Auto merge of #119878 - scottmcm:inline-always-unwrap, r=<try>

Tune the inlinability of `unwrap`

Fixes #115463
cc `@thomcc`

This tweaks `unwrap` on `Option` & `Result` to be two parts:
- `#[inline(always)]` for checking the discriminant
- `#[cold]` for actually panicking

The idea here is that checking the discriminant on a `Result` or `Option` should always be trivial enough to be worth inlining, even in `opt-level=z`, especially compared to passing it to a function.

As seen in the issue and codegen test, this will hopefully help particularly for things like `.try_into().unwrap()`s that are actually infallible, but in a way that's only visible with the inlining.

library/core/src/option.rs

@@ -921,14 +921,14 @@ impl<T> Option<T> {
     /// let x: Option<&str> = None;
     /// assert_eq!(x.unwrap(), "air"); // fails
     /// ```
-    #[inline]
+    #[inline(always)]
     #[track_caller]
     #[stable(feature = "rust1", since = "1.0.0")]
     #[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"),
+            None => unwrap_failed(),
         }
     }
 
@@ -1970,6 +1970,14 @@ impl<T, E> Option<Result<T, E>> {
     }
 }
 
+#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
+#[cfg_attr(feature = "panic_immediate_abort", inline)]
+#[cold]
+#[track_caller]
+const fn unwrap_failed() -> ! {
+    panic("called `Option::unwrap()` on a `None` value")
+}
+
 // This is a separate function to reduce the code size of .expect() itself.
 #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
 #[cfg_attr(feature = "panic_immediate_abort", inline)]

library/core/src/result.rs

@@ -1061,7 +1061,7 @@ impl<T, E> Result<T, E> {
     /// let x: Result<u32, &str> = Err("emergency failure");
     /// x.unwrap(); // panics with `emergency failure`
     /// ```
-    #[inline]
+    #[inline(always)]
     #[track_caller]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn unwrap(self) -> T

tests/codegen/debuginfo-inline-callsite-location.rs

@@ -4,9 +4,9 @@
 // can correctly merge the debug info if it merges the inlined code (e.g., for merging of tail
 // calls to panic.
 
-// CHECK:       tail call void @_ZN4core9panicking5panic17h{{([0-9a-z]{16})}}E
+// CHECK:       tail call void @_ZN4core6option13unwrap_failed17h{{([0-9a-z]{16})}}E
 // CHECK-SAME:  !dbg ![[#first_dbg:]]
-// CHECK:       tail call void @_ZN4core9panicking5panic17h{{([0-9a-z]{16})}}E
+// CHECK:       tail call void @_ZN4core6option13unwrap_failed17h{{([0-9a-z]{16})}}E
 // CHECK-SAME:  !dbg ![[#second_dbg:]]
 
 // CHECK-DAG:   ![[#func_dbg:]] = distinct !DISubprogram(name: "unwrap<i32>"

tests/codegen/infallible-unwrap-in-opt-z.rs

@@ -0,0 +1,26 @@
+// compile-flags: -C opt-level=z --edition=2021
+// ignore-debug
+
+#![crate_type = "lib"]
+
+// From <https://github.com/rust-lang/rust/issues/115463>
+
+// CHECK-LABEL: @read_up_to_8(
+#[no_mangle]
+pub fn read_up_to_8(buf: &[u8]) -> u64 {
+    // CHECK-NOT: unwrap_failed
+    if buf.len() < 4 {
+        // actual instance has more code.
+        return 0;
+    }
+    let lo = u32::from_le_bytes(buf[..4].try_into().unwrap()) as u64;
+    let hi = u32::from_le_bytes(buf[buf.len() - 4..][..4].try_into().unwrap()) as u64;
+    lo | (hi << 8 * (buf.len() as u64 - 4))
+}
+
+// CHECK-LABEL: @checking_unwrap_expectation(
+#[no_mangle]
+pub fn checking_unwrap_expectation(buf: &[u8]) -> &[u8; 4] {
+    // CHECK: call void @_ZN4core6result13unwrap_failed17h
+    buf.try_into().unwrap()
+}