Make String a must_use type

library/alloc/src/fmt.rs

@@ -12,6 +12,7 @@
 //! Some examples of the [`format!`] extension are:
 //!
 //! ```
+//! # #![allow(unused_must_use)]
 //! format!("Hello");                 // => "Hello"
 //! format!("Hello, {}!", "world");   // => "Hello, world!"
 //! format!("The number is {}", 1);   // => "The number is 1"
@@ -44,7 +45,7 @@
 //! the iterator advances. This leads to behavior like this:
 //!
 //! ```
-//! format!("{1} {} {0} {}", 1, 2); // => "2 1 1 2"
+//! println!("{1} {} {0} {}", 1, 2); // => "2 1 1 2"
 //! ```
 //!
 //! The internal iterator over the argument has not been advanced by the time
@@ -71,6 +72,7 @@
 //! For example, the following [`format!`] expressions all use named argument:
 //!
 //! ```
+//! # #![allow(unused_must_use)]
 //! format!("{argument}", argument = "test");   // => "test"
 //! format!("{name} {}", 1, name = 2);          // => "2 1"
 //! format!("{a} {c} {b}", a="a", b='b', c=3);  // => "a 3 b"

library/alloc/src/macros.rs

@@ -95,6 +95,7 @@ macro_rules! vec {
 /// # Examples
 ///
 /// ```
+/// # #![allow(unused_must_use)]
 /// format!("test");
 /// format!("hello {}", "world!");
 /// format!("x = {}, y = {y}", 10, y = 30);

library/alloc/src/str.rs

@@ -487,6 +487,7 @@ impl str {
     /// A panic upon overflow:
     ///
     /// ```should_panic
+    /// # #![allow(unused_must_use)]
     /// // this will panic at runtime
     /// "0123456789abcdef".repeat(usize::MAX);
     /// ```

library/alloc/src/string.rs

@@ -275,6 +275,7 @@ use crate::vec::Vec;
 #[derive(PartialOrd, Eq, Ord)]
 #[cfg_attr(not(test), rustc_diagnostic_item = "string_type")]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[must_use = "unused allocation might not be optimized out by compiler"]
 pub struct String {
     vec: Vec<u8>,
 }
@@ -949,6 +950,7 @@ impl String {
     /// # Examples
     ///
     /// ```
+    /// # #![allow(unused_must_use)]
     /// #![feature(try_reserve)]
     /// use std::collections::TryReserveError;
     ///
@@ -987,6 +989,7 @@ impl String {
     /// # Examples
     ///
     /// ```
+    /// # #![allow(unused_must_use)]
     /// #![feature(try_reserve)]
     /// use std::collections::TryReserveError;
     ///

library/core/tests/fmt/builders.rs

@@ -446,7 +446,7 @@ mod debug_map {
             }
         }
 
-        format!("{:?}", Foo);
+        println!("{:?}", Foo);
     }
 
     #[test]
@@ -460,7 +460,7 @@ mod debug_map {
             }
         }
 
-        format!("{:?}", Foo);
+        println!("{:?}", Foo);
     }
 
     #[test]
@@ -474,7 +474,7 @@ mod debug_map {
             }
         }
 
-        format!("{:?}", Foo);
+        println!("{:?}", Foo);
     }
 }
 

library/core/tests/option.rs

@@ -139,7 +139,7 @@ fn test_unwrap_panic1() {
 #[should_panic]
 fn test_unwrap_panic2() {
     let x: Option<String> = None;
-    x.unwrap();
+    drop(x.unwrap());
 }
 
 #[test]

src/librustdoc/formats/cache.rs

@@ -472,7 +472,7 @@ impl DocFolder for Cache {
         });
 
         if pushed {
-            self.stack.pop().expect("stack already empty");
+            let _ = self.stack.pop().expect("stack already empty");
         }
         if parent_pushed {
             self.parent_stack.pop().expect("parent stack already empty");

src/test/ui/block-result/consider-removing-last-semi.fixed

@@ -5,6 +5,7 @@ pub fn f() -> String {  //~ ERROR mismatched types
     "bla".to_string()
 }
 
+#[allow(unused_must_use)]
 pub fn g() -> String {  //~ ERROR mismatched types
     "this won't work".to_string();
     "removeme".to_string()

src/test/ui/block-result/consider-removing-last-semi.rs

@@ -5,6 +5,7 @@ pub fn f() -> String {  //~ ERROR mismatched types
     "bla".to_string();
 }
 
+#[allow(unused_must_use)]
 pub fn g() -> String {  //~ ERROR mismatched types
     "this won't work".to_string();
     "removeme".to_string();

src/test/ui/block-result/consider-removing-last-semi.stderr

@@ -10,7 +10,7 @@ LL |     "bla".to_string();
    |                      - help: consider removing this semicolon
 
 error[E0308]: mismatched types
-  --> $DIR/consider-removing-last-semi.rs:8:15
+  --> $DIR/consider-removing-last-semi.rs:9:15
    |
 LL | pub fn g() -> String {
    |        -      ^^^^^^ expected struct `std::string::String`, found `()`

src/test/ui/deriving/deriving-in-fn.rs

@@ -6,5 +6,5 @@ pub fn main() {
     }
 
     let f = Foo { foo: 10 };
-    format!("{:?}", f);
+    println!("{:?}", f);
 }

src/test/ui/issues/issue-2063.rs

@@ -14,8 +14,8 @@ impl ToStr2 for T {
 }
 
 #[allow(dead_code)]
-fn new_t(x: T) {
-    x.my_to_string();
+fn new_t(x: T) -> String {
+    x.my_to_string()
 }
 
 fn main() {

src/test/ui/issues/issue-20676.rs

@@ -8,5 +8,5 @@ use std::fmt;
 
 fn main() {
     let a: &dyn fmt::Debug = &1;
-    format!("{:?}", a);
+    println!("{:?}", a);
 }

src/test/ui/istr.rs

@@ -11,6 +11,7 @@ fn test_stack_assign() {
     assert!((s != u));
 }
 
+#[allow(unused_must_use)]
 fn test_heap_lit() { "a big string".to_string(); }
 
 fn test_heap_assign() {

src/test/ui/nll/issue-61424.fixed

@@ -5,5 +5,5 @@
 fn main() {
     let x; //~ ERROR: variable does not need to be mutable
     x = String::new();
-    dbg!(x);
+    let _ = dbg!(x);
 }

src/test/ui/nll/issue-61424.rs

@@ -5,5 +5,5 @@
 fn main() {
     let mut x; //~ ERROR: variable does not need to be mutable
     x = String::new();
-    dbg!(x);
+    let _ = dbg!(x);
 }

src/test/ui/overloaded/overloaded-deref-count.rs

@@ -64,7 +64,7 @@ pub fn main() {
 
     // Immutable deref used for calling a method taking &self. (The
     // typechecker is smarter now about doing this.)
-    (*n).to_string();
+    let _ = (*n).to_string();
     assert_eq!(n.counts(), (3, 3));
 
     // Mutable deref used for calling a method taking &mut self.

src/tools/clippy/src/driver.rs

@@ -348,7 +348,7 @@ pub fn main() {
         // for example `clippy-driver --rustc --version` will print the rustc version that clippy-driver
         // uses
         if let Some(pos) = orig_args.iter().position(|arg| arg == "--rustc") {
-            orig_args.remove(pos);
+            drop(orig_args.remove(pos));
             orig_args[0] = "rustc".to_string();
 
             // if we call "rustc", we need to pass --sysroot here as well
@@ -372,7 +372,7 @@ pub fn main() {
 
         if wrapper_mode {
             // we still want to be able to invoke it normally though
-            orig_args.remove(1);
+            drop(orig_args.remove(1));
         }
 
         if !wrapper_mode && (orig_args.iter().any(|a| a == "--help" || a == "-h") || orig_args.len() == 1) {

src/tools/clippy/tests/ui/format.fixed

@@ -1,6 +1,6 @@
 // run-rustfix
 
-#![allow(clippy::print_literal, clippy::redundant_clone)]
+#![allow(unused_must_use, clippy::print_literal, clippy::redundant_clone)]
 #![warn(clippy::useless_format)]
 
 struct Foo(pub String);

src/tools/clippy/tests/ui/format.rs

@@ -1,6 +1,6 @@
 // run-rustfix
 
-#![allow(clippy::print_literal, clippy::redundant_clone)]
+#![allow(unused_must_use, clippy::print_literal, clippy::redundant_clone)]
 #![warn(clippy::useless_format)]
 
 struct Foo(pub String);

src/tools/clippy/tests/ui/question_mark.fixed

@@ -94,8 +94,8 @@ impl MoveStruct {
 }
 
 fn func() -> Option<i32> {
-    fn f() -> Option<String> {
-        Some(String::new())
+    fn f() -> Option<u32> {
+        Some(42)
     }
 
     f()?;

src/tools/clippy/tests/ui/question_mark.rs

@@ -122,8 +122,8 @@ impl MoveStruct {
 }
 
 fn func() -> Option<i32> {
-    fn f() -> Option<String> {
-        Some(String::new())
+    fn f() -> Option<u32> {
+        Some(42)
     }
 
     if f().is_none() {

src/tools/clippy/tests/ui/redundant_clone.fixed

@@ -48,7 +48,7 @@ fn main() {
 
     with_branch(Alpha, true);
     cannot_double_move(Alpha);
-    cannot_move_from_type_with_drop();
+    drop(cannot_move_from_type_with_drop());
     borrower_propagation();
     not_consumed();
     issue_5405();

src/tools/clippy/tests/ui/redundant_clone.rs

@@ -48,7 +48,7 @@ fn main() {
 
     with_branch(Alpha, true);
     cannot_double_move(Alpha);
-    cannot_move_from_type_with_drop();
+    drop(cannot_move_from_type_with_drop());
     borrower_propagation();
     not_consumed();
     issue_5405();