Make GridPlacement's fields non-zero and add accessor functions. (#9486)

# Objective

* There is no way to read the fields of `GridPlacement` once set. 
* Values of `0` for `GridPlacement`'s fields are invalid but can be set.
* A non-zero representation would be half the size.

fixes #9474

## Solution
* Add `get_start`, `get_end` and `get_span` accessor methods.
* Change`GridPlacement`'s constructor functions to panic on arguments of
zero.
* Use non-zero types instead of primitives for `GridPlacement`'s fields.

---

## Changelog

`bevy_ui::ui_node::GridPlacement`:
* Field types have been changed to `Option<NonZeroI16>` and
`Option<NonZeroU16>`. This is because zero values are not valid for
`GridPlacement`. Previously, Taffy interpreted these as auto variants.
* Constructor functions for `GridPlacement` panic on arguments of `0`.
* Added accessor functions: `get_start`, `get_end`, and `get_span`.
These return the inner primitive value (if present) of the respective
fields.

## Migration Guide
`GridPlacement`'s constructor functions no longer accept values of `0`.
Given any argument of `0` they will panic with a `GridPlacementError`.

crates/bevy_ui/src/layout/convert.rs

@@ -278,8 +278,8 @@ impl From<GridAutoFlow> for taffy::style::GridAutoFlow {
 
 impl From<GridPlacement> for taffy::geometry::Line<taffy::style::GridPlacement> {
     fn from(value: GridPlacement) -> Self {
-        let span = value.span.unwrap_or(1).max(1);
+        let span = value.get_span().unwrap_or(1);
-        match (value.start, value.end) {
+        match (value.get_start(), value.get_end()) {
             (Some(start), Some(end)) => taffy::geometry::Line {
                 start: style_helpers::line(start),
                 end: style_helpers::line(end),

crates/bevy_ui/src/ui_node.rs

@@ -10,7 +10,10 @@ use bevy_render::{
 use bevy_transform::prelude::GlobalTransform;
 use serde::{Deserialize, Serialize};
 use smallvec::SmallVec;
-use std::ops::{Div, DivAssign, Mul, MulAssign};
+use std::{
+    num::{NonZeroI16, NonZeroU16},
+    ops::{Div, DivAssign, Mul, MulAssign},
+};
 use thiserror::Error;
 
 /// Describes the size of a UI node
@@ -1470,100 +1473,145 @@ impl From<RepeatedGridTrack> for Vec<RepeatedGridTrack> {
 /// <https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Grid_Layout/Line-based_Placement_with_CSS_Grid>
 pub struct GridPlacement {
     /// The grid line at which the item should start. Lines are 1-indexed. Negative indexes count backwards from the end of the grid. Zero is not a valid index.
-    pub(crate) start: Option<i16>,
+    pub(crate) start: Option<NonZeroI16>,
     /// How many grid tracks the item should span. Defaults to 1.
-    pub(crate) span: Option<u16>,
+    pub(crate) span: Option<NonZeroU16>,
-    /// The grid line at which the node should end. Lines are 1-indexed. Negative indexes count backwards from the end of the grid. Zero is not a valid index.
+    /// The grid line at which the item should end. Lines are 1-indexed. Negative indexes count backwards from the end of the grid. Zero is not a valid index.
-    pub(crate) end: Option<i16>,
+    pub(crate) end: Option<NonZeroI16>,
 }
 
 impl GridPlacement {
     pub const DEFAULT: Self = Self {
         start: None,
-        span: Some(1),
+        span: Some(unsafe { NonZeroU16::new_unchecked(1) }),
         end: None,
     };
 
     /// Place the grid item automatically (letting the `span` default to `1`).
     pub fn auto() -> Self {
-        Self {
+        Self::DEFAULT
-            start: None,
-            end: None,
-            span: Some(1),
-        }
     }
 
     /// Place the grid item automatically, specifying how many tracks it should `span`.
+    ///
+    /// # Panics
+    ///
+    /// Panics if `span` is `0`
     pub fn span(span: u16) -> Self {
         Self {
             start: None,
             end: None,
-            span: Some(span),
+            span: try_into_grid_span(span).expect("Invalid span value of 0."),
         }
     }
 
     /// Place the grid item specifying the `start` grid line (letting the `span` default to `1`).
+    ///
+    /// # Panics
+    ///
+    /// Panics if `start` is `0`
     pub fn start(start: i16) -> Self {
         Self {
-            start: Some(start),
+            start: try_into_grid_index(start).expect("Invalid start value of 0."),
-            end: None,
+            ..Self::DEFAULT
-            span: Some(1),
         }
     }
 
     /// Place the grid item specifying the `end` grid line (letting the `span` default to `1`).
+    ///
+    /// # Panics
+    ///
+    /// Panics if `end` is `0`
     pub fn end(end: i16) -> Self {
         Self {
-            start: None,
+            end: try_into_grid_index(end).expect("Invalid end value of 0."),
-            end: Some(end),
+            ..Self::DEFAULT
-            span: Some(1),
         }
     }
 
     /// Place the grid item specifying the `start` grid line and how many tracks it should `span`.
+    ///
+    /// # Panics
+    ///
+    /// Panics if `start` or `span` is `0`
     pub fn start_span(start: i16, span: u16) -> Self {
         Self {
-            start: Some(start),
+            start: try_into_grid_index(start).expect("Invalid start value of 0."),
             end: None,
-            span: Some(span),
+            span: try_into_grid_span(span).expect("Invalid span value of 0."),
         }
     }
 
     /// Place the grid item specifying `start` and `end` grid lines (`span` will be inferred)
+    ///
+    /// # Panics
+    ///
+    /// Panics if `start` or `end` is `0`
     pub fn start_end(start: i16, end: i16) -> Self {
         Self {
-            start: Some(start),
+            start: try_into_grid_index(start).expect("Invalid start value of 0."),
-            end: Some(end),
+            end: try_into_grid_index(end).expect("Invalid end value of 0."),
             span: None,
         }
     }
 
     /// Place the grid item specifying the `end` grid line and how many tracks it should `span`.
+    ///
+    /// # Panics
+    ///
+    /// Panics if `end` or `span` is `0`
     pub fn end_span(end: i16, span: u16) -> Self {
         Self {
             start: None,
-            end: Some(end),
+            end: try_into_grid_index(end).expect("Invalid end value of 0."),
-            span: Some(span),
+            span: try_into_grid_span(span).expect("Invalid span value of 0."),
         }
     }
 
     /// Mutate the item, setting the `start` grid line
+    ///
+    /// # Panics
+    ///
+    /// Panics if `start` is `0`
     pub fn set_start(mut self, start: i16) -> Self {
-        self.start = Some(start);
+        self.start = try_into_grid_index(start).expect("Invalid start value of 0.");
         self
     }
 
     /// Mutate the item, setting the `end` grid line
+    ///
+    /// # Panics
+    ///
+    /// Panics if `end` is `0`
     pub fn set_end(mut self, end: i16) -> Self {
-        self.end = Some(end);
+        self.end = try_into_grid_index(end).expect("Invalid end value of 0.");
         self
     }
 
     /// Mutate the item, setting the number of tracks the item should `span`
+    ///
+    /// # Panics
+    ///
+    /// Panics if `span` is `0`
     pub fn set_span(mut self, span: u16) -> Self {
-        self.span = Some(span);
+        self.span = try_into_grid_span(span).expect("Invalid span value of 0.");
         self
     }
+
+    /// Returns the grid line at which the item should start, or `None` if not set.
+    pub fn get_start(self) -> Option<i16> {
+        self.start.map(NonZeroI16::get)
+    }
+
+    /// Returns the grid line at which the item should end, or `None` if not set.
+    pub fn get_end(self) -> Option<i16> {
+        self.end.map(NonZeroI16::get)
+    }
+
+    /// Returns span for this grid item, or `None` if not set.
+    pub fn get_span(self) -> Option<u16> {
+        self.span.map(NonZeroU16::get)
+    }
 }
 
 impl Default for GridPlacement {
@@ -1572,6 +1620,29 @@ impl Default for GridPlacement {
     }
 }
 
+/// Convert an `i16` to `NonZeroI16`, fails on `0` and returns the `InvalidZeroIndex` error.
+fn try_into_grid_index(index: i16) -> Result<Option<NonZeroI16>, GridPlacementError> {
+    Ok(Some(
+        NonZeroI16::new(index).ok_or(GridPlacementError::InvalidZeroIndex)?,
+    ))
+}
+
+/// Convert a `u16` to `NonZeroU16`, fails on `0` and returns the `InvalidZeroSpan` error.
+fn try_into_grid_span(span: u16) -> Result<Option<NonZeroU16>, GridPlacementError> {
+    Ok(Some(
+        NonZeroU16::new(span).ok_or(GridPlacementError::InvalidZeroSpan)?,
+    ))
+}
+
+/// Errors that occur when setting contraints for a `GridPlacement`
+#[derive(Debug, Eq, PartialEq, Clone, Copy, Error)]
+pub enum GridPlacementError {
+    #[error("Zero is not a valid grid position")]
+    InvalidZeroIndex,
+    #[error("Spans cannot be zero length")]
+    InvalidZeroSpan,
+}
+
 /// The background color of the node
 ///
 /// This serves as the "fill" color.
@@ -1719,6 +1790,7 @@ impl Default for ZIndex {
 
 #[cfg(test)]
 mod tests {
+    use crate::GridPlacement;
     use crate::ValArithmeticError;
 
     use super::Val;
@@ -1867,4 +1939,30 @@ mod tests {
     fn default_val_equals_const_default_val() {
         assert_eq!(Val::default(), Val::DEFAULT);
     }
+
+    #[test]
+    fn invalid_grid_placement_values() {
+        assert!(std::panic::catch_unwind(|| GridPlacement::span(0)).is_err());
+        assert!(std::panic::catch_unwind(|| GridPlacement::start(0)).is_err());
+        assert!(std::panic::catch_unwind(|| GridPlacement::end(0)).is_err());
+        assert!(std::panic::catch_unwind(|| GridPlacement::start_end(0, 1)).is_err());
+        assert!(std::panic::catch_unwind(|| GridPlacement::start_end(-1, 0)).is_err());
+        assert!(std::panic::catch_unwind(|| GridPlacement::start_span(1, 0)).is_err());
+        assert!(std::panic::catch_unwind(|| GridPlacement::start_span(0, 1)).is_err());
+        assert!(std::panic::catch_unwind(|| GridPlacement::end_span(0, 1)).is_err());
+        assert!(std::panic::catch_unwind(|| GridPlacement::end_span(1, 0)).is_err());
+        assert!(std::panic::catch_unwind(|| GridPlacement::default().set_start(0)).is_err());
+        assert!(std::panic::catch_unwind(|| GridPlacement::default().set_end(0)).is_err());
+        assert!(std::panic::catch_unwind(|| GridPlacement::default().set_span(0)).is_err());
+    }
+
+    #[test]
+    fn grid_placement_accessors() {
+        assert_eq!(GridPlacement::start(5).get_start(), Some(5));
+        assert_eq!(GridPlacement::end(-4).get_end(), Some(-4));
+        assert_eq!(GridPlacement::span(2).get_span(), Some(2));
+        assert_eq!(GridPlacement::start_end(11, 21).get_span(), None);
+        assert_eq!(GridPlacement::start_span(3, 5).get_end(), None);
+        assert_eq!(GridPlacement::end_span(-4, 12).get_start(), None);
+    }
 }