only extend diagnostic end for words

Diagnostics are currently extended if text is inserted at their end. This is
desirable when inserting text after an identifier. For example consider:

let foo = 2;
    --- unused variable

Renaming the identifier should extend the diagnostic:

let foobar = 2;
    ------ unused variable

This is currently implemented in helix bug as a consequence adding whitespaces
or a type hint also extends the diagnostic:

let foo      = 2;
    -------- unused variable
let foo: Bar = 2;
    -------- unused variable

In these cases the diagnostic should remain unchanged:

let foo      = 2;
    --- unused variable
let foo: Bar = 2;
    --- unused variable

As a heuristic helix will now only extend diagnostics that end on a word char
if new chars are appended to the word (so not for punctuation/ whitespace).
The idea for this mapping was inspired for the word level tracking vscode uses
for many positions. While VSCode doesn't currently update diagnostics after
receiving publishDiagnostic it does use this system for inlay hints for example.
Similarly, the new association mechanism implemented here can be used for word
level tracking of inlay hints.

helix-core/src/diagnostic.rs

@@ -39,6 +39,8 @@ pub enum DiagnosticTag {
 #[derive(Debug, Clone)]
 pub struct Diagnostic {
     pub range: Range,
+    // whether this diagnostic ends at the end of(or inside) a word
+    pub ends_at_word: bool,
     pub line: usize,
     pub message: String,
     pub severity: Option<Severity>,

helix-core/src/transaction.rs

@@ -1,6 +1,6 @@
 use smallvec::SmallVec;
 
-use crate::{Range, Rope, Selection, Tendril};
+use crate::{chars::char_is_word, Range, Rope, Selection, Tendril};
 use std::borrow::Cow;
 
 /// (from, to, replacement)
@@ -22,6 +22,20 @@ pub enum Operation {
 pub enum Assoc {
     Before,
     After,
+    /// Acts like `After` if a word character is inserted
+    /// after the position, otherwise acts like `Before`
+    AfterWord,
+}
+
+impl Assoc {
+    fn insert_offset(self, s: &str) -> usize {
+        match self {
+            Assoc::After => s.chars().count(),
+            Assoc::AfterWord => s.chars().take_while(|&c| char_is_word(c)).count(),
+            // return position before inserted text
+            Assoc::Before => 0,
+        }
+    }
 }
 
 #[derive(Debug, Default, Clone, PartialEq, Eq)]
@@ -360,37 +374,28 @@ impl ChangeSet {
                     }
                 }
                 Insert(s) => {
-                    let ins = s.chars().count();
-
                     // a subsequent delete means a replace, consume it
                     if let Some(Delete(len)) = iter.peek() {
                         iter.next();
 
                         old_end = old_pos + len;
                         // in range of replaced text
                         if old_end > pos {
-                            // at point or tracking before
-                            if pos == old_pos || assoc == Assoc::Before {
+                            // replacement starting at the point doesn't
+                            // move the point forward
+                            if pos == old_pos {
                                 return new_pos;
-                            } else {
-                                // place to end of insert
-                                return new_pos + ins;
                             }
+                            return new_pos + assoc.insert_offset(s);
                         }
                     } else {
                         // at insert point
                         if old_pos == pos {
-                            // return position before inserted text
-                            if assoc == Assoc::Before {
-                                return new_pos;
-                            } else {
-                                // after text
-                                return new_pos + ins;
-                            }
+                            return new_pos + assoc.insert_offset(s);
                         }
                     }
 
-                    new_pos += ins;
+                    new_pos += s.chars().count();
                 }
             }
             old_pos = old_end;

helix-term/src/application.rs

@@ -1,6 +1,7 @@
 use arc_swap::{access::Map, ArcSwap};
 use futures_util::Stream;
 use helix_core::{
+    chars::char_is_word,
     diagnostic::{DiagnosticTag, NumberOrString},
     path::get_relative_path,
     pos_at_coords, syntax, Selection,
@@ -777,7 +778,6 @@ impl Application {
                                         }
                                     }
                                 }
-
                             }
 
                             let unchaged_diag_sources = &unchaged_diag_sources_;
@@ -858,8 +858,13 @@ impl Application {
                                         Vec::new()
                                     };
 
+                                    let ends_at_word = start != end
+                                        && end != 0
+                                        && text.get_char(end - 1).map_or(false, char_is_word);
+
                                     Some(Diagnostic {
                                         range: Range { start, end },
+                                        ends_at_word,
                                         line: diagnostic.range.start.line as usize,
                                         message: diagnostic.message.clone(),
                                         severity,

helix-view/src/document.rs

@@ -1119,7 +1119,12 @@ impl Document {
             // map state.diagnostics over changes::map_pos too
             for diagnostic in &mut self.diagnostics {
                 diagnostic.range.start = changes.map_pos(diagnostic.range.start, Assoc::After);
-                diagnostic.range.end = changes.map_pos(diagnostic.range.end, Assoc::After);
+                let assoc = if diagnostic.ends_at_word {
+                    Assoc::AfterWord
+                } else {
+                    Assoc::Before
+                };
+                diagnostic.range.end = changes.map_pos(diagnostic.range.end, assoc);
                 diagnostic.line = self.text.char_to_line(diagnostic.range.start);
             }
             self.diagnostics