Add a MIR pre-codegen test for Vec::deref

Author: scottmcm

tests/mir-opt/pre-codegen/vec_deref.rs

@@ -0,0 +1,13 @@
+// skip-filecheck
+//@ compile-flags: -O -Zmir-opt-level=2 -Cdebuginfo=2
+// EMIT_MIR_FOR_EACH_PANIC_STRATEGY
+
+#![crate_type = "lib"]
+
+// Added after it stopped inlining in a nightly; see
+// <https://github.com./rust-lang/rust/issues/123174>
+
+// EMIT_MIR vec_deref.vec_deref_to_slice.PreCodegen.after.mir
+pub fn vec_deref_to_slice(v: &Vec<u8>) -> &[u8] {
+    v
+}

tests/mir-opt/pre-codegen/vec_deref.vec_deref_to_slice.PreCodegen.after.panic-abort.mir

@@ -0,0 +1,14 @@
+// MIR for `vec_deref_to_slice` after PreCodegen
+
+fn vec_deref_to_slice(_1: &Vec<u8>) -> &[u8] {
+    debug v => _1;
+    let mut _0: &[u8];
+
+    bb0: {
+        _0 = <Vec<u8> as Deref>::deref(move _1) -> [return: bb1, unwind unreachable];
+    }
+
+    bb1: {
+        return;
+    }
+}

tests/mir-opt/pre-codegen/vec_deref.vec_deref_to_slice.PreCodegen.after.panic-unwind.mir

@@ -0,0 +1,14 @@
+// MIR for `vec_deref_to_slice` after PreCodegen
+
+fn vec_deref_to_slice(_1: &Vec<u8>) -> &[u8] {
+    debug v => _1;
+    let mut _0: &[u8];
+
+    bb0: {
+        _0 = <Vec<u8> as Deref>::deref(move _1) -> [return: bb1, unwind continue];
+    }
+
+    bb1: {
+        return;
+    }
+}