Auto merge of #23272 - tdelacour:ISSUE-20455, r=SimonSapin

ISSUE-20455: introduce stronger types for textinput indexing

<!-- Please describe your changes on the following line: -->
Added two new types:
- ByteOffset
- UTF16CodeUnitOffset

I've replaced any instance of `usize` that would be better represented by one or the other of these. I also updated any downstream code, including the unit tests for `textinput.rs`. Along the way, I tried to add or edit comments to better reflect my understanding of this file - happy to revisit if I have misrepresented anything.

I did not end up finding any places where types were very obviously being mixed, as the issue description suggested I should do... LMK if I should re-audit the file for that (might need a bit of guidance)!

---
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #20455 (GitHub issue number if applicable)

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- [X] There are tests for these changes OR
- [ ] These changes do not require tests because ___

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components/script/dom/htmlinputelement.rs

@@ -45,7 +45,7 @@ use crate::textinput::KeyReaction::{
     DispatchInput, Nothing, RedrawSelection, TriggerDefaultAction,
 };
 use crate::textinput::Lines::Single;
-use crate::textinput::{Direction, SelectionDirection, TextInput};
+use crate::textinput::{Direction, SelectionDirection, TextInput, UTF16CodeUnits, UTF8Bytes};
 use caseless::compatibility_caseless_match_str;
 use dom_struct::dom_struct;
 use embedder_traits::FilterPattern;
@@ -434,7 +434,7 @@ impl LayoutHTMLInputElementHelpers for LayoutDom<HTMLInputElement> {
         match (*self.unsafe_get()).input_type() {
             InputType::Password => {
                 let text = get_raw_textinput_value(self);
-                let sel = textinput.sorted_selection_offsets_range();
+                let sel = UTF8Bytes::unwrap_range(textinput.sorted_selection_offsets_range());
 
                 // Translate indices from the raw value to indices in the replacement value.
                 let char_start = text[..sel.start].chars().count();
@@ -443,9 +443,9 @@ impl LayoutHTMLInputElementHelpers for LayoutDom<HTMLInputElement> {
                 let bytes_per_char = PASSWORD_REPLACEMENT_CHAR.len_utf8();
                 Some(char_start * bytes_per_char..char_end * bytes_per_char)
             },
-            input_type if input_type.is_textual() => {
-                Some(textinput.sorted_selection_offsets_range())
-            },
+            input_type if input_type.is_textual() => Some(UTF8Bytes::unwrap_range(
+                textinput.sorted_selection_offsets_range(),
+            )),
             _ => None,
         }
     }
@@ -1357,7 +1357,7 @@ impl VirtualMethods for HTMLInputElement {
                     if value < 0 {
                         textinput.set_max_length(None);
                     } else {
-                        textinput.set_max_length(Some(value as usize))
+                        textinput.set_max_length(Some(UTF16CodeUnits(value as usize)))
                     }
                 },
                 _ => panic!("Expected an AttrValue::Int"),
@@ -1369,7 +1369,7 @@ impl VirtualMethods for HTMLInputElement {
                     if value < 0 {
                         textinput.set_min_length(None);
                     } else {
-                        textinput.set_min_length(Some(value as usize))
+                        textinput.set_min_length(Some(UTF16CodeUnits(value as usize)))
                     }
                 },
                 _ => panic!("Expected an AttrValue::Int"),

components/script/dom/htmltextareaelement.rs

@@ -31,7 +31,9 @@ use crate::dom::nodelist::NodeList;
 use crate::dom::textcontrol::{TextControlElement, TextControlSelection};
 use crate::dom::validation::Validatable;
 use crate::dom::virtualmethods::VirtualMethods;
-use crate::textinput::{Direction, KeyReaction, Lines, SelectionDirection, TextInput};
+use crate::textinput::{
+    Direction, KeyReaction, Lines, SelectionDirection, TextInput, UTF16CodeUnits, UTF8Bytes,
+};
 use dom_struct::dom_struct;
 use html5ever::{LocalName, Prefix};
 use script_traits::ScriptToConstellationChan;
@@ -89,7 +91,9 @@ impl LayoutHTMLTextAreaElementHelpers for LayoutDom<HTMLTextAreaElement> {
             return None;
         }
         let textinput = (*self.unsafe_get()).textinput.borrow_for_layout();
-        Some(textinput.sorted_selection_offsets_range())
+        Some(UTF8Bytes::unwrap_range(
+            textinput.sorted_selection_offsets_range(),
+        ))
     }
 
     #[allow(unsafe_code)]
@@ -307,7 +311,8 @@ impl HTMLTextAreaElementMethods for HTMLTextAreaElement {
 
     // https://html.spec.whatwg.org/multipage/#dom-textarea-textlength
     fn TextLength(&self) -> u32 {
-        self.textinput.borrow().utf16_len() as u32
+        let UTF16CodeUnits(num_units) = self.textinput.borrow().utf16_len();
+        num_units as u32
     }
 
     // https://html.spec.whatwg.org/multipage/#dom-lfe-labels
@@ -423,7 +428,7 @@ impl VirtualMethods for HTMLTextAreaElement {
                     if value < 0 {
                         textinput.set_max_length(None);
                     } else {
-                        textinput.set_max_length(Some(value as usize))
+                        textinput.set_max_length(Some(UTF16CodeUnits(value as usize)))
                     }
                 },
                 _ => panic!("Expected an AttrValue::Int"),
@@ -435,7 +440,7 @@ impl VirtualMethods for HTMLTextAreaElement {
                     if value < 0 {
                         textinput.set_min_length(None);
                     } else {
-                        textinput.set_min_length(Some(value as usize))
+                        textinput.set_min_length(Some(UTF16CodeUnits(value as usize)))
                     }
                 },
                 _ => panic!("Expected an AttrValue::Int"),

components/script/dom/textcontrol.rs

@@ -15,7 +15,7 @@ use crate::dom::bindings::str::DOMString;
 use crate::dom::event::{EventBubbles, EventCancelable};
 use crate::dom::eventtarget::EventTarget;
 use crate::dom::node::{window_from_node, Node, NodeDamage};
-use crate::textinput::{SelectionDirection, SelectionState, TextInput};
+use crate::textinput::{SelectionDirection, SelectionState, TextInput, UTF8Bytes};
 use script_traits::ScriptToConstellationChan;
 
 pub trait TextControlElement: DerivedFrom<EventTarget> + DerivedFrom<Node> {
@@ -177,7 +177,8 @@ impl<'a, E: TextControlElement> TextControlSelection<'a, E> {
         // change the selection state in order to replace the text in the range.
         let original_selection_state = self.textinput.borrow().selection_state();
 
-        let content_length = self.textinput.borrow().len() as u32;
+        let UTF8Bytes(content_length) = self.textinput.borrow().len_utf8();
+        let content_length = content_length as u32;
 
         // Step 5
         if start > content_length {
@@ -262,11 +263,13 @@ impl<'a, E: TextControlElement> TextControlSelection<'a, E> {
     }
 
     fn start(&self) -> u32 {
-        self.textinput.borrow().selection_start_offset() as u32
+        let UTF8Bytes(offset) = self.textinput.borrow().selection_start_offset();
+        offset as u32
     }
 
     fn end(&self) -> u32 {
-        self.textinput.borrow().selection_end_offset() as u32
+        let UTF8Bytes(offset) = self.textinput.borrow().selection_end_offset();
+        offset as u32
     }
 
     fn direction(&self) -> SelectionDirection {