Rollup merge of rust-lang#57106 - matthiaskrgr:trim_must_use, r=sfackler

mark str::string::String.trim.* functions as #[must_use].

The functions return a reference to a new object and do not modify in-place
as the following code shows:
````
let s = String::from("   hello   ");
s.trim();
assert_eq!(s, "   hello   ");
````

The new reference should be bound to a variable as now indicated by #[must_use].

src/libcore/str/mod.rs

@@ -3489,6 +3489,8 @@ impl str {
     ///
     /// assert_eq!("Hello\tworld", s.trim());
     /// ```
+    #[must_use = "this returns the trimmed string as a slice, \
+                  without modifying the original"]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn trim(&self) -> &str {
         self.trim_matches(|c: char| c.is_whitespace())
@@ -3524,6 +3526,8 @@ impl str {
     /// let s = "  עברית  ";
     /// assert!(Some('ע') == s.trim_start().chars().next());
     /// ```
+    #[must_use = "this returns the trimmed string as a new slice, \
+                  without modifying the original"]
     #[stable(feature = "trim_direction", since = "1.30.0")]
     pub fn trim_start(&self) -> &str {
         self.trim_start_matches(|c: char| c.is_whitespace())
@@ -3559,6 +3563,8 @@ impl str {
     /// let s = "  עברית  ";
     /// assert!(Some('ת') == s.trim_end().chars().rev().next());
     /// ```
+    #[must_use = "this returns the trimmed string as a new slice, \
+                  without modifying the original"]
     #[stable(feature = "trim_direction", since = "1.30.0")]
     pub fn trim_end(&self) -> &str {
         self.trim_end_matches(|c: char| c.is_whitespace())
@@ -3661,6 +3667,8 @@ impl str {
     /// ```
     /// assert_eq!("1foo1barXX".trim_matches(|c| c == '1' || c == 'X'), "foo1bar");
     /// ```
+    #[must_use = "this returns the trimmed string as a new slice, \
+                  without modifying the original"]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn trim_matches<'a, P: Pattern<'a>>(&'a self, pat: P) -> &'a str
         where P::Searcher: DoubleEndedSearcher<'a>
@@ -3706,6 +3714,8 @@ impl str {
     /// let x: &[_] = &['1', '2'];
     /// assert_eq!("12foo1bar12".trim_start_matches(x), "foo1bar12");
     /// ```
+    #[must_use = "this returns the trimmed string as a new slice, \
+                  without modifying the original"]
     #[stable(feature = "trim_direction", since = "1.30.0")]
     pub fn trim_start_matches<'a, P: Pattern<'a>>(&'a self, pat: P) -> &'a str {
         let mut i = self.len();
@@ -3749,6 +3759,8 @@ impl str {
     /// ```
     /// assert_eq!("1fooX".trim_end_matches(|c| c == '1' || c == 'X'), "1foo");
     /// ```
+    #[must_use = "this returns the trimmed string as a new slice, \
+                  without modifying the original"]
     #[stable(feature = "trim_direction", since = "1.30.0")]
     pub fn trim_end_matches<'a, P: Pattern<'a>>(&'a self, pat: P) -> &'a str
         where P::Searcher: ReverseSearcher<'a>