Lint against more useless #[must_use] attributes

This expands the existing `#[must_use]` check in `unused_attributes`
to lint against pretty much everything `#[must_use]` doesn't support.
Fixes #93906.

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

@@ -1111,7 +1111,7 @@ 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() {
@@ -1131,6 +1131,39 @@ impl CheckAttrVisitor<'_> {
             }
         }
 
+        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
         true
     }

src/test/ui/feature-gates/issue-43106-gating-of-builtin-attrs.rs

@@ -71,6 +71,7 @@
 //~^^ WARN this was previously accepted by the compiler
 // see issue-43106-gating-of-rustc_deprecated.rs
 #![must_use]
+//~^ WARN `#[must_use]` has no effect
 // see issue-43106-gating-of-stable.rs
 // see issue-43106-gating-of-unstable.rs
 // see issue-43106-gating-of-deprecated.rs
@@ -597,17 +598,17 @@ mod deprecated {
     #[deprecated] impl super::StructForDeprecated { }
 }
 
-#[must_use]
+#[must_use] //~ WARN `#[must_use]` has no effect
 mod must_use {
-    mod inner { #![must_use] }
+    mod inner { #![must_use] } //~ WARN `#[must_use]` has no effect
 
     #[must_use] fn f() { }
 
     #[must_use] struct S;
 
-    #[must_use] type T = S;
+    #[must_use] type T = S; //~ WARN `#[must_use]` has no effect
 
-    #[must_use] impl S { }
+    #[must_use] impl S { } //~ WARN `#[must_use]` has no effect
 }
 
 #[windows_subsystem = "windows"]

src/test/ui/lint/unused/unused_attributes-must_use.rs

@@ -0,0 +1,125 @@
+#![allow(dead_code, path_statements)]
+#![deny(unused_attributes, unused_must_use)]
+#![feature(asm_experimental_arch, stmt_expr_attributes, trait_alias)]
+
+#[must_use] //~ ERROR `#[must_use]` has no effect
+extern crate std as std2;
+
+#[must_use] //~ ERROR `#[must_use]` has no effect
+mod test_mod {}
+
+#[must_use] //~ ERROR `#[must_use]` has no effect
+use std::arch::global_asm;
+
+#[must_use] //~ ERROR `#[must_use]` has no effect
+const CONST: usize = 4;
+#[must_use] //~ ERROR `#[must_use]` has no effect
+#[no_mangle]
+static STATIC: usize = 4;
+
+#[must_use]
+struct X;
+
+#[must_use]
+enum Y {
+    Z,
+}
+
+#[must_use]
+union U {
+    unit: (),
+}
+
+#[must_use] //~ ERROR `#[must_use]` has no effect
+impl U {
+    #[must_use]
+    fn method() -> i32 {
+        4
+    }
+}
+
+#[must_use]
+#[no_mangle]
+fn foo() -> i64 {
+    4
+}
+
+#[must_use] //~ ERROR `#[must_use]` has no effect
+extern "Rust" {
+    #[link_name = "STATIC"]
+    #[must_use] //~ ERROR `#[must_use]` has no effect
+    static FOREIGN_STATIC: usize;
+
+    #[link_name = "foo"]
+    #[must_use]
+    fn foreign_foo() -> i64;
+}
+
+#[must_use] //~ ERROR unused attribute
+global_asm!("");
+
+#[must_use] //~ ERROR `#[must_use]` has no effect
+type UseMe = ();
+
+fn qux<#[must_use] T>(_: T) {} //~ ERROR `#[must_use]` has no effect
+
+#[must_use]
+trait Use {
+    #[must_use] //~ ERROR `#[must_use]` has no effect
+    const ASSOC_CONST: usize = 4;
+    #[must_use] //~ ERROR `#[must_use]` has no effect
+    type AssocTy;
+
+    #[must_use]
+    fn get_four(&self) -> usize {
+        4
+    }
+}
+
+#[must_use] //~ ERROR `#[must_use]` has no effect
+impl Use for () {
+    type AssocTy = ();
+}
+
+#[must_use] //~ ERROR `#[must_use]` has no effect
+trait Alias = Use;
+
+#[must_use] //~ ERROR `#[must_use]` has no effect
+macro_rules! cool_macro {
+    () => {
+        4
+    };
+}
+
+fn main() {
+    #[must_use] //~ ERROR `#[must_use]` has no effect
+    let x = || {};
+    x();
+
+    let x = #[must_use] //~ ERROR `#[must_use]` has no effect
+    || {};
+    x();
+
+    X; //~ ERROR that must be used
+    Y::Z; //~ ERROR that must be used
+    U { unit: () }; //~ ERROR that must be used
+    U::method(); //~ ERROR that must be used
+    foo(); //~ ERROR that must be used
+
+    unsafe {
+        foreign_foo(); //~ ERROR that must be used
+    };
+
+    CONST;
+    STATIC;
+    unsafe { FOREIGN_STATIC };
+    cool_macro!();
+    qux(4);
+    ().get_four(); //~ ERROR that must be used
+
+    match Some(4) {
+        #[must_use] //~ ERROR `#[must_use]` has no effect
+        Some(res) => res,
+        None => 0,
+    };
+}

src/test/ui/lint/unused/unused_attributes-must_use.stderr

@@ -0,0 +1,175 @@
+error: unused attribute `must_use`
+  --> $DIR/unused_attributes-must_use.rs:58:1
+   |
+LL | #[must_use]
+   | ^^^^^^^^^^^
+   |
+note: the lint level is defined here
+  --> $DIR/unused_attributes-must_use.rs:2:9
+   |
+LL | #![deny(unused_attributes, unused_must_use)]
+   |         ^^^^^^^^^^^^^^^^^
+note: the built-in attribute `must_use` will be ignored, since it's applied to the macro invocation `global_asm`
+  --> $DIR/unused_attributes-must_use.rs:59:1
+   |
+LL | global_asm!("");
+   | ^^^^^^^^^^
+
+error: `#[must_use]` has no effect when applied to an extern crate
+  --> $DIR/unused_attributes-must_use.rs:5:1
+   |
+LL | #[must_use]
+   | ^^^^^^^^^^^
+
+error: `#[must_use]` has no effect when applied to a module
+  --> $DIR/unused_attributes-must_use.rs:8:1
+   |
+LL | #[must_use]
+   | ^^^^^^^^^^^
+
+error: `#[must_use]` has no effect when applied to a use
+  --> $DIR/unused_attributes-must_use.rs:11:1
+   |
+LL | #[must_use]
+   | ^^^^^^^^^^^
+
+error: `#[must_use]` has no effect when applied to a constant item
+  --> $DIR/unused_attributes-must_use.rs:14:1
+   |
+LL | #[must_use]
+   | ^^^^^^^^^^^
+
+error: `#[must_use]` has no effect when applied to a static item
+  --> $DIR/unused_attributes-must_use.rs:16:1
+   |
+LL | #[must_use]
+   | ^^^^^^^^^^^
+
+error: `#[must_use]` has no effect when applied to an item
+  --> $DIR/unused_attributes-must_use.rs:33:1
+   |
+LL | #[must_use]
+   | ^^^^^^^^^^^
+
+error: `#[must_use]` has no effect when applied to a foreign module
+  --> $DIR/unused_attributes-must_use.rs:47:1
+   |
+LL | #[must_use]
+   | ^^^^^^^^^^^
+
+error: `#[must_use]` has no effect when applied to a type alias
+  --> $DIR/unused_attributes-must_use.rs:61:1
+   |
+LL | #[must_use]
+   | ^^^^^^^^^^^
+
+error: `#[must_use]` has no effect when applied to a type parameter
+  --> $DIR/unused_attributes-must_use.rs:64:8
+   |
+LL | fn qux<#[must_use] T>(_: T) {}
+   |        ^^^^^^^^^^^
+
+error: `#[must_use]` has no effect when applied to an item
+  --> $DIR/unused_attributes-must_use.rs:79:1
+   |
+LL | #[must_use]
+   | ^^^^^^^^^^^
+
+error: `#[must_use]` has no effect when applied to a trait alias
+  --> $DIR/unused_attributes-must_use.rs:84:1
+   |
+LL | #[must_use]
+   | ^^^^^^^^^^^
+
+error: `#[must_use]` has no effect when applied to a macro def
+  --> $DIR/unused_attributes-must_use.rs:87:1
+   |
+LL | #[must_use]
+   | ^^^^^^^^^^^
+
+error: `#[must_use]` has no effect when applied to a statement
+  --> $DIR/unused_attributes-must_use.rs:95:5
+   |
+LL |     #[must_use]
+   |     ^^^^^^^^^^^
+
+error: `#[must_use]` has no effect when applied to a closure
+  --> $DIR/unused_attributes-must_use.rs:99:13
+   |
+LL |     let x = #[must_use]
+   |             ^^^^^^^^^^^
+
+error: `#[must_use]` has no effect when applied to an match arm
+  --> $DIR/unused_attributes-must_use.rs:121:9
+   |
+LL |         #[must_use]
+   |         ^^^^^^^^^^^
+
+error: `#[must_use]` has no effect when applied to an associated const
+  --> $DIR/unused_attributes-must_use.rs:68:5
+   |
+LL |     #[must_use]
+   |     ^^^^^^^^^^^
+
+error: `#[must_use]` has no effect when applied to an associated type
+  --> $DIR/unused_attributes-must_use.rs:70:5
+   |
+LL |     #[must_use]
+   |     ^^^^^^^^^^^
+
+error: `#[must_use]` has no effect when applied to a foreign static item
+  --> $DIR/unused_attributes-must_use.rs:50:5
+   |
+LL |     #[must_use]
+   |     ^^^^^^^^^^^
+
+error: unused `X` that must be used
+  --> $DIR/unused_attributes-must_use.rs:103:5
+   |
+LL |     X;
+   |     ^^
+   |
+note: the lint level is defined here
+  --> $DIR/unused_attributes-must_use.rs:2:28
+   |
+LL | #![deny(unused_attributes, unused_must_use)]
+   |                            ^^^^^^^^^^^^^^^
+
+error: unused `Y` that must be used
+  --> $DIR/unused_attributes-must_use.rs:104:5
+   |
+LL |     Y::Z;
+   |     ^^^^^
+
+error: unused `U` that must be used
+  --> $DIR/unused_attributes-must_use.rs:105:5
+   |
+LL |     U { unit: () };
+   |     ^^^^^^^^^^^^^^^
+
+error: unused return value of `U::method` that must be used
+  --> $DIR/unused_attributes-must_use.rs:106:5
+   |
+LL |     U::method();
+   |     ^^^^^^^^^^^^
+
+error: unused return value of `foo` that must be used
+  --> $DIR/unused_attributes-must_use.rs:107:5
+   |
+LL |     foo();
+   |     ^^^^^^
+
+error: unused return value of `foreign_foo` that must be used
+  --> $DIR/unused_attributes-must_use.rs:110:9
+   |
+LL |         foreign_foo();
+   |         ^^^^^^^^^^^^^^
+
+error: unused return value of `Use::get_four` that must be used
+  --> $DIR/unused_attributes-must_use.rs:118:5
+   |
+LL |     ().get_four();
+   |     ^^^^^^^^^^^^^^
+
+error: aborting due to 26 previous errors
+