Rollup merge of #126154 - RalfJung:storage-live, r=compiler-errors

StorageLive: refresh storage (instead of UB) when local is already live

Blocked on [this FCP](rust-lang/rust#99160 (comment)), which also contains the motivation.

Fixes rust-lang/rust#99160
Fixes rust-lang/rust#98896 (by declaring it not-a-bug)
Fixes rust-lang/rust#119366
Fixes rust-lang/unsafe-code-guidelines#129

tests/fail/storage-live-dead-var.rs

@@ -0,0 +1,14 @@
+#![feature(core_intrinsics, custom_mir)]
+use std::intrinsics::mir::*;
+
+#[custom_mir(dialect = "runtime")]
+fn main() {
+    mir! {
+        let val: i32;
+        {
+            val = 42; //~ERROR: accessing a dead local variable
+            StorageLive(val); // too late... (but needs to be here to make `val` not implicitly live)
+            Return()
+        }
+    }
+}

tests/fail/storage-live-dead-var.stderr

@@ -0,0 +1,15 @@
+error: Undefined Behavior: accessing a dead local variable
+  --> $DIR/storage-live-dead-var.rs:LL:CC
+   |
+LL |             val = 42;
+   |             ^^^^^^^^ accessing a dead local variable
+   |
+   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
+   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
+   = note: BACKTRACE:
+   = note: inside `main` at $DIR/storage-live-dead-var.rs:LL:CC
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to 1 previous error
+

tests/fail/storage-live-resets-var.rs

@@ -0,0 +1,17 @@
+#![feature(core_intrinsics, custom_mir)]
+use std::intrinsics::mir::*;
+
+#[custom_mir(dialect = "runtime")]
+fn main() {
+    mir! {
+        let val: i32;
+        let _val2: i32;
+        {
+            StorageLive(val);
+            val = 42;
+            StorageLive(val); // reset val to `uninit`
+            _val2 = val; //~ERROR: uninitialized
+            Return()
+        }
+    }
+}

tests/fail/storage-live-resets-var.stderr

@@ -0,0 +1,15 @@
+error: Undefined Behavior: constructing invalid value: encountered uninitialized memory, but expected an integer
+  --> $DIR/storage-live-resets-var.rs:LL:CC
+   |
+LL |             _val2 = val;
+   |             ^^^^^^^^^^^ constructing invalid value: encountered uninitialized memory, but expected an integer
+   |
+   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
+   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
+   = note: BACKTRACE:
+   = note: inside `main` at $DIR/storage-live-resets-var.rs:LL:CC
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to 1 previous error
+