Auto merge of #98914 - fee1-dead-contrib:min-deref-patterns, r=compiler-errors

Minimal implementation of implicit deref patterns for Strings

cc `@compiler-errors` `@BoxyUwU` https://github.com/rust-lang/lang-team/issues/88 #87121

~~I forgot to add a feature gate, will do so in a minute~~ Done

9 files changed, 151 insertions, 0 deletions

diff --git a/src/test/mir-opt/deref-patterns/string.foo.PreCodegen.after.mir b/src/test/mir-opt/deref-patterns/string.foo.PreCodegen.after.mir
new file mode 100644
index 00000000000..5b185082d4d
--- /dev/null
+++ b/src/test/mir-opt/deref-patterns/string.foo.PreCodegen.after.mir
@@ -0,0 +1,74 @@
+// MIR for `foo` after PreCodegen
+
+fn foo(_1: Option<String>) -> i32 {
+    debug s => _1;                       // in scope 0 at $DIR/string.rs:+0:12: +0:13
+    let mut _0: i32;                     // return place in scope 0 at $DIR/string.rs:+0:34: +0:37
+    let mut _2: &std::string::String;    // in scope 0 at $DIR/string.rs:+2:14: +2:17
+    let mut _3: &str;                    // in scope 0 at $DIR/string.rs:+2:14: +2:17
+    let mut _4: bool;                    // in scope 0 at $DIR/string.rs:+2:14: +2:17
+    let mut _5: isize;                   // in scope 0 at $DIR/string.rs:+2:9: +2:18
+    let _6: std::option::Option<std::string::String>; // in scope 0 at $DIR/string.rs:+3:9: +3:10
+    let mut _7: bool;                    // in scope 0 at $DIR/string.rs:+5:1: +5:2
+    scope 1 {
+        debug s => _6;                   // in scope 1 at $DIR/string.rs:+3:9: +3:10
+    }
+
+    bb0: {
+        _7 = const false;                // scope 0 at $DIR/string.rs:+1:11: +1:12
+        _7 = const true;                 // scope 0 at $DIR/string.rs:+1:11: +1:12
+        _5 = discriminant(_1);           // scope 0 at $DIR/string.rs:+1:11: +1:12
+        switchInt(move _5) -> [1_isize: bb2, otherwise: bb1]; // scope 0 at $DIR/string.rs:+1:5: +1:12
+    }
+
+    bb1: {
+        StorageLive(_6);                 // scope 0 at $DIR/string.rs:+3:9: +3:10
+        _7 = const false;                // scope 0 at $DIR/string.rs:+3:9: +3:10
+        _6 = move _1;                    // scope 0 at $DIR/string.rs:+3:9: +3:10
+        _0 = const 4321_i32;             // scope 1 at $DIR/string.rs:+3:14: +3:18
+        drop(_6) -> bb6;                 // scope 0 at $DIR/string.rs:+3:17: +3:18
+    }
+
+    bb2: {
+        _2 = &((_1 as Some).0: std::string::String); // scope 0 at $DIR/string.rs:+2:14: +2:17
+        _3 = <String as Deref>::deref(move _2) -> bb3; // scope 0 at $DIR/string.rs:+2:14: +2:17
+                                         // mir::Constant
+                                         // + span: $DIR/string.rs:9:14: 9:17
+                                         // + literal: Const { ty: for<'a> fn(&'a String) -> &'a <String as Deref>::Target {<String as Deref>::deref}, val: Value(<ZST>) }
+    }
+
+    bb3: {
+        _4 = <str as PartialEq>::eq(_3, const "a") -> bb4; // scope 0 at $DIR/string.rs:+2:14: +2:17
+                                         // mir::Constant
+                                         // + span: $DIR/string.rs:9:14: 9:17
+                                         // + literal: Const { ty: for<'a, 'b> fn(&'a str, &'b str) -> bool {<str as PartialEq>::eq}, val: Value(<ZST>) }
+                                         // mir::Constant
+                                         // + span: $DIR/string.rs:9:14: 9:17
+                                         // + literal: Const { ty: &str, val: Value(Slice(..)) }
+    }
+
+    bb4: {
+        switchInt(move _4) -> [false: bb1, otherwise: bb5]; // scope 0 at $DIR/string.rs:+2:14: +2:17
+    }
+
+    bb5: {
+        _0 = const 1234_i32;             // scope 0 at $DIR/string.rs:+2:22: +2:26
+        goto -> bb9;                     // scope 0 at $DIR/string.rs:+2:22: +2:26
+    }
+
+    bb6: {
+        StorageDead(_6);                 // scope 0 at $DIR/string.rs:+3:17: +3:18
+        goto -> bb9;                     // scope 0 at $DIR/string.rs:+3:17: +3:18
+    }
+
+    bb7: {
+        return;                          // scope 0 at $DIR/string.rs:+5:2: +5:2
+    }
+
+    bb8: {
+        drop(_1) -> bb7;                 // scope 0 at $DIR/string.rs:+5:1: +5:2
+    }
+
+    bb9: {
+        switchInt(_7) -> [false: bb7, otherwise: bb8]; // scope 0 at $DIR/string.rs:+5:1: +5:2
+    }
+}
diff --git a/src/test/mir-opt/deref-patterns/string.rs b/src/test/mir-opt/deref-patterns/string.rs
new file mode 100644
index 00000000000..3a99c44aa0e
--- /dev/null
+++ b/src/test/mir-opt/deref-patterns/string.rs
@@ -0,0 +1,12 @@
+// compile-flags: -Z mir-opt-level=0 -C panic=abort
+
+#![feature(string_deref_patterns)]
+#![crate_type = "lib"]
+
+// EMIT_MIR string.foo.PreCodegen.after.mir
+pub fn foo(s: Option<String>) -> i32 {
+    match s {
+        Some("a") => 1234,
+        s => 4321,
+    }
+}
diff --git a/src/test/ui/deref-patterns/basic.rs b/src/test/ui/deref-patterns/basic.rs
new file mode 100644
index 00000000000..249716040a1
--- /dev/null
+++ b/src/test/ui/deref-patterns/basic.rs
@@ -0,0 +1,17 @@
+// run-pass
+// check-run-results
+#![feature(string_deref_patterns)]
+
+fn main() {
+    test(Some(String::from("42")));
+    test(Some(String::new()));
+    test(None);
+}
+
+fn test(o: Option<String>) {
+    match o {
+        Some("42") => println!("the answer"),
+        Some(_) => println!("something else?"),
+        None => println!("nil"),
+    }
+}
diff --git a/src/test/ui/deref-patterns/basic.run.stdout b/src/test/ui/deref-patterns/basic.run.stdout
new file mode 100644
index 00000000000..e50df058281
--- /dev/null
+++ b/src/test/ui/deref-patterns/basic.run.stdout
@@ -0,0 +1,3 @@
+the answer
+something else?
+nil
diff --git a/src/test/ui/deref-patterns/default-infer.rs b/src/test/ui/deref-patterns/default-infer.rs
new file mode 100644
index 00000000000..050b847305b
--- /dev/null
+++ b/src/test/ui/deref-patterns/default-infer.rs
@@ -0,0 +1,9 @@
+// check-pass
+#![feature(string_deref_patterns)]
+
+fn main() {
+    match <_ as Default>::default() {
+        "" => (),
+        _ => unreachable!(),
+    }
+}
diff --git a/src/test/ui/deref-patterns/gate.rs b/src/test/ui/deref-patterns/gate.rs
new file mode 100644
index 00000000000..ff50e30dea8
--- /dev/null
+++ b/src/test/ui/deref-patterns/gate.rs
@@ -0,0 +1,7 @@
+// gate-test-string_deref_patterns
+fn main() {
+    match String::new() {
+        "" | _ => {}
+        //~^ mismatched types
+    }
+}
diff --git a/src/test/ui/deref-patterns/gate.stderr b/src/test/ui/deref-patterns/gate.stderr
new file mode 100644
index 00000000000..993468b5e82
--- /dev/null
+++ b/src/test/ui/deref-patterns/gate.stderr
@@ -0,0 +1,11 @@
+error[E0308]: mismatched types
+  --> $DIR/gate.rs:4:9
+   |
+LL |     match String::new() {
+   |           ------------- this expression has type `String`
+LL |         "" | _ => {}
+   |         ^^ expected struct `String`, found `&str`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/deref-patterns/refs.rs b/src/test/ui/deref-patterns/refs.rs
new file mode 100644
index 00000000000..97e260d2752
--- /dev/null
+++ b/src/test/ui/deref-patterns/refs.rs
@@ -0,0 +1,18 @@
+// check-pass
+#![feature(string_deref_patterns)]
+
+fn foo(s: &String) -> i32 {
+    match *s {
+        "a" => 42,
+        _ => -1,
+    }
+}
+
+fn bar(s: Option<&&&&String>) -> i32 {
+    match s {
+        Some(&&&&"&&&&") => 1,
+        _ => -1,
+    }
+}
+
+fn main() {}
diff --git a/src/test/ui/blind-item-local-shadow.rs b/src/test/ui/resolve/blind-item-local-shadow.rs
index 942aeb6fdf4..942aeb6fdf4 100644
--- a/src/test/ui/blind-item-local-shadow.rs
+++ b/src/test/ui/resolve/blind-item-local-shadow.rs