Fix #103320, add explanatory message for [#must_use]

compiler/rustc_error_messages/locales/en-US/borrowck.ftl

@@ -33,7 +33,7 @@ borrowck_var_here_defined = variable defined here
 
 borrowck_var_here_captured = variable captured here
 
-borrowck_closure_inferred_mut =  inferred to be a `FnMut` closure
+borrowck_closure_inferred_mut = inferred to be a `FnMut` closure
 
 borrowck_returned_closure_escaped =
     returns a closure that contains a reference to a captured variable, which then escapes the closure body

compiler/rustc_error_messages/locales/en-US/lint.ftl

@@ -309,6 +309,7 @@ lint_unused_generator =
     .note = generators are lazy and do nothing unless resumed
 
 lint_unused_def = unused {$pre}`{$def}`{$post} that must be used
+    .suggestion = use `let _ = ...` to ignore the resulting value
 
 lint_path_statement_drop = path statement drops value
     .suggestion = use `drop` to clarify the intent

compiler/rustc_lint/src/lints.rs

@@ -1402,6 +1402,21 @@ pub struct UnusedDef<'a, 'b> {
     pub cx: &'a LateContext<'b>,
     pub def_id: DefId,
     pub note: Option<Symbol>,
+    pub suggestion: Option<UnusedDefSuggestion>,
+}
+
+#[derive(Subdiagnostic)]
+pub enum UnusedDefSuggestion {
+    #[suggestion(
+        suggestion,
+        style = "verbose",
+        code = "let _ = ",
+        applicability = "machine-applicable"
+    )]
+    Default {
+        #[primary_span]
+        span: Span,
+    },
 }
 
 // Needed because of def_path_str
@@ -1417,6 +1432,9 @@ fn decorate_lint<'b>(
         if let Some(note) = self.note {
             diag.note(note.as_str());
         }
+        if let Some(sugg) = self.suggestion {
+            diag.subdiagnostic(sugg);
+        }
         diag
     }
 

compiler/rustc_lint/src/unused.rs

@@ -1,7 +1,7 @@
 use crate::lints::{
     PathStatementDrop, PathStatementDropSub, PathStatementNoEffect, UnusedAllocationDiag,
-    UnusedAllocationMutDiag, UnusedClosure, UnusedDef, UnusedDelim, UnusedDelimSuggestion,
-    UnusedGenerator, UnusedImportBracesDiag, UnusedOp, UnusedResult,
+    UnusedAllocationMutDiag, UnusedClosure, UnusedDef, UnusedDefSuggestion, UnusedDelim,
+    UnusedDelimSuggestion, UnusedGenerator, UnusedImportBracesDiag, UnusedOp, UnusedResult,
 };
 use crate::Lint;
 use crate::{EarlyContext, EarlyLintPass, LateContext, LateLintPass, LintContext};
@@ -418,6 +418,19 @@ fn emit_must_use_untranslated(
                     );
                 }
                 MustUsePath::Def(span, def_id, reason) => {
+                    let suggestion = if matches!(
+                        cx.tcx.get_diagnostic_name(*def_id),
+                        Some(sym::add)
+                            | Some(sym::sub)
+                            | Some(sym::mul)
+                            | Some(sym::div)
+                            | Some(sym::rem)
+                            | Some(sym::neg),
+                    ) {
+                        Some(UnusedDefSuggestion::Default { span: span.shrink_to_lo() })
+                    } else {
+                        None
+                    };
                     cx.emit_spanned_lint(
                         UNUSED_MUST_USE,
                         *span,
@@ -427,6 +440,7 @@ fn emit_must_use_untranslated(
                             cx,
                             def_id: *def_id,
                             note: *reason,
+                            suggestion,
                         },
                     );
                 }

library/core/src/ops/arith.rs

@@ -86,7 +86,8 @@ pub trait Add<Rhs = Self> {
     /// ```
     /// assert_eq!(12 + 1, 13);
     /// ```
-    #[must_use]
+    #[must_use = "this returns the result of the operation, without modifying the original"]
+    #[rustc_diagnostic_item = "add"]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn add(self, rhs: Rhs) -> Self::Output;
 }
@@ -195,7 +196,8 @@ pub trait Sub<Rhs = Self> {
     /// ```
     /// assert_eq!(12 - 1, 11);
     /// ```
-    #[must_use]
+    #[must_use = "this returns the result of the operation, without modifying the original"]
+    #[rustc_diagnostic_item = "sub"]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn sub(self, rhs: Rhs) -> Self::Output;
 }
@@ -325,7 +327,8 @@ pub trait Mul<Rhs = Self> {
     /// ```
     /// assert_eq!(12 * 2, 24);
     /// ```
-    #[must_use]
+    #[must_use = "this returns the result of the operation, without modifying the original"]
+    #[rustc_diagnostic_item = "mul"]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn mul(self, rhs: Rhs) -> Self::Output;
 }
@@ -459,7 +462,8 @@ pub trait Div<Rhs = Self> {
     /// ```
     /// assert_eq!(12 / 2, 6);
     /// ```
-    #[must_use]
+    #[must_use = "this returns the result of the operation, without modifying the original"]
+    #[rustc_diagnostic_item = "div"]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn div(self, rhs: Rhs) -> Self::Output;
 }
@@ -562,7 +566,8 @@ pub trait Rem<Rhs = Self> {
     /// ```
     /// assert_eq!(12 % 10, 2);
     /// ```
-    #[must_use]
+    #[must_use = "this returns the result of the operation, without modifying the original"]
+    #[rustc_diagnostic_item = "rem"]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn rem(self, rhs: Rhs) -> Self::Output;
 }
@@ -678,7 +683,8 @@ pub trait Neg {
     /// let x: i32 = 12;
     /// assert_eq!(-x, -12);
     /// ```
-    #[must_use]
+    #[must_use = "this returns the result of the operation, without modifying the original"]
+    #[rustc_diagnostic_item = "neg"]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn neg(self) -> Self::Output;
 }

tests/ui/lint/unused/issue-103320-must-use-ops.rs

@@ -0,0 +1,27 @@
+// check-pass
+
+#![warn(unused_must_use)]
+#![feature(never_type)]
+
+use std::ops::Add;
+use std::ops::Sub;
+use std::ops::Mul;
+use std::ops::Div;
+use std::ops::Rem;
+
+fn main() {
+    let x = 2_u32;
+    (x.add(4), x.sub(4), x.mul(4), x.div(4), x.rem(4));
+
+    x.add(4); //~ WARN unused return value of `add` that must be used
+
+    x.sub(4); //~ WARN unused return value of `sub` that must be used
+
+    x.mul(4); //~ WARN unused return value of `mul` that must be used
+
+    x.div(4); //~ WARN unused return value of `div` that must be used
+
+    x.rem(4); //~ WARN unused return value of `rem` that must be used
+
+    println!("{}", x);
+}

tests/ui/lint/unused/issue-103320-must-use-ops.stderr

@@ -0,0 +1,67 @@
+warning: unused return value of `add` that must be used
+  --> $DIR/issue-103320-must-use-ops.rs:16:5
+   |
+LL |     x.add(4);
+   |     ^^^^^^^^
+   |
+   = note: this returns the result of the operation, without modifying the original
+note: the lint level is defined here
+  --> $DIR/issue-103320-must-use-ops.rs:3:9
+   |
+LL | #![warn(unused_must_use)]
+   |         ^^^^^^^^^^^^^^^
+help: use `let _ = ...` to ignore the resulting value
+   |
+LL |     let _ = x.add(4);
+   |     +++++++
+
+warning: unused return value of `sub` that must be used
+  --> $DIR/issue-103320-must-use-ops.rs:18:5
+   |
+LL |     x.sub(4);
+   |     ^^^^^^^^
+   |
+   = note: this returns the result of the operation, without modifying the original
+help: use `let _ = ...` to ignore the resulting value
+   |
+LL |     let _ = x.sub(4);
+   |     +++++++
+
+warning: unused return value of `mul` that must be used
+  --> $DIR/issue-103320-must-use-ops.rs:20:5
+   |
+LL |     x.mul(4);
+   |     ^^^^^^^^
+   |
+   = note: this returns the result of the operation, without modifying the original
+help: use `let _ = ...` to ignore the resulting value
+   |
+LL |     let _ = x.mul(4);
+   |     +++++++
+
+warning: unused return value of `div` that must be used
+  --> $DIR/issue-103320-must-use-ops.rs:22:5
+   |
+LL |     x.div(4);
+   |     ^^^^^^^^
+   |
+   = note: this returns the result of the operation, without modifying the original
+help: use `let _ = ...` to ignore the resulting value
+   |
+LL |     let _ = x.div(4);
+   |     +++++++
+
+warning: unused return value of `rem` that must be used
+  --> $DIR/issue-103320-must-use-ops.rs:24:5
+   |
+LL |     x.rem(4);
+   |     ^^^^^^^^
+   |
+   = note: this returns the result of the operation, without modifying the original
+help: use `let _ = ...` to ignore the resulting value
+   |
+LL |     let _ = x.rem(4);
+   |     +++++++
+
+warning: 5 warnings emitted
+