Rollup merge of rust-lang#52116 - Pazzaz:match-str-case, r=SimonSapin

Handle array manually in str case conversion methods

Avoiding the overhead incurred from `String.extend(char.to_lowercase())` showed a notable performance improvement when I benchmarked it.

I tested on these strings:
```rust
ALL_LOWER:       "loremipsumdolorsitametduosensibusmnesarchumabcdefgh"
ALL_UPPER:       "LOREMIPSUMDOLORSITAMETDUOSENSIBUSMNESARCHUMABCDEFGH"
REALISTIC_UPPER: "LOREM IPSUM DOLOR SIT AMET, DUO SENSIBUS MNESARCHUM"
SIGMAS:          "ΣΣΣΣΣ ΣΣΣΣΣ ΣΣΣΣΣ ΣΣΣ ΣΣΣΣ, ΣΣΣ ΣΣΣΣΣΣΣΣ ΣΣΣΣΣΣΣΣΣΣ"
WORD_UPPER:      "Lorem Ipsum Dolor Sit Amet, Duo Sensibus Mnesarchum"
```
the performance improvements of `to_lowercase()` were
```
running 10 tests
test tests::all_lower           ... bench:       1,752 ns/iter (+/- 49)
test tests::all_lower_new       ... bench:       1,266 ns/iter (+/- 15)   -28%
test tests::all_upper           ... bench:       1,832 ns/iter (+/- 39)
test tests::all_upper_new       ... bench:       1,337 ns/iter (+/- 18)   -27%
test tests::realistic_upper     ... bench:       1,993 ns/iter (+/- 14)
test tests::realistic_upper_new ... bench:       1,445 ns/iter (+/- 22)   -27%
test tests::sigmas              ... bench:       1,342 ns/iter (+/- 39)
test tests::sigmas_new          ... bench:       1,226 ns/iter (+/- 16)    -9%
test tests::word_upper          ... bench:       1,899 ns/iter (+/- 12)
test tests::word_upper_new      ... bench:       1,381 ns/iter (+/- 26)   -27%
```
and of `to_uppercase()`
```
running 10 tests
test tests::all_lower           ... bench:       1,813 ns/iter (+/- 20)
test tests::all_lower_new       ... bench:       1,321 ns/iter (+/- 16)   -27%
test tests::all_upper           ... bench:       1,629 ns/iter (+/- 22)
test tests::all_upper_new       ... bench:       1,241 ns/iter (+/- 9)    -24%
test tests::realistic_upper     ... bench:       1,670 ns/iter (+/- 24)
test tests::realistic_upper_new ... bench:       1,241 ns/iter (+/- 17)   -26%
test tests::sigmas              ... bench:       2,053 ns/iter (+/- 20)
test tests::sigmas_new          ... bench:       1,753 ns/iter (+/- 23)   -15%
test tests::word_upper          ... bench:       1,873 ns/iter (+/- 30)
test tests::word_upper_new      ... bench:       1,412 ns/iter (+/- 25)   -25%
```
I gave up on the more advanced method from rust-lang#52061 as it wasn't always a clear improvement and would help in even less cases if this PR was merged.

src/liballoc/str.rs

@@ -45,6 +45,7 @@ use core::str::pattern::{Searcher, ReverseSearcher, DoubleEndedSearcher};
 use core::mem;
 use core::ptr;
 use core::iter::FusedIterator;
+use core::unicode::conversions;
 
 use borrow::{Borrow, ToOwned};
 use boxed::Box;
@@ -369,7 +370,18 @@ impl str {
                 // See https://github.com/rust-lang/rust/issues/26035
                 map_uppercase_sigma(self, i, &mut s)
             } else {
-                s.extend(c.to_lowercase());
+                match conversions::to_lower(c) {
+                    [a, '\0', _] => s.push(a),
+                    [a, b, '\0'] => {
+                        s.push(a);
+                        s.push(b);
+                    }
+                    [a, b, c] => {
+                        s.push(a);
+                        s.push(b);
+                        s.push(c);
+                    }
+                }
             }
         }
         return s;
@@ -423,7 +435,20 @@ impl str {
     #[stable(feature = "unicode_case_mapping", since = "1.2.0")]
     pub fn to_uppercase(&self) -> String {
         let mut s = String::with_capacity(self.len());
-        s.extend(self.chars().flat_map(|c| c.to_uppercase()));
+        for c in self[..].chars() {
+            match conversions::to_upper(c) {
+                [a, '\0', _] => s.push(a),
+                [a, b, '\0'] => {
+                    s.push(a);
+                    s.push(b);
+                }
+                [a, b, c] => {
+                    s.push(a);
+                    s.push(b);
+                    s.push(c);
+                }
+            }
+        }
         return s;
     }
 

src/libcore/unicode/mod.rs

@@ -20,6 +20,9 @@ pub(crate) mod version;
 pub mod derived_property {
     pub use unicode::tables::derived_property::{Case_Ignorable, Cased};
 }
+pub mod conversions {
+    pub use unicode::tables::conversions::{to_lower, to_upper};
+}
 
 // For use in libsyntax
 pub mod property {