diff --git a/src/liballoc/str.rs b/src/liballoc/str.rs
index 9e693c89be90d..56c48465fb9ec 100644
--- a/src/liballoc/str.rs
+++ b/src/liballoc/str.rs
@@ -305,6 +305,7 @@ impl str {
     ///
     /// assert_eq!(new_year, new_year.to_lowercase());
     /// ```
+    #[must_use]
     #[stable(feature = "unicode_case_mapping", since = "1.2.0")]
     pub fn to_lowercase(&self) -> String {
         let mut s = String::with_capacity(self.len());
@@ -368,6 +369,7 @@ impl str {
     ///
     /// assert_eq!(new_year, new_year.to_uppercase());
     /// ```
+    #[must_use]
     #[stable(feature = "unicode_case_mapping", since = "1.2.0")]
     pub fn to_uppercase(&self) -> String {
         let mut s = String::with_capacity(self.len());
@@ -378,6 +380,7 @@ impl str {
     /// Escapes each char in `s` with [`char::escape_debug`].
     ///
     /// [`char::escape_debug`]: primitive.char.html#method.escape_debug
+    #[must_use]
     #[unstable(feature = "str_escape",
                reason = "return type may change to be an iterator",
                issue = "27791")]
@@ -388,6 +391,7 @@ impl str {
     /// Escapes each char in `s` with [`char::escape_default`].
     ///
     /// [`char::escape_default`]: primitive.char.html#method.escape_default
+    #[must_use]
     #[unstable(feature = "str_escape",
                reason = "return type may change to be an iterator",
                issue = "27791")]
@@ -398,6 +402,7 @@ impl str {
     /// Escapes each char in `s` with [`char::escape_unicode`].
     ///
     /// [`char::escape_unicode`]: primitive.char.html#method.escape_unicode
+    #[must_use]
     #[unstable(feature = "str_escape",
                reason = "return type may change to be an iterator",
                issue = "27791")]
@@ -420,6 +425,7 @@ impl str {
     ///
     /// assert_eq!(boxed_str.into_string(), string);
     /// ```
+    #[must_use]
     #[stable(feature = "box_str", since = "1.4.0")]
     #[inline]
     pub fn into_string(self: Box<str>) -> String {
@@ -438,6 +444,7 @@ impl str {
     /// ```
     /// assert_eq!("abc".repeat(4), String::from("abcabcabcabc"));
     /// ```
+    #[must_use]
     #[stable(feature = "repeat_str", since = "1.16.0")]
     pub fn repeat(&self, n: usize) -> String {
         unsafe { String::from_utf8_unchecked(self.as_bytes().repeat(n)) }
@@ -464,6 +471,7 @@ impl str {
     ///
     /// [`make_ascii_uppercase`]: #method.make_ascii_uppercase
     /// [`to_uppercase`]: #method.to_uppercase
+    #[must_use]
     #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
     #[inline]
     pub fn to_ascii_uppercase(&self) -> String {
@@ -494,6 +502,7 @@ impl str {
     ///
     /// [`make_ascii_lowercase`]: #method.make_ascii_lowercase
     /// [`to_lowercase`]: #method.to_lowercase
+    #[must_use]
     #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
     #[inline]
     pub fn to_ascii_lowercase(&self) -> String {
diff --git a/src/liballoc/string.rs b/src/liballoc/string.rs
index da9afdd2ca37b..02a7deb00e813 100644
--- a/src/liballoc/string.rs
+++ b/src/liballoc/string.rs
@@ -422,6 +422,7 @@ impl String {
     /// // ...but this may make the vector reallocate
     /// s.push('a');
     /// ```
+    #[must_use]
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn with_capacity(capacity: usize) -> String {
@@ -432,6 +433,7 @@ impl String {
     // required for this method definition, is not available. Since we don't
     // require this method for testing purposes, I'll just stub it
     // NB see the slice::hack module in slice.rs for more information
+    #[must_use]
     #[inline]
     #[cfg(test)]
     pub fn from_str(_: &str) -> String {
@@ -643,6 +645,7 @@ impl String {
     /// assert_eq!(String::from("𝄞mus\u{FFFD}ic\u{FFFD}"),
     ///            String::from_utf16_lossy(v));
     /// ```
+    #[must_use]
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn from_utf16_lossy(v: &[u16]) -> String {
@@ -690,6 +693,7 @@ impl String {
     ///     assert_eq!(String::from("hello"), s);
     /// }
     /// ```
+    #[must_use]
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub unsafe fn from_raw_parts(buf: *mut u8, length: usize, capacity: usize) -> String {
@@ -724,6 +728,7 @@ impl String {
     ///
     /// assert_eq!("💖", sparkle_heart);
     /// ```
+    #[must_use]
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub unsafe fn from_utf8_unchecked(bytes: Vec<u8>) -> String {
@@ -1420,6 +1425,7 @@ impl String {
     /// assert_eq!(world, "World!");
     /// # }
     /// ```
+    #[must_use]
     #[inline]
     #[stable(feature = "string_split_off", since = "1.16.0")]
     pub fn split_off(&mut self, at: usize) -> String {
diff --git a/src/liballoc/tests/string.rs b/src/liballoc/tests/string.rs
index befb36baeef1e..17f646f1391d0 100644
--- a/src/liballoc/tests/string.rs
+++ b/src/liballoc/tests/string.rs
@@ -244,14 +244,14 @@ fn test_split_off_empty() {
 fn test_split_off_past_end() {
     let orig = "Hello, world!";
     let mut split = String::from(orig);
-    split.split_off(orig.len() + 1);
+    let _ = split.split_off(orig.len() + 1);
 }
 
 #[test]
 #[should_panic]
 fn test_split_off_mid_char() {
     let mut orig = String::from("山");
-    orig.split_off(1);
+    let _ = orig.split_off(1);
 }
 
 #[test]