Auto merge of #101168 - jachris:dataflow-const-prop, r=oli-obk

Add new MIR constant propagation based on dataflow analysis

The current constant propagation in `rustc_mir_transform/src/const_prop.rs` fails to handle many cases that would be expected from a constant propagation optimization. For example:
```rust
let x = if true { 0 } else { 0 };
```
This pull request adds a new constant propagation MIR optimization pass based on the existing dataflow analysis framework. Since most of the analysis is not unique to constant propagation, a generic framework has been extracted. It works on top of the existing framework and could be reused for other optimzations.

Closes #80038. Closes #81605.

## Todo
### Essential
- [x] [Writes to inactive enum variants](#101168 (review)). Resolved by rejecting the registration of places with downcast projections for now. Could be improved by flooding other variants if mutable access to a variant is observed.
- [X] Handle [`StatementKind::CopyNonOverlapping`](#101168 (comment)). Resolved by flooding the destination.
- [x] Handle `UnsafeCell` / `!Freeze` correctly.
- [X] Overflow propagation of `CheckedBinaryOp`: Decided to not propagate if overflow flag is `true` (`false` will still be propagated)
- [x] More documentation in general.
- [x] Arguments for correctness, documentation of necessary assumptions.
- [x] Better performance, or alternatively, require `-Zmir-opt-level=3` for now.

### Extra
- [x]  Add explicit unreachability, i.e. upgrading the lattice from $\mathbb{P} \to \mathbb{V}$ to $\set{\bot} \cup (\mathbb{P} \to \mathbb{V})$.
- [x] Use storage statements to improve precision.
- [ ] Consider opening issue for duplicate diagnostics: #101168 (comment)
- [ ] Flood moved-from places with $\bot$ (requires some changes for places with tracked projections).
- [ ] Add downcast projections back in.
- [ ] [Algebraic simplifications](#101168 (comment)) (possibly with a shared API; done by old const prop).
- [ ] Propagation through slices / arrays.
- [ ] Find other optimizations that are done by old `const_prop.rs`, but not by this one.

Author: bors

compiler/rustc_graphviz/src/lib.rs

@@ -471,7 +471,11 @@ pub trait Labeller<'a> {
 /// Escape tags in such a way that it is suitable for inclusion in a
 /// Graphviz HTML label.
 pub fn escape_html(s: &str) -> String {
-    s.replace('&', "&amp;").replace('\"', "&quot;").replace('<', "&lt;").replace('>', "&gt;")
+    s.replace('&', "&amp;")
+        .replace('\"', "&quot;")
+        .replace('<', "&lt;")
+        .replace('>', "&gt;")
+        .replace('\n', "<br align=\"left\"/>")
 }
 
 impl<'a> LabelText<'a> {

compiler/rustc_middle/src/mir/generic_graphviz.rs

@@ -126,7 +126,7 @@ impl<
             write!(
                 w,
                 r#"<tr><td align="left" balign="left">{}</td></tr>"#,
-                dot::escape_html(&section).replace('\n', "<br/>")
+                dot::escape_html(&section)
             )?;
         }
 
@@ -147,7 +147,7 @@ impl<
             let src = self.node(source);
             let trg = self.node(target);
             let escaped_edge_label = if let Some(edge_label) = edge_labels.get(index) {
-                dot::escape_html(edge_label).replace('\n', r#"<br align="left"/>"#)
+                dot::escape_html(edge_label)
             } else {
                 "".to_owned()
             };
@@ -162,8 +162,7 @@ impl<
     where
         W: Write,
     {
-        let lines = label.split('\n').map(|s| dot::escape_html(s)).collect::<Vec<_>>();
-        let escaped_label = lines.join(r#"<br align="left"/>"#);
+        let escaped_label = dot::escape_html(label);
         writeln!(w, r#"    label=<<br/><br/>{}<br align="left"/><br/><br/><br/>>;"#, escaped_label)
     }
 

compiler/rustc_middle/src/mir/visit.rs

@@ -1320,6 +1320,15 @@ impl PlaceContext {
         )
     }
 
+    /// Returns `true` if this place context represents an address-of.
+    pub fn is_address_of(&self) -> bool {
+        matches!(
+            self,
+            PlaceContext::NonMutatingUse(NonMutatingUseContext::AddressOf)
+                | PlaceContext::MutatingUse(MutatingUseContext::AddressOf)
+        )
+    }
+
     /// Returns `true` if this place context represents a storage live or storage dead marker.
     #[inline]
     pub fn is_storage_marker(&self) -> bool {

compiler/rustc_mir_dataflow/src/framework/graphviz.rs

@@ -475,7 +475,10 @@ where
                 r#"<td colspan="{colspan}" {fmt} align="left">{state}</td>"#,
                 colspan = this.style.num_state_columns(),
                 fmt = fmt,
-                state = format!("{:?}", DebugWithAdapter { this: state, ctxt: analysis }),
+                state = dot::escape_html(&format!(
+                    "{:?}",
+                    DebugWithAdapter { this: state, ctxt: analysis }
+                )),
             )
         })
     }

compiler/rustc_mir_dataflow/src/framework/lattice.rs

@@ -73,6 +73,16 @@ pub trait MeetSemiLattice: Eq {
     fn meet(&mut self, other: &Self) -> bool;
 }
 
+/// A set that has a "bottom" element, which is less than or equal to any other element.
+pub trait HasBottom {
+    fn bottom() -> Self;
+}
+
+/// A set that has a "top" element, which is greater than or equal to any other element.
+pub trait HasTop {
+    fn top() -> Self;
+}
+
 /// A `bool` is a "two-point" lattice with `true` as the top element and `false` as the bottom:
 ///
 /// ```text
@@ -102,6 +112,18 @@ impl MeetSemiLattice for bool {
     }
 }
 
+impl HasBottom for bool {
+    fn bottom() -> Self {
+        false
+    }
+}
+
+impl HasTop for bool {
+    fn top() -> Self {
+        true
+    }
+}
+
 /// A tuple (or list) of lattices is itself a lattice whose least upper bound is the concatenation
 /// of the least upper bounds of each element of the tuple (or list).
 ///
@@ -250,3 +272,15 @@ impl<T: Clone + Eq> MeetSemiLattice for FlatSet<T> {
         true
     }
 }
+
+impl<T> HasBottom for FlatSet<T> {
+    fn bottom() -> Self {
+        Self::Bottom
+    }
+}
+
+impl<T> HasTop for FlatSet<T> {
+    fn top() -> Self {
+        Self::Top
+    }
+}

compiler/rustc_mir_dataflow/src/lib.rs

@@ -41,6 +41,7 @@ pub mod move_paths;
 pub mod rustc_peek;
 pub mod storage;
 pub mod un_derefer;
+pub mod value_analysis;
 
 pub(crate) mod indexes {
     pub(crate) use super::move_paths::MovePathIndex;