12.0.2

🚀 New Feature

• Reanalyze: add parallel processing for CMT file analysis with new -parallel and -timing flags, plus benchmark infrastructure for performance testing. #8089

🐛 Bug fix

• Reanalyze: make optional args analysis liveness-aware, preventing false positives when functions are only called from dead code. #8082
• Fix: do not warn for "editor" field in rescript.json. #8084
• Fix @val shadowing (rewrite using globalThis). #8098
• Fix @scope shadowing (rewrite using globalThis). #8100
• Formatter: normalize underscore placeholders in pipe expressions to canonical form (e.g., a->map2(_, fn) formats to a->map2(fn)). #8033
• Fix rewatch panic on duplicate module name. #8102
• Fix let? unwrap to use actual variable names from pattern instead of hardcoded "x"/"e". #8085
• Fix "Ill-formed list of warnings" errors for PPXs with rescript-legacy. #8103
• Fix gentype generating invalid syntax for exotic / escaped record field names and type names. #8087

🏠 Internal

• Reanalyze: refactor DCE to pure pipeline architecture for order-independence and incremental update support. #8043

12.0.1

🐛 Bug fix

• Fix exponential compilation blowup with large unboxed variants and dict pattern matching. #8078
• Rewatch: warnings for unsupported/unknown rescript.json fields. #8031
• Fix missing ignore function in some Stdlib modules. #8060
• Fix signature matching for externals when abstract alias hides function arity. #8045
• Fix arity detection for arrows returning nested generics. #8064
• Fix error handling when rescript.json parsing fails and improve error message. #8067
• Fix invalid JSX being generated for empty fragments. #8077

💅 Polish

• Add missing deprecation and migration for Exn.Error. #8036

12.0.0

ReScript’s third major community-led release has arrived! This milestone wraps up a multi-year push to free the compiler from legacy limitations—and it comes loaded with new features and enhancements. 🎉

🚀 Highlights

• New Build System
• Improved Standard Library
• Unified Operators
• F#-style binary operators
• Dict Literals and Pattern Matching
• Variant Pattern Spreads
• JSX Preserve Mode
• Function-Level Directives
• Regex Literals
• Experimental let? Syntax

📋 More Information

• ReScript 12 Release Blog Post
• Changelog

12.0.0-rc.5

🐛 Bug fix

• Fix printing of tagged template literals. #8018
• Fix printing of optional record fields in pattern matching errors. #8019

🏠 Internal

• Editor: resolve @rescript/runtime via environment variable RESCRIPT_RUNTIME. #8023
• Remove leftovers from @meth attribute. #8022

12.0.0-rc.4

💥 Breaking Change

• Fix some Intl bindings (Intl.Collator.supportedLocalesOf, Intl.DateTimeFormat.supportedLocalesOf, Intl.ListFormat.supportedLocalesOf, Intl.NumberFormat.supportedLocalesOf, Intl.PluralRules.supportedLocalesOf, Intl.RelativeTimeFormat.supportedLocalesOf, Intl.Segmenter.supportedLocalesOf) which return array<string> and not their corresponding main type t. Also remove Intl.PluralRules.selectBigInt and Intl.PluralRules.selectRangeBigInt which don't work in many JS runtimes. #7995

🐛 Bug fix

• Fix fatal compiler error that occurred when an %ffi extension point contained invalid JavaScript. #7998

💅 Polish

• Dedicated error messages for old Reason array literal syntax ([| and |]), and for the old pipe (|>). Primarly intended to help LLMs that might try to use old code patterns. #8010

🏠 Internal

• Rename Core to Stdlib in tests/tests. #8005
• CI: Build on windows-2025 runners. #8006
• Rewatch: upgrade Rust to 1.91.0. #8007

12.0.0-rc.3

🐛 Bug fix

• Fix @directive on function level with async and multiple parameters. #7977
• Fix fatal error for external with @as. #7978

💅 Polish

• Rewatch: plain output when not running in tty. #7970
• Streamline rewatch help texts. #7973
• Rewatch: Reduced build progress output from 7 steps to 3 for cleaner, less verbose logging. #7971

🏠 Internal

• Clean up usage of Ast_uncurried helpers. #7987

12.0.0-rc.2

💥 Breaking Change

• Replace binary operators with F#-style ~~~, ^^^, &&&, |||. #7894

🐛 Bug fix

• Fix: use configured Jsx module for constraining component return type. #7945
• Undeprecate Js_OO module since it is still used with the @this attribute. #7955
• Fix crash when using bitwise not (~~~) on an incompatible type. #7965

🏠 Internal

• JSX PPX: use React.component instead of React.componentLike for externals. #7952

12.0.0-rc.1

💅 Polish

• Improve circular dependency errors, and make sure they end up in the compiler log so the editor tooling can surface them. #7940
• JSX PPX: add Jsx.element return constraint. #7939

🏠 Internal

• Use AST nodes with locations for fn arguments in the typed tree. #7873

12.0.0-beta.14

💥 Breaking Change

• Removed rescript legacy subcommand in favor of separate rescript-legacy binary. #7928
• Add comparison function for Error in Result.equal and compare. #7933
• Rewatch: "type": "dev" and dev-dependencies will be compiled by default for local packages. The --dev flag no longer does anything. #7934

🐛 Bug fix

• Prevent lockfile creation when project folder is missing. #7927
• Fix parameter type / return type ambiguity error for unit case. #7930

💅 Polish

• ReScript cli: do not show build command options in the root help. #7715
• Deprecate reanalyze @raises in favor of @throws. #7932

🏠 Internal

• CI: run macOS builds on macOS 15. #7935

12.0.0-beta.13

🐛 Bug fix

• Fix result examples. #7914
• Make inline record fields that overlap with a variant's tag a compile error. #7875

💅 Polish

• Keep track of compiler info during build. #7889
• Improve option optimization for constants. #7913
• Option optimization: do not create redundant local vars. #7915
• Js output: remove superfluous newline after every if. #7920
• Rewatch: Traverse upwards for package resolution in single context projects. #7896
• Rewatch: Add --warn-error flag to build. #7916