feat(rust): System.Uri (PR 7 of 7 Rust improvements) by Thorium · Pull Request #4790 · fable-compiler/Fable

Thorium · 2026-07-14T21:48:46Z

Adds System.Uri to Rust target.
Last part of the dependency chain for Rust improvements.

MangelMaxime · 2026-07-15T08:29:53Z

Thanks for the contributions! Using AI to assist with PRs is fine, but it's crucial that you thoroughly review and curate the output before submitting, keeping the maintainers' workload in mind. Right now, with this batch of PRs, it's difficult for us to even know where to start reviewing.

To help us help you, please keep these guidelines in mind for future contributions:

Keep PRs independent and focused. Chained PRs (where one requires another to be merged first) are very hard to review. This PR, for instance, likely only needed your final commit.

Chained PRs can happens some times but they are the exceptions.
Use meaningful PR name: The name of the PR is used for generating the CHANGELOG.md
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We maintain Fable in our free time. Small and focused contributions are much easier for us to merge than large batches of complex changes.

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Thorium · 2026-07-15T08:58:31Z

Thanks. The chain reason is that all of these PRs focus on bringing the Rust target closer to the current TS/JS reference implementation, so they touch the same files (mainly src\Fable.Transforms\Rust\Fable2Rust.fs and src\Fable.Transforms\Rust\Replacements.fs). So, if they build on without a chain, there will be many merge conflicts.

The non-Rust commits are independent and not chained (except "feat(all): quotations" #4780, let's start from that please), which is needed by many quotation PRs). The main feature for that PR is to bring more types supported to quotations, which right now seems to be quite much like "number or string, and everything else is an obj".

Either you start from the bottom of the chain, or you only use the top one.

…4781) === changelog === - Cast `bool` to integer types; support `**` via `.powf` - Add `Decimal.IsInteger/IsEvenInteger/IsOddInteger/IsCanonical` - Fix two-switch decision trees dispatching to the wrong target - Fix reference-typed match bindings panicking via `mem::zeroed` - Fix string and non-ident option pattern matches emitting invalid Rust `match` arms - Fix `Convert.ToXxx(bool)` - Fix `Char.ToUpper/ToLower` truncating multi-char case mappings - Fix `Random.nextDouble` panic message - Fix `Map.find/tryFind` panicking on reference-typed values === changelog === Co-authored-by: Mangel Maxime <me@mangelmaxime.fr>

Copilot

Pull request overview

This PR extends the Rust target runtime and compiler replacements to support System.Uri (and related runtime features needed by the dependency chain), and enables/expands Rust test coverage for these newly supported APIs.

Changes:

Add a Rust runtime Uri implementation plus compiler routing for System.Uri constructors/members, and enable Rust UriTests.
Introduce/expand Rust runtime support for reflection + quotations + events/observables, and add corresponding Rust tests.
Add std-only Rust runtime modules and tests for System.Environment and System.IO (Path/File), gated for no_std.

Reviewed changes

Copilot reviewed 25 out of 26 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
tests/Rust/tests/src/UriTests.fs	Adds regression tests for authority-less absolute schemes and dot-segment collapsing.
tests/Rust/tests/src/SystemIOTests.fs	Adds std-only tests for `System.IO.Path`/`System.IO.File`.
tests/Rust/tests/src/RecordReflectionTests.fs	Adds std-only tests for record reflection MVP (`FSharpType`/`FSharpValue`).
tests/Rust/tests/src/QuotationTests.fs	Adds quotation evaluation/deconstruction tests for Rust.
tests/Rust/tests/src/ObservableTests.fs	Adjusts observable test helper for Rust backend limitations.
tests/Rust/tests/src/EventTests.fs	Disables unsupported Rust event scenarios; keeps core module-function tests.
tests/Rust/tests/src/EnvironmentTests.fs	Adds std-only tests for `System.Environment` basics.
tests/Rust/tests/src/EnumerableTests.fs	Re-enables enumerable tests and adjusts type/interface layout formatting.
tests/Rust/Fable.Tests.Rust.fsproj	Enables newly added Rust test sources (Uri/IO/Env/Quotations/etc.).
src/Fable.Transforms/Rust/Replacements.fs	Routes Rust compiler replacements for `System.Uri`, `System.Environment`, `System.IO.Path/File`, and reflection/quotation carriers.
src/Fable.Transforms/Rust/Fable2Rust.fs	Updates Rust type/boxing rules for `System.Type`, quotation carriers, `System.Uri`, and object-expression generic handling.
src/fable-library-rust/src/Util.fs	Adds Uri escape/unescape helpers (currently stubs).
src/fable-library-rust/src/Uri.rs	Adds Rust runtime `Uri` implementation and constructors/tryCreate helpers.
src/fable-library-rust/src/Reflection.rs	Expands reflection runtime: record registry/type equality/property accessors/GetType-from-obj.
src/fable-library-rust/src/QuotationTypes.fs	Adds Rust-target quotation AST carrier types under `Microsoft.FSharp.Quotations`.
src/fable-library-rust/src/Quotation.fs	Adds Rust-target quotation runtime constructors, pattern helpers, and evaluator subset.
src/fable-library-rust/src/Path.rs	Adds std-only Rust runtime `System.IO.Path` helpers.
src/fable-library-rust/src/Observable.rs	Adds Rust runtime `Observable`/`Observer`/combinators used by F# Observable APIs.
src/fable-library-rust/src/Native.rs	Adds `box_lrc`/`unbox_lrc` helpers for boxing reference-typed records/unions into `obj`.
src/fable-library-rust/src/lib.fs	Registers additional runtime imports (Uri/Path/File/Observable/Event/Environment).
src/fable-library-rust/src/Interfaces.fs	Adds `System.IObservable/IObserver` and `Microsoft.FSharp.Control` event interfaces needed by runtime.
src/fable-library-rust/src/File.rs	Adds std-only Rust runtime `System.IO.File` helpers.
src/fable-library-rust/src/Fable.Library.Rust.fsproj	Includes quotation runtime sources in the Rust library project.
src/fable-library-rust/src/Event.rs	Adds Rust runtime event implementation (`FSharpEvent\`1` and event combinators).
src/fable-library-rust/src/Environment.rs	Adds std-only Rust runtime `System.Environment` helpers.
src/fable-library-rust/src/Async.rs	Adjusts monitor lock addressing to support `obj`/fat pointers.

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+        if !baseUri.isAbsolute {
+            panic!("Invalid URI: A relative URI cannot be created because the 'uriString' parameter represents an absolute URI.");
+        }


+// System.Uri escape/unescape helpers (percent-encode/decode).
+// NOTE: minimal identity stubs; not currently exercised by the enabled
+// Rust UriTests. Provided so the `uris` replacement routing resolves.
+// Replace with real percent-encoding if/when Uri escape members are tested.
+let escapeDataString (s: string) : string = s
+let unescapeDataString (s: string) : string = s
+let escapeUriString (s: string) : string = s
+let unescapeUriString (s: string) : string = s


+    #[cfg(not(feature = "lrc_ptr"))]
+    pub fn box_lrc<T: 'static>(o: LrcPtr<T>) -> LrcPtr<dyn Any> {
+        o
+    }
+
+    #[cfg(not(feature = "lrc_ptr"))]
+    pub fn unbox_lrc<T: Clone + 'static>(o: LrcPtr<dyn Any>) -> LrcPtr<T> {
+        LrcPtr::new((*o).downcast_ref::<T>().expect("Invalid unbox").clone())
+    }


Thorium · 2026-07-16T10:26:22Z

System.Uri gives some parity to Fable reference implementation TS/JS. It makes sense in fronted-web-target (original goal of Fable). But for F# -> Rust, I think there would be more important namespaces like for example System.Data (or ADO in general).

Any point of view from the maintainers? (As it'd be pointless to implement if that's not the direction wanted.)

ncave · 2026-07-16T16:42:08Z

@Thorium We probably don't want to start onboarding new large surface namespaces from BCL, as we have limited resources to maintain that, but System.Uri is fine for parity.

This PR looks ok, perhaps you can consider the Copilot suggestion for a fall-back for Native::box_lrc/unbox_lrc, or perhaps just specialize the existing Native::box_/unbox with the more efficient implementation, when available, instead of new functions.

MangelMaxime · 2026-07-16T19:00:20Z

We probably don't want to start onboarding new large surface namespaces from BCL, as we have limited resources to maintain that, but System.Uri is fine for parity.

To complete this answer, I have a working prototype locally which would allows people to use Fable Plugins to add new BCL API support. I will open a PR with it so we can discuss more about it.

The need for it, come from my need to improve the cross target story for Fable and the need to support more BCL API which are niche and so not worth maintaining in Fable.

Thorium force-pushed the pr/7c-rust-uri branch 4 times, most recently from 682fcb0 to 623d897 Compare July 14, 2026 23:50

Thorium and others added 3 commits July 15, 2026 21:58

chore: undo changes to QuickTest project

0ab2efe

feat(rust): quotations

1a52cd9

Thorium force-pushed the pr/7c-rust-uri branch from 623d897 to 24ffc57 Compare July 15, 2026 21:58

Thorium added 3 commits July 15, 2026 23:58

feat(rust): reflection

6a1e767

feat(rust): System.Environment and System.IO

8565626

feat(rust): Observables, Events and IEnumerable

db63a2c

Thorium force-pushed the pr/7c-rust-uri branch from 24ffc57 to c662846 Compare July 15, 2026 23:02

ncave requested a review from Copilot July 16, 2026 06:04

Copilot started reviewing on behalf of ncave July 16, 2026 06:04 View session

Copilot AI reviewed Jul 16, 2026

View reviewed changes

MangelMaxime force-pushed the main branch from 29408a5 to 55b9d56 Compare July 16, 2026 08:01

feat(rust): System.Uri

eb322f2

Thorium force-pushed the pr/7c-rust-uri branch from c662846 to eb322f2 Compare July 16, 2026 10:44

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feat(rust): System.Uri (PR 7 of 7 Rust improvements)#4790

feat(rust): System.Uri (PR 7 of 7 Rust improvements)#4790
Thorium wants to merge 7 commits into
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Thorium:pr/7c-rust-uri

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