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
+