom { hash: 5 }), 5);
+
+    const { assert!(hash(&Custom { hash: 6 }) == 6) };
 }
 
 // FIXME: Instantiated functions with i128 in the signature is not supported in Emscripten.
diff --git a/library/core/tests/hash/sip.rs b/library/core/tests/hash/sip.rs
index 877d0841830..3abf6efcfa9 100644
--- a/library/core/tests/hash/sip.rs
+++ b/library/core/tests/hash/sip.rs
@@ -8,7 +8,6 @@ use core::{mem, slice};
 struct Bytes<'a>(&'a [u8]);
 
 impl<'a> Hash for Bytes<'a> {
-    #[allow(unused_must_use)]
     fn hash<H: Hasher>(&self, state: &mut H) {
         let Bytes(v) = *self;
         state.write(v);
@@ -25,6 +24,20 @@ fn hash<T: Hash>(x: &T) -> u64 {
 }
 
 #[test]
+const fn test_const_sip() {
+    let val1 = 0x45;
+    let val2 = 0xfeed;
+
+    const fn const_hash<T: ~const Hash>(x: &T) -> u64 {
+        let mut st = SipHasher::new();
+        x.hash(&mut st);
+        st.finish()
+    }
+
+    assert!(const_hash(&(val1)) != const_hash(&(val2)));
+}
+
+#[test]
 #[allow(unused_must_use)]
 fn test_siphash_1_3() {
     let vecs: [[u8; 8]; 64] = [
diff --git a/library/core/tests/lib.rs b/library/core/tests/lib.rs
index a7db2a02bd7..61d31b34487 100644
--- a/library/core/tests/lib.rs
+++ b/library/core/tests/lib.rs
@@ -11,6 +11,7 @@
 #![feature(const_caller_location)]
 #![feature(const_cell_into_inner)]
 #![feature(const_convert)]
+#![feature(const_hash)]
 #![feature(const_heap)]
 #![feature(const_maybe_uninit_as_mut_ptr)]
 #![feature(const_maybe_uninit_assume_init_read)]
diff --git a/library/std/src/collections/hash/map.rs b/library/std/src/collections/hash/map.rs
index 708edc5de47..df490358827 100644
--- a/library/std/src/collections/hash/map.rs
+++ b/library/std/src/collections/hash/map.rs
@@ -3161,14 +3161,16 @@ impl DefaultHasher {
     #[stable(feature = "hashmap_default_hasher", since = "1.13.0")]
     #[inline]
     #[allow(deprecated)]
+    #[rustc_const_unstable(feature = "const_hash", issue = "104061")]
     #[must_use]
-    pub fn new() -> DefaultHasher {
+    pub const fn new() -> DefaultHasher {
         DefaultHasher(SipHasher13::new_with_keys(0, 0))
     }
 }
 
 #[stable(feature = "hashmap_default_hasher", since = "1.13.0")]
-impl Default for DefaultHasher {
+#[rustc_const_unstable(feature = "const_hash", issue = "104061")]
+impl const Default for DefaultHasher {
     /// Creates a new `DefaultHasher` using [`new`].
     /// See its documentation for more.
     ///
@@ -3180,7 +3182,8 @@ impl Default for DefaultHasher {
 }
 
 #[stable(feature = "hashmap_default_hasher", since = "1.13.0")]
-impl Hasher for DefaultHasher {
+#[rustc_const_unstable(feature = "const_hash", issue = "104061")]
+impl const Hasher for DefaultHasher {
     // The underlying `SipHasher13` doesn't override the other
     // `write_*` methods, so it's ok not to forward them here.
 
diff --git a/library/std/src/lib.rs b/library/std/src/lib.rs
index 385585dada8..9334c833bb6 100644
--- a/library/std/src/lib.rs
+++ b/library/std/src/lib.rs
@@ -352,6 +352,7 @@
 //
 // Only for const-ness:
 #![feature(const_collections_with_hasher)]
+#![feature(const_hash)]
 #![feature(const_io_structs)]
 #![feature(const_ip)]
 #![feature(const_ipv4)]
diff --git a/src/test/codegen/abi-efiapi.rs b/src/test/codegen/abi-efiapi.rs
index b4fda5f8c84..9061d7432a3 100644
--- a/src/test/codegen/abi-efiapi.rs
+++ b/src/test/codegen/abi-efiapi.rs
@@ -27,7 +27,7 @@ trait Copy { }
 //x86_64: define win64cc void @has_efiapi
 //i686: define void @has_efiapi
 //aarch64: define dso_local void @has_efiapi
-//arm: define dso_local void @has_efiapi
+//arm: define dso_local arm_aapcscc void @has_efiapi
 //riscv: define dso_local void @has_efiapi
 #[no_mangle]
 pub extern "efiapi" fn has_efiapi() {}
diff --git a/src/test/ui/chalkify/trait-objects.stderr b/src/test/ui/chalkify/trait-objects.stderr
index 098bd2d3226..422d39742eb 100644
--- a/src/test/ui/chalkify/trait-objects.stderr
+++ b/src/test/ui/chalkify/trait-objects.stderr
@@ -22,6 +22,10 @@ LL |     f(2);
    |     ^^^^ expected an `Fn<(i32,)>` closure, found `dyn Fn(i32) -> i32`
    |
    = help: the trait `Fn<(i32,)>` is not implemented for `dyn Fn(i32) -> i32`
+help: consider introducing a `where` clause, but there might be an alternative better way to express this requirement
+   |
+LL | fn main() where dyn Fn(i32) -> i32: Fn<(i32,)> {
+   |           ++++++++++++++++++++++++++++++++++++
 
 error: aborting due to 3 previous errors
 
diff --git a/src/test/ui/consts/invalid-const-in-body.rs b/src/test/ui/consts/invalid-const-in-body.rs
new file mode 100644
index 00000000000..f0fa3bb7bd1
--- /dev/null
+++ b/src/test/ui/consts/invalid-const-in-body.rs
@@ -0,0 +1,6 @@
+fn f() -> impl Sized {
+    2.0E
+    //~^ ERROR expected at least one digit in exponent
+}
+
+fn main() {}
diff --git a/src/test/ui/consts/invalid-const-in-body.stderr b/src/test/ui/consts/invalid-const-in-body.stderr
new file mode 100644
index 00000000000..3be65835946
--- /dev/null
+++ b/src/test/ui/consts/invalid-const-in-body.stderr
@@ -0,0 +1,8 @@
+error: expected at least one digit in exponent
+  --> $DIR/invalid-const-in-body.rs:2:5
+   |
+LL |     2.0E
+   |     ^^^^
+
+error: aborting due to previous error
+
diff --git a/src/test/ui/suggestions/issue-104086-suggest-let.rs b/src/test/ui/suggestions/issue-104086-suggest-let.rs
new file mode 100644
index 00000000000..d22ad27d0e0
--- /dev/null
+++ b/src/test/ui/suggestions/issue-104086-suggest-let.rs
@@ -0,0 +1,30 @@
+fn main() {
+    x = x = x;
+    //~^ ERROR cannot find value `x` in this scope
+    //~| ERROR cannot find value `x` in this scope
+    //~| ERROR cannot find value `x` in this scope
+
+    x = y = y = y;
+    //~^ ERROR cannot find value `y` in this scope
+    //~| ERROR cannot find value `y` in this scope
+    //~| ERROR cannot find value `y` in this scope
+    //~| ERROR cannot find value `x` in this scope
+
+    x = y = y;
+    //~^ ERROR cannot find value `x` in this scope
+    //~| ERROR cannot find value `y` in this scope
+    //~| ERROR cannot find value `y` in this scope
+
+    x = x = y;
+    //~^ ERROR cannot find value `x` in this scope
+    //~| ERROR cannot find value `x` in this scope
+    //~| ERROR cannot find value `y` in this scope
+
+    x = x; // will suggest add `let`
+    //~^ ERROR cannot find value `x` in this scope
+    //~| ERROR cannot find value `x` in this scope
+
+    x = y // will suggest add `let`
+    //~^ ERROR cannot find value `x` in this scope
+    //~| ERROR cannot find value `y` in this scope
+}
diff --git a/src/test/ui/suggestions/issue-104086-suggest-let.stderr b/src/test/ui/suggestions/issue-104086-suggest-let.stderr
new file mode 100644
index 00000000000..fb4ea3121ac
--- /dev/null
+++ b/src/test/ui/suggestions/issue-104086-suggest-let.stderr
@@ -0,0 +1,135 @@
+error[E0425]: cannot find value `x` in this scope
+  --> $DIR/issue-104086-suggest-let.rs:2:5
+   |
+LL |     x = x = x;
+   |     ^
+   |
+help: you might have meant to introduce a new binding
+   |
+LL |     let x = x = x;
+   |     +++
+
+error[E0425]: cannot find value `x` in this scope
+  --> $DIR/issue-104086-suggest-let.rs:2:9
+   |
+LL |     x = x = x;
+   |         ^ not found in this scope
+
+error[E0425]: cannot find value `x` in this scope
+  --> $DIR/issue-104086-suggest-let.rs:2:13
+   |
+LL |     x = x = x;
+   |             ^ not found in this scope
+
+error[E0425]: cannot find value `x` in this scope
+  --> $DIR/issue-104086-suggest-let.rs:7:5
+   |
+LL |     x = y = y = y;
+   |     ^
+   |
+help: you might have meant to introduce a new binding
+   |
+LL |     let x = y = y = y;
+   |     +++
+
+error[E0425]: cannot find value `y` in this scope
+  --> $DIR/issue-104086-suggest-let.rs:7:9
+   |
+LL |     x = y = y = y;
+   |         ^ not found in this scope
+
+error[E0425]: cannot find value `y` in this scope
+  --> $DIR/issue-104086-suggest-let.rs:7:13
+   |
+LL |     x = y = y = y;
+   |             ^ not found in this scope
+
+error[E0425]: cannot find value `y` in this scope
+  --> $DIR/issue-104086-suggest-let.rs:7:17
+   |
+LL |     x = y = y = y;
+   |                 ^ not found in this scope
+
+error[E0425]: cannot find value `x` in this scope
+  --> $DIR/issue-104086-suggest-let.rs:13:5
+   |
+LL |     x = y = y;
+   |     ^
+   |
+help: you might have meant to introduce a new binding
+   |
+LL |     let x = y = y;
+   |     +++
+
+error[E0425]: cannot find value `y` in this scope
+  --> $DIR/issue-104086-suggest-let.rs:13:9
+   |
+LL |     x = y = y;
+   |         ^ not found in this scope
+
+error[E0425]: cannot find value `y` in this scope
+  --> $DIR/issue-104086-suggest-let.rs:13:13
+   |
+LL |     x = y = y;
+   |             ^ not found in this scope
+
+error[E0425]: cannot find value `x` in this scope
+  --> $DIR/issue-104086-suggest-let.rs:18:5
+   |
+LL |     x = x = y;
+   |     ^
+   |
+help: you might have meant to introduce a new binding
+   |
+LL |     let x = x = y;
+   |     +++
+
+error[E0425]: cannot find value `x` in this scope
+  --> $DIR/issue-104086-suggest-let.rs:18:9
+   |
+LL |     x = x = y;
+   |         ^ not found in this scope
+
+error[E0425]: cannot find value `y` in this scope
+  --> $DIR/issue-104086-suggest-let.rs:18:13
+   |
+LL |     x = x = y;
+   |             ^ not found in this scope
+
+error[E0425]: cannot find value `x` in this scope
+  --> $DIR/issue-104086-suggest-let.rs:23:5
+   |
+LL |     x = x; // will suggest add `let`
+   |     ^
+   |
+help: you might have meant to introduce a new binding
+   |
+LL |     let x = x; // will suggest add `let`
+   |     +++
+
+error[E0425]: cannot find value `x` in this scope
+  --> $DIR/issue-104086-suggest-let.rs:23:9
+   |
+LL |     x = x; // will suggest add `let`
+   |         ^ not found in this scope
+
+error[E0425]: cannot find value `x` in this scope
+  --> $DIR/issue-104086-suggest-let.rs:27:5
+   |
+LL |     x = y // will suggest add `let`
+   |     ^
+   |
+help: you might have meant to introduce a new binding
+   |
+LL |     let x = y // will suggest add `let`
+   |     +++
+
+error[E0425]: cannot find value `y` in this scope
+  --> $DIR/issue-104086-suggest-let.rs:27:9
+   |
+LL |     x = y // will suggest add `let`
+   |         ^ not found in this scope
+
+error: aborting due to 17 previous errors
+
+For more information about this error, try `rustc --explain E0425`.
diff --git a/src/test/ui/suggestions/option-to-bool.rs b/src/test/ui/suggestions/option-to-bool.rs
new file mode 100644
index 00000000000..2a1823b15f5
--- /dev/null
+++ b/src/test/ui/suggestions/option-to-bool.rs
@@ -0,0 +1,9 @@
+#![cfg_attr(let_chains, feature(let_chains))]
+
+fn foo(x: Option<i32>) {
+    if true && x {}
+    //~^ ERROR mismatched types
+    //~| HELP use `Option::is_some` to test if the `Option` has a value
+}
+
+fn main() {}
diff --git a/src/test/ui/suggestions/option-to-bool.stderr b/src/test/ui/suggestions/option-to-bool.stderr
new file mode 100644
index 00000000000..57a934b8342
--- /dev/null
+++ b/src/test/ui/suggestions/option-to-bool.stderr
@@ -0,0 +1,16 @@
+error[E0308]: mismatched types
+  --> $DIR/option-to-bool.rs:4:16
+   |
+LL |     if true && x {}
+   |                ^ expected `bool`, found enum `Option`
+   |
+   = note: expected type `bool`
+              found enum `Option<i32>`
+help: use `Option::is_some` to test if the `Option` has a value
+   |
+LL |     if true && x.is_some() {}
+   |                 ++++++++++
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.