Auto merge of #75573 - Aaron1011:feature/const-mutation-lint, r=oli-obk

Add CONST_ITEM_MUTATION lint

Fixes #74053
Fixes #55721

This PR adds a new lint `CONST_ITEM_MUTATION`.
Given an item `const FOO: SomeType = ..`, this lint fires on:

* Attempting to write directly to a field (`FOO.field = some_val`) or
  array entry (`FOO.array_field[0] = val`)
* Taking a mutable reference to the `const` item (`&mut FOO`), including
  through an autoderef `FOO.some_mut_self_method()`

The lint message explains that since each use of a constant creates a
new temporary, the original `const` item will not be modified.

compiler/rustc_mir/src/util/find_self_call.rs

@@ -0,0 +1,35 @@
+use rustc_middle::mir::*;
+use rustc_middle::ty::{self, TyCtxt};
+use rustc_span::def_id::DefId;
+
+/// Checks if the specified `local` is used as the `self` prameter of a method call
+/// in the provided `BasicBlock`. If it is, then the `DefId` of the called method is
+/// returned.
+pub fn find_self_call(
+    tcx: TyCtxt<'_>,
+    body: &Body<'_>,
+    local: Local,
+    block: BasicBlock,
+) -> Option<DefId> {
+    debug!("find_self_call(local={:?}): terminator={:?}", local, &body[block].terminator);
+    if let Some(Terminator { kind: TerminatorKind::Call { func, args, .. }, .. }) =
+        &body[block].terminator
+    {
+        debug!("find_self_call: func={:?}", func);
+        if let Operand::Constant(box Constant { literal: ty::Const { ty, .. }, .. }) = func {
+            if let ty::FnDef(def_id, _) = *ty.kind() {
+                if let Some(ty::AssocItem { fn_has_self_parameter: true, .. }) =
+                    tcx.opt_associated_item(def_id)
+                {
+                    debug!("find_self_call: args={:?}", args);
+                    if let [Operand::Move(self_place) | Operand::Copy(self_place), ..] = **args {
+                        if self_place.as_local() == Some(local) {
+                            return Some(def_id);
+                        }
+                    }
+                }
+            }
+        }
+    }
+    None
+}

compiler/rustc_mir/src/util/mod.rs

@@ -7,12 +7,14 @@ pub mod storage;
 
 mod alignment;
 pub mod collect_writes;
+mod find_self_call;
 mod graphviz;
 pub(crate) mod pretty;
 pub(crate) mod spanview;
 
 pub use self::aggregate::expand_aggregate;
 pub use self::alignment::is_disaligned;
+pub use self::find_self_call::find_self_call;
 pub use self::graphviz::write_node_label as write_graphviz_node_label;
 pub use self::graphviz::{graphviz_safe_def_name, write_mir_graphviz};
 pub use self::pretty::{dump_enabled, dump_mir, write_mir_pretty, PassWhere};