Rollup of 7 pull requests

compiler/rustc_macros/src/session_diagnostic.rs

@@ -587,7 +587,6 @@ impl<'a> SessionDiagnosticDeriveBuilder<'a> {
         // next call to `it.next()` retrieves the next character.
         while let Some(c) = it.next() {
             if c == '{' && *it.peek().unwrap_or(&'\0') != '{' {
-                #[must_use]
                 let mut eat_argument = || -> Option<String> {
                     let mut result = String::new();
                     // Format specifiers look like

compiler/rustc_passes/src/check_attr.rs

@@ -1111,24 +1111,55 @@ impl CheckAttrVisitor<'_> {
     }
 
     /// Warns against some misuses of `#[must_use]`
-    fn check_must_use(&self, hir_id: HirId, attr: &Attribute, span: Span, _target: Target) -> bool {
+    fn check_must_use(&self, hir_id: HirId, attr: &Attribute, span: Span, target: Target) -> bool {
         let node = self.tcx.hir().get(hir_id);
-        if let Some(fn_node) = node.fn_kind() {
-            if let rustc_hir::IsAsync::Async = fn_node.asyncness() {
-                self.tcx.struct_span_lint_hir(UNUSED_ATTRIBUTES, hir_id, attr.span, |lint| {
-                    lint.build(
-                        "`must_use` attribute on `async` functions \
-                              applies to the anonymous `Future` returned by the \
-                              function, not the value within",
-                    )
-                    .span_label(
-                        span,
-                        "this attribute does nothing, the `Future`s \
-                                returned by async functions are already `must_use`",
-                    )
-                    .emit();
-                });
-            }
+        if let Some(kind) = node.fn_kind() && let rustc_hir::IsAsync::Async = kind.asyncness() {
+            self.tcx.struct_span_lint_hir(UNUSED_ATTRIBUTES, hir_id, attr.span, |lint| {
+                lint.build(
+                    "`must_use` attribute on `async` functions \
+                    applies to the anonymous `Future` returned by the \
+                    function, not the value within",
+                )
+                .span_label(
+                    span,
+                    "this attribute does nothing, the `Future`s \
+                    returned by async functions are already `must_use`",
+                )
+                .emit();
+            });
+        }
+
+        if !matches!(
+            target,
+            Target::Fn
+                | Target::Enum
+                | Target::Struct
+                | Target::Union
+                | Target::Method(_)
+                | Target::ForeignFn
+                // `impl Trait` in return position can trip
+                // `unused_must_use` if `Trait` is marked as
+                // `#[must_use]`
+                | Target::Trait
+        ) {
+            let article = match target {
+                Target::ExternCrate
+                | Target::OpaqueTy
+                | Target::Enum
+                | Target::Impl
+                | Target::Expression
+                | Target::Arm
+                | Target::AssocConst
+                | Target::AssocTy => "an",
+                _ => "a",
+            };
+
+            self.tcx.struct_span_lint_hir(UNUSED_ATTRIBUTES, hir_id, attr.span, |lint| {
+                lint.build(&format!(
+                    "`#[must_use]` has no effect when applied to {article} {target}"
+                ))
+                .emit();
+            });
         }
 
         // For now, its always valid

compiler/rustc_passes/src/dead.rs

@@ -683,34 +683,33 @@ impl<'tcx> DeadVisitor<'tcx> {
                 let descr = self.tcx.def_kind(def_id).descr(def_id.to_def_id());
                 let mut err = lint.build(&format!("{} is never {}: `{}`", descr, participle, name));
                 let hir = self.tcx.hir();
-                if let Some(encl_scope) = hir.get_enclosing_scope(id) {
-                    if let Some(encl_def_id) = hir.opt_local_def_id(encl_scope) {
-                        if let Some(ign_traits) = self.ignored_derived_traits.get(&encl_def_id) {
-                            let traits_str = ign_traits
-                                .iter()
-                                .map(|(trait_id, _)| format!("`{}`", self.tcx.item_name(*trait_id)))
-                                .collect::<Vec<_>>()
-                                .join(" and ");
-                            let plural_s = pluralize!(ign_traits.len());
-                            let article = if ign_traits.len() > 1 { "" } else { "a " };
-                            let is_are = if ign_traits.len() > 1 { "these are" } else { "this is" };
-                            let msg = format!(
-                                "`{}` has {}derived impl{} for the trait{} {}, but {} \
-                                 intentionally ignored during dead code analysis",
-                                self.tcx.item_name(encl_def_id.to_def_id()),
-                                article,
-                                plural_s,
-                                plural_s,
-                                traits_str,
-                                is_are
-                            );
-                            let multispan = ign_traits
-                                .iter()
-                                .map(|(_, impl_id)| self.tcx.def_span(*impl_id))
-                                .collect::<Vec<_>>();
-                            err.span_note(multispan, &msg);
-                        }
-                    }
+                if let Some(encl_scope) = hir.get_enclosing_scope(id)
+                    && let Some(encl_def_id) = hir.opt_local_def_id(encl_scope)
+                    && let Some(ign_traits) = self.ignored_derived_traits.get(&encl_def_id)
+                {
+                    let traits_str = ign_traits
+                        .iter()
+                        .map(|(trait_id, _)| format!("`{}`", self.tcx.item_name(*trait_id)))
+                        .collect::<Vec<_>>()
+                        .join(" and ");
+                    let plural_s = pluralize!(ign_traits.len());
+                    let article = if ign_traits.len() > 1 { "" } else { "a " };
+                    let is_are = if ign_traits.len() > 1 { "these are" } else { "this is" };
+                    let msg = format!(
+                        "`{}` has {}derived impl{} for the trait{} {}, but {} \
+                        intentionally ignored during dead code analysis",
+                        self.tcx.item_name(encl_def_id.to_def_id()),
+                        article,
+                        plural_s,
+                        plural_s,
+                        traits_str,
+                        is_are
+                    );
+                    let multispan = ign_traits
+                        .iter()
+                        .map(|(_, impl_id)| self.tcx.def_span(*impl_id))
+                        .collect::<Vec<_>>();
+                    err.span_note(multispan, &msg);
                 }
                 err.emit();
             });

compiler/rustc_passes/src/lib.rs

@@ -4,16 +4,17 @@
 //!
 //! This API is completely unstable and subject to change.
 
+#![allow(rustc::potential_query_instability)]
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![feature(crate_visibility_modifier)]
 #![feature(iter_intersperse)]
 #![feature(let_else)]
+#![feature(let_chains)]
 #![feature(map_try_insert)]
 #![feature(min_specialization)]
 #![feature(nll)]
 #![feature(try_blocks)]
 #![recursion_limit = "256"]
-#![allow(rustc::potential_query_instability)]
 
 #[macro_use]
 extern crate rustc_middle;

compiler/rustc_passes/src/liveness.rs

@@ -332,12 +332,11 @@ impl<'tcx> Visitor<'tcx> for IrMaps<'tcx> {
         let def_id = local_def_id.to_def_id();
 
         // Don't run unused pass for #[derive()]
-        if let Some(parent) = self.tcx.parent(def_id) {
-            if let DefKind::Impl = self.tcx.def_kind(parent.expect_local()) {
-                if self.tcx.has_attr(parent, sym::automatically_derived) {
-                    return;
-                }
-            }
+        if let Some(parent) = self.tcx.parent(def_id)
+            && let DefKind::Impl = self.tcx.def_kind(parent.expect_local())
+            && self.tcx.has_attr(parent, sym::automatically_derived)
+        {
+            return;
         }
 
         // Don't run unused pass for #[naked]

compiler/rustc_passes/src/reachable.rs

@@ -94,24 +94,22 @@ impl<'tcx> Visitor<'tcx> for ReachableContext<'tcx> {
             _ => None,
         };
 
-        if let Some(res) = res {
-            if let Some(def_id) = res.opt_def_id().and_then(|def_id| def_id.as_local()) {
-                if self.def_id_represents_local_inlined_item(def_id.to_def_id()) {
-                    self.worklist.push(def_id);
-                } else {
-                    match res {
-                        // If this path leads to a constant, then we need to
-                        // recurse into the constant to continue finding
-                        // items that are reachable.
-                        Res::Def(DefKind::Const | DefKind::AssocConst, _) => {
-                            self.worklist.push(def_id);
-                        }
+        if let Some(res) = res && let Some(def_id) = res.opt_def_id().and_then(|el| el.as_local()) {
+            if self.def_id_represents_local_inlined_item(def_id.to_def_id()) {
+                self.worklist.push(def_id);
+            } else {
+                match res {
+                    // If this path leads to a constant, then we need to
+                    // recurse into the constant to continue finding
+                    // items that are reachable.
+                    Res::Def(DefKind::Const | DefKind::AssocConst, _) => {
+                        self.worklist.push(def_id);
+                    }
 
-                        // If this wasn't a static, then the destination is
-                        // surely reachable.
-                        _ => {
-                            self.reachable_symbols.insert(def_id);
-                        }
+                    // If this wasn't a static, then the destination is
+                    // surely reachable.
+                    _ => {
+                        self.reachable_symbols.insert(def_id);
                     }
                 }
             }