Rollup of 5 pull requests

compiler/rustc_error_codes/src/error_codes/E0404.md

@@ -8,14 +8,15 @@ struct Foo;
 struct Bar;
 
 impl Foo for Bar {} // error: `Foo` is not a trait
+fn baz<T: Foo>(t: T) {} // error: `Foo` is not a trait
 ```
 
 Another erroneous code example:
 
 ```compile_fail,E0404
-struct Foo;
+type Foo = Iterator<Item=String>;
 
-fn bar<T: Foo>(t: T) {} // error: `Foo` is not a trait
+fn bar<T: Foo>(t: T) {} // error: `Foo` is a type alias
 ```
 
 Please verify that the trait's name was not misspelled or that the right
@@ -30,14 +31,27 @@ struct Bar;
 impl Foo for Bar { // ok!
     // functions implementation
 }
+
+fn baz<T: Foo>(t: T) {} // ok!
 ```
 
-or:
+Alternatively, you could introduce a new trait with your desired restrictions
+as a super trait:
 
 ```
-trait Foo {
-    // some functions
-}
+# trait Foo {}
+# struct Bar;
+# impl Foo for Bar {}
+trait Qux: Foo {} // Anything that implements Qux also needs to implement Foo
+fn baz<T: Qux>(t: T) {} // also ok!
+```
+
+Finally, if you are on nightly and want to use a trait alias
+instead of a type alias, you should use `#![feature(trait_alias)]`:
+
+```
+#![feature(trait_alias)]
+trait Foo = Iterator<Item=String>;
 
 fn bar<T: Foo>(t: T) {} // ok!
 ```

compiler/rustc_passes/src/dead.rs

@@ -3,6 +3,7 @@
 // from live codes are live, and everything else is dead.
 
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
+use rustc_errors::Applicability;
 use rustc_hir as hir;
 use rustc_hir::def::{CtorOf, DefKind, Res};
 use rustc_hir::def_id::{DefId, LOCAL_CRATE};
@@ -15,7 +16,7 @@ use rustc_middle::middle::privacy;
 use rustc_middle::ty::{self, DefIdTree, TyCtxt};
 use rustc_session::lint;
 
-use rustc_span::symbol::{sym, Symbol};
+use rustc_span::symbol::{sym, Ident, Symbol};
 
 // Any local node that may call something in its body block should be
 // explored. For example, if it's a live Node::Item that is a
@@ -505,6 +506,13 @@ fn find_live<'tcx>(
     symbol_visitor.live_symbols
 }
 
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+enum ExtraNote {
+    /// Use this to provide some examples in the diagnostic of potential other purposes for a value
+    /// or field that is dead code
+    OtherPurposeExamples,
+}
+
 struct DeadVisitor<'tcx> {
     tcx: TyCtxt<'tcx>,
     live_symbols: FxHashSet<hir::HirId>,
@@ -570,14 +578,42 @@ impl DeadVisitor<'tcx> {
         &mut self,
         id: hir::HirId,
         span: rustc_span::Span,
-        name: Symbol,
+        name: Ident,
         participle: &str,
+        extra_note: Option<ExtraNote>,
     ) {
         if !name.as_str().starts_with('_') {
             self.tcx.struct_span_lint_hir(lint::builtin::DEAD_CODE, id, span, |lint| {
                 let def_id = self.tcx.hir().local_def_id(id);
                 let descr = self.tcx.def_kind(def_id).descr(def_id.to_def_id());
-                lint.build(&format!("{} is never {}: `{}`", descr, participle, name)).emit()
+
+                let prefixed = vec![(name.span, format!("_{}", name))];
+
+                let mut diag =
+                    lint.build(&format!("{} is never {}: `{}`", descr, participle, name));
+
+                diag.multipart_suggestion(
+                    "if this is intentional, prefix it with an underscore",
+                    prefixed,
+                    Applicability::MachineApplicable,
+                );
+
+                let mut note = format!(
+                    "the leading underscore signals that this {} serves some other \
+                    purpose even if it isn't used in a way that we can detect.",
+                    descr,
+                );
+                if matches!(extra_note, Some(ExtraNote::OtherPurposeExamples)) {
+                    note += " (e.g. for its effect when dropped or in foreign code)";
+                }
+
+                diag.note(&note);
+
+                // Force the note we added to the front, before any other subdiagnostics
+                // added in lint.build(...)
+                diag.children.rotate_right(1);
+
+                diag.emit()
             });
         }
     }
@@ -623,7 +659,7 @@ impl Visitor<'tcx> for DeadVisitor<'tcx> {
                 hir::ItemKind::Struct(..) => "constructed", // Issue #52325
                 _ => "used",
             };
-            self.warn_dead_code(item.hir_id(), span, item.ident.name, participle);
+            self.warn_dead_code(item.hir_id(), span, item.ident, participle, None);
         } else {
             // Only continue if we didn't warn
             intravisit::walk_item(self, item);
@@ -637,22 +673,28 @@ impl Visitor<'tcx> for DeadVisitor<'tcx> {
         id: hir::HirId,
     ) {
         if self.should_warn_about_variant(&variant) {
-            self.warn_dead_code(variant.id, variant.span, variant.ident.name, "constructed");
+            self.warn_dead_code(variant.id, variant.span, variant.ident, "constructed", None);
         } else {
             intravisit::walk_variant(self, variant, g, id);
         }
     }
 
     fn visit_foreign_item(&mut self, fi: &'tcx hir::ForeignItem<'tcx>) {
         if self.should_warn_about_foreign_item(fi) {
-            self.warn_dead_code(fi.hir_id(), fi.span, fi.ident.name, "used");
+            self.warn_dead_code(fi.hir_id(), fi.span, fi.ident, "used", None);
         }
         intravisit::walk_foreign_item(self, fi);
     }
 
     fn visit_field_def(&mut self, field: &'tcx hir::FieldDef<'tcx>) {
         if self.should_warn_about_field(&field) {
-            self.warn_dead_code(field.hir_id, field.span, field.ident.name, "read");
+            self.warn_dead_code(
+                field.hir_id,
+                field.span,
+                field.ident,
+                "read",
+                Some(ExtraNote::OtherPurposeExamples),
+            );
         }
         intravisit::walk_field_def(self, field);
     }
@@ -664,8 +706,9 @@ impl Visitor<'tcx> for DeadVisitor<'tcx> {
                     self.warn_dead_code(
                         impl_item.hir_id(),
                         impl_item.span,
-                        impl_item.ident.name,
+                        impl_item.ident,
                         "used",
+                        None,
                     );
                 }
                 self.visit_nested_body(body_id)
@@ -683,7 +726,7 @@ impl Visitor<'tcx> for DeadVisitor<'tcx> {
                     } else {
                         impl_item.ident.span
                     };
-                    self.warn_dead_code(impl_item.hir_id(), span, impl_item.ident.name, "used");
+                    self.warn_dead_code(impl_item.hir_id(), span, impl_item.ident, "used", None);
                 }
                 self.visit_nested_body(body_id)
             }

compiler/rustc_resolve/src/late/diagnostics.rs

@@ -930,7 +930,14 @@ impl<'a: 'ast, 'ast> LateResolutionVisitor<'a, '_, 'ast> {
                     let msg = "you might have meant to use `#![feature(trait_alias)]` instead of a \
                                `type` alias";
                     if let Some(span) = self.def_span(def_id) {
-                        err.span_help(span, msg);
+                        if let Ok(snip) = self.r.session.source_map().span_to_snippet(span) {
+                            // The span contains a type alias so we should be able to
+                            // replace `type` with `trait`.
+                            let snip = snip.replacen("type", "trait", 1);
+                            err.span_suggestion(span, msg, snip, Applicability::MaybeIncorrect);
+                        } else {
+                            err.span_help(span, msg);
+                        }
                     } else {
                         err.help(msg);
                     }

compiler/rustc_session/src/config.rs

@@ -75,19 +75,21 @@ impl_stable_hash_via_hash!(OptLevel);
 
 /// This is what the `LtoCli` values get mapped to after resolving defaults and
 /// and taking other command line options into account.
+///
+/// Note that linker plugin-based LTO is a different mechanism entirely.
 #[derive(Clone, PartialEq)]
 pub enum Lto {
-    /// Don't do any LTO whatsoever
+    /// Don't do any LTO whatsoever.
     No,
 
-    /// Do a full crate graph LTO with ThinLTO
+    /// Do a full-crate-graph (inter-crate) LTO with ThinLTO.
     Thin,
 
-    /// Do a local graph LTO with ThinLTO (only relevant for multiple codegen
-    /// units).
+    /// Do a local ThinLTO (intra-crate, over the CodeGen Units of the local crate only). This is
+    /// only relevant if multiple CGUs are used.
     ThinLocal,
 
-    /// Do a full crate graph LTO with "fat" LTO
+    /// Do a full-crate-graph (inter-crate) LTO with "fat" LTO.
     Fat,
 }
 

src/librustdoc/html/render/mod.rs

@@ -1284,6 +1284,7 @@ fn render_impl(
     let cache = cx.cache();
     let traits = &cache.traits;
     let trait_ = i.trait_did_full(cache).map(|did| &traits[&did]);
+    let mut close_tags = String::new();
 
     if render_mode == RenderMode::Normal {
         let id = cx.derive_id(match i.inner_impl().trait_ {
@@ -1302,7 +1303,12 @@ fn render_impl(
             format!(" aliases=\"{}\"", aliases.join(","))
         };
         if let Some(use_absolute) = use_absolute {
-            write!(w, "<h3 id=\"{}\" class=\"impl\"{}><code class=\"in-band\">", id, aliases);
+            write!(
+                w,
+                "<details class=\"rustdoc-toggle implementors-toggle\"><summary><h3 id=\"{}\" class=\"impl\"{}><code class=\"in-band\">",
+                id, aliases
+            );
+            close_tags.insert_str(0, "</details>");
             write!(w, "{}", i.inner_impl().print(use_absolute, cx));
             if show_def_docs {
                 for it in &i.inner_impl().items {
@@ -1325,11 +1331,12 @@ fn render_impl(
         } else {
             write!(
                 w,
-                "<h3 id=\"{}\" class=\"impl\"{}><code class=\"in-band\">{}</code>",
+                "<details class=\"rustdoc-toggle implementors-toggle\"><summary><h3 id=\"{}\" class=\"impl\"{}><code class=\"in-band\">{}</code>",
                 id,
                 aliases,
                 i.inner_impl().print(false, cx)
             );
+            close_tags.insert_str(0, "</details>");
         }
         write!(w, "<a href=\"#{}\" class=\"anchor\"></a>", id);
         render_stability_since_raw(
@@ -1341,6 +1348,7 @@ fn render_impl(
         );
         write_srclink(cx, &i.impl_item, w);
         w.write_str("</h3>");
+        w.write_str("</summary>");
 
         if trait_.is_some() {
             if let Some(portability) = portability(&i.impl_item, Some(parent)) {
@@ -1542,6 +1550,7 @@ fn render_impl(
     }
 
     w.write_str("<div class=\"impl-items\">");
+    close_tags.insert_str(0, "</div>");
     for trait_item in &i.inner_impl().items {
         doc_impl_item(
             w,
@@ -1612,7 +1621,7 @@ fn render_impl(
             );
         }
     }
-    w.write_str("</div>");
+    w.write_str(&close_tags);
 }
 
 fn print_sidebar(cx: &Context<'_>, it: &clean::Item, buffer: &mut Buffer) {

src/librustdoc/html/static/main.js

@@ -1196,31 +1196,18 @@ function hideThemeButtonState() {
             if (!next) {
                 return;
             }
-            if (hasClass(e, "impl") &&
-                (next.getElementsByClassName("method").length > 0 ||
-                 next.getElementsByClassName("associatedconstant").length > 0)) {
-                var newToggle = toggle.cloneNode(true);
-                insertAfter(newToggle, e.childNodes[e.childNodes.length - 1]);
-                // In case the option "auto-collapse implementors" is not set to false, we collapse
-                // all implementors.
-                if (hideImplementors === true && e.parentNode.id === "implementors-list") {
-                    collapseDocs(newToggle, "hide");
-                }
-            }
         };
 
         onEachLazy(document.getElementsByClassName("method"), func);
         onEachLazy(document.getElementsByClassName("associatedconstant"), func);
-        onEachLazy(document.getElementsByClassName("impl"), funcImpl);
         var impl_call = function() {};
-        // Large items are hidden by default in the HTML. If the setting overrides that, show 'em.
-        if (!hideLargeItemContents) {
-            onEachLazy(document.getElementsByTagName("details"), function (e) {
-                if (hasClass(e, "type-contents-toggle")) {
-                    e.open = true;
-                }
-            });
-        }
+        onEachLazy(document.getElementsByTagName("details"), function (e) {
+            var showLargeItem = !hideLargeItemContents && hasClass(e, "type-contents-toggle");
+            var showImplementor = !hideImplementors && hasClass(e, "implementors-toggle");
+            if (showLargeItem || showImplementor) {
+                e.open = true;
+            }
+        });
         if (hideMethodDocs === true) {
             impl_call = function(e, newToggle) {
                 if (e.id.match(/^impl(?:-\d+)?$/) === null) {

src/librustdoc/html/static/rustdoc.css

@@ -1567,6 +1567,10 @@ h4 > .notable-traits {
 		left: -10px;
 	}
 
+	.item-list > details.rustdoc-toggle > summary:not(.hideme)::before {
+		left: -10px;
+	}
+
 	#all-types {
 		margin: 10px;
 	}
@@ -1781,14 +1785,16 @@ details.rustdoc-toggle > summary::before {
 	font-weight: 300;
 	font-size: 0.8em;
 	letter-spacing: 1px;
+	cursor: pointer;
 }
 
 details.rustdoc-toggle > summary.hideme::before {
 	position: relative;
 }
 
 details.rustdoc-toggle > summary:not(.hideme)::before {
-	float: left;
+	position: absolute;
+	left: -23px;
 }
 
 /* When a "hideme" summary is open and the "Expand description" or "Show

src/test/rustdoc/const-generics/add-impl.rs

@@ -8,7 +8,7 @@ pub struct Simd<T, const WIDTH: usize> {
     inner: T,
 }
 
-// @has foo/struct.Simd.html '//div[@id="trait-implementations-list"]/h3/code' 'impl Add<Simd<u8, 16_usize>> for Simd<u8, 16>'
+// @has foo/struct.Simd.html '//div[@id="trait-implementations-list"]//h3/code' 'impl Add<Simd<u8, 16_usize>> for Simd<u8, 16>'
 impl Add for Simd<u8, 16> {
     type Output = Self;
 

src/test/rustdoc/duplicate_impls/issue-33054.rs

@@ -1,8 +1,8 @@
 // @has issue_33054/impls/struct.Foo.html
 // @has - '//code' 'impl Foo'
 // @has - '//code' 'impl Bar for Foo'
-// @count - '//*[@id="trait-implementations-list"]/*[@class="impl"]' 1
-// @count - '//*[@id="main"]/*[@class="impl"]' 1
+// @count - '//*[@id="trait-implementations-list"]//*[@class="impl"]' 1
+// @count - '//*[@id="main"]/details/summary/*[@class="impl"]' 1
 // @has issue_33054/impls/bar/trait.Bar.html
 // @has - '//code' 'impl Bar for Foo'
 // @count - '//*[@class="struct"]' 1

src/test/rustdoc/issue-21474.rs

@@ -7,5 +7,5 @@ mod inner {
 pub trait Blah { }
 
 // @count issue_21474/struct.What.html \
-//        '//*[@id="trait-implementations-list"]/*[@class="impl"]' 1
+//        '//*[@id="trait-implementations-list"]//*[@class="impl"]' 1
 pub struct What;

src/test/rustdoc/issue-29503.rs

@@ -5,7 +5,7 @@ pub trait MyTrait {
     fn my_string(&self) -> String;
 }
 
-// @has - "//div[@id='implementors-list']/h3[@id='impl-MyTrait']//code" "impl<T> MyTrait for T where T: Debug"
+// @has - "//div[@id='implementors-list']//h3[@id='impl-MyTrait']//code" "impl<T> MyTrait for T where T: Debug"
 impl<T> MyTrait for T where T: fmt::Debug {
     fn my_string(&self) -> String {
         format!("{:?}", self)