Rollup merge of #97325 - tmiasko:capture-enum-field, r=arora-aman

Fix precise field capture of univariant enums

When constructing a MIR from a THIR field expression, introduce an
additional downcast projection before accessing a field of an enum.

When rebasing a place builder on top of a captured place, account for
the fact that a single HIR enum field projection corresponds to two MIR
projection elements: a downcast element and a field element.

Fixes #95271.
Fixes #96299.
Fixes #96512.
Fixes #97378.

r? ``@nikomatsakis`` ``@arora-aman``

src/test/ui/closures/2229_closure_analysis/capture-enum-field.rs

@@ -0,0 +1,27 @@
+// edition:2021
+// run-pass
+
+#[derive(Debug, PartialEq, Eq)]
+pub enum Color {
+    RGB(u8, u8, u8),
+}
+
+fn main() {
+    let mut color = Color::RGB(0, 0, 0);
+    let mut red = |v| {
+        let Color::RGB(ref mut r, _, _) = color;
+        *r = v;
+    };
+    let mut green = |v| {
+        let Color::RGB(_, ref mut g, _) = color;
+        *g = v;
+    };
+    let mut blue = |v| {
+        let Color::RGB(_, _, ref mut b) = color;
+        *b = v;
+    };
+    red(1);
+    green(2);
+    blue(3);
+    assert_eq!(Color::RGB(1, 2, 3), color);
+}