code to reproduce undefined behaviour detected by Miri when using SWC. Run with this command `RUSTCFLAGS="-C debug-assertions" RUSTFLAGS="-Zrandomize-layout" MIRIFLAGS="-Zmiri-symbolic-alignment-check -Zmiri-check-number-validity -Zmiri-tag-raw-pointers -Zmiri-disable-isolation" cargo miri run`

Cargo.toml

[package]
name = "swc-miri-ub"
version = "0.1.0"
edition = "2021"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[dependencies]
swc = "0.102.2"
swc_ecmascript = { version="0.104.3", features = ["visit", "parser", "typescript"] }
anyhow = "1.0.52"

[dependencies.swc_ecma_parser]
version = "0.83.0"

main.rs

use std::{fs, sync::Arc};

use anyhow::Context;
use swc::{
    common::{sync::Lazy, FileName, FilePathMapping, SourceMap},
    config::ParseOptions,
    ecmascript::ast::{TsKeywordType, TsKeywordTypeKind, TsType},
    try_with_handler, Compiler,
};
use swc_ecma_parser::{Syntax, TsConfig};

pub fn main() {
    let c = compiler();

    let str = "console.log('foo')";

    let prog = try_with_handler(c.cm.clone(), false, |handler| {
        let opts: ParseOptions = ParseOptions {
            syntax: Syntax::Typescript(TsConfig::default()),
            ..ParseOptions::default()
        };

        let fm = c.cm.new_source_file(FileName::Anon, str.to_string());
        let program = c
            .parse_js(
                fm,
                handler,
                opts.target,
                opts.syntax,
                opts.is_module,
                opts.comments,
            )
            .context("failed to parse code")?;

        Ok(program)
    })
    .expect("Program failed");

    println!("Parsed program: {:?}", prog);
}

/// Get global sourcemap
fn compiler() -> Arc<Compiler> {
    static C: Lazy<Arc<Compiler>> = Lazy::new(|| {
        let cm = Arc::new(SourceMap::new(FilePathMapping::empty()));

        Arc::new(Compiler::new(cm))
    });

    C.clone()
}