Incremental-safety based approach

src/parse/mod.rs

@@ -25,18 +25,36 @@ pub fn parse(src: &str) -> Rc<GreenNode> {
     }
 }
 
-/// Parse a block. Returns `Some` if there was only one block.
+/// Parse an atomic primary. Returns `Some` if all of the input was consumed.
-pub fn parse_block(source: &str) -> Option<Rc<GreenNode>> {
+pub fn parse_atomic(source: &str, _: bool) -> Option<Vec<Green>> {
     let mut p = Parser::new(source);
-    block(&mut p);
+    primary(&mut p, true).ok()?;
-    if p.eof() {
+    p.eject()
-        match p.finish().into_iter().next() {
+}
-            Some(Green::Node(node)) => Some(node),
+
-            _ => unreachable!(),
+/// Parse some markup. Returns `Some` if all of the input was consumed.
-        }
+pub fn parse_markup(source: &str, _: bool) -> Option<Vec<Green>> {
-    } else {
+    let mut p = Parser::new(source);
-        None
+    markup(&mut p);
+    p.eject()
+}
+
+/// Parse some markup without the topmost node. Returns `Some` if all of the
+/// input was consumed.
+pub fn parse_markup_elements(source: &str, mut at_start: bool) -> Option<Vec<Green>> {
+    let mut p = Parser::new(source);
+    while !p.eof() {
+        markup_node(&mut p, &mut at_start);
     }
+    p.eject()
+}
+
+/// Parse some code. Returns `Some` if all of the input was consumed.
+pub fn parse_code(source: &str, _: bool) -> Option<Vec<Green>> {
+    let mut p = Parser::new(source);
+    p.set_mode(TokenMode::Code);
+    expr_list(&mut p);
+    p.eject()
 }
 
 /// Parse markup.
@@ -118,7 +136,7 @@ fn markup_node(p: &mut Parser, at_start: &mut bool) {
 
         // Line-based markup that is not currently at the start of the line.
         NodeKind::Eq | NodeKind::Minus | NodeKind::EnumNumbering(_) => {
-            p.convert(NodeKind::Text(p.peek_src().into()));
+            p.convert(NodeKind::TextInLine(p.peek_src().into()))
         }
 
         // Hashtag + keyword / identifier.
@@ -196,7 +214,7 @@ fn expr_prec(p: &mut Parser, atomic: bool, min_prec: usize) -> ParseResult {
     let marker = p.marker();
 
     // Start the unary expression.
-    match p.peek().and_then(UnOp::from_token) {
+    match (!atomic).then(|| p.peek().and_then(UnOp::from_token)).flatten() {
         Some(op) => {
             p.eat();
             let prec = op.precedence();
@@ -268,7 +286,7 @@ fn primary(p: &mut Parser, atomic: bool) -> ParseResult {
         }
 
         // Structures.
-        Some(NodeKind::LeftParen) => parenthesized(p),
+        Some(NodeKind::LeftParen) => parenthesized(p, atomic),
         Some(NodeKind::LeftBracket) => {
             template(p);
             Ok(())
@@ -329,7 +347,7 @@ fn literal(p: &mut Parser) -> bool {
 /// - Dictionary literal
 /// - Parenthesized expression
 /// - Parameter list of closure expression
-fn parenthesized(p: &mut Parser) -> ParseResult {
+fn parenthesized(p: &mut Parser, atomic: bool) -> ParseResult {
     let marker = p.marker();
 
     p.start_group(Group::Paren);
@@ -344,7 +362,7 @@ fn parenthesized(p: &mut Parser) -> ParseResult {
     }
 
     // Arrow means this is a closure's parameter list.
-    if p.at(&NodeKind::Arrow) {
+    if !atomic && p.at(&NodeKind::Arrow) {
         params(p, marker);
         p.eat_assert(&NodeKind::Arrow);
         return marker.perform(p, NodeKind::Closure, expr);
@@ -507,20 +525,25 @@ fn template(p: &mut Parser) {
 fn block(p: &mut Parser) {
     p.perform(NodeKind::Block, |p| {
         p.start_group(Group::Brace);
-        while !p.eof() {
+        expr_list(p);
-            p.start_group(Group::Stmt);
-            if expr(p).is_ok() && !p.eof() {
-                p.expected_at("semicolon or line break");
-            }
-            p.end_group();
-
-            // Forcefully skip over newlines since the group's contents can't.
-            p.eat_while(|t| matches!(t, NodeKind::Space(_)));
-        }
         p.end_group();
     });
 }
 
+/// Parse a number of code expressions.
+fn expr_list(p: &mut Parser) {
+    while !p.eof() {
+        p.start_group(Group::Stmt);
+        if expr(p).is_ok() && !p.eof() {
+            p.expected_at("semicolon or line break");
+        }
+        p.end_group();
+
+        // Forcefully skip over newlines since the group's contents can't.
+        p.eat_while(|t| matches!(t, NodeKind::Space(_)));
+    }
+}
+
 /// Parse a function call.
 fn call(p: &mut Parser, callee: Marker) -> ParseResult {
     callee.perform(p, NodeKind::Call, |p| args(p, true, true))

src/parse/parser.rs

@@ -21,6 +21,8 @@ pub struct Parser<'s> {
     groups: Vec<GroupEntry>,
     /// The children of the currently built node.
     children: Vec<Green>,
+    /// Whether the last group was terminated.
+    last_group_terminated: bool,
 }
 
 impl<'s> Parser<'s> {
@@ -36,6 +38,7 @@ impl<'s> Parser<'s> {
             current_start: 0,
             groups: vec![],
             children: vec![],
+            last_group_terminated: true,
         }
     }
 
@@ -44,6 +47,15 @@ impl<'s> Parser<'s> {
         self.children
     }
 
+    /// End the parsing process and return multiple children.
+    pub fn eject(self) -> Option<Vec<Green>> {
+        if self.eof() && self.group_success() {
+            Some(self.children)
+        } else {
+            None
+        }
+    }
+
     /// Create a new marker.
     pub fn marker(&mut self) -> Marker {
         Marker(self.children.len())
@@ -190,6 +202,11 @@ impl<'s> Parser<'s> {
         self.tokens.scanner().column(index)
     }
 
+    /// Set the tokenizer's mode.
+    pub fn set_mode(&mut self, mode: TokenMode) {
+        self.tokens.set_mode(mode);
+    }
+
     /// Continue parsing in a group.
     ///
     /// When the end delimiter of the group is reached, all subsequent calls to
@@ -225,6 +242,7 @@ impl<'s> Parser<'s> {
         let group = self.groups.pop().expect("no started group");
         self.tokens.set_mode(group.prev_mode);
         self.repeek();
+        self.last_group_terminated = true;
 
         let mut rescan = self.tokens.mode() != group_mode;
 
@@ -243,6 +261,7 @@ impl<'s> Parser<'s> {
                 rescan = false;
             } else if required {
                 self.push_error(format_eco!("expected {}", end));
+                self.last_group_terminated = false;
             }
         }
 
@@ -260,6 +279,11 @@ impl<'s> Parser<'s> {
         }
     }
 
+    /// Check if the group processing was successfully terminated.
+    pub fn group_success(&self) -> bool {
+        self.last_group_terminated && self.groups.is_empty()
+    }
+
     /// Low-level bump that consumes exactly one token without special trivia
     /// handling.
     fn bump(&mut self) {

src/source.rs

@@ -2,7 +2,7 @@
 
 use std::collections::HashMap;
 use std::io;
-use std::ops::{Not, Range};
+use std::ops::Range;
 use std::path::{Path, PathBuf};
 use std::rc::Rc;
 
@@ -10,9 +10,9 @@ use serde::{Deserialize, Serialize};
 
 use crate::diag::TypResult;
 use crate::loading::{FileHash, Loader};
-use crate::parse::{is_newline, parse, parse_block, Scanner};
+use crate::parse::{is_newline, parse, Scanner};
 use crate::syntax::ast::Markup;
-use crate::syntax::{self, Category, GreenNode, NodeKind, RedNode, Span};
+use crate::syntax::{self, Category, GreenNode, RedNode, Span};
 use crate::util::PathExt;
 
 #[cfg(feature = "codespan-reporting")]
@@ -285,27 +285,10 @@ impl SourceFile {
 
         // Update the root node.
         let insertion_span = Span::new(self.id, replace.start, replace.end);
-        let incremental_target =
+        let source = self.src().to_string();
-            Rc::make_mut(&mut self.root).incremental_parent(insertion_span);
+        if !Rc::make_mut(&mut self.root).incremental(&source, insertion_span, with.len())
-
+        {
-        match incremental_target {
+            self.root = parse(self.src());
-            Some((child, offset)) => {
-                let src = &self.src[offset .. offset + child.len()];
-                let parse_res = match child.kind() {
-                    NodeKind::Markup => Some(parse(src)),
-                    _ => parse_block(src),
-                }
-                .and_then(|x| x.erroneous.not().then(|| x));
-
-                if let Some(parse_res) = parse_res {
-                    *child = Rc::try_unwrap(parse_res).unwrap();
-                } else {
-                    self.root = parse(&self.src);
-                }
-            }
-            None => {
-                self.root = parse(&self.src);
-            }
         }
     }
 
@@ -511,6 +494,6 @@ mod tests {
         }
 
         // Test inserting at the begining.
-        test("abc #f()[def] ghi", 10 .. 11, "xyz", "abc #f()[dxyzf] ghi");
+        test("abc #f()[def] ghi", 5 .. 6, "g", "abc #g()[def] ghi");
     }
 }

src/syntax/ast.rs

@@ -65,7 +65,9 @@ impl Markup {
             NodeKind::Parbreak => Some(MarkupNode::Parbreak),
             NodeKind::Strong => Some(MarkupNode::Strong),
             NodeKind::Emph => Some(MarkupNode::Emph),
-            NodeKind::Text(s) => Some(MarkupNode::Text(s.clone())),
+            NodeKind::Text(s) | NodeKind::TextInLine(s) => {
+                Some(MarkupNode::Text(s.clone()))
+            }
             NodeKind::Escape(c) => Some(MarkupNode::Text((*c).into())),
             NodeKind::EnDash => Some(MarkupNode::Text('\u{2013}'.into())),
             NodeKind::EmDash => Some(MarkupNode::Text('\u{2014}'.into())),

src/syntax/highlight.rs

@@ -158,6 +158,7 @@ impl Category {
             NodeKind::Space(_) => None,
             NodeKind::Parbreak => None,
             NodeKind::Text(_) => None,
+            NodeKind::TextInLine(_) => None,
             NodeKind::List => None,
             NodeKind::Enum => None,
             NodeKind::Array => None,

src/syntax/mod.rs

@@ -15,6 +15,9 @@ pub use span::*;
 use self::ast::{MathNode, RawNode, TypedNode};
 use crate::diag::Error;
 use crate::geom::{AngularUnit, LengthUnit};
+use crate::parse::{
+    parse_atomic, parse_code, parse_markup, parse_markup_elements, TokenMode,
+};
 use crate::source::SourceId;
 use crate::util::EcoString;
 
@@ -73,6 +76,43 @@ impl Green {
             Self::Token(data) => data.kind = kind,
         }
     }
+
+    /// Find the innermost child that is incremental safe.
+    pub fn incremental_int(
+        &mut self,
+        edit: &str,
+        replace: Span,
+        replacement_len: usize,
+        offset: usize,
+        parent_mode: TokenMode,
+        outermost: bool,
+    ) -> bool {
+        match self {
+            Green::Node(n) => Rc::make_mut(n).incremental_int(
+                edit,
+                replace,
+                replacement_len,
+                offset,
+                parent_mode,
+                outermost,
+            ),
+            Green::Token(_) => false,
+        }
+    }
+
+    /// The error messages for this node and its descendants.
+    pub fn errors(&self) -> Vec<NodeKind> {
+        match self {
+            Green::Node(n) => n.errors(),
+            Green::Token(t) => {
+                if t.kind().is_error() {
+                    vec![t.kind().clone()]
+                } else {
+                    vec![]
+                }
+            }
+        }
+    }
 }
 
 impl Default for Green {
@@ -148,60 +188,181 @@ impl GreenNode {
         self.data().len()
     }
 
-    /// Find the deepest incremental-safe node and its offset in the source
+    /// The error messages for this node and its descendants.
-    /// code.
+    pub fn errors(&self) -> Vec<NodeKind> {
-    pub fn incremental_parent(&mut self, span: Span) -> Option<(&mut GreenNode, usize)> {
+        let mut res = self.children.iter().flat_map(|c| c.errors()).collect::<Vec<_>>();
-        self.incremental_parent_internal(span, 0)
+
+        if self.kind().is_error() {
+            res.push(self.kind().clone());
+        }
+
+        res
     }
 
-    fn incremental_parent_internal(
+    /// Find the innermost child that is incremental safe.
+    pub fn incremental(
         &mut self,
-        span: Span,
+        edit: &str,
+        replace: Span,
+        replacement_len: usize,
+    ) -> bool {
+        self.incremental_int(edit, replace, replacement_len, 0, TokenMode::Markup, true)
+    }
+
+    fn incremental_int(
+        &mut self,
+        src: &str,
+        replace: Span,
+        replacement_len: usize,
         mut offset: usize,
-    ) -> Option<(&mut GreenNode, usize)> {
+        parent_mode: TokenMode,
-        for child in self.children.iter_mut() {
+        outermost: bool,
-            match child {
+    ) -> bool {
-                Green::Token(n) => {
+        let kind = self.kind().clone();
-                    if offset < span.start {
+        let mode = kind.mode().apply(parent_mode);
-                        // the token is strictly before the span
+        eprintln!("in {:?} (mode {:?})", kind, mode);
-                        offset += n.len();
+
-                    } else {
+        let mut loop_result = None;
-                        // the token is within or after the span; tokens are
+        let mut child_at_start = true;
-                        // never safe, so we return.
+        let last = self.children.len() - 1;
-                        return None;
+        for (i, child) in self.children.iter_mut().enumerate() {
-                    }
+            let child_span = Span::new(replace.source, offset, offset + child.len());
+            if child_span.surrounds(replace) {
+                eprintln!("found correct child");
+
+                // First, we try if the child has another, more specific applicable child.
+                if kind.incremental_safety() != IncrementalSafety::Unsafe
+                    && child.incremental_int(
+                        src,
+                        replace,
+                        replacement_len,
+                        offset,
+                        mode,
+                        i == last && outermost,
+                    )
+                {
+                    eprintln!("child was successful");
+                    return true;
                 }
-                Green::Node(n) => {
+
-                    let end = n.len() + offset;
+                // This didn't work, so we try to replace the child at this
-                    if offset < span.start && end < span.start {
+                // level.
-                        // the node is strictly before the span
+                let (function, policy) =
-                        offset += n.len();
+                    if let Some(p) = child.kind().reparsing_function(mode) {
-                    } else if span.start >= offset
+                        p
-                        && span.start < end
-                        && span.end <= end
-                        && span.end > offset
-                    {
-                        // the node is within the span.
-                        let safe = n.kind().is_incremental_safe();
-                        let mut_n = Rc::make_mut(n);
-                        if safe {
-                            let res = mut_n.incremental_parent_internal(span, offset);
-                            if res.is_none() {
-                                return Some((mut_n, offset));
-                            }
-                        } else {
-                            return mut_n.incremental_parent_internal(span, offset);
-                        }
                     } else {
-                        // the node is overlapping or after after the span; nodes are
+                        return false;
-                        // never safe, so we return.
+                    };
-                        return None;
+                loop_result = Some((i, child_span, function, policy));
-                    }
+                break;
+            }
+
+            offset += child.len();
+            child_at_start = child.kind().is_at_start(child_at_start);
+        }
+
+
+        // We now have a child that we can replace and a function to do so if
+        // the loop found any results at all.
+        let (child_idx, child_span, func, policy) = if let Some(loop_result) = loop_result
+        {
+            loop_result
+        } else {
+            // No child fully contains the replacement.
+            eprintln!("no child match");
+            return false;
+        };
+
+        eprintln!("aquired function, policy {:?}", policy);
+
+        let src_span = child_span.inserted(replace, replacement_len);
+
+        let new_children =
+            if let Some(new_children) = func(&src[src_span.to_range()], child_at_start) {
+                new_children
+            } else {
+                eprintln!("function failed");
+                return false;
+            };
+        let child_mode = self.children[child_idx].kind().mode().apply(mode);
+        eprintln!("child mode {:?}", child_mode);
+
+        // Check if the children / child has the right type.
+        let require_single = match policy {
+            IncrementalSafety::AtomicPrimary | IncrementalSafety::SameKind => true,
+            IncrementalSafety::SameKindInCode if child_mode == TokenMode::Code => true,
+            _ => false,
+        };
+
+        if require_single {
+            eprintln!("must be a single replacement");
+            if new_children.len() != 1 {
+                eprintln!("not a single replacement");
+                return false;
+            }
+
+            if match policy {
+                IncrementalSafety::SameKind => true,
+                IncrementalSafety::SameKindInCode if child_mode == TokenMode::Code => {
+                    true
+                }
+                _ => false,
+            } {
+                if self.children[child_idx].kind() != new_children[0].kind() {
+                    eprintln!("not the same kind");
+                    return false;
                 }
             }
         }
 
-        return None;
+        // Do not accept unclosed nodes if the old node did not use to be at the
+        // right edge of the tree.
+        if !outermost
+            && new_children
+                .iter()
+                .flat_map(|x| x.errors())
+                .any(|x| matches!(x, NodeKind::Error(ErrorPos::End, _)))
+        {
+            eprintln!("unclosed node");
+            return false;
+        }
+
+        // Check if the neighbor invariants are still true.
+        if mode == TokenMode::Markup {
+            if child_idx > 0 {
+                if self.children[child_idx - 1].kind().incremental_safety()
+                    == IncrementalSafety::EnsureRightWhitespace
+                    && !new_children[0].kind().is_whitespace()
+                {
+                    eprintln!("left whitespace missing");
+                    return false;
+                }
+            }
+
+            let mut new_at_start = child_at_start;
+            for child in &new_children {
+                new_at_start = child.kind().is_at_start(new_at_start);
+            }
+
+            for child in &self.children[child_idx + 1 ..] {
+                if child.kind().is_trivia() {
+                    new_at_start = child.kind().is_at_start(new_at_start);
+                    continue;
+                }
+
+                match child.kind().incremental_safety() {
+                    IncrementalSafety::EnsureAtStart if !new_at_start => return false,
+                    IncrementalSafety::EnsureNotAtStart if new_at_start => return false,
+                    _ => {}
+                }
+                break;
+            }
+        }
+
+        eprintln!("... replacing");
+
+        self.children.splice(child_idx .. child_idx + 1, new_children);
+        true
     }
 }
 
@@ -397,6 +558,7 @@ impl<'a> RedRef<'a> {
         }
     }
 
+
     /// The error messages for this node and its descendants.
     pub fn errors(self) -> Vec<Error> {
         if !self.green.erroneous() {
@@ -593,6 +755,8 @@ pub enum NodeKind {
     Parbreak,
     /// A consecutive non-markup string.
     Text(EcoString),
+    /// A text node that cannot appear at the beginning of a source line.
+    TextInLine(EcoString),
     /// A non-breaking space: `~`.
     NonBreakingSpace,
     /// An en-dash: `--`.
@@ -729,19 +893,249 @@ impl NodeKind {
         matches!(self, Self::LeftParen | Self::RightParen)
     }
 
+    /// Whether this is whitespace.
+    pub fn is_whitespace(&self) -> bool {
+        match self {
+            Self::Space(_) | Self::Parbreak => true,
+            _ => false,
+        }
+    }
+
+    /// Whether this is trivia.
+    pub fn is_trivia(&self) -> bool {
+        match self {
+            _ if self.is_whitespace() => true,
+            Self::LineComment | Self::BlockComment => true,
+            _ => false,
+        }
+    }
+
     /// Whether this is some kind of error.
     pub fn is_error(&self) -> bool {
         matches!(self, NodeKind::Error(_, _) | NodeKind::Unknown(_))
     }
 
-    /// Whether it is safe to do incremental parsing on this node.
+    /// Whether this node is `at_start` given the previous value of the property.
-    pub fn is_incremental_safe(&self) -> bool {
+    pub fn is_at_start(&self, prev: bool) -> bool {
         match self {
-            Self::Block | Self::Markup => true,
+            Self::Space(n) if *n > 0 => true,
+            Self::Parbreak => true,
+            Self::LineComment | Self::BlockComment => prev,
             _ => false,
         }
     }
 
+    /// Whether this token appears in Markup.
+    pub fn mode(&self) -> NodeMode {
+        match self {
+            Self::Markup
+            | Self::Space(_)
+            | Self::Parbreak
+            | Self::Text(_)
+            | Self::TextInLine(_)
+            | Self::NonBreakingSpace
+            | Self::EnDash
+            | Self::EmDash
+            | Self::Escape(_)
+            | Self::Strong
+            | Self::Emph
+            | Self::Math(_) => NodeMode::Markup,
+            Self::Template
+            | Self::Block
+            | Self::None
+            | Self::Auto
+            | Self::Ident(_)
+            | Self::Bool(_)
+            | Self::Int(_)
+            | Self::Float(_)
+            | Self::Length(_, _)
+            | Self::Angle(_, _)
+            | Self::Percentage(_)
+            | Self::Str(_)
+            | Self::Fraction(_)
+            | Self::Array
+            | Self::Dict
+            | Self::Group
+            | Self::Call
+            | Self::LineComment
+            | Self::BlockComment
+            | Self::Error(_, _)
+            | Self::Minus
+            | Self::Eq => NodeMode::Universal,
+            _ => NodeMode::Code,
+        }
+    }
+
+    pub fn reparsing_function(
+        &self,
+        parent_mode: TokenMode,
+    ) -> Option<(fn(&str, bool) -> Option<Vec<Green>>, IncrementalSafety)> {
+        let policy = self.incremental_safety();
+        if policy == IncrementalSafety::Unsafe {
+            return None;
+        }
+
+        let mode = self.mode();
+        if mode == NodeMode::Code && policy == IncrementalSafety::UnsafeLayer {
+            return None;
+        }
+
+        if mode != NodeMode::Markup
+            && parent_mode == TokenMode::Code
+            && policy == IncrementalSafety::AtomicPrimary
+        {
+            return Some((parse_atomic, policy));
+        }
+
+        let parser: fn(&str, bool) -> _ = match mode {
+            NodeMode::Code => parse_code,
+            NodeMode::Markup if self == &Self::Markup => parse_markup,
+            NodeMode::Markup => parse_markup_elements,
+            NodeMode::Universal if parent_mode == TokenMode::Code => parse_code,
+            NodeMode::Universal => parse_markup_elements,
+        };
+
+        Some((parser, policy))
+    }
+
+    /// Whether it is safe to do incremental parsing on this node. Never allow
+    /// non-termination errors if this is not already the last leaf node.
+    pub fn incremental_safety(&self) -> IncrementalSafety {
+        match self {
+            // Replacing parenthesis changes if the expression is balanced and
+            // is therefore not safe.
+            Self::LeftBracket
+            | Self::RightBracket
+            | Self::LeftBrace
+            | Self::RightBrace
+            | Self::LeftParen
+            | Self::RightParen => IncrementalSafety::Unsafe,
+
+            // Replacing an operator can change whether the parent is an
+            // operation which makes it unsafe. The star can appear in markup.
+            Self::Star
+            | Self::Comma
+            | Self::Semicolon
+            | Self::Colon
+            | Self::Plus
+            | Self::Minus
+            | Self::Slash
+            | Self::Eq
+            | Self::EqEq
+            | Self::ExclEq
+            | Self::Lt
+            | Self::LtEq
+            | Self::Gt
+            | Self::GtEq
+            | Self::PlusEq
+            | Self::HyphEq
+            | Self::StarEq
+            | Self::SlashEq
+            | Self::Not
+            | Self::And
+            | Self::Or
+            | Self::With
+            | Self::Dots
+            | Self::Arrow => IncrementalSafety::Unsafe,
+
+            // These keywords are literals and can be safely be substituted with
+            // other expressions.
+            Self::None | Self::Auto => IncrementalSafety::AtomicPrimary,
+
+            // These keywords change what kind of expression the parent is.
+            Self::Let
+            | Self::If
+            | Self::Else
+            | Self::For
+            | Self::In
+            | Self::While
+            | Self::Break
+            | Self::Continue
+            | Self::Return
+            | Self::Set
+            | Self::Import
+            | Self::Include
+            | Self::From => IncrementalSafety::Unsafe,
+
+            // This is a backslash followed by a space. But changing it to
+            // anything else is fair game.
+            Self::Linebreak => IncrementalSafety::EnsureRightWhitespace,
+
+            Self::Markup => IncrementalSafety::SameKind,
+
+            Self::Space(_) => IncrementalSafety::SameKindInCode,
+
+            // These are all replaceable by other tokens.
+            Self::Parbreak
+            | Self::Text(_)
+            | Self::NonBreakingSpace
+            | Self::EnDash
+            | Self::EmDash
+            | Self::Escape(_)
+            | Self::Strong
+            | Self::Emph => IncrementalSafety::Safe,
+
+            // This is text that needs to be not `at_start`, otherwise it would
+            // start one of the below items.
+            Self::TextInLine(_) => IncrementalSafety::EnsureNotAtStart,
+
+            // These have to be `at_start` so they must be preceeded with a
+            // Space(n) with n > 0 or a Parbreak.
+            Self::Heading | Self::Enum | Self::List => IncrementalSafety::EnsureAtStart,
+
+            // Changing the heading level, enum numbering, or list bullet
+            // changes the next layer.
+            Self::EnumNumbering(_) => IncrementalSafety::Unsafe,
+
+            Self::Raw(_) | Self::Math(_) => IncrementalSafety::Safe,
+
+            // These are expressions that can be replaced by other expressions.
+            Self::Ident(_)
+            | Self::Bool(_)
+            | Self::Int(_)
+            | Self::Float(_)
+            | Self::Length(_, _)
+            | Self::Angle(_, _)
+            | Self::Percentage(_)
+            | Self::Str(_)
+            | Self::Fraction(_)
+            | Self::Array
+            | Self::Dict
+            | Self::Group => IncrementalSafety::AtomicPrimary,
+
+            Self::Call | Self::Unary | Self::Binary | Self::SetExpr => {
+                IncrementalSafety::UnsafeLayer
+            }
+
+            Self::CallArgs | Self::Named | Self::Spread => IncrementalSafety::UnsafeLayer,
+
+            // The closure is a bit magic with the let expression, and also it
+            // is not atomic.
+            Self::Closure | Self::ClosureParams => IncrementalSafety::UnsafeLayer,
+
+            // These can appear as bodies and would trigger an error if they
+            // became something else.
+            Self::Template | Self::Block => IncrementalSafety::SameKindInCode,
+
+            Self::ForExpr
+            | Self::WhileExpr
+            | Self::IfExpr
+            | Self::LetExpr
+            | Self::ImportExpr
+            | Self::IncludeExpr => IncrementalSafety::AtomicPrimary,
+
+            Self::WithExpr | Self::ForPattern | Self::ImportItems => {
+                IncrementalSafety::UnsafeLayer
+            }
+
+            // These can appear everywhere and must not change to other stuff
+            // because that could change the outer expression.
+            Self::LineComment | Self::BlockComment => IncrementalSafety::SameKind,
+
+            Self::Error(_, _) | Self::Unknown(_) => IncrementalSafety::Unsafe,
+        }
+    }
+
     /// A human-readable name for the kind.
     pub fn as_str(&self) -> &'static str {
         match self {
@@ -794,7 +1188,7 @@ impl NodeKind {
             Self::Space(_) => "space",
             Self::Linebreak => "forced linebreak",
             Self::Parbreak => "paragraph break",
-            Self::Text(_) => "text",
+            Self::Text(_) | Self::TextInLine(_) => "text",
             Self::NonBreakingSpace => "non-breaking space",
             Self::EnDash => "en dash",
             Self::EmDash => "em dash",
@@ -855,3 +1249,87 @@ impl Display for NodeKind {
         f.pad(self.as_str())
     }
 }
+
+/// This enum describes what conditions a node has for being replaced by a new
+/// parse result.
+///
+/// Safe nodes are replaced by the new parse result from the respective mode.
+/// They can be replaced by multiple tokens. If a token is inserted in Markup
+/// mode and the next token would not be `at_start` there needs to be a forward
+/// check for a `EnsureAtStart` node. If this fails, the parent has to be
+/// reparsed. if the direct whitespace sibling of a `EnsureRightWhitespace` is
+/// `Unsafe`. Similarly, if a `EnsureRightWhitespace` token is one of the last
+/// tokens to be inserted, the edit is invalidated if there is no following
+/// whitespace. The atomic nodes may only be replaced by other atomic nodes. The
+/// unsafe layers cannot be used but allow children access, the unsafe nodes do
+/// neither.
+///
+/// *Procedure:*
+/// 1. Check if the node is safe - if unsafe layer recurse, if unsafe, return
+///    None.
+/// 2. Reparse with appropriate node kind and `at_start`.
+/// 3. Check whether the topmost group is terminated and the range was
+///    completely consumed, otherwise return None.
+/// 4. Check if the type criteria are met.
+/// 5. If the node is not at the end of the tree, check if Strings etc. are
+///    terminated.
+/// 6. If this is markup, check the following things:
+///   - The `at_start` conditions of the next non-comment and non-space(0) node
+///     are met.
+///   - The first node is whitespace or the previous siblings are not
+///     `EnsureRightWhitespace`.
+///   - If any of those fails, return None.
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Hash)]
+pub enum IncrementalSafety {
+    /// Changing this node can never have an influence on the other nodes.
+    Safe,
+    /// This node has to be replaced with a single token of the same kind.
+    SameKind,
+    /// This node has to be replaced with a single token of the same kind if in
+    /// code mode.
+    SameKindInCode,
+    /// These nodes depend on being at the start of a line. Reparsing of safe
+    /// left neighbors has to check this invariant. Otherwise, this node is
+    /// safe.
+    EnsureAtStart,
+    /// These nodes depend on not being at the start of a line. Reparsing of
+    /// safe left neighbors has to check this invariant. Otherwise, this node is
+    /// safe.
+    EnsureNotAtStart,
+    /// These nodes must be followed by whitespace.
+    EnsureRightWhitespace,
+    /// Changing this node into a single atomic expression is allowed if it
+    /// appears in code mode, otherwise it is safe.
+    AtomicPrimary,
+    /// Changing an unsafe layer node changes what the parents or the
+    /// surrounding nodes would be and is therefore disallowed. Change the
+    /// parents or children instead. If it appears in Markup, however, it is
+    /// safe to change.
+    UnsafeLayer,
+    /// Changing an unsafe node or any of its children will trigger undefined
+    /// behavior. Change the parents instead.
+    Unsafe,
+}
+
+/// This enum describes which mode a token of [`NodeKind`] can appear in.
+#[derive(Debug, Copy, Clone, Eq, PartialEq)]
+pub enum NodeMode {
+    /// The token can only appear in markup mode.
+    Markup,
+    /// The token can only appear in code mode.
+    Code,
+    /// The token can appear in either mode. Look at the parent node to decide
+    /// which mode it is in.
+    Universal,
+}
+
+impl NodeMode {
+    /// Returns the new [`TokenMode`] given the old one.
+    pub fn apply(&self, old: TokenMode) -> TokenMode {
+        match self {
+            Self::Markup => TokenMode::Markup,
+            Self::Code => TokenMode::Code,
+            Self::Universal => old,
+        }
+    }
+}

src/syntax/span.rs

@@ -125,7 +125,8 @@ impl Span {
         *self = self.join(other)
     }
 
-    /// Create a new span with n characters inserted inside of this span.
+    /// Create a new span by specifying a span in which a modification happened
+    /// and how many characters are now in that span.
     pub fn inserted(mut self, other: Self, n: usize) -> Self {
         if !self.surrounds(other) {
             panic!();