Auto merge of #50546 - kennytm:rollup, r=kennytm

Rollup of 11 pull requests

Successful merges:

 - #49988 (Mention Result<!, E> in never docs.)
 - #50148 (turn `ManuallyDrop::new` into a constant function)
 - #50456 (Update the Cargo submodule)
 - #50460 (Make `String::new()` const)
 - #50464 (Remove some transmutes)
 - #50505 (Added regression function match value test)
 - #50511 (Add some explanations for #[must_use])
 - #50525 (Optimize string handling in lit_token().)
 - #50527 (Cleanup a `use` in a raw_vec test)
 - #50539 (Add more logarithm constants)
 - #49523 (Update RELEASES.md for 1.26.0)

Failed merges:

1 files changed, 17 insertions, 6 deletions

diff --git a/src/libsyntax/parse/mod.rs b/src/libsyntax/parse/mod.rs
index f252020bc31..f26a6a53074 100644
--- a/src/libsyntax/parse/mod.rs
+++ b/src/libsyntax/parse/mod.rs
@@ -419,13 +419,24 @@ pub fn lit_token(lit: token::Lit, suf: Option<Symbol>, diag: Option<(Span, &Hand
         token::Integer(s) => (false, integer_lit(&s.as_str(), suf, diag)),
         token::Float(s) => (false, float_lit(&s.as_str(), suf, diag)),
 
-        token::Str_(s) => {
-            let s = Symbol::intern(&str_lit(&s.as_str(), diag));
-            (true, Some(LitKind::Str(s, ast::StrStyle::Cooked)))
+        token::Str_(mut sym) => {
+            // If there are no characters requiring special treatment we can
+            // reuse the symbol from the Token. Otherwise, we must generate a
+            // new symbol because the string in the LitKind is different to the
+            // string in the Token.
+            let s = &sym.as_str();
+            if s.as_bytes().iter().any(|&c| c == b'\\' || c == b'\r') {
+                sym = Symbol::intern(&str_lit(s, diag));
+            }
+            (true, Some(LitKind::Str(sym, ast::StrStyle::Cooked)))
         }
-        token::StrRaw(s, n) => {
-            let s = Symbol::intern(&raw_str_lit(&s.as_str()));
-            (true, Some(LitKind::Str(s, ast::StrStyle::Raw(n))))
+        token::StrRaw(mut sym, n) => {
+            // Ditto.
+            let s = &sym.as_str();
+            if s.contains('\r') {
+                sym = Symbol::intern(&raw_str_lit(s));
+            }
+            (true, Some(LitKind::Str(sym, ast::StrStyle::Raw(n))))
         }
         token::ByteStr(i) => {
             (true, Some(LitKind::ByteStr(byte_str_lit(&i.as_str()))))