Auto merge of #126965 - matthiaskrgr:rollup-x3kamn8, r=matthiaskrgr

Rollup of 8 pull requests

Successful merges:

 - #126302 (Detect unused structs which derived Default)
 - #126885 (Remove internal `PathBuf::as_mut_vec`)
 - #126916 (Specify target specific linker for `riscv64gc-gnu` job)
 - #126926 (Tweak a confusing comment in `create_match_candidates`)
 - #126927 (core: VaArgSafe is an unsafe trait)
 - #126932 (Tweak `FlatPat::new` to avoid a temporarily-invalid state)
 - #126946 (Add missing slash in `const_eval_select` doc comment)
 - #126947 (Delegation: ast lowering refactor)

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

19 files changed, 227 insertions, 116 deletions

diff --git a/compiler/rustc_ast_lowering/src/delegation.rs b/compiler/rustc_ast_lowering/src/delegation.rs
index d9dd0b3bca5..678cac210f4 100644
--- a/compiler/rustc_ast_lowering/src/delegation.rs
+++ b/compiler/rustc_ast_lowering/src/delegation.rs
@@ -66,14 +66,18 @@ impl<'hir> LoweringContext<'_, 'hir> {
         let Ok(sig_id) = sig_id else {
             return false;
         };
-        if let Some(local_sig_id) = sig_id.as_local() {
+        self.has_self(sig_id, span)
+    }
+
+    fn has_self(&self, def_id: DefId, span: Span) -> bool {
+        if let Some(local_sig_id) = def_id.as_local() {
             // The value may be missing due to recursive delegation.
             // Error will be emmited later during HIR ty lowering.
             self.resolver.delegation_fn_sigs.get(&local_sig_id).map_or(false, |sig| sig.has_self)
         } else {
-            match self.tcx.def_kind(sig_id) {
+            match self.tcx.def_kind(def_id) {
                 DefKind::Fn => false,
-                DefKind::AssocFn => self.tcx.associated_item(sig_id).fn_has_self_parameter,
+                DefKind::AssocFn => self.tcx.associated_item(def_id).fn_has_self_parameter,
                 _ => span_bug!(span, "unexpected DefKind for delegation item"),
             }
         }
@@ -107,12 +111,17 @@ impl<'hir> LoweringContext<'_, 'hir> {
         span: Span,
     ) -> Result<DefId, ErrorGuaranteed> {
         let sig_id = if self.is_in_trait_impl { item_id } else { path_id };
-        let sig_id =
-            self.resolver.get_partial_res(sig_id).and_then(|r| r.expect_full_res().opt_def_id());
-        sig_id.ok_or_else(|| {
-            self.tcx
-                .dcx()
-                .span_delayed_bug(span, "LoweringContext: couldn't resolve delegation item")
+        self.get_resolution_id(sig_id, span)
+    }
+
+    fn get_resolution_id(&self, node_id: NodeId, span: Span) -> Result<DefId, ErrorGuaranteed> {
+        let def_id =
+            self.resolver.get_partial_res(node_id).and_then(|r| r.expect_full_res().opt_def_id());
+        def_id.ok_or_else(|| {
+            self.tcx.dcx().span_delayed_bug(
+                span,
+                format!("LoweringContext: couldn't resolve node {:?} in delegation item", node_id),
+            )
         })
     }
 
@@ -122,7 +131,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
             predicates: &[],
             has_where_clause_predicates: false,
             where_clause_span: span,
-            span: span,
+            span,
         })
     }
 
@@ -222,12 +231,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         }));
 
         let path = self.arena.alloc(hir::Path { span, res: Res::Local(param_id), segments });
-
-        hir::Expr {
-            hir_id: self.next_id(),
-            kind: hir::ExprKind::Path(hir::QPath::Resolved(None, path)),
-            span,
-        }
+        self.mk_expr(hir::ExprKind::Path(hir::QPath::Resolved(None, path)), span)
     }
 
     fn lower_delegation_body(
@@ -236,19 +240,11 @@ impl<'hir> LoweringContext<'_, 'hir> {
         param_count: usize,
         span: Span,
     ) -> BodyId {
-        let path = self.lower_qpath(
-            delegation.id,
-            &delegation.qself,
-            &delegation.path,
-            ParamMode::Optional,
-            ImplTraitContext::Disallowed(ImplTraitPosition::Path),
-            None,
-        );
         let block = delegation.body.as_deref();
 
         self.lower_body(|this| {
-            let mut parameters: Vec<hir::Param<'_>> = Vec::new();
-            let mut args: Vec<hir::Expr<'hir>> = Vec::new();
+            let mut parameters: Vec<hir::Param<'_>> = Vec::with_capacity(param_count);
+            let mut args: Vec<hir::Expr<'_>> = Vec::with_capacity(param_count);
 
             for idx in 0..param_count {
                 let (param, pat_node_id) = this.generate_param(span);
@@ -264,11 +260,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     };
                     self_resolver.visit_block(block);
                     let block = this.lower_block(block, false);
-                    hir::Expr {
-                        hir_id: this.next_id(),
-                        kind: hir::ExprKind::Block(block, None),
-                        span: block.span,
-                    }
+                    this.mk_expr(hir::ExprKind::Block(block, None), block.span)
                 } else {
                     let pat_hir_id = this.lower_node_id(pat_node_id);
                     this.generate_arg(pat_hir_id, span)
@@ -276,43 +268,41 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 args.push(arg);
             }
 
-            let args = self.arena.alloc_from_iter(args);
-            let final_expr = this.generate_call(path, args);
+            let final_expr = this.finalize_body_lowering(delegation, args, span);
             (this.arena.alloc_from_iter(parameters), final_expr)
         })
     }
 
-    fn generate_call(
+    // Generates fully qualified call for the resulting body.
+    fn finalize_body_lowering(
         &mut self,
-        path: hir::QPath<'hir>,
-        args: &'hir [hir::Expr<'hir>],
+        delegation: &Delegation,
+        args: Vec<hir::Expr<'hir>>,
+        span: Span,
     ) -> hir::Expr<'hir> {
-        let callee = self.arena.alloc(hir::Expr {
-            hir_id: self.next_id(),
-            kind: hir::ExprKind::Path(path),
-            span: path.span(),
-        });
+        let path = self.lower_qpath(
+            delegation.id,
+            &delegation.qself,
+            &delegation.path,
+            ParamMode::Optional,
+            ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+            None,
+        );
 
-        let expr = self.arena.alloc(hir::Expr {
-            hir_id: self.next_id(),
-            kind: hir::ExprKind::Call(callee, args),
-            span: path.span(),
-        });
+        let args = self.arena.alloc_from_iter(args);
+        let path_expr = self.arena.alloc(self.mk_expr(hir::ExprKind::Path(path), span));
+        let call = self.arena.alloc(self.mk_expr(hir::ExprKind::Call(path_expr, args), span));
 
         let block = self.arena.alloc(hir::Block {
             stmts: &[],
-            expr: Some(expr),
+            expr: Some(call),
             hir_id: self.next_id(),
             rules: hir::BlockCheckMode::DefaultBlock,
-            span: path.span(),
+            span,
             targeted_by_break: false,
         });
 
-        hir::Expr {
-            hir_id: self.next_id(),
-            kind: hir::ExprKind::Block(block, None),
-            span: path.span(),
-        }
+        self.mk_expr(hir::ExprKind::Block(block, None), span)
     }
 
     fn generate_delegation_error(
@@ -333,11 +323,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         let header = self.generate_header_error();
         let sig = hir::FnSig { decl, header, span };
 
-        let body_id = self.lower_body(|this| {
-            let expr =
-                hir::Expr { hir_id: this.next_id(), kind: hir::ExprKind::Err(err), span: span };
-            (&[], expr)
-        });
+        let body_id = self.lower_body(|this| (&[], this.mk_expr(hir::ExprKind::Err(err), span)));
         DelegationResults { generics, body_id, sig }
     }
 
@@ -349,6 +335,11 @@ impl<'hir> LoweringContext<'_, 'hir> {
             abi: abi::Abi::Rust,
         }
     }
+
+    #[inline]
+    fn mk_expr(&mut self, kind: hir::ExprKind<'hir>, span: Span) -> hir::Expr<'hir> {
+        hir::Expr { hir_id: self.next_id(), kind, span }
+    }
 }
 
 struct SelfResolver<'a> {
diff --git a/compiler/rustc_mir_build/src/build/matches/mod.rs b/compiler/rustc_mir_build/src/build/matches/mod.rs
index 68244136d1a..932406fd1aa 100644
--- a/compiler/rustc_mir_build/src/build/matches/mod.rs
+++ b/compiler/rustc_mir_build/src/build/matches/mod.rs
@@ -358,8 +358,11 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
     where
         'a: 'pat,
     {
-        // Assemble a list of candidates: there is one candidate per pattern,
-        // which means there may be more than one candidate *per arm*.
+        // Assemble the initial list of candidates. These top-level candidates
+        // are 1:1 with the original match arms, but other parts of match
+        // lowering also introduce subcandidates (for subpatterns), and will
+        // also flatten candidates in some cases. So in general a list of
+        // candidates does _not_ necessarily correspond to a list of arms.
         arms.iter()
             .copied()
             .map(|arm| {
@@ -1031,6 +1034,12 @@ impl<'tcx> PatternExtraData<'tcx> {
 }
 
 /// A pattern in a form suitable for generating code.
+///
+/// Here, "flat" indicates that the pattern's match pairs have been recursively
+/// simplified by [`Builder::simplify_match_pairs`]. They are not necessarily
+/// flat in an absolute sense.
+///
+/// Will typically be incorporated into a [`Candidate`].
 #[derive(Debug, Clone)]
 struct FlatPat<'pat, 'tcx> {
     /// To match the pattern, all of these must be satisfied...
@@ -1042,23 +1051,25 @@ struct FlatPat<'pat, 'tcx> {
 }
 
 impl<'tcx, 'pat> FlatPat<'pat, 'tcx> {
+    /// Creates a `FlatPat` containing a simplified [`MatchPair`] list/forest
+    /// for the given pattern.
     fn new(
         place: PlaceBuilder<'tcx>,
         pattern: &'pat Pat<'tcx>,
         cx: &mut Builder<'_, 'tcx>,
     ) -> Self {
-        let is_never = pattern.is_never_pattern();
-        let mut flat_pat = FlatPat {
-            match_pairs: vec![MatchPair::new(place, pattern, cx)],
-            extra_data: PatternExtraData {
-                span: pattern.span,
-                bindings: Vec::new(),
-                ascriptions: Vec::new(),
-                is_never,
-            },
+        // First, recursively build a tree of match pairs for the given pattern.
+        let mut match_pairs = vec![MatchPair::new(place, pattern, cx)];
+        let mut extra_data = PatternExtraData {
+            span: pattern.span,
+            bindings: Vec::new(),
+            ascriptions: Vec::new(),
+            is_never: pattern.is_never_pattern(),
         };
-        cx.simplify_match_pairs(&mut flat_pat.match_pairs, &mut flat_pat.extra_data);
-        flat_pat
+        // Partly-flatten and sort the match pairs, while recording extra data.
+        cx.simplify_match_pairs(&mut match_pairs, &mut extra_data);
+
+        Self { match_pairs, extra_data }
     }
 }
 
@@ -1104,9 +1115,12 @@ impl<'tcx, 'pat> Candidate<'pat, 'tcx> {
         has_guard: bool,
         cx: &mut Builder<'_, 'tcx>,
     ) -> Self {
+        // Use `FlatPat` to build simplified match pairs, then immediately
+        // incorporate them into a new candidate.
         Self::from_flat_pat(FlatPat::new(place, pattern, cx), has_guard)
     }
 
+    /// Incorporates an already-simplified [`FlatPat`] into a new candidate.
     fn from_flat_pat(flat_pat: FlatPat<'pat, 'tcx>, has_guard: bool) -> Self {
         Candidate {
             match_pairs: flat_pat.match_pairs,
diff --git a/compiler/rustc_mir_build/src/build/matches/util.rs b/compiler/rustc_mir_build/src/build/matches/util.rs
index 50f4ca2d819..630d0b9438d 100644
--- a/compiler/rustc_mir_build/src/build/matches/util.rs
+++ b/compiler/rustc_mir_build/src/build/matches/util.rs
@@ -95,6 +95,8 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
 }
 
 impl<'pat, 'tcx> MatchPair<'pat, 'tcx> {
+    /// Recursively builds a `MatchPair` tree for the given pattern and its
+    /// subpatterns.
     pub(in crate::build) fn new(
         mut place_builder: PlaceBuilder<'tcx>,
         pattern: &'pat Pat<'tcx>,
diff --git a/compiler/rustc_passes/src/dead.rs b/compiler/rustc_passes/src/dead.rs
index 7c7700dd859..2c86949685b 100644
--- a/compiler/rustc_passes/src/dead.rs
+++ b/compiler/rustc_passes/src/dead.rs
@@ -399,6 +399,31 @@ impl<'tcx> MarkSymbolVisitor<'tcx> {
                 return false;
             }
 
+            // don't ignore impls for Enums and pub Structs whose methods don't have self receiver,
+            // cause external crate may call such methods to construct values of these types
+            if let Some(local_impl_of) = impl_of.as_local()
+                && let Some(local_def_id) = def_id.as_local()
+                && let Some(fn_sig) =
+                    self.tcx.hir().fn_sig_by_hir_id(self.tcx.local_def_id_to_hir_id(local_def_id))
+                && matches!(fn_sig.decl.implicit_self, hir::ImplicitSelfKind::None)
+                && let TyKind::Path(hir::QPath::Resolved(_, path)) =
+                    self.tcx.hir().expect_item(local_impl_of).expect_impl().self_ty.kind
+                && let Res::Def(def_kind, did) = path.res
+            {
+                match def_kind {
+                    // for example, #[derive(Default)] pub struct T(i32);
+                    // external crate can call T::default() to construct T,
+                    // so that don't ignore impl Default for pub Enum and Structs
+                    DefKind::Struct | DefKind::Union if self.tcx.visibility(did).is_public() => {
+                        return false;
+                    }
+                    // don't ignore impl Default for Enums,
+                    // cause we don't know which variant is constructed
+                    DefKind::Enum => return false,
+                    _ => (),
+                };
+            }
+
             if let Some(trait_of) = self.tcx.trait_id_of_impl(impl_of)
                 && self.tcx.has_attr(trait_of, sym::rustc_trivial_field_reads)
             {
diff --git a/library/alloc/src/sync/tests.rs b/library/alloc/src/sync/tests.rs
index 49eae718c16..1b123aa58f2 100644
--- a/library/alloc/src/sync/tests.rs
+++ b/library/alloc/src/sync/tests.rs
@@ -396,7 +396,7 @@ fn show_arc() {
 
 // Make sure deriving works with Arc<T>
 #[derive(Eq, Ord, PartialEq, PartialOrd, Clone, Debug, Default)]
-struct Foo {
+struct _Foo {
     inner: Arc<i32>,
 }
 
diff --git a/library/core/src/default.rs b/library/core/src/default.rs
index 4524b352ec8..5cacedcb241 100644
--- a/library/core/src/default.rs
+++ b/library/core/src/default.rs
@@ -103,6 +103,7 @@ use crate::ascii::Char as AsciiChar;
 /// ```
 #[cfg_attr(not(test), rustc_diagnostic_item = "Default")]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[cfg_attr(not(bootstrap), rustc_trivial_field_reads)]
 pub trait Default: Sized {
     /// Returns the "default value" for a type.
     ///
diff --git a/library/core/src/ffi/mod.rs b/library/core/src/ffi/mod.rs
index 618897b3aba..6d1f10f5211 100644
--- a/library/core/src/ffi/mod.rs
+++ b/library/core/src/ffi/mod.rs
@@ -484,7 +484,7 @@ mod sealed_trait {
                   all supported platforms",
         issue = "44930"
     )]
-    pub trait VaArgSafe {}
+    pub unsafe trait VaArgSafe {}
 }
 
 macro_rules! impl_va_arg_safe {
@@ -494,7 +494,7 @@ macro_rules! impl_va_arg_safe {
                        reason = "the `c_variadic` feature has not been properly tested on \
                                  all supported platforms",
                        issue = "44930")]
-            impl sealed_trait::VaArgSafe for $t {}
+            unsafe impl sealed_trait::VaArgSafe for $t {}
         )+
     }
 }
@@ -509,14 +509,15 @@ impl_va_arg_safe! {f64}
               all supported platforms",
     issue = "44930"
 )]
-impl<T> sealed_trait::VaArgSafe for *mut T {}
+unsafe impl<T> sealed_trait::VaArgSafe for *mut T {}
+
 #[unstable(
     feature = "c_variadic",
     reason = "the `c_variadic` feature has not been properly tested on \
               all supported platforms",
     issue = "44930"
 )]
-impl<T> sealed_trait::VaArgSafe for *const T {}
+unsafe impl<T> sealed_trait::VaArgSafe for *const T {}
 
 #[unstable(
     feature = "c_variadic",
diff --git a/library/core/src/intrinsics.rs b/library/core/src/intrinsics.rs
index 6b5054a9f06..9ba1c6a4154 100644
--- a/library/core/src/intrinsics.rs
+++ b/library/core/src/intrinsics.rs
@@ -2579,7 +2579,7 @@ extern "rust-intrinsic" {
 ///     fn runtime() -> i32 { 1 }
 ///     const fn compiletime() -> i32 { 2 }
 ///
-//      // ⚠ This code violates the required equivalence of `compiletime`
+///     // ⚠ This code violates the required equivalence of `compiletime`
 ///     // and `runtime`.
 ///     const_eval_select((), compiletime, runtime)
 /// }
diff --git a/library/std/src/ffi/os_str.rs b/library/std/src/ffi/os_str.rs
index f6b9de26c1c..4a417c84a30 100644
--- a/library/std/src/ffi/os_str.rs
+++ b/library/std/src/ffi/os_str.rs
@@ -552,10 +552,20 @@ impl OsString {
         OsStr::from_inner_mut(self.inner.leak())
     }
 
-    /// Part of a hack to make PathBuf::push/pop more efficient.
+    /// Provides plumbing to core `Vec::truncate`.
+    /// More well behaving alternative to allowing outer types
+    /// full mutable access to the core `Vec`.
     #[inline]
-    pub(crate) fn as_mut_vec_for_path_buf(&mut self) -> &mut Vec<u8> {
-        self.inner.as_mut_vec_for_path_buf()
+    pub(crate) fn truncate(&mut self, len: usize) {
+        self.inner.truncate(len);
+    }
+
+    /// Provides plumbing to core `Vec::extend_from_slice`.
+    /// More well behaving alternative to allowing outer types
+    /// full mutable access to the core `Vec`.
+    #[inline]
+    pub(crate) fn extend_from_slice(&mut self, other: &[u8]) {
+        self.inner.extend_from_slice(other);
     }
 }
 
diff --git a/library/std/src/path.rs b/library/std/src/path.rs
index 72073d13280..caae8f924d2 100644
--- a/library/std/src/path.rs
+++ b/library/std/src/path.rs
@@ -1163,11 +1163,6 @@ pub struct PathBuf {
 }
 
 impl PathBuf {
-    #[inline]
-    fn as_mut_vec(&mut self) -> &mut Vec<u8> {
-        self.inner.as_mut_vec_for_path_buf()
-    }
-
     /// Allocates an empty `PathBuf`.
     ///
     /// # Examples
@@ -1290,7 +1285,8 @@ impl PathBuf {
 
     fn _push(&mut self, path: &Path) {
         // in general, a separator is needed if the rightmost byte is not a separator
-        let mut need_sep = self.as_mut_vec().last().map(|c| !is_sep_byte(*c)).unwrap_or(false);
+        let buf = self.inner.as_encoded_bytes();
+        let mut need_sep = buf.last().map(|c| !is_sep_byte(*c)).unwrap_or(false);
 
         // in the special case of `C:` on Windows, do *not* add a separator
         let comps = self.components();
@@ -1304,7 +1300,7 @@ impl PathBuf {
 
         // absolute `path` replaces `self`
         if path.is_absolute() || path.prefix().is_some() {
-            self.as_mut_vec().truncate(0);
+            self.inner.truncate(0);
 
         // verbatim paths need . and .. removed
         } else if comps.prefix_verbatim() && !path.inner.is_empty() {
@@ -1349,7 +1345,7 @@ impl PathBuf {
         // `path` has a root but no prefix, e.g., `\windows` (Windows only)
         } else if path.has_root() {
             let prefix_len = self.components().prefix_remaining();
-            self.as_mut_vec().truncate(prefix_len);
+            self.inner.truncate(prefix_len);
 
         // `path` is a pure relative path
         } else if need_sep {
@@ -1382,7 +1378,7 @@ impl PathBuf {
     pub fn pop(&mut self) -> bool {
         match self.parent().map(|p| p.as_u8_slice().len()) {
             Some(len) => {
-                self.as_mut_vec().truncate(len);
+                self.inner.truncate(len);
                 true
             }
             None => false,
@@ -1510,15 +1506,14 @@ impl PathBuf {
         // truncate until right after the file stem
         let end_file_stem = file_stem[file_stem.len()..].as_ptr().addr();
         let start = self.inner.as_encoded_bytes().as_ptr().addr();
-        let v = self.as_mut_vec();
-        v.truncate(end_file_stem.wrapping_sub(start));
+        self.inner.truncate(end_file_stem.wrapping_sub(start));
 
         // add the new extension, if any
-        let new = extension.as_encoded_bytes();
+        let new = extension;
         if !new.is_empty() {
-            v.reserve_exact(new.len() + 1);
-            v.push(b'.');
-            v.extend_from_slice(new);
+            self.inner.reserve_exact(new.len() + 1);
+            self.inner.push(OsStr::new("."));
+            self.inner.push(new);
         }
 
         true
@@ -2645,18 +2640,18 @@ impl Path {
             None => {
                 // Enough capacity for the extension and the dot
                 let capacity = self_len + extension.len() + 1;
-                let whole_path = self_bytes.iter();
+                let whole_path = self_bytes;
                 (capacity, whole_path)
             }
             Some(previous_extension) => {
                 let capacity = self_len + extension.len() - previous_extension.len();
-                let path_till_dot = self_bytes[..self_len - previous_extension.len()].iter();
+                let path_till_dot = &self_bytes[..self_len - previous_extension.len()];
                 (capacity, path_till_dot)
             }
         };
 
         let mut new_path = PathBuf::with_capacity(new_capacity);
-        new_path.as_mut_vec().extend(slice_to_copy);
+        new_path.inner.extend_from_slice(slice_to_copy);
         new_path.set_extension(extension);
         new_path
     }
diff --git a/library/std/src/sys/os_str/bytes.rs b/library/std/src/sys/os_str/bytes.rs
index f7c6b0877aa..2a7477e3afc 100644
--- a/library/std/src/sys/os_str/bytes.rs
+++ b/library/std/src/sys/os_str/bytes.rs
@@ -202,10 +202,20 @@ impl Buf {
         self.as_slice().into_rc()
     }
 
-    /// Part of a hack to make PathBuf::push/pop more efficient.
+    /// Provides plumbing to core `Vec::truncate`.
+    /// More well behaving alternative to allowing outer types
+    /// full mutable access to the core `Vec`.
     #[inline]
-    pub(crate) fn as_mut_vec_for_path_buf(&mut self) -> &mut Vec<u8> {
-        &mut self.inner
+    pub(crate) fn truncate(&mut self, len: usize) {
+        self.inner.truncate(len);
+    }
+
+    /// Provides plumbing to core `Vec::extend_from_slice`.
+    /// More well behaving alternative to allowing outer types
+    /// full mutable access to the core `Vec`.
+    #[inline]
+    pub(crate) fn extend_from_slice(&mut self, other: &[u8]) {
+        self.inner.extend_from_slice(other);
     }
 }
 
diff --git a/library/std/src/sys/os_str/wtf8.rs b/library/std/src/sys/os_str/wtf8.rs
index 96690f8c44e..edb923a4750 100644
--- a/library/std/src/sys/os_str/wtf8.rs
+++ b/library/std/src/sys/os_str/wtf8.rs
@@ -165,10 +165,20 @@ impl Buf {
         self.as_slice().into_rc()
     }
 
-    /// Part of a hack to make PathBuf::push/pop more efficient.
+    /// Provides plumbing to core `Vec::truncate`.
+    /// More well behaving alternative to allowing outer types
+    /// full mutable access to the core `Vec`.
     #[inline]
-    pub(crate) fn as_mut_vec_for_path_buf(&mut self) -> &mut Vec<u8> {
-        self.inner.as_mut_vec_for_path_buf()
+    pub(crate) fn truncate(&mut self, len: usize) {
+        self.inner.truncate(len);
+    }
+
+    /// Provides plumbing to core `Vec::extend_from_slice`.
+    /// More well behaving alternative to allowing outer types
+    /// full mutable access to the core `Vec`.
+    #[inline]
+    pub(crate) fn extend_from_slice(&mut self, other: &[u8]) {
+        self.inner.extend_from_slice(other);
     }
 }
 
diff --git a/library/std/src/sys_common/wtf8.rs b/library/std/src/sys_common/wtf8.rs
index 84128a4b595..708f62f476e 100644
--- a/library/std/src/sys_common/wtf8.rs
+++ b/library/std/src/sys_common/wtf8.rs
@@ -474,13 +474,13 @@ impl Wtf8Buf {
         Wtf8Buf { bytes: bytes.into_vec(), is_known_utf8: false }
     }
 
-    /// Part of a hack to make PathBuf::push/pop more efficient.
+    /// Provides plumbing to core `Vec::extend_from_slice`.
+    /// More well behaving alternative to allowing outer types
+    /// full mutable access to the core `Vec`.
     #[inline]
-    pub(crate) fn as_mut_vec_for_path_buf(&mut self) -> &mut Vec<u8> {
-        // FIXME: this function should not even exist, as it implies violating Wtf8Buf invariants
-        // For now, simply assume that is about to happen.
-        self.is_known_utf8 = false;
-        &mut self.bytes
+    pub(crate) fn extend_from_slice(&mut self, other: &[u8]) {
+        self.bytes.extend_from_slice(other);
+        self.is_known_utf8 = self.is_known_utf8 || self.next_surrogate(0).is_none();
     }
 }
 
diff --git a/src/ci/docker/host-x86_64/disabled/riscv64gc-gnu/Dockerfile b/src/ci/docker/host-x86_64/disabled/riscv64gc-gnu/Dockerfile
index a9ffa5918b5..a52c3839196 100644
--- a/src/ci/docker/host-x86_64/disabled/riscv64gc-gnu/Dockerfile
+++ b/src/ci/docker/host-x86_64/disabled/riscv64gc-gnu/Dockerfile
@@ -91,7 +91,9 @@ RUN sh /scripts/sccache.sh
 # Avoid "fatal: detected dubious ownership in repository at '/checkout'" error
 RUN git config --global --add safe.directory /checkout
 
-ENV RUST_CONFIGURE_ARGS --qemu-riscv64-rootfs=/tmp/rootfs
+ENV RUST_CONFIGURE_ARGS \
+    --qemu-riscv64-rootfs=/tmp/rootfs \
+    --set target.riscv64gc-unknown-linux-gnu.linker=riscv64-linux-gnu-gcc
 ENV SCRIPT python3 ../x.py --stage 2 test --host='' --target riscv64gc-unknown-linux-gnu
 
 ENV NO_CHANGE_USER=1
diff --git a/tests/ui/delegation/explicit-paths.stderr b/tests/ui/delegation/explicit-paths.stderr
index 30891c94c0e..d33c5da4377 100644
--- a/tests/ui/delegation/explicit-paths.stderr
+++ b/tests/ui/delegation/explicit-paths.stderr
@@ -110,10 +110,10 @@ error[E0308]: mismatched types
   --> $DIR/explicit-paths.rs:78:30
    |
 LL |         reuse <S2 as Trait>::foo1;
-   |               ---------------^^^^
-   |               |              |
-   |               |              expected `&S2`, found `&S`
-   |               arguments to this function are incorrect
+   |                              ^^^^
+   |                              |
+   |                              expected `&S2`, found `&S`
+   |                              arguments to this function are incorrect
    |
    = note: expected reference `&S2`
               found reference `&S`
diff --git a/tests/ui/deriving/deriving-default-enum.rs b/tests/ui/deriving/deriving-default-enum.rs
index 96eba258c97..6b59f39a67d 100644
--- a/tests/ui/deriving/deriving-default-enum.rs
+++ b/tests/ui/deriving/deriving-default-enum.rs
@@ -22,6 +22,6 @@ enum MyOption<T> {
 }
 
 fn main() {
-    assert_eq!(Foo::default(), Foo::Alpha);
+    assert!(matches!(Foo::default(), Foo::Alpha));
     assert!(matches!(MyOption::<NotDefault>::default(), MyOption::None));
 }
diff --git a/tests/ui/issues/issue-68696-catch-during-unwind.rs b/tests/ui/issues/issue-68696-catch-during-unwind.rs
index 2368cccef0d..80d63b0cde7 100644
--- a/tests/ui/issues/issue-68696-catch-during-unwind.rs
+++ b/tests/ui/issues/issue-68696-catch-during-unwind.rs
@@ -7,6 +7,7 @@
 
 use std::panic::catch_unwind;
 
+#[allow(dead_code)]
 #[derive(Default)]
 struct Guard;
 
diff --git a/tests/ui/lint/dead-code/unused-struct-derive-default.rs b/tests/ui/lint/dead-code/unused-struct-derive-default.rs
new file mode 100644
index 00000000000..330ad32dd57
--- /dev/null
+++ b/tests/ui/lint/dead-code/unused-struct-derive-default.rs
@@ -0,0 +1,25 @@
+#![deny(dead_code)]
+
+#[derive(Default)]
+struct T; //~ ERROR struct `T` is never constructed
+
+#[derive(Default)]
+struct Used;
+
+#[derive(Default)]
+enum E {
+    #[default]
+    A,
+    B, //~ ERROR variant `B` is never constructed
+}
+
+// external crate can call T2::default() to construct T2,
+// so that no warnings for pub adts
+#[derive(Default)]
+pub struct T2 {
+    _unread: i32,
+}
+
+fn main() {
+    let _x: Used = Default::default();
+}
diff --git a/tests/ui/lint/dead-code/unused-struct-derive-default.stderr b/tests/ui/lint/dead-code/unused-struct-derive-default.stderr
new file mode 100644
index 00000000000..bbb0bd7be70
--- /dev/null
+++ b/tests/ui/lint/dead-code/unused-struct-derive-default.stderr
@@ -0,0 +1,24 @@
+error: struct `T` is never constructed
+  --> $DIR/unused-struct-derive-default.rs:4:8
+   |
+LL | struct T;
+   |        ^
+   |
+   = note: `T` has a derived impl for the trait `Default`, but this is intentionally ignored during dead code analysis
+note: the lint level is defined here
+  --> $DIR/unused-struct-derive-default.rs:1:9
+   |
+LL | #![deny(dead_code)]
+   |         ^^^^^^^^^
+
+error: variant `B` is never constructed
+  --> $DIR/unused-struct-derive-default.rs:13:5
+   |
+LL | enum E {
+   |      - variant in this enum
+...
+LL |     B,
+   |     ^
+
+error: aborting due to 2 previous errors
+