Add `Align2::anchor_size` (#3863)

crates/egui/src/painter.rs

@@ -229,7 +229,7 @@ impl Painter {
         text: impl ToString,
     ) -> Rect {
         let galley = self.layout_no_wrap(text.to_string(), FontId::monospace(12.0), color);
-        let rect = anchor.anchor_rect(Rect::from_min_size(pos, galley.size()));
+        let rect = anchor.anchor_size(pos, galley.size());
         let frame_rect = rect.expand(2.0);
         self.add(Shape::rect_filled(
             frame_rect,
@@ -378,7 +378,7 @@ impl Painter {
         text_color: Color32,
     ) -> Rect {
         let galley = self.layout_no_wrap(text.to_string(), font_id, text_color);
-        let rect = anchor.anchor_rect(Rect::from_min_size(pos, galley.size()));
+        let rect = anchor.anchor_size(pos, galley.size());
         self.galley(rect.min, galley, text_color);
         rect
     }

crates/egui_plot/src/items/mod.rs

@@ -728,9 +728,7 @@ impl PlotItem for Text {
                 .into_galley(ui, Some(false), f32::INFINITY, TextStyle::Small);
 
         let pos = transform.position_from_point(&self.position);
-        let rect = self
-            .anchor
-            .anchor_rect(Rect::from_min_size(pos, galley.size()));
+        let rect = self.anchor.anchor_size(pos, galley.size());
 
         shapes.push(epaint::TextShape::new(rect.min, galley, color).into());
 

crates/emath/src/align.rs

@@ -186,6 +186,23 @@ impl Align2 {
         Rect::from_min_size(pos2(x, y), rect.size())
     }
 
+    /// Use this anchor to position something around `pos`,
+    /// e.g. [`Self::RIGHT_TOP`] means the right-top of the rect
+    /// will end up at `pos`.
+    pub fn anchor_size(self, pos: Pos2, size: Vec2) -> Rect {
+        let x = match self.x() {
+            Align::Min => pos.x,
+            Align::Center => pos.x - 0.5 * size.x,
+            Align::Max => pos.x - size.x,
+        };
+        let y = match self.y() {
+            Align::Min => pos.y,
+            Align::Center => pos.y - 0.5 * size.y,
+            Align::Max => pos.y - size.y,
+        };
+        Rect::from_min_size(pos2(x, y), size)
+    }
+
     /// e.g. center a size within a given frame
     pub fn align_size_within_rect(self, size: Vec2, frame: Rect) -> Rect {
         let x_range = self.x().align_size_within_range(size.x, frame.x_range());

crates/epaint/src/shape.rs

@@ -264,7 +264,7 @@ impl Shape {
         color: Color32,
     ) -> Self {
         let galley = fonts.layout_no_wrap(text.to_string(), font_id, color);
-        let rect = anchor.anchor_rect(Rect::from_min_size(pos, galley.size()));
+        let rect = anchor.anchor_size(pos, galley.size());
         Self::galley(rect.min, galley, color)
     }