Auto merge of #70672 - Dylan-DPC:rollup-cco9bbd, r=Dylan-DPC

Rollup of 6 pull requests

Successful merges:

 - #70535 (Track the finalizing node in the specialization graph)
 - #70590 (Miri: make backtrace function names and spans match up)
 - #70616 (rustc_target::abi: rename FieldPlacement to FieldsShape.)
 - #70626 (cargotest: remove webrender)
 - #70649 (clean up E0468 explanation)
 - #70662 (compiletest: don't use `std::io::stdout()`, as it bypasses `set_print`.)

Failed merges:

r? @ghost

Author: bors

Diff for: src/librustc_codegen_llvm/type_of.rs

@@ -79,7 +79,7 @@ fn uncached_llvm_type<'a, 'tcx>(
     };
 
     match layout.fields {
-        layout::FieldPlacement::Union(_) => {
+        layout::FieldsShape::Union(_) => {
             let fill = cx.type_padding_filler(layout.size, layout.align.abi);
             let packed = false;
             match name {
@@ -91,10 +91,10 @@ fn uncached_llvm_type<'a, 'tcx>(
                 }
             }
         }
-        layout::FieldPlacement::Array { count, .. } => {
+        layout::FieldsShape::Array { count, .. } => {
             cx.type_array(layout.field(cx, 0).llvm_type(cx), count)
         }
-        layout::FieldPlacement::Arbitrary { .. } => match name {
+        layout::FieldsShape::Arbitrary { .. } => match name {
             None => {
                 let (llfields, packed) = struct_llfields(cx, layout);
                 cx.type_struct(&llfields, packed)
@@ -371,13 +371,13 @@ impl<'tcx> LayoutLlvmExt<'tcx> for TyAndLayout<'tcx> {
             _ => {}
         }
         match self.fields {
-            layout::FieldPlacement::Union(_) => {
+            layout::FieldsShape::Union(_) => {
                 bug!("TyAndLayout::llvm_field_index({:?}): not applicable", self)
             }
 
-            layout::FieldPlacement::Array { .. } => index as u64,
+            layout::FieldsShape::Array { .. } => index as u64,
 
-            layout::FieldPlacement::Arbitrary { .. } => {
+            layout::FieldsShape::Arbitrary { .. } => {
                 1 + (self.fields.memory_index(index) as u64) * 2
             }
         }

Diff for: src/librustc_codegen_ssa/mir/place.rs

@@ -66,7 +66,7 @@ impl<'a, 'tcx, V: CodegenObject> PlaceRef<'tcx, V> {
     }
 
     pub fn len<Cx: ConstMethods<'tcx, Value = V>>(&self, cx: &Cx) -> V {
-        if let layout::FieldPlacement::Array { count, .. } = self.layout.fields {
+        if let layout::FieldsShape::Array { count, .. } = self.layout.fields {
             if self.layout.is_unsized() {
                 assert_eq!(count, 0);
                 self.llextra.unwrap()

Diff for: src/librustc_error_codes/error_codes/E0468.md

@@ -1,4 +1,4 @@
-A non-root module attempts to import macros from another crate.
+A non-root module tried to import macros from another crate.
 
 Example of erroneous code:
 
@@ -17,7 +17,7 @@ Either move the macro import to crate root or do without the foreign macros.
 This will work:
 
 ```
-#[macro_use(debug_assert)]
+#[macro_use(debug_assert)] // ok!
 extern crate core;
 
 mod foo {

Diff for: src/librustc_middle/mir/interpret/error.rs

@@ -53,9 +53,8 @@ pub struct ConstEvalErr<'tcx> {
 
 #[derive(Debug)]
 pub struct FrameInfo<'tcx> {
-    /// This span is in the caller.
-    pub call_site: Span,
     pub instance: ty::Instance<'tcx>,
+    pub span: Span,
     pub lint_root: Option<hir::HirId>,
 }
 
@@ -65,12 +64,12 @@ impl<'tcx> fmt::Display for FrameInfo<'tcx> {
             if tcx.def_key(self.instance.def_id()).disambiguated_data.data
                 == DefPathData::ClosureExpr
             {
-                write!(f, "inside call to closure")?;
+                write!(f, "inside closure")?;
             } else {
-                write!(f, "inside call to `{}`", self.instance)?;
+                write!(f, "inside `{}`", self.instance)?;
             }
-            if !self.call_site.is_dummy() {
-                let lo = tcx.sess.source_map().lookup_char_pos(self.call_site.lo());
+            if !self.span.is_dummy() {
+                let lo = tcx.sess.source_map().lookup_char_pos(self.span.lo());
                 write!(f, " at {}:{}:{}", lo.file.name, lo.line, lo.col.to_usize() + 1)?;
             }
             Ok(())
@@ -168,13 +167,10 @@ impl<'tcx> ConstEvalErr<'tcx> {
             if let Some(span_msg) = span_msg {
                 err.span_label(self.span, span_msg);
             }
-            // Add spans for the stacktrace.
-            // Skip the last, which is just the environment of the constant.  The stacktrace
-            // is sometimes empty because we create "fake" eval contexts in CTFE to do work
-            // on constant values.
-            if !self.stacktrace.is_empty() {
-                for frame_info in &self.stacktrace[..self.stacktrace.len() - 1] {
-                    err.span_label(frame_info.call_site, frame_info.to_string());
+            // Add spans for the stacktrace. Don't print a single-line backtrace though.
+            if self.stacktrace.len() > 1 {
+                for frame_info in &self.stacktrace {
+                    err.span_label(frame_info.span, frame_info.to_string());
                 }
             }
             // Let the caller finish the job.

Diff for: src/librustc_middle/traits/specialization_graph.rs

@@ -154,14 +154,44 @@ impl Iterator for Ancestors<'_> {
     }
 }
 
-pub struct NodeItem<T> {
-    pub node: Node,
-    pub item: T,
+/// Information about the most specialized definition of an associated item.
+pub struct LeafDef {
+    /// The associated item described by this `LeafDef`.
+    pub item: ty::AssocItem,
+
+    /// The node in the specialization graph containing the definition of `item`.
+    pub defining_node: Node,
+
+    /// The "top-most" (ie. least specialized) specialization graph node that finalized the
+    /// definition of `item`.
+    ///
+    /// Example:
+    ///
+    /// ```
+    /// trait Tr {
+    ///     fn assoc(&self);
+    /// }
+    ///
+    /// impl<T> Tr for T {
+    ///     default fn assoc(&self) {}
+    /// }
+    ///
+    /// impl Tr for u8 {}
+    /// ```
+    ///
+    /// If we start the leaf definition search at `impl Tr for u8`, that impl will be the
+    /// `finalizing_node`, while `defining_node` will be the generic impl.
+    ///
+    /// If the leaf definition search is started at the generic impl, `finalizing_node` will be
+    /// `None`, since the most specialized impl we found still allows overriding the method
+    /// (doesn't finalize it).
+    pub finalizing_node: Option<Node>,
 }
 
-impl<T> NodeItem<T> {
-    pub fn map<U, F: FnOnce(T) -> U>(self, f: F) -> NodeItem<U> {
-        NodeItem { node: self.node, item: f(self.item) }
+impl LeafDef {
+    /// Returns whether this definition is known to not be further specializable.
+    pub fn is_final(&self) -> bool {
+        self.finalizing_node.is_some()
     }
 }
 
@@ -173,18 +203,36 @@ impl<'tcx> Ancestors<'tcx> {
         tcx: TyCtxt<'tcx>,
         trait_item_name: Ident,
         trait_item_kind: ty::AssocKind,
-    ) -> Option<NodeItem<ty::AssocItem>> {
+    ) -> Option<LeafDef> {
         let trait_def_id = self.trait_def_id;
+        let mut finalizing_node = None;
+
         self.find_map(|node| {
-            node.item(tcx, trait_item_name, trait_item_kind, trait_def_id)
-                .map(|item| NodeItem { node, item })
+            if let Some(item) = node.item(tcx, trait_item_name, trait_item_kind, trait_def_id) {
+                if finalizing_node.is_none() {
+                    let is_specializable = item.defaultness.is_default()
+                        || tcx.impl_defaultness(node.def_id()).is_default();
+
+                    if !is_specializable {
+                        finalizing_node = Some(node);
+                    }
+                }
+
+                Some(LeafDef { item, defining_node: node, finalizing_node })
+            } else {
+                // Item not mentioned. This "finalizes" any defaulted item provided by an ancestor.
+                finalizing_node = Some(node);
+                None
+            }
         })
     }
 }
 
 /// Walk up the specialization ancestors of a given impl, starting with that
-/// impl itself. Returns `None` if an error was reported while building the
-/// specialization graph.
+/// impl itself.
+///
+/// Returns `Err` if an error was reported while building the specialization
+/// graph.
 pub fn ancestors(
     tcx: TyCtxt<'tcx>,
     trait_def_id: DefId,