Add support for parsing MDX ESM, expressions

This commit adds support for hooks that lets a user integrate another
parser with `micromark-rs`, to parse ESM and expressions according to
a particular grammar (such as a programming language, typically
JavaScript).

For an example integrating with SWC, see `tests/test_utils/mod.rs`.

The integration occurs with two functions passed in `options`:
`mdx_expression_parse` and `mdx_esm_parse`.
The can signal back to micromark when they are successful,
whether there is an error at the end (in which case micromark will
try to parse more), or whether there is a syntax error (in which case
micromark will crash).

12 files changed, 628 insertions, 27 deletions

diff --git a/src/compiler.rs b/src/compiler.rs
index d1ac774..eaa15ee 100644
--- a/src/compiler.rs
+++ b/src/compiler.rs
@@ -364,6 +364,7 @@ fn enter(context: &mut CompileContext) {
         | Name::HeadingAtxText
         | Name::HeadingSetextText
         | Name::Label
+        | Name::MdxEsm
         | Name::MdxFlowExpression
         | Name::MdxTextExpression
         | Name::MdxJsxFlowTag
@@ -412,7 +413,7 @@ fn exit(context: &mut CompileContext) {
         | Name::Resource => {
             on_exit_drop(context);
         }
-        Name::MdxFlowExpression | Name::MdxJsxFlowTag => on_exit_drop_slurp(context),
+        Name::MdxEsm | Name::MdxFlowExpression | Name::MdxJsxFlowTag => on_exit_drop_slurp(context),
         Name::CharacterEscapeValue | Name::CodeTextData | Name::Data | Name::MathTextData => {
             on_exit_data(context);
         }
diff --git a/src/construct/flow.rs b/src/construct/flow.rs
index 08e0466..d6a79d8 100644
--- a/src/construct/flow.rs
+++ b/src/construct/flow.rs
@@ -15,6 +15,7 @@
 //! *   [Heading (atx)][crate::construct::heading_atx]
 //! *   [Heading (setext)][crate::construct::heading_setext]
 //! *   [HTML (flow)][crate::construct::html_flow]
+//! *   [MDX esm][crate::construct::mdx_esm]
 //! *   [MDX expression (flow)][crate::construct::mdx_expression_flow]
 //! *   [MDX JSX (flow)][crate::construct::mdx_jsx_flow]
 //! *   [Raw (flow)][crate::construct::raw_flow] (code (fenced), math (flow))
@@ -66,6 +67,13 @@ pub fn start(tokenizer: &mut Tokenizer) -> State {
             );
             State::Retry(StateName::HtmlFlowStart)
         }
+        Some(b'e' | b'i') => {
+            tokenizer.attempt(
+                State::Next(StateName::FlowAfter),
+                State::Next(StateName::FlowBeforeContent),
+            );
+            State::Retry(StateName::MdxEsmStart)
+        }
         Some(b'{') => {
             tokenizer.attempt(
                 State::Next(StateName::FlowAfter),
diff --git a/src/construct/mdx_esm.rs b/src/construct/mdx_esm.rs
new file mode 100644
index 0000000..53f8beb
--- /dev/null
+++ b/src/construct/mdx_esm.rs
@@ -0,0 +1,224 @@
+//! MDX ESM occurs in the [flow][] content type.
+//!
+//! ## Grammar
+//!
+//! MDX expression (flow) forms with the following BNF
+//! (<small>see [construct][crate::construct] for character groups</small>):
+//!
+//! ```bnf
+//! mdx_esm ::= word *line *(eol *line)
+//!
+//! word ::= 'e' 'x' 'p' 'o' 'r' 't' | 'i' 'm' 'p' 'o' 'r' 't'
+//! ```
+//!
+//! This construct must be followed by a blank line or eof (end of file).
+//! It can include blank lines if [`MdxEsmParse`][crate::MdxEsmParse] passed in
+//! `options.mdx_esm_parse` allows it.
+//!
+//! ## Tokens
+//!
+//! *   [`LineEnding`][Name::LineEnding]
+//! *   [`MdxEsm`][Name::MdxEsm]
+//! *   [`MdxEsmData`][Name::MdxEsmData]
+//!
+//! ## References
+//!
+//! *   [`syntax.js` in `micromark-extension-mdxjs-esm`](https://github.com/micromark/micromark-extension-mdxjs-esm/blob/main/dev/lib/syntax.js)
+//! *   [`mdxjs.com`](https://mdxjs.com)
+//!
+//! [flow]: crate::construct::flow
+
+use crate::event::Name;
+use crate::state::{Name as StateName, State};
+use crate::tokenizer::Tokenizer;
+use crate::util::{
+    mdx_collect::{collect, place_to_point},
+    slice::Slice,
+};
+use crate::MdxSignal;
+use alloc::format;
+
+/// Start of MDX ESM.
+///
+/// ```markdown
+/// > | import a from 'b'
+///     ^
+/// ```
+pub fn start(tokenizer: &mut Tokenizer) -> State {
+    // If it’s turned on.
+    if tokenizer.parse_state.options.constructs.mdx_esm
+        // If there is a gnostic parser.
+        && tokenizer.parse_state.options.mdx_esm_parse.is_some()
+        // When not interrupting.
+        && !tokenizer.interrupt
+        // Only at the start of a line, not at whitespace or in a container.
+        && tokenizer.point.column == 1
+        && matches!(tokenizer.current, Some(b'e' | b'i'))
+    {
+        // Place where keyword starts.
+        tokenizer.tokenize_state.start = tokenizer.point.index;
+        tokenizer.enter(Name::MdxEsm);
+        tokenizer.enter(Name::MdxEsmData);
+        tokenizer.consume();
+        State::Next(StateName::MdxEsmWord)
+    } else {
+        State::Nok
+    }
+}
+
+/// In keyword.
+///
+/// ```markdown
+/// > | import a from 'b'
+///     ^^^^^^
+/// ```
+pub fn word(tokenizer: &mut Tokenizer) -> State {
+    if matches!(tokenizer.current, Some(b'a'..=b'z')) {
+        tokenizer.consume();
+        State::Next(StateName::MdxEsmWord)
+    } else {
+        let slice = Slice::from_indices(
+            tokenizer.parse_state.bytes,
+            tokenizer.tokenize_state.start,
+            tokenizer.point.index,
+        );
+
+        if matches!(slice.as_str(), "export" | "import") && tokenizer.current == Some(b' ') {
+            tokenizer.concrete = true;
+            tokenizer.tokenize_state.start = tokenizer.events.len() - 1;
+            tokenizer.consume();
+            State::Next(StateName::MdxEsmInside)
+        } else {
+            tokenizer.tokenize_state.start = 0;
+            State::Nok
+        }
+    }
+}
+
+/// In data.
+///
+/// ```markdown
+/// > | import a from 'b'
+///           ^
+/// ```
+pub fn inside(tokenizer: &mut Tokenizer) -> State {
+    match tokenizer.current {
+        None | Some(b'\n') => {
+            tokenizer.exit(Name::MdxEsmData);
+            State::Retry(StateName::MdxEsmLineStart)
+        }
+        _ => {
+            tokenizer.consume();
+            State::Next(StateName::MdxEsmInside)
+        }
+    }
+}
+
+/// At start of line.
+///
+/// ```markdown
+///   | import a from 'b'
+/// > | export {a}
+///     ^
+/// ```
+pub fn line_start(tokenizer: &mut Tokenizer) -> State {
+    match tokenizer.current {
+        None => State::Retry(StateName::MdxEsmAtEnd),
+        Some(b'\n') => {
+            tokenizer.check(
+                State::Next(StateName::MdxEsmAtEnd),
+                State::Next(StateName::MdxEsmContinuationStart),
+            );
+            State::Retry(StateName::MdxEsmBlankLineBefore)
+        }
+        _ => {
+            tokenizer.enter(Name::MdxEsmData);
+            tokenizer.consume();
+            State::Next(StateName::MdxEsmInside)
+        }
+    }
+}
+
+/// At start of line that continues.
+///
+/// ```markdown
+///   | import a from 'b'
+/// > | export {a}
+///     ^
+/// ```
+pub fn continuation_start(tokenizer: &mut Tokenizer) -> State {
+    tokenizer.enter(Name::LineEnding);
+    tokenizer.consume();
+    tokenizer.exit(Name::LineEnding);
+    State::Next(StateName::MdxEsmLineStart)
+}
+
+/// At start of a potentially blank line.
+///
+/// ```markdown
+///   | import a from 'b'
+/// > | export {a}
+///     ^
+/// ```
+pub fn blank_line_before(tokenizer: &mut Tokenizer) -> State {
+    tokenizer.enter(Name::LineEnding);
+    tokenizer.consume();
+    tokenizer.exit(Name::LineEnding);
+    State::Next(StateName::BlankLineStart)
+}
+
+/// At end of line (blank or eof).
+///
+/// ```markdown
+/// > | import a from 'b'
+///                      ^
+/// ```
+pub fn at_end(tokenizer: &mut Tokenizer) -> State {
+    let result = parse_esm(tokenizer);
+
+    // Done!.
+    if matches!(result, State::Ok) {
+        tokenizer.concrete = false;
+        tokenizer.exit(Name::MdxEsm);
+    }
+
+    result
+}
+
+/// Parse ESM with a given function.
+fn parse_esm(tokenizer: &mut Tokenizer) -> State {
+    // We can `unwrap` because we don’t parse if this is `None`.
+    let parse = tokenizer
+        .parse_state
+        .options
+        .mdx_esm_parse
+        .as_ref()
+        .unwrap();
+
+    // Collect the body of the ESM and positional info for each run of it.
+    let result = collect(
+        tokenizer,
+        tokenizer.tokenize_state.start,
+        &[Name::MdxEsmData, Name::LineEnding],
+    );
+
+    // Parse and handle what was signaled back.
+    match parse(&result.value) {
+        MdxSignal::Ok => State::Ok,
+        MdxSignal::Error(message, place) => {
+            let point = place_to_point(&result, place);
+            State::Error(format!("{}:{}: {}", point.line, point.column, message))
+        }
+        MdxSignal::Eof(message) => {
+            if tokenizer.current == None {
+                State::Error(format!(
+                    "{}:{}: {}",
+                    tokenizer.point.line, tokenizer.point.column, message
+                ))
+            } else {
+                tokenizer.tokenize_state.mdx_last_parse_error = Some(message);
+                State::Retry(StateName::MdxEsmContinuationStart)
+            }
+        }
+    }
+}
diff --git a/src/construct/mod.rs b/src/construct/mod.rs
index ae6facf..88f3050 100644
--- a/src/construct/mod.rs
+++ b/src/construct/mod.rs
@@ -66,6 +66,7 @@
 //! *   [gfm task list item check][gfm_task_list_item_check]
 //! *   [mdx expression (flow)][mdx_expression_flow]
 //! *   [mdx expression (text)][mdx_expression_text]
+//! *   [mdx esm][mdx_esm]
 //! *   [mdx jsx (flow)][mdx_jsx_flow]
 //! *   [mdx jsx (text)][mdx_jsx_text]
 //!
@@ -169,6 +170,7 @@ pub mod label_end;
 pub mod label_start_image;
 pub mod label_start_link;
 pub mod list_item;
+pub mod mdx_esm;
 pub mod mdx_expression_flow;
 pub mod mdx_expression_text;
 pub mod mdx_jsx_flow;
diff --git a/src/construct/partial_mdx_expression.rs b/src/construct/partial_mdx_expression.rs
index 31a9af8..3ebd0f0 100644
--- a/src/construct/partial_mdx_expression.rs
+++ b/src/construct/partial_mdx_expression.rs
@@ -14,7 +14,6 @@
 //! ## Tokens
 //!
 //! *   [`LineEnding`][Name::LineEnding]
-//! *   [`SpaceOrTab`][Name::SpaceOrTab]
 //! *   [`MdxExpressionMarker`][Name::MdxExpressionMarker]
 //! *   [`MdxExpressionData`][Name::MdxExpressionData]
 //!
@@ -61,7 +60,12 @@ use crate::construct::partial_space_or_tab::space_or_tab_min_max;
 use crate::event::Name;
 use crate::state::{Name as StateName, State};
 use crate::tokenizer::Tokenizer;
-use alloc::format;
+use crate::util::{
+    constant::TAB_SIZE,
+    mdx_collect::{collect, place_to_point},
+};
+use crate::{MdxExpressionKind, MdxExpressionParse, MdxSignal};
+use alloc::{format, string::ToString};
 
 /// Start of an MDX expression.
 ///
@@ -75,6 +79,7 @@ pub fn start(tokenizer: &mut Tokenizer) -> State {
     tokenizer.enter(Name::MdxExpressionMarker);
     tokenizer.consume();
     tokenizer.exit(Name::MdxExpressionMarker);
+    tokenizer.tokenize_state.start = tokenizer.events.len() - 1;
     State::Next(StateName::MdxExpressionBefore)
 }
 
@@ -88,8 +93,10 @@ pub fn before(tokenizer: &mut Tokenizer) -> State {
     match tokenizer.current {
         None => {
             State::Error(format!(
-                "{}:{}: Unexpected end of file in expression, expected a corresponding closing brace for `{{`",
-                tokenizer.point.line, tokenizer.point.column
+                "{}:{}: {}",
+                tokenizer.point.line, tokenizer.point.column,
+                tokenizer.tokenize_state.mdx_last_parse_error.take()
+                    .unwrap_or_else(|| "Unexpected end of file in expression, expected a corresponding closing brace for `{`".to_string())
             ))
         }
         Some(b'\n') => {
@@ -97,24 +104,26 @@ pub fn before(tokenizer: &mut Tokenizer) -> State {
             tokenizer.consume();
             tokenizer.exit(Name::LineEnding);
             State::Next(StateName::MdxExpressionEolAfter)
-        },
+        }
         Some(b'}') if tokenizer.tokenize_state.size == 0 => {
-            if tokenizer.tokenize_state.token_1 == Name::MdxJsxTagAttributeValueExpression && !tokenizer.tokenize_state.seen {
-                State::Error(format!(
-                    "{}:{}: Unexpected empty expression, expected a value between braces",
-                    tokenizer.point.line, tokenizer.point.column
-                ))
+            let state = if let Some(ref parse) = tokenizer.parse_state.options.mdx_expression_parse
+            {
+                parse_expression(tokenizer, parse)
             } else {
-                tokenizer.tokenize_state.seen = false;
+                State::Ok
+            };
+
+            if state == State::Ok {
+                tokenizer.tokenize_state.start = 0;
                 tokenizer.enter(Name::MdxExpressionMarker);
                 tokenizer.consume();
                 tokenizer.exit(Name::MdxExpressionMarker);
                 tokenizer.exit(tokenizer.tokenize_state.token_1.clone());
-                State::Ok
             }
-        },
+
+            state
+        }
         Some(_) => {
-            tokenizer.tokenize_state.seen = true;
             tokenizer.enter(Name::MdxExpressionData);
             State::Retry(StateName::MdxExpressionInside)
         }
@@ -134,8 +143,10 @@ pub fn inside(tokenizer: &mut Tokenizer) -> State {
         tokenizer.exit(Name::MdxExpressionData);
         State::Retry(StateName::MdxExpressionBefore)
     } else {
-        // To do: don’t count if gnostic.
-        if tokenizer.current == Some(b'{') {
+        // Don’t count if gnostic.
+        if tokenizer.current == Some(b'{')
+            && tokenizer.parse_state.options.mdx_expression_parse.is_none()
+        {
             tokenizer.tokenize_state.size += 1;
         } else if tokenizer.current == Some(b'}') {
             tokenizer.tokenize_state.size -= 1;
@@ -165,9 +176,60 @@ pub fn eol_after(tokenizer: &mut Tokenizer) -> State {
         ))
     } else if matches!(tokenizer.current, Some(b'\t' | b' ')) {
         tokenizer.attempt(State::Next(StateName::MdxExpressionBefore), State::Nok);
-        // To do: use `start_column` + constants.tabSize for max space to eat.
-        State::Next(space_or_tab_min_max(tokenizer, 0, usize::MAX))
+        // Idea: investigate if we’d need to use more complex stripping.
+        // Take this example:
+        //
+        // ```markdown
+        // >  aaa <b c={`
+        // >      d
+        // >  `} /> eee
+        // ```
+        //
+        // Currently, the “paragraph” starts at `> | aaa`, so for the next line
+        // here we split it into `>␠|␠␠␠␠|␠d` (prefix, this indent here,
+        // expression data).
+        // The intention above is likely for the split to be as `>␠␠|␠␠␠␠|d`,
+        // which is impossible, but we can mimick it with `>␠|␠␠␠␠␠|d`.
+        //
+        // To improve the situation, we could take `tokenizer.line_start` at
+        // the start of the expression and move past whitespace.
+        // For future lines, we’d move at most to
+        // `line_start_shifted.column + 4`.
+        State::Retry(space_or_tab_min_max(tokenizer, 0, TAB_SIZE))
     } else {
         State::Retry(StateName::MdxExpressionBefore)
     }
 }
+
+/// Parse an expression with a given function.
+fn parse_expression(tokenizer: &mut Tokenizer, parse: &MdxExpressionParse) -> State {
+    // Collect the body of the expression and positional info for each run of it.
+    let result = collect(
+        tokenizer,
+        tokenizer.tokenize_state.start,
+        &[Name::MdxExpressionData, Name::LineEnding],
+    );
+
+    // Turn the name of the expression into a kind.
+    let kind = match tokenizer.tokenize_state.token_1 {
+        Name::MdxFlowExpression | Name::MdxTextExpression => MdxExpressionKind::Expression,
+        Name::MdxJsxTagAttributeExpression => MdxExpressionKind::AttributeExpression,
+        Name::MdxJsxTagAttributeValueExpression => MdxExpressionKind::AttributeValueExpression,
+        _ => unreachable!("cannot handle unknown expression name"),
+    };
+
+    // Parse and handle what was signaled back.
+    match parse(&result.value, kind) {
+        MdxSignal::Ok => State::Ok,
+        MdxSignal::Error(message, place) => {
+            let point = place_to_point(&result, place);
+            State::Error(format!("{}:{}: {}", point.line, point.column, message))
+        }
+        MdxSignal::Eof(message) => {
+            tokenizer.tokenize_state.mdx_last_parse_error = Some(message);
+            tokenizer.enter(Name::MdxExpressionData);
+            tokenizer.consume();
+            State::Next(StateName::MdxExpressionInside)
+        }
+    }
+}
diff --git a/src/construct/partial_mdx_jsx.rs b/src/construct/partial_mdx_jsx.rs
index 9177b5b..e49a8e0 100644
--- a/src/construct/partial_mdx_jsx.rs
+++ b/src/construct/partial_mdx_jsx.rs
@@ -611,8 +611,6 @@ pub fn attribute_before(tokenizer: &mut Tokenizer) -> State {
         Some(b'>') => State::Retry(StateName::MdxJsxTagEnd),
         // Attribute expression.
         Some(b'{') => {
-            // To do: force `spread: true` if gnostic.
-            // To do: pass `start_point` if gnostic.
             tokenizer.tokenize_state.token_2 = tokenizer.tokenize_state.token_1.clone();
             tokenizer.tokenize_state.token_1 = Name::MdxJsxTagAttributeExpression;
             tokenizer.attempt(
@@ -886,7 +884,6 @@ pub fn attribute_value_before(tokenizer: &mut Tokenizer) -> State {
         }
         // Attribute value expression.
         Some(b'{') => {
-            // To do: pass `start_point` if gnostic.
             tokenizer.tokenize_state.token_2 = tokenizer.tokenize_state.token_1.clone();
             tokenizer.tokenize_state.token_1 = Name::MdxJsxTagAttributeValueExpression;
             tokenizer.attempt(
diff --git a/src/event.rs b/src/event.rs
index a2626ee..b3fa9ae 100644
--- a/src/event.rs
+++ b/src/event.rs
@@ -2391,6 +2391,45 @@ pub enum Name {
     ///       ^ ^
     /// ```
     MathTextSequence,
+    /// MDX extension: ESM.
+    ///
+    /// ## Info
+    ///
+    /// *   **Context**:
+    ///     [flow content][crate::construct::flow]
+    /// *   **Content model**:
+    ///     void
+    ///     [`MdxEsmData`][Name::MdxEsmData],
+    ///     [`SpaceOrTab`][Name::SpaceOrTab],
+    ///     [`LineEnding`][Name::LineEnding]
+    /// *   **Construct**:
+    ///     [`mdx_esm`][crate::construct::mdx_esm]
+    ///
+    /// ## Example
+    ///
+    /// ```markdown
+    /// > | import a from 'b'
+    ///     ^^^^^^^^^^^^^^^^^
+    /// ```
+    MdxEsm,
+    /// MDX extension: ESM data.
+    ///
+    /// ## Info
+    ///
+    /// *   **Context**:
+    ///     [`MdxEsm`][Name::MdxEsm]
+    /// *   **Content model**:
+    ///     void
+    /// *   **Construct**:
+    ///     [`mdx_esm`][crate::construct::mdx_esm]
+    ///
+    /// ## Example
+    ///
+    /// ```markdown
+    /// > | import a from 'b'
+    ///     ^^^^^^^^^^^^^^^^^
+    /// ```
+    MdxEsmData,
     /// MDX extension: expression marker.
     ///
     /// ## Info
@@ -3336,7 +3375,7 @@ pub enum Name {
 }
 
 /// List of void events, used to make sure everything is working well.
-pub const VOID_EVENTS: [Name; 75] = [
+pub const VOID_EVENTS: [Name; 76] = [
     Name::AttentionSequence,
     Name::AutolinkEmail,
     Name::AutolinkMarker,
@@ -3391,6 +3430,7 @@ pub const VOID_EVENTS: [Name; 75] = [
     Name::MathFlowChunk,
     Name::MathTextData,
     Name::MathTextSequence,
+    Name::MdxEsmData,
     Name::MdxExpressionMarker,
     Name::MdxExpressionData,
     Name::MdxJsxTagMarker,
diff --git a/src/lib.rs b/src/lib.rs
index 47a125f..0b1a571 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -20,7 +20,7 @@ mod util;
 
 use crate::compiler::compile;
 use crate::parser::parse;
-use alloc::string::String;
+use alloc::{boxed::Box, fmt, string::String};
 
 /// Type of line endings in markdown.
 #[derive(Clone, Debug, Default, Eq, PartialEq)]
@@ -79,6 +79,71 @@ impl LineEnding {
     }
 }
 
+/// Signal used as feedback when parsing MDX esm/expressions.
+#[derive(Clone, Debug)]
+pub enum MdxSignal {
+    /// A syntax error.
+    ///
+    /// `micromark-rs` will crash with error message `String`, and convert the
+    /// `usize` (byte offset into `&str` passed to `MdxExpressionParse` or
+    /// `MdxEsmParse`) to where it happened in the whole document.
+    ///
+    /// ## Examples
+    ///
+    /// ```rust ignore
+    /// MdxSignal::Error("Unexpected `\"`, expected identifier".to_string(), 1)
+    /// ```
+    Error(String, usize),
+    /// An error at the end of the (partial?) expression.
+    ///
+    /// `micromark-rs` will either crash with error message `String` if it
+    /// doesn’t have any more text, or it will try again later when more text
+    /// is available.
+    ///
+    /// ## Examples
+    ///
+    /// ```rust ignore
+    /// MdxSignal::Eof("Unexpected end of file in string literal".to_string())
+    /// ```
+    Eof(String),
+    /// Done, successfully.
+    ///
+    /// `micromark-rs` knows that this is the end of a valid expression/esm and
+    /// continues with markdown.
+    ///
+    /// ## Examples
+    ///
+    /// ```rust ignore
+    /// MdxSignal::Ok
+    /// ```
+    Ok,
+}
+
+/// Expression kind.
+#[derive(Clone, Debug)]
+pub enum MdxExpressionKind {
+    /// Kind of expressions in prose: `# {Math.PI}` and `{Math.PI}`.
+    Expression,
+    /// Kind of expressions as attributes: `<a {...b}>`
+    AttributeExpression,
+    /// Kind of expressions as attribute values: `<a b={c}>`.
+    AttributeValueExpression,
+}
+
+/// Signature of a function that parses expressions.
+///
+/// Can be passed as `mdx_expression_parse` in [`Options`][] to support
+/// expressions according to a certain grammar (typically, a programming
+/// language).
+pub type MdxExpressionParse = dyn Fn(&str, MdxExpressionKind) -> MdxSignal;
+
+/// Signature of a function that parses ESM.
+///
+/// Can be passed as `mdx_esm_parse` in [`Options`][] to support
+/// ESM according to a certain grammar (typically, a programming
+/// language).
+pub type MdxEsmParse = dyn Fn(&str) -> MdxSignal;
+
 /// Control which constructs are enabled.
 ///
 /// Not all constructs can be configured.
@@ -301,12 +366,28 @@ pub struct Constructs {
     ///       ^^^
     /// ```
     pub math_text: bool,
+    /// MDX: ESM.
+    ///
+    /// ```markdown
+    /// > | import a from 'b'
+    ///     ^^^^^^^^^^^^^^^^^
+    /// ```
+    ///
+    /// > 👉 **Note**: you *must* pass [`options.mdx_esm_parse`][MdxEsmParse]
+    /// > too.
+    /// > Otherwise, this option has no affect.
+    pub mdx_esm: bool,
     /// MDX: expression (flow).
     ///
     /// ```markdown
     /// > | {Math.PI}
     ///     ^^^^^^^^^
     /// ```
+    ///
+    /// > 👉 **Note**: you *can* pass
+    /// > [`options.mdx_expression_parse`][MdxExpressionParse]
+    /// > to parse expressions according to a certain grammar (typically, a
+    /// > programming language).
     pub mdx_expression_flow: bool,
     /// MDX: expression (text).
     ///
@@ -314,6 +395,11 @@ pub struct Constructs {
     /// > | a {Math.PI} c
     ///       ^^^^^^^^^
     /// ```
+    ///
+    /// > 👉 **Note**: you *can* pass
+    /// > [`options.mdx_expression_parse`][MdxExpressionParse]
+    /// > to parse expressions according to a certain grammar (typically, a
+    /// > programming language).
     pub mdx_expression_text: bool,
     /// MDX: JSX (flow).
     ///
@@ -321,6 +407,11 @@ pub struct Constructs {
     /// > | <Component />
     ///     ^^^^^^^^^^^^^
     /// ```
+    ///
+    /// > 👉 **Note**: you *can* pass
+    /// > [`options.mdx_expression_parse`][MdxExpressionParse]
+    /// > to parse expressions in JSX according to a certain grammar
+    /// > (typically, a programming language).
     pub mdx_jsx_flow: bool,
     /// MDX: JSX (text).
     ///
@@ -328,6 +419,11 @@ pub struct Constructs {
     /// > | a <Component /> c
     ///       ^^^^^^^^^^^^^
     /// ```
+    ///
+    /// > 👉 **Note**: you *can* pass
+    /// > [`options.mdx_expression_parse`][MdxExpressionParse]
+    /// > to parse expressions in JSX according to a certain grammar
+    /// > (typically, a programming language).
     pub mdx_jsx_text: bool,
     /// Thematic break.
     ///
@@ -370,6 +466,7 @@ impl Default for Constructs {
             list_item: true,
             math_flow: false,
             math_text: false,
+            mdx_esm: false,
             mdx_expression_flow: false,
             mdx_expression_text: false,
             mdx_jsx_flow: false,
@@ -405,6 +502,13 @@ impl Constructs {
     /// This turns on `CommonMark`, turns off some conflicting constructs
     /// (autolinks, code (indented), html), and turns on MDX (JSX,
     /// expressions, ESM).
+    ///
+    /// > 👉 **Note**: you *must* pass [`options.mdx_esm_parse`][MdxEsmParse]
+    /// > to support ESM.
+    /// > You *can* pass
+    /// > [`options.mdx_expression_parse`][MdxExpressionParse]
+    /// > to parse expressions according to a certain grammar (typically, a
+    /// > programming language).
     #[must_use]
     pub fn mdx() -> Self {
         Self {
@@ -412,6 +516,7 @@ impl Constructs {
             code_indented: false,
             html_flow: false,
             html_text: false,
+            mdx_esm: true,
             mdx_expression_flow: true,
             mdx_expression_text: true,
             mdx_jsx_flow: true,
@@ -423,8 +528,8 @@ impl Constructs {
 
 /// Configuration (optional).
 #[allow(clippy::struct_excessive_bools)]
-#[derive(Clone, Debug)]
 pub struct Options {
+    // Note: when adding fields, don’t forget to add them to `fmt::Debug` below.
     /// Whether to allow (dangerous) HTML.
     /// The default is `false`, you can turn it on to `true` for trusted
     /// content.
@@ -913,6 +1018,75 @@ pub struct Options {
     /// # }
     /// ```
     pub math_text_single_dollar: bool,
+
+    /// Function to parse expressions with.
+    ///
+    /// This can be used to parse expressions with a parser.
+    /// It can be used to support for arbitrary programming languages within
+    /// expressions.
+    ///
+    /// For an example that adds support for JavaScript with SWC, see
+    /// `tests/test_utils/mod.rs`.
+    pub mdx_expression_parse: Option<Box<MdxExpressionParse>>,
+
+    /// Function to parse ESM with.
+    ///
+    /// This can be used to parse ESM with a parser.
+    /// It can be used to support for arbitrary programming languages within
+    /// ESM, however, the keywords (`export`, `import`) are currently hardcoded
+    /// JavaScript-specific.
+    ///
+    /// For an example that adds support for JavaScript with SWC, see
+    /// `tests/test_utils/mod.rs`.
+    pub mdx_esm_parse: Option<Box<MdxEsmParse>>,
+    // Note: when adding fields, don’t forget to add them to `fmt::Debug` below.
+}
+
+impl fmt::Debug for Options {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Options")
+            .field("allow_dangerous_html", &self.allow_dangerous_html)
+            .field("allow_dangerous_protocol", &self.allow_dangerous_protocol)
+            .field("constructs", &self.constructs)
+            .field("default_line_ending", &self.default_line_ending)
+            .field("gfm_footnote_label", &self.gfm_footnote_label)
+            .field(
+                "gfm_footnote_label_tag_name",
+                &self.gfm_footnote_label_tag_name,
+            )
+            .field(
+                "gfm_footnote_label_attributes",
+                &self.gfm_footnote_label_attributes,
+            )
+            .field("gfm_footnote_back_label", &self.gfm_footnote_back_label)
+            .field(
+                "gfm_footnote_clobber_prefix",
+                &self.gfm_footnote_clobber_prefix,
+            )
+            .field(
+                "gfm_strikethrough_single_tilde",
+                &self.gfm_strikethrough_single_tilde,
+            )
+            .field("gfm_tagfilter", &self.gfm_tagfilter)
+            .field("math_text_single_dollar", &self.math_text_single_dollar)
+            .field(
+                "mdx_expression_parse",
+                if self.mdx_expression_parse.is_none() {
+                    &"None"
+                } else {
+                    &"Some([Function])"
+                },
+            )
+            .field(
+                "mdx_esm_parse",
+                if self.mdx_esm_parse.is_none() {
+                    &"None"
+                } else {
+                    &"Some([Function])"
+                },
+            )
+            .finish()
+    }
 }
 
 impl Default for Options {
@@ -931,6 +1105,8 @@ impl Default for Options {
             gfm_strikethrough_single_tilde: true,
             gfm_tagfilter: false,
             math_text_single_dollar: true,
+            mdx_expression_parse: None,
+            mdx_esm_parse: None,
         }
     }
 }
diff --git a/src/state.rs b/src/state.rs
index 896761e..1cc2720 100644
--- a/src/state.rs
+++ b/src/state.rs
@@ -344,6 +344,14 @@ pub enum Name {
     ListItemContBlank,
     ListItemContFilled,
 
+    MdxEsmStart,
+    MdxEsmWord,
+    MdxEsmInside,
+    MdxEsmLineStart,
+    MdxEsmBlankLineBefore,
+    MdxEsmContinuationStart,
+    MdxEsmAtEnd,
+
     MdxExpressionTextStart,
     MdxExpressionTextAfter,
 
@@ -356,8 +364,6 @@ pub enum Name {
     MdxExpressionBefore,
     MdxExpressionInside,
     MdxExpressionEolAfter,
-    MdxJsxAttributeValueExpressionAfter,
-    MdxJsxAttributeExpressionAfter,
 
     MdxJsxFlowStart,
     MdxJsxFlowBefore,
@@ -385,6 +391,7 @@ pub enum Name {
     MdxJsxLocalNameAfter,
     MdxJsxAttributeBefore,
     MdxJsxSelfClosing,
+    MdxJsxAttributeExpressionAfter,
     MdxJsxAttributePrimaryName,
     MdxJsxAttributePrimaryNameAfter,
     MdxJsxAttributeLocalNameBefore,
@@ -393,6 +400,7 @@ pub enum Name {
     MdxJsxAttributeValueBefore,
     MdxJsxAttributeValueQuotedStart,
     MdxJsxAttributeValueQuoted,
+    MdxJsxAttributeValueExpressionAfter,
 
     NonLazyContinuationStart,
     NonLazyContinuationAfter,
@@ -822,6 +830,14 @@ pub fn call(tokenizer: &mut Tokenizer, name: Name) -> State {
         Name::ListItemContBlank => construct::list_item::cont_blank,
         Name::ListItemContFilled => construct::list_item::cont_filled,
 
+        Name::MdxEsmStart => construct::mdx_esm::start,
+        Name::MdxEsmWord => construct::mdx_esm::word,
+        Name::MdxEsmInside => construct::mdx_esm::inside,
+        Name::MdxEsmLineStart => construct::mdx_esm::line_start,
+        Name::MdxEsmBlankLineBefore => construct::mdx_esm::blank_line_before,
+        Name::MdxEsmContinuationStart => construct::mdx_esm::continuation_start,
+        Name::MdxEsmAtEnd => construct::mdx_esm::at_end,
+
         Name::MdxExpressionStart => construct::partial_mdx_expression::start,
         Name::MdxExpressionBefore => construct::partial_mdx_expression::before,
         Name::MdxExpressionInside => construct::partial_mdx_expression::inside,
diff --git a/src/tokenizer.rs b/src/tokenizer.rs
index 8441f7e..84d3d6d 100644
--- a/src/tokenizer.rs
+++ b/src/tokenizer.rs
@@ -227,6 +227,9 @@ pub struct TokenizeState<'a> {
     /// List of defined GFM footnote definition identifiers.
     pub gfm_footnote_definitions: Vec<String>,
 
+    // Last error message provided at an EOF of an expression.
+    pub mdx_last_parse_error: Option<String>,
+
     /// Whether to connect events.
     pub connect: bool,
     /// Marker.
@@ -343,6 +346,7 @@ impl<'a> Tokenizer<'a> {
                 document_at_first_paragraph_of_list_item: false,
                 definitions: vec![],
                 gfm_footnote_definitions: vec![],
+                mdx_last_parse_error: None,
                 end: 0,
                 label_starts: vec![],
                 label_starts_loose: vec![],
diff --git a/src/util/mdx_collect.rs b/src/util/mdx_collect.rs
new file mode 100644
index 0000000..73ead51
--- /dev/null
+++ b/src/util/mdx_collect.rs
@@ -0,0 +1,70 @@
+//! Collect info for MDX.
+
+use crate::event::{Kind, Name, Point};
+use crate::tokenizer::Tokenizer;
+use crate::util::slice::{Position, Slice};
+use alloc::{string::String, vec, vec::Vec};
+
+pub type Location<'a> = (usize, &'a Point);
+
+pub struct Result<'a> {
+    pub start: &'a Point,
+    pub value: String,
+    pub locations: Vec<Location<'a>>,
+}
+
+pub fn collect<'a>(tokenizer: &'a Tokenizer, from: usize, names: &[Name]) -> Result<'a> {
+    let mut result = Result {
+        start: &tokenizer.events[from].point,
+        value: String::new(),
+        locations: vec![],
+    };
+    let mut index = from;
+    let mut acc = 0;
+
+    while index < tokenizer.events.len() {
+        if tokenizer.events[index].kind == Kind::Enter
+            && names.contains(&tokenizer.events[index].name)
+        {
+            // Include virtual spaces.
+            let value = Slice::from_position(
+                tokenizer.parse_state.bytes,
+                &Position {
+                    start: &tokenizer.events[index].point,
+                    end: &tokenizer.events[index + 1].point,
+                },
+            )
+            .serialize();
+            acc += value.len();
+            result.locations.push((acc, &tokenizer.events[index].point));
+            result.value.push_str(&value);
+        }
+
+        index += 1;
+    }
+
+    result
+}
+
+// Turn an index of `result.value` into a point in the whole document.
+pub fn place_to_point(result: &Result, place: usize) -> Point {
+    let mut index = 0;
+    let mut point = result.start;
+    let mut rest = place;
+
+    while index < result.locations.len() {
+        point = result.locations[index].1;
+
+        if result.locations[index].0 > place {
+            break;
+        }
+
+        rest = place - result.locations[index].0;
+        index += 1;
+    }
+
+    let mut point = point.clone();
+    point.column += rest;
+    point.index += rest;
+    point
+}
diff --git a/src/util/mod.rs b/src/util/mod.rs
index 2ea372c..6281356 100644
--- a/src/util/mod.rs
+++ b/src/util/mod.rs
@@ -6,6 +6,7 @@ pub mod decode_character_reference;
 pub mod edit_map;
 pub mod encode;
 pub mod gfm_tagfilter;
+pub mod mdx_collect;
 pub mod normalize_identifier;
 pub mod sanitize_uri;
 pub mod skip;