Auto merge of #86525 - shamatar:array_len_opt, r=oli-obk

Array `.len()` MIR optimization pass

This pass kind-of works back the `[T; N].len()` call that at the moment is first coerced as `&[T; N]` -> `&[T]` and then uses `&[T].len()`. Depends on #86383

1 files changed, 25 insertions, 0 deletions

diff --git a/src/test/mir-opt/lower_array_len.array_len.SimplifyLocals.diff b/src/test/mir-opt/lower_array_len.array_len.SimplifyLocals.diff
new file mode 100644
index 00000000000..20e2685aba5
--- /dev/null
+++ b/src/test/mir-opt/lower_array_len.array_len.SimplifyLocals.diff
@@ -0,0 +1,25 @@
+- // MIR for `array_len` before SimplifyLocals
++ // MIR for `array_len` after SimplifyLocals
+  
+  fn array_len(_1: &[u8; N]) -> usize {
+      debug arr => _1;                     // in scope 0 at $DIR/lower_array_len.rs:30:34: 30:37
+      let mut _0: usize;                   // return place in scope 0 at $DIR/lower_array_len.rs:30:52: 30:57
+-     let mut _2: &[u8];                   // in scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
+-     let mut _3: &[u8; N];                // in scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
+-     let mut _4: &[u8; N];                // in scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
+  
+      bb0: {
+-         StorageLive(_2);                 // scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
+-         StorageLive(_3);                 // scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
+-         _3 = _1;                         // scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
+-         StorageLive(_4);                 // scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
+-         _4 = _3;                         // scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
+-         _2 = move _3 as &[u8] (Pointer(Unsize)); // scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
+-         StorageDead(_3);                 // scope 0 at $DIR/lower_array_len.rs:31:7: 31:8
+          _0 = const N;                    // scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
+-         StorageDead(_4);                 // scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
+-         StorageDead(_2);                 // scope 0 at $DIR/lower_array_len.rs:31:13: 31:14
+          return;                          // scope 0 at $DIR/lower_array_len.rs:32:2: 32:2
+      }
+  }
+