Auto merge of #74956 - ecstatic-morse:const-option-unwrap, r=oli-obk

Make `Option::unwrap` unstably const

This is lumped into the `const_option` feature gate (#67441), which enables a potpourri of `Option` methods.

cc @rust-lang/wg-const-eval

r? @oli-obk

Author: bors

library/core/src/lib.rs

@@ -81,6 +81,7 @@
 #![feature(const_fn_union)]
 #![feature(const_generics)]
 #![feature(const_option)]
+#![feature(const_precise_live_drops)]
 #![feature(const_ptr_offset)]
 #![feature(const_ptr_offset_from)]
 #![feature(const_raw_ptr_comparison)]

library/core/src/option.rs

@@ -380,7 +380,8 @@ impl<T> Option<T> {
     #[inline]
     #[track_caller]
     #[stable(feature = "rust1", since = "1.0.0")]
-    pub fn unwrap(self) -> T {
+    #[rustc_const_unstable(feature = "const_option", issue = "67441")]
+    pub const fn unwrap(self) -> T {
         match self {
             Some(val) => val,
             None => panic!("called `Option::unwrap()` on a `None` value"),

src/test/ui/consts/const-unwrap.rs

@@ -0,0 +1,14 @@
+// check-fail
+
+#![feature(const_option)]
+
+const FOO: i32 = Some(42i32).unwrap();
+
+// This causes an error, but it is attributed to the `panic` *inside* `Option::unwrap` (maybe due
+// to `track_caller`?). A note points to the originating `const`.
+const BAR: i32 = Option::<i32>::None.unwrap(); //~ NOTE
+
+fn main() {
+    println!("{}", FOO);
+    println!("{}", BAR);
+}

src/test/ui/consts/const-unwrap.stderr

@@ -0,0 +1,20 @@
+error: any use of this value will cause an error
+  --> $SRC_DIR/core/src/option.rs:LL:COL
+   |
+LL |             None => panic!("called `Option::unwrap()` on a `None` value"),
+   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |                     |
+   |                     the evaluated program panicked at 'called `Option::unwrap()` on a `None` value', $DIR/const-unwrap.rs:9:38
+   |                     inside `std::option::Option::<i32>::unwrap` at $SRC_DIR/core/src/macros/mod.rs:LL:COL
+   |                     inside `BAR` at $DIR/const-unwrap.rs:9:18
+   | 
+  ::: $DIR/const-unwrap.rs:9:1
+   |
+LL | const BAR: i32 = Option::<i32>::None.unwrap();
+   | ----------------------------------------------
+   |
+   = note: `#[deny(const_err)]` on by default
+   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to previous error
+