Add support for rewriting JSX

4 files changed, 1194 insertions, 2 deletions

diff --git a/tests/test_utils/jsx_rewrite.rs b/tests/test_utils/jsx_rewrite.rs
new file mode 100644
index 0000000..fbce344
--- /dev/null
+++ b/tests/test_utils/jsx_rewrite.rs
@@ -0,0 +1,1181 @@
+extern crate swc_common;
+extern crate swc_ecma_ast;
+use crate::test_utils::to_swc::Program;
+use swc_ecma_visit::{noop_visit_mut_type, VisitMut, VisitMutWith};
+use unicode_id::UnicodeID;
+
+/// Configuration.
+#[derive(Debug, Default, Clone)]
+pub struct Options {
+    /// Place to import a provider from.
+    ///
+    /// See [MDX provider](https://mdxjs.com/docs/using-mdx/#mdx-provider)
+    /// on the MDX website for more info.
+    pub provider_import_source: Option<String>,
+    /// Whether to add extra information to error messages in generated code.
+    /// This is not yet supported.
+    pub development: bool,
+}
+
+/// Rewrite JSX in an MDX file so that components can be passed in and provided.
+#[allow(dead_code)]
+pub fn jsx_rewrite(mut program: Program, options: &Options) -> Program {
+    let mut state = State {
+        scopes: vec![],
+        provider: options.provider_import_source.is_some(),
+        create_provider_import: false,
+        create_error_helper: false,
+    };
+    state.enter(Some(Info::default()));
+    program.module.visit_mut_with(&mut state);
+
+    // If a provider is used (and can be used), import it.
+    if let Some(source) = &options.provider_import_source {
+        if state.create_provider_import {
+            program
+                .module
+                .body
+                .insert(0, create_import_provider(source))
+        }
+    }
+
+    // If potentially missing components are used, add the helper used for
+    // errors.
+    if state.create_error_helper {
+        program.module.body.push(create_error_helper());
+    }
+
+    program
+}
+
+/// Collection of different SWC functions.
+#[derive(Debug)]
+enum Func<'a> {
+    /// Function declaration.
+    Decl(&'a mut swc_ecma_ast::FnDecl),
+    /// Function expression.
+    Expr(&'a mut swc_ecma_ast::FnExpr),
+    /// Arrow function.
+    Arrow(&'a mut swc_ecma_ast::ArrowExpr),
+}
+
+/// Info for a function scope.
+#[derive(Debug, Default, Clone)]
+struct Info {
+    /// Function name.
+    name: Option<String>,
+    /// Used objects (`a` in `<a.b />`).
+    objects: Vec<String>,
+    /// Used components (`<A />`).
+    components: Vec<String>,
+    /// Used literals (`<a />`).
+    tags: Vec<String>,
+    /// List of JSX identifiers of literal tags that are not valid JS
+    /// identifiers in the shape of `Vec<(invalid, valid)>`.
+    ///
+    /// Example:
+    ///
+    /// ```
+    /// vec![("a-b".into(), "_component0".into())]
+    /// ```
+    aliases: Vec<(String, String)>,
+    /// Non-literal references in the shape of `Vec<(name, is_component)>`.
+    ///
+    /// Example:
+    ///
+    /// ```
+    /// vec![("a".into(), false), ("a.b".into(), true)]
+    /// ```
+    // To do: add positional info later.
+    references: Vec<(String, bool)>,
+}
+
+/// Scope (block or function/global).
+#[derive(Debug, Clone)]
+struct Scope {
+    /// If this is a function (or global) scope, we track info.
+    info: Option<Info>,
+    /// Things that are defined in this scope.
+    defined: Vec<String>,
+}
+
+/// Context.
+#[derive(Debug, Default, Clone)]
+struct State {
+    /// List of current scopes.
+    scopes: Vec<Scope>,
+    /// Whether the user uses a provider.
+    provider: bool,
+    /// Whether a provider is referenced.
+    create_provider_import: bool,
+    /// Whether a missing component helper is referenced.
+    ///
+    /// When things are referenced that might not be defined, we reference a
+    /// helper function to throw when they are missing.
+    create_error_helper: bool,
+}
+
+impl State {
+    /// Open a new scope.
+    fn enter(&mut self, info: Option<Info>) {
+        self.scopes.push(Scope {
+            info,
+            defined: vec![],
+        });
+    }
+
+    /// Close the current scope.
+    fn exit(&mut self) -> Scope {
+        self.scopes.pop().expect("expected scope")
+    }
+
+    /// Close a function.
+    fn exit_func(&mut self, func: Func) {
+        let mut scope = self.exit();
+        let mut defaults = vec![];
+        let mut info = scope.info.take().unwrap();
+        let mut index = 0;
+
+        // Create defaults for tags.
+        //
+        // ```jsx
+        // {h1: 'h1'}
+        // ```
+        while index < info.tags.len() {
+            let name = &info.tags[index];
+
+            defaults.push(swc_ecma_ast::PropOrSpread::Prop(Box::new(
+                swc_ecma_ast::Prop::KeyValue(swc_ecma_ast::KeyValueProp {
+                    key: if is_identifier_name(name) {
+                        swc_ecma_ast::PropName::Ident(create_ident(name))
+                    } else {
+                        swc_ecma_ast::PropName::Str(swc_ecma_ast::Str {
+                            value: name.clone().into(),
+                            span: swc_common::DUMMY_SP,
+                            raw: None,
+                        })
+                    },
+                    value: Box::new(swc_ecma_ast::Expr::Lit(swc_ecma_ast::Lit::Str(
+                        swc_ecma_ast::Str {
+                            value: name.clone().into(),
+                            span: swc_common::DUMMY_SP,
+                            raw: None,
+                        },
+                    ))),
+                }),
+            )));
+
+            index += 1;
+        }
+
+        let mut actual = info.components.split_off(0);
+        let mut index = 0;
+
+        // In some cases, a component is used directly (`<X>`) but it’s also
+        // used as an object (`<X.Y>`).
+        while index < info.objects.len() {
+            if !actual.contains(&info.objects[index]) {
+                actual.push(info.objects[index].clone());
+            }
+            index += 1;
+        }
+
+        let mut statements = vec![];
+
+        if !defaults.is_empty() || !actual.is_empty() || !info.aliases.is_empty() {
+            let mut parameters = vec![];
+
+            // Use a provider, if configured.
+            //
+            // ```jsx
+            // _provideComponents()
+            // ```
+            if self.provider {
+                self.create_provider_import = true;
+                parameters.push(swc_ecma_ast::Expr::Call(swc_ecma_ast::CallExpr {
+                    callee: swc_ecma_ast::Callee::Expr(Box::new(create_ident_expression(
+                        "_provideComponents",
+                    ))),
+                    args: vec![],
+                    type_args: None,
+                    span: swc_common::DUMMY_SP,
+                }));
+            }
+
+            // Accept `components` as a prop if this is the `MDXContent` or
+            // `_createMdxContent` function.
+            //
+            // ```jsx
+            // props.components
+            // ```
+            if is_props_receiving_fn(&info.name) {
+                parameters.push(swc_ecma_ast::Expr::Member(swc_ecma_ast::MemberExpr {
+                    obj: Box::new(create_ident_expression("props")),
+                    prop: swc_ecma_ast::MemberProp::Ident(create_ident("components")),
+                    span: swc_common::DUMMY_SP,
+                }));
+            }
+
+            // Inject an object at the start, when:
+            // - there are defaults,
+            // - there are two sources
+            //
+            // ```jsx
+            // (_provideComponents(), props.components)
+            // ()
+            // ```
+            //
+            // To:
+            //
+            // ```jsx
+            // ({}, _provideComponents(), props.components)
+            // ({h1: 'h1'})
+            // ```
+            if !defaults.is_empty() || parameters.len() > 1 {
+                parameters.insert(
+                    0,
+                    swc_ecma_ast::Expr::Object(swc_ecma_ast::ObjectLit {
+                        props: defaults,
+                        span: swc_common::DUMMY_SP,
+                    }),
+                );
+            }
+
+            // Merge things and prevent errors.
+            //
+            // ```jsx
+            // {}, _provideComponents(), props.components
+            // props.components
+            // _provideComponents()
+            // ```
+            //
+            // To:
+            //
+            // ```jsx
+            // Object.assign({}, _provideComponents(), props.components)
+            // props.components || {}
+            // _provideComponents()
+            // ```
+            let mut components_init = if parameters.len() > 1 {
+                let mut args = vec![];
+                parameters.reverse();
+                while let Some(param) = parameters.pop() {
+                    args.push(swc_ecma_ast::ExprOrSpread {
+                        spread: None,
+                        expr: Box::new(param),
+                    });
+                }
+                swc_ecma_ast::Expr::Call(swc_ecma_ast::CallExpr {
+                    callee: swc_ecma_ast::Callee::Expr(Box::new(swc_ecma_ast::Expr::Member(
+                        swc_ecma_ast::MemberExpr {
+                            obj: Box::new(create_ident_expression("Object")),
+                            prop: swc_ecma_ast::MemberProp::Ident(create_ident("assign")),
+                            span: swc_common::DUMMY_SP,
+                        },
+                    ))),
+                    args,
+                    type_args: None,
+                    span: swc_common::DUMMY_SP,
+                })
+            } else {
+                // Always one.
+                let param = parameters.pop().unwrap();
+
+                if let swc_ecma_ast::Expr::Member(_) = param {
+                    create_binary_expression(
+                        vec![
+                            param,
+                            swc_ecma_ast::Expr::Object(swc_ecma_ast::ObjectLit {
+                                props: vec![],
+                                span: swc_common::DUMMY_SP,
+                            }),
+                        ],
+                        swc_ecma_ast::BinaryOp::LogicalOr,
+                    )
+                } else {
+                    param
+                }
+            };
+
+            // Add components to scope.
+            //
+            // For `['MyComponent', 'MDXLayout']` this generates:
+            //
+            // ```js
+            // const {MyComponent, wrapper: MDXLayout} = _components
+            // ```
+            //
+            // Note that MDXLayout is special as it’s taken from
+            // `_components.wrapper`.
+            let components_pattern = if actual.is_empty() {
+                None
+            } else {
+                let mut props = vec![];
+                actual.reverse();
+                while let Some(key) = actual.pop() {
+                    // `wrapper: MDXLayout`
+                    if key == "MDXLayout" {
+                        props.push(swc_ecma_ast::ObjectPatProp::KeyValue(
+                            swc_ecma_ast::KeyValuePatProp {
+                                key: swc_ecma_ast::PropName::Ident(create_ident("wrapper")),
+                                value: Box::new(swc_ecma_ast::Pat::Ident(
+                                    swc_ecma_ast::BindingIdent {
+                                        id: create_ident(&key),
+                                        type_ann: None,
+                                    },
+                                )),
+                            },
+                        ))
+                    }
+                    // `MyComponent`
+                    else {
+                        props.push(swc_ecma_ast::ObjectPatProp::Assign(
+                            swc_ecma_ast::AssignPatProp {
+                                key: create_ident(&key),
+                                value: None,
+                                span: swc_common::DUMMY_SP,
+                            },
+                        ))
+                    }
+                }
+
+                Some(swc_ecma_ast::ObjectPat {
+                    props,
+                    optional: false,
+                    span: swc_common::DUMMY_SP,
+                    type_ann: None,
+                })
+            };
+
+            let mut declarators = vec![];
+
+            // If there are tags, they take them from `_components`, so we need
+            // to make it defined.
+            if !info.tags.is_empty() {
+                declarators.push(swc_ecma_ast::VarDeclarator {
+                    span: swc_common::DUMMY_SP,
+                    name: swc_ecma_ast::Pat::Ident(swc_ecma_ast::BindingIdent {
+                        id: create_ident("_components"),
+                        type_ann: None,
+                    }),
+                    init: Some(Box::new(components_init)),
+                    definite: false,
+                });
+                components_init = create_ident_expression("_components");
+            }
+
+            // For JSX IDs that can’t be represented as JavaScript IDs (as in,
+            // those with dashes, such as `custom-element`), we generated a
+            // separate variable that is a valid JS ID (such as `_component0`),
+            // and here we take it from components:
+            // ```js
+            // const _component0 = _components['custom-element']
+            // ```
+            if !info.aliases.is_empty() {
+                info.aliases.reverse();
+
+                while let Some((id, name)) = info.aliases.pop() {
+                    declarators.push(swc_ecma_ast::VarDeclarator {
+                        span: swc_common::DUMMY_SP,
+                        name: swc_ecma_ast::Pat::Ident(swc_ecma_ast::BindingIdent {
+                            id: create_ident(&name),
+                            type_ann: None,
+                        }),
+                        init: Some(Box::new(swc_ecma_ast::Expr::Member(
+                            swc_ecma_ast::MemberExpr {
+                                obj: Box::new(create_ident_expression("_components")),
+                                prop: swc_ecma_ast::MemberProp::Computed(
+                                    swc_ecma_ast::ComputedPropName {
+                                        expr: Box::new(swc_ecma_ast::Expr::Lit(
+                                            swc_ecma_ast::Lit::Str(swc_ecma_ast::Str {
+                                                value: id.into(),
+                                                span: swc_common::DUMMY_SP,
+                                                raw: None,
+                                            }),
+                                        )),
+                                        span: swc_common::DUMMY_SP,
+                                    },
+                                ),
+                                span: swc_common::DUMMY_SP,
+                            },
+                        ))),
+                        definite: false,
+                    });
+                }
+            }
+
+            if let Some(pat) = components_pattern {
+                declarators.push(swc_ecma_ast::VarDeclarator {
+                    name: swc_ecma_ast::Pat::Object(pat),
+                    init: Some(Box::new(components_init)),
+                    span: swc_common::DUMMY_SP,
+                    definite: false,
+                });
+            }
+
+            // Add the variable declaration.
+            statements.push(swc_ecma_ast::Stmt::Decl(swc_ecma_ast::Decl::Var(Box::new(
+                swc_ecma_ast::VarDecl {
+                    kind: swc_ecma_ast::VarDeclKind::Const,
+                    decls: declarators,
+                    span: swc_common::DUMMY_SP,
+                    declare: false,
+                },
+            ))));
+        }
+
+        // Add checks at runtime to verify that object/components are passed.
+        //
+        // ```js
+        // if (!a) _missingMdxReference("a", false);
+        // if (!a.b) _missingMdxReference("a.b", true);
+        // ```
+        for (id, component) in info.references {
+            self.create_error_helper = true;
+            statements.push(swc_ecma_ast::Stmt::If(swc_ecma_ast::IfStmt {
+                test: Box::new(swc_ecma_ast::Expr::Unary(swc_ecma_ast::UnaryExpr {
+                    op: swc_ecma_ast::UnaryOp::Bang,
+                    arg: Box::new(create_member_expression(&id)),
+                    span: swc_common::DUMMY_SP,
+                })),
+                cons: Box::new(swc_ecma_ast::Stmt::Expr(swc_ecma_ast::ExprStmt {
+                    span: swc_common::DUMMY_SP,
+                    expr: Box::new(swc_ecma_ast::Expr::Call(swc_ecma_ast::CallExpr {
+                        callee: swc_ecma_ast::Callee::Expr(Box::new(create_ident_expression(
+                            "_missingMdxReference",
+                        ))),
+                        args: vec![
+                            swc_ecma_ast::ExprOrSpread {
+                                spread: None,
+                                expr: Box::new(swc_ecma_ast::Expr::Lit(swc_ecma_ast::Lit::Str(
+                                    swc_ecma_ast::Str {
+                                        value: id.into(),
+                                        span: swc_common::DUMMY_SP,
+                                        raw: None,
+                                    },
+                                ))),
+                            },
+                            swc_ecma_ast::ExprOrSpread {
+                                spread: None,
+                                expr: Box::new(swc_ecma_ast::Expr::Lit(swc_ecma_ast::Lit::Bool(
+                                    swc_ecma_ast::Bool {
+                                        value: component,
+                                        span: swc_common::DUMMY_SP,
+                                    },
+                                ))),
+                            },
+                        ],
+                        type_args: None,
+                        span: swc_common::DUMMY_SP,
+                    })),
+                })),
+                alt: None,
+                span: swc_common::DUMMY_SP,
+            }));
+        }
+
+        // Add statements to functions.
+        if !statements.is_empty() {
+            let mut body: &mut swc_ecma_ast::BlockStmt = match func {
+                Func::Expr(expr) => {
+                    if expr.function.body.is_none() {
+                        expr.function.body = Some(swc_ecma_ast::BlockStmt {
+                            stmts: vec![],
+                            span: swc_common::DUMMY_SP,
+                        });
+                    }
+                    expr.function.body.as_mut().unwrap()
+                }
+                Func::Decl(decl) => {
+                    if decl.function.body.is_none() {
+                        decl.function.body = Some(swc_ecma_ast::BlockStmt {
+                            stmts: vec![],
+                            span: swc_common::DUMMY_SP,
+                        });
+                    }
+                    decl.function.body.as_mut().unwrap()
+                }
+                Func::Arrow(arr) => {
+                    if let swc_ecma_ast::BlockStmtOrExpr::Expr(expr) = &mut arr.body {
+                        arr.body =
+                            swc_ecma_ast::BlockStmtOrExpr::BlockStmt(swc_ecma_ast::BlockStmt {
+                                stmts: vec![swc_ecma_ast::Stmt::Return(swc_ecma_ast::ReturnStmt {
+                                    // To do: figure out non-clone.
+                                    arg: Some(expr.clone()),
+                                    span: swc_common::DUMMY_SP,
+                                })],
+                                span: swc_common::DUMMY_SP,
+                            });
+                    }
+                    arr.body.as_mut_block_stmt().unwrap()
+                }
+            };
+
+            statements.append(&mut body.stmts.split_off(0));
+            body.stmts = statements;
+        }
+    }
+
+    /// Get the current function scope.
+    fn current_fn_scope_mut(&mut self) -> &mut Scope {
+        let mut index = self.scopes.len();
+
+        while index > 0 {
+            index -= 1;
+            if self.scopes[index].info.is_some() {
+                return &mut self.scopes[index];
+            }
+        }
+
+        unreachable!("expected scope")
+    }
+
+    /// Get the current scope.
+    fn current_scope_mut(&mut self) -> &mut Scope {
+        self.scopes.last_mut().expect("expected scope")
+    }
+
+    /// Get the top-level scope’s info.
+    fn current_top_level_info(&self) -> Option<&Info> {
+        if let Some(scope) = self.scopes.get(1) {
+            scope.info.as_ref()
+        } else {
+            None
+        }
+    }
+    /// Get the top-level scope’s info, mutably.
+    fn current_top_level_info_mut(&mut self) -> Option<&mut Info> {
+        if let Some(scope) = self.scopes.get_mut(1) {
+            scope.info.as_mut()
+        } else {
+            None
+        }
+    }
+
+    /// Check if `id` is in scope.
+    fn in_scope(&self, id: &String) -> bool {
+        let mut index = self.scopes.len();
+
+        while index > 0 {
+            index -= 1;
+            if self.scopes[index].defined.contains(id) {
+                return true;
+            }
+        }
+
+        false
+    }
+
+    /// Add an identifier to a scope.
+    fn add_id(&mut self, id: String, block: bool) {
+        let scope = if block {
+            self.current_scope_mut()
+        } else {
+            self.current_fn_scope_mut()
+        };
+        scope.defined.push(id);
+    }
+
+    // Add a pattern to a scope.
+    fn add_pat(&mut self, pat: &swc_ecma_ast::Pat, block: bool) {
+        match pat {
+            // `x`
+            swc_ecma_ast::Pat::Ident(d) => self.add_id(d.id.sym.to_string(), block),
+            // `...x`
+            swc_ecma_ast::Pat::Array(d) => {
+                let mut index = 0;
+                while index < d.elems.len() {
+                    if let Some(d) = &d.elems[index] {
+                        self.add_pat(d, block);
+                    }
+                    index += 1;
+                }
+            }
+            // `...x`
+            swc_ecma_ast::Pat::Rest(d) => self.add_pat(&d.arg, block),
+            // `{x=y}`
+            swc_ecma_ast::Pat::Assign(d) => self.add_pat(&d.left, block),
+            swc_ecma_ast::Pat::Object(d) => {
+                let mut index = 0;
+                while index < d.props.len() {
+                    match &d.props[index] {
+                        // `{...x}`
+                        swc_ecma_ast::ObjectPatProp::Rest(d) => {
+                            self.add_pat(&d.arg, block);
+                        }
+                        // `{key: value}`
+                        swc_ecma_ast::ObjectPatProp::KeyValue(d) => {
+                            self.add_pat(&d.value, block);
+                        }
+                        // `{key}` or `{key = value}`
+                        swc_ecma_ast::ObjectPatProp::Assign(d) => {
+                            self.add_id(d.key.to_string(), block);
+                        }
+                    }
+                    index += 1;
+                }
+            }
+            // Ignore `Invalid` / `Expr`.
+            _ => {}
+        }
+    }
+}
+
+impl VisitMut for State {
+    noop_visit_mut_type!();
+
+    /// Rewrite JSX identifiers.
+    fn visit_mut_jsx_element(&mut self, node: &mut swc_ecma_ast::JSXElement) {
+        // If there is a top-level, non-global, scope which is a function.
+        if let Some(info) = self.current_top_level_info() {
+            // Rewrite only if we can rewrite.
+            if is_props_receiving_fn(&info.name) || self.provider {
+                match &node.opening.name {
+                    // `<x.y>`, `<Foo.Bar>`, `<x.y.z>`.
+                    swc_ecma_ast::JSXElementName::JSXMemberExpr(d) => {
+                        let mut ids = vec![];
+                        let mut mem = d;
+                        loop {
+                            ids.push(mem.prop.sym.to_string());
+                            match &mem.obj {
+                                swc_ecma_ast::JSXObject::Ident(d) => {
+                                    ids.push(d.sym.to_string());
+                                    break;
+                                }
+                                swc_ecma_ast::JSXObject::JSXMemberExpr(d) => {
+                                    mem = d;
+                                }
+                            }
+                        }
+                        ids.reverse();
+                        let primary_id = ids.first().unwrap().clone();
+                        let in_scope = self.in_scope(&primary_id);
+
+                        if !in_scope {
+                            let info_mut = self.current_top_level_info_mut().unwrap();
+
+                            // To do: add positional info.
+                            let mut index = 1;
+                            while index <= ids.len() {
+                                let full_id = ids[0..index].join(".");
+                                let component = index == ids.len();
+                                if let Some(reference) =
+                                    info_mut.references.iter_mut().find(|d| d.0 == full_id)
+                                {
+                                    if component {
+                                        reference.1 = true;
+                                    }
+                                } else {
+                                    info_mut.references.push((full_id, component))
+                                }
+                                index += 1;
+                            }
+
+                            if !info_mut.objects.contains(&primary_id) {
+                                info_mut.objects.push(primary_id);
+                            }
+                        }
+                    }
+                    // `<foo>`, `<Foo>`, `<$>`, `<_bar>`, `<a_b>`.
+                    swc_ecma_ast::JSXElementName::Ident(d) => {
+                        // If the name is a valid ES identifier, and it doesn’t
+                        // start with a lowercase letter, it’s a component.
+                        // For example, `$foo`, `_bar`, `Baz` are all component
+                        // names.
+                        // But `foo` and `b-ar` are tag names.
+                        let id = d.sym.to_string();
+
+                        if is_literal_name(&id) {
+                            // To do: ignore explicit JSX?
+
+                            let mut invalid = None;
+
+                            let name = if is_identifier_name(&id) {
+                                swc_ecma_ast::JSXElementName::JSXMemberExpr(
+                                    swc_ecma_ast::JSXMemberExpr {
+                                        obj: swc_ecma_ast::JSXObject::Ident(create_ident(
+                                            "_components",
+                                        )),
+                                        prop: create_ident(&id),
+                                    },
+                                )
+                            } else {
+                                let name = if let Some(invalid_ref) =
+                                    info.aliases.iter().find(|d| d.0 == id)
+                                {
+                                    invalid_ref.1.clone()
+                                } else {
+                                    let name = format!("_component{}", info.aliases.len());
+                                    invalid = Some((id.clone(), name.clone()));
+                                    name
+                                };
+
+                                swc_ecma_ast::JSXElementName::Ident(create_ident(&name))
+                            };
+
+                            let info_mut = self.current_top_level_info_mut().unwrap();
+
+                            if !info_mut.tags.contains(&id) {
+                                info_mut.tags.push(id);
+                            }
+
+                            if let Some(invalid) = invalid {
+                                info_mut.aliases.push(invalid)
+                            }
+
+                            if let Some(closing) = node.closing.as_mut() {
+                                closing.name = name.clone();
+                            }
+
+                            node.opening.name = name;
+                        } else {
+                            let mut is_layout = false;
+
+                            // The MDXLayout is wrapped in a
+                            if let Some(name) = &info.name {
+                                if name == "MDXContent" && id == "MDXLayout" {
+                                    is_layout = true;
+                                }
+                            }
+
+                            if !self.in_scope(&id) {
+                                let info_mut = self.current_top_level_info_mut().unwrap();
+
+                                if !is_layout {
+                                    if let Some(reference) =
+                                        info_mut.references.iter_mut().find(|d| d.0 == id)
+                                    {
+                                        reference.1 = true;
+                                    } else {
+                                        info_mut.references.push((id.clone(), true))
+                                    }
+                                }
+
+                                if !info_mut.components.contains(&id) {
+                                    info_mut.components.push(id);
+                                }
+                            }
+                        }
+                    }
+                    // `<xml:thing>`.
+                    swc_ecma_ast::JSXElementName::JSXNamespacedName(_) => {
+                        // Ignore.
+                    }
+                }
+            }
+        }
+
+        node.visit_mut_children_with(self);
+    }
+
+    /// Add specifiers of import declarations.
+    fn visit_mut_import_decl(&mut self, node: &mut swc_ecma_ast::ImportDecl) {
+        let mut index = 0;
+        while index < node.specifiers.len() {
+            let ident = match &node.specifiers[index] {
+                swc_ecma_ast::ImportSpecifier::Default(x) => &x.local.sym,
+                swc_ecma_ast::ImportSpecifier::Namespace(x) => &x.local.sym,
+                swc_ecma_ast::ImportSpecifier::Named(x) => &x.local.sym,
+            };
+            self.add_id(ident.to_string(), false);
+            index += 1;
+        }
+
+        node.visit_mut_children_with(self);
+    }
+
+    /// Add patterns of variable declarations.
+    fn visit_mut_var_decl(&mut self, node: &mut swc_ecma_ast::VarDecl) {
+        let block = node.kind != swc_ecma_ast::VarDeclKind::Var;
+        let mut index = 0;
+        while index < node.decls.len() {
+            self.add_pat(&node.decls[index].name, block);
+            index += 1;
+        }
+        node.visit_mut_children_with(self);
+    }
+
+    /// Add identifier of class declaration.
+    fn visit_mut_class_decl(&mut self, node: &mut swc_ecma_ast::ClassDecl) {
+        self.add_id(node.ident.sym.to_string(), false);
+        node.visit_mut_children_with(self);
+    }
+
+    /// On function declarations, add name, create scope, add parameters.
+    fn visit_mut_fn_decl(&mut self, node: &mut swc_ecma_ast::FnDecl) {
+        let id = node.ident.sym.to_string();
+        self.add_id(id.clone(), false);
+        self.enter(Some(Info {
+            name: Some(id),
+            ..Default::default()
+        }));
+        let mut index = 0;
+        while index < node.function.params.len() {
+            self.add_pat(&node.function.params[index].pat, false);
+            index += 1;
+        }
+        node.visit_mut_children_with(self);
+        // Rewrite.
+        self.exit_func(Func::Decl(node));
+    }
+
+    /// On function expressions, add name, create scope, add parameters.
+    fn visit_mut_fn_expr(&mut self, node: &mut swc_ecma_ast::FnExpr) {
+        // Note: `periscopic` adds the ID to the newly generated scope, for
+        // fn expressions.
+        // That seems wrong?
+        let name = if let Some(ident) = &node.ident {
+            let id = ident.sym.to_string();
+            self.add_id(id.clone(), false);
+            Some(id)
+        } else {
+            None
+        };
+
+        self.enter(Some(Info {
+            name,
+            ..Default::default()
+        }));
+        let mut index = 0;
+        while index < node.function.params.len() {
+            self.add_pat(&node.function.params[index].pat, false);
+            index += 1;
+        }
+        node.visit_mut_children_with(self);
+        self.exit_func(Func::Expr(node));
+    }
+
+    /// On arrow functions, create scope, add parameters.
+    fn visit_mut_arrow_expr(&mut self, node: &mut swc_ecma_ast::ArrowExpr) {
+        self.enter(Some(Info::default()));
+        let mut index = 0;
+        while index < node.params.len() {
+            self.add_pat(&node.params[index], false);
+            index += 1;
+        }
+        node.visit_mut_children_with(self);
+        self.exit_func(Func::Arrow(node));
+    }
+
+    // Blocks.
+    // Not sure why `periscopic` only does `For`/`ForIn`/`ForOf`/`Block`.
+    // I added `While`/`DoWhile` here just to be sure.
+    // But there are more.
+    /// On for statements, create scope.
+    fn visit_mut_for_stmt(&mut self, node: &mut swc_ecma_ast::ForStmt) {
+        self.enter(None);
+        node.visit_mut_children_with(self);
+        self.exit();
+    }
+    /// On for/in statements, create scope.
+    fn visit_mut_for_in_stmt(&mut self, node: &mut swc_ecma_ast::ForInStmt) {
+        self.enter(None);
+        node.visit_mut_children_with(self);
+        self.exit();
+    }
+    /// On for/of statements, create scope.
+    fn visit_mut_for_of_stmt(&mut self, node: &mut swc_ecma_ast::ForOfStmt) {
+        self.enter(None);
+        node.visit_mut_children_with(self);
+        self.exit();
+    }
+    /// On while statements, create scope.
+    fn visit_mut_while_stmt(&mut self, node: &mut swc_ecma_ast::WhileStmt) {
+        self.enter(None);
+        node.visit_mut_children_with(self);
+        self.exit();
+    }
+    /// On do/while statements, create scope.
+    fn visit_mut_do_while_stmt(&mut self, node: &mut swc_ecma_ast::DoWhileStmt) {
+        self.enter(None);
+        node.visit_mut_children_with(self);
+        self.exit();
+    }
+    /// On block statements, create scope.
+    fn visit_mut_block_stmt(&mut self, node: &mut swc_ecma_ast::BlockStmt) {
+        self.enter(None);
+        node.visit_mut_children_with(self);
+        self.exit();
+    }
+
+    /// On catch clauses, create scope, add param.
+    fn visit_mut_catch_clause(&mut self, node: &mut swc_ecma_ast::CatchClause) {
+        self.enter(None);
+        if let Some(pat) = &node.param {
+            self.add_pat(pat, true);
+        }
+        node.visit_mut_children_with(self);
+        self.exit();
+    }
+}
+
+/// Generate an import provider.
+///
+/// ```js
+/// import { useMDXComponents as _provideComponents } from "x"
+/// ```
+fn create_import_provider(source: &str) -> swc_ecma_ast::ModuleItem {
+    swc_ecma_ast::ModuleItem::ModuleDecl(swc_ecma_ast::ModuleDecl::Import(
+        swc_ecma_ast::ImportDecl {
+            specifiers: vec![swc_ecma_ast::ImportSpecifier::Named(
+                swc_ecma_ast::ImportNamedSpecifier {
+                    local: create_ident("_provideComponents"),
+                    imported: Some(swc_ecma_ast::ModuleExportName::Ident(create_ident(
+                        "useMDXComponents",
+                    ))),
+                    span: swc_common::DUMMY_SP,
+                    is_type_only: false,
+                },
+            )],
+            src: Box::new(swc_ecma_ast::Str {
+                value: source.into(),
+                span: swc_common::DUMMY_SP,
+                raw: None,
+            }),
+            type_only: false,
+            asserts: None,
+            span: swc_common::DUMMY_SP,
+        },
+    ))
+}
+
+/// Generate an error helper.
+///
+/// ```js
+/// function _missingMdxReference(id, component) {
+///   throw new Error("Expected " + (component ? "component" : "object") + " `" + id + "` to be defined: you likely forgot to import, pass, or provide it.");
+/// }
+/// ```
+fn create_error_helper() -> swc_ecma_ast::ModuleItem {
+    let parameters = vec![
+        swc_ecma_ast::Param {
+            pat: swc_ecma_ast::Pat::Ident(swc_ecma_ast::BindingIdent {
+                id: create_ident("id"),
+                type_ann: None,
+            }),
+            decorators: vec![],
+            span: swc_common::DUMMY_SP,
+        },
+        swc_ecma_ast::Param {
+            pat: swc_ecma_ast::Pat::Ident(swc_ecma_ast::BindingIdent {
+                id: create_ident("component"),
+                type_ann: None,
+            }),
+            decorators: vec![],
+            span: swc_common::DUMMY_SP,
+        },
+    ];
+
+    let message = vec![
+        swc_ecma_ast::Expr::Lit(swc_ecma_ast::Lit::Str(swc_ecma_ast::Str {
+            value: "Expected ".into(),
+            span: swc_common::DUMMY_SP,
+            raw: None,
+        })),
+        // `component ? "component" : "object"`
+        swc_ecma_ast::Expr::Paren(swc_ecma_ast::ParenExpr {
+            expr: Box::new(swc_ecma_ast::Expr::Cond(swc_ecma_ast::CondExpr {
+                test: Box::new(create_ident_expression("component")),
+                cons: Box::new(swc_ecma_ast::Expr::Lit(swc_ecma_ast::Lit::Str(
+                    swc_ecma_ast::Str {
+                        value: "component".into(),
+                        span: swc_common::DUMMY_SP,
+                        raw: None,
+                    },
+                ))),
+                alt: Box::new(swc_ecma_ast::Expr::Lit(swc_ecma_ast::Lit::Str(
+                    swc_ecma_ast::Str {
+                        value: "object".into(),
+                        span: swc_common::DUMMY_SP,
+                        raw: None,
+                    },
+                ))),
+                span: swc_common::DUMMY_SP,
+            })),
+            span: swc_common::DUMMY_SP,
+        }),
+        swc_ecma_ast::Expr::Lit(swc_ecma_ast::Lit::Str(swc_ecma_ast::Str {
+            value: " `".into(),
+            span: swc_common::DUMMY_SP,
+            raw: None,
+        })),
+        create_ident_expression("id"),
+        swc_ecma_ast::Expr::Lit(swc_ecma_ast::Lit::Str(swc_ecma_ast::Str {
+            value: "` to be defined: you likely forgot to import, pass, or provide it.".into(),
+            span: swc_common::DUMMY_SP,
+            raw: None,
+        })),
+    ];
+
+    // To do: in development, add `place` param, and use the positional info.
+    // Also, then, add file path.
+
+    swc_ecma_ast::ModuleItem::Stmt(swc_ecma_ast::Stmt::Decl(swc_ecma_ast::Decl::Fn(
+        swc_ecma_ast::FnDecl {
+            ident: create_ident("_missingMdxReference"),
+            declare: false,
+            function: Box::new(swc_ecma_ast::Function {
+                params: parameters,
+                decorators: vec![],
+                body: Some(swc_ecma_ast::BlockStmt {
+                    stmts: vec![swc_ecma_ast::Stmt::Throw(swc_ecma_ast::ThrowStmt {
+                        arg: Box::new(swc_ecma_ast::Expr::New(swc_ecma_ast::NewExpr {
+                            callee: Box::new(create_ident_expression("Error")),
+                            args: Some(vec![swc_ecma_ast::ExprOrSpread {
+                                spread: None,
+                                expr: Box::new(create_binary_expression(
+                                    message,
+                                    swc_ecma_ast::BinaryOp::Add,
+                                )),
+                            }]),
+                            span: swc_common::DUMMY_SP,
+                            type_args: None,
+                        })),
+                        span: swc_common::DUMMY_SP,
+                    })],
+                    span: swc_common::DUMMY_SP,
+                }),
+                is_generator: false,
+                is_async: false,
+                type_params: None,
+                return_type: None,
+                span: swc_common::DUMMY_SP,
+            }),
+        },
+    )))
+}
+
+/// Generate a binary expression.
+///
+/// ```js
+/// a + b + c
+/// a || b
+/// ```
+fn create_binary_expression(
+    mut exprs: Vec<swc_ecma_ast::Expr>,
+    op: swc_ecma_ast::BinaryOp,
+) -> swc_ecma_ast::Expr {
+    exprs.reverse();
+
+    let mut left = None;
+
+    while let Some(right_expr) = exprs.pop() {
+        left = Some(if let Some(left_expr) = left {
+            swc_ecma_ast::Expr::Bin(swc_ecma_ast::BinExpr {
+                left: Box::new(left_expr),
+                right: Box::new(right_expr),
+                op,
+                span: swc_common::DUMMY_SP,
+            })
+        } else {
+            right_expr
+        });
+    }
+
+    left.expect("expected one or more expressions")
+}
+
+/// Generate a member expression.
+///
+/// ```js
+/// a.b
+/// a
+/// ```
+fn create_member_expression(name: &str) -> swc_ecma_ast::Expr {
+    let bytes = name.as_bytes();
+    let mut index = 0;
+    let mut start = 0;
+    let mut parts = vec![];
+
+    while index < bytes.len() {
+        if bytes[index] == b'.' {
+            parts.push(&name[start..index]);
+            start = index + 1;
+        }
+
+        index += 1;
+    }
+
+    if parts.len() > 1 {
+        let mut member = swc_ecma_ast::MemberExpr {
+            obj: Box::new(create_ident_expression(parts[0])),
+            prop: swc_ecma_ast::MemberProp::Ident(create_ident(parts[1])),
+            span: swc_common::DUMMY_SP,
+        };
+        let mut index = 2;
+        while index < parts.len() {
+            member = swc_ecma_ast::MemberExpr {
+                obj: Box::new(swc_ecma_ast::Expr::Member(member)),
+                prop: swc_ecma_ast::MemberProp::Ident(create_ident(parts[1])),
+                span: swc_common::DUMMY_SP,
+            };
+            index += 1;
+        }
+        swc_ecma_ast::Expr::Member(member)
+    } else {
+        create_ident_expression(name)
+    }
+}
+
+/// Generate an ident expression.
+///
+/// ```js
+/// a
+/// ```
+fn create_ident_expression(sym: &str) -> swc_ecma_ast::Expr {
+    swc_ecma_ast::Expr::Ident(create_ident(sym))
+}
+
+/// Generate an ident.
+///
+/// ```js
+/// a
+/// ```
+fn create_ident(sym: &str) -> swc_ecma_ast::Ident {
+    swc_ecma_ast::Ident {
+        sym: sym.into(),
+        optional: false,
+        span: swc_common::DUMMY_SP,
+    }
+}
+
+/// Check if this function is a props receiving component: it’s one of ours.
+fn is_props_receiving_fn(name: &Option<String>) -> bool {
+    if let Some(name) = name {
+        name == "_createMdxContent" || name == "MDXContent"
+    } else {
+        false
+    }
+}
+
+/// Check if a name is a literal tag name or an identifier to a component.
+fn is_literal_name(name: &str) -> bool {
+    matches!(name.as_bytes().first(), Some(b'a'..=b'z')) || !is_identifier_name(name)
+}
+
+// Check if a name is a valid identifier name.
+fn is_identifier_name(name: &str) -> bool {
+    for (index, char) in name.chars().enumerate() {
+        if if index == 0 {
+            !id_start(char)
+        } else {
+            !id_cont(char)
+        } {
+            return false;
+        }
+    }
+
+    true
+}
+
+// To do: share with `partial_mdx_jsx`.
+/// Check if a character can start a JS identifier.
+fn id_start(char: char) -> bool {
+    UnicodeID::is_id_start(char) || matches!(char, '$' | '_')
+}
+
+// To do: share with `partial_mdx_jsx`.
+/// Check if a character can continue a JS identifier.
+fn id_cont(char: char) -> bool {
+    UnicodeID::is_id_continue(char) || matches!(char, '\u{200c}' | '\u{200d}')
+}
diff --git a/tests/test_utils/mod.rs b/tests/test_utils/mod.rs
index aa1d509..339992c 100644
--- a/tests/test_utils/mod.rs
+++ b/tests/test_utils/mod.rs
@@ -1,4 +1,5 @@
 pub mod hast;
+pub mod jsx_rewrite;
 pub mod swc;
 pub mod to_document;
 pub mod to_hast;
diff --git a/tests/test_utils/swc.rs b/tests/test_utils/swc.rs
index 80376b8..fb91a3b 100644
--- a/tests/test_utils/swc.rs
+++ b/tests/test_utils/swc.rs
@@ -40,6 +40,7 @@ pub fn parse_esm(value: &str) -> MdxSignal {
 
 /// Parse ESM in MDX with SWC.
 /// To do: figure out how to fix positional info.
+/// See `drop_span` in `swc_ecma_utils` for inspiration?
 #[allow(dead_code)]
 pub fn parse_esm_to_tree(value: &str) -> Result<swc_ecma_ast::Module, String> {
     let (file, syntax, version) = create_config(value.to_string());
@@ -108,6 +109,7 @@ pub fn parse_expression(value: &str, kind: &MdxExpressionKind) -> MdxSignal {
 
 /// Parse ESM in MDX with SWC.
 /// To do: figure out how to fix positional info.
+/// See `drop_span` in `swc_ecma_utils` for inspiration?
 #[allow(dead_code)]
 pub fn parse_expression_to_tree(
     value: &str,
diff --git a/tests/test_utils/to_document.rs b/tests/test_utils/to_document.rs
index 96e9d7f..91fc134 100644
--- a/tests/test_utils/to_document.rs
+++ b/tests/test_utils/to_document.rs
@@ -300,8 +300,16 @@ pub fn to_document(mut program: Program, options: &Options) -> Result<Program, S
                     ));
                 }
             }
-            swc_ecma_ast::ModuleItem::ModuleDecl(swc_ecma_ast::ModuleDecl::Import(_))
-            | swc_ecma_ast::ModuleItem::ModuleDecl(swc_ecma_ast::ModuleDecl::ExportDecl(_))
+            swc_ecma_ast::ModuleItem::ModuleDecl(swc_ecma_ast::ModuleDecl::Import(mut x)) => {
+                // SWC is currently crashing when generating code, w/o source
+                // map, if an actual location is set on this node.
+                x.span = swc_common::DUMMY_SP;
+                // Pass through.
+                replacements.push(swc_ecma_ast::ModuleItem::ModuleDecl(
+                    swc_ecma_ast::ModuleDecl::Import(x),
+                ));
+            }
+            swc_ecma_ast::ModuleItem::ModuleDecl(swc_ecma_ast::ModuleDecl::ExportDecl(_))
             | swc_ecma_ast::ModuleItem::ModuleDecl(swc_ecma_ast::ModuleDecl::ExportAll(_))
             | swc_ecma_ast::ModuleItem::ModuleDecl(swc_ecma_ast::ModuleDecl::TsImportEquals(_))
             | swc_ecma_ast::ModuleItem::ModuleDecl(swc_ecma_ast::ModuleDecl::TsExportAssignment(