fix(lang): support `return` as a diverging expression in catch/match arms by antoniosarosi · Pull Request #3881 · BoundaryML/baml

antoniosarosi · 2026-06-28T23:53:19Z

Fixes B-246.

Problem

Writing a braceless return as a catch/match arm value fails because return is a statement, not an expression. The compiler emits a cascade of misleading errors where only the first identifies the real problem:

function foo() -> float {
    let x = risky() catch (e) {
        _ => return 0.0,   // <- braceless return
    };
    x + 1.0
}

error: Expected expression, found return            (E0010)  <- the real issue
error: Float literal values are not supported: 0.0  (E0010)  <- recovery artifact
error: Expected '=>' after catch pattern, found ','  (E0010) <- recovery artifact
error: operator `Add` cannot be applied to ...       (E0004) <- recovery artifact

The last three are downstream parse failures from recovery misalignment, not real bugs — they read as "this expression has four bugs."

Fix

Adopt the Rust model the ticket recommends: return now also parses as a RETURN_EXPR — a diverging expression of type never, exactly mirroring how throw already works. Since never unifies with any arm type, the arm type-checks without special-casing, while the returned value is still checked against the function's declared return type (identical typing to the block form => { return x }).

throw was already a diverging expression and worked braceless in arm position; this makes return symmetric.

Pipeline (mirrors `throw` at every stage)

Parser — parse_return_expr produces RETURN_EXPR, recognized in parse_primary_expr. Statement-position return is still taken by parse_stmt first, so it stays a RETURN_STMT and no existing snapshots move.
AST — new Expr::Return { value: Option<ExprId> }; value extraction shared with Stmt::Return.
HIR / TIR / MIR — walk, type as never (+ check value against the declared return type, + defer-escape), and lower to a function-exit return (not routed through catch_context, so it returns from the function rather than being caught).
Formatter — wraps a braceless arm body into a block (as it already does for throw) and appends the ; a block-position return needs, so output round-trips (idempotent) for both catch and match arms.

Tests

baml_src/ns_catch_arm_return — executable catch- and match-arm cases asserting runtime behavior (early-return fires on throw; non-throw path flows normally; mixed arm types).
projects/compiles/catch_arm_return — clean compile + pins the formatter wrap for both arm kinds.
projects/diagnostic_errors/catch_arm_braceless_return — a wrong-typed braceless return now yields a single clear type mismatch: expected bool, got Errors, identical to the block form, instead of the four-error cascade.

Full baml_tests lib suite (1703) green; baml_src suite (1954 tests) green; parser/fmt/defer/exceptions suites green; clippy clean on all touched crates.

Summary by CodeRabbit

New Features
- Added support for using return in expression positions, including match and catch arms, with correct control-flow behavior.
- Braceless return arm bodies are now formatted appropriately when wrapped into blocks.
Bug Fixes
- Improved scope traversal, throw handling, type-checking, and control-flow lowering for return expressions.
- Enhanced diagnostics to avoid cascading errors and report clearer return-type mismatches in braceless return catch arms.
Tests
- Added new compile/format and diagnostic regression coverage for return used as arm values.

…arms Writing a braceless `return` as a `catch`/`match` arm value (`_ => return 0`) used to fail: `return` was a statement, so the expression parser rejected it and emitted a cascade of misleading downstream errors (B-246): error: Expected expression, found return error: Float literal values are not supported: 0.0 error: Expected '=>' after catch pattern, found ',' error: operator `Add` cannot be applied to ... Only the first pointed at the real problem; the rest were recovery artifacts. `return` now also parses as a `RETURN_EXPR` — a diverging expression of type `never`, mirroring `throw`. `never` unifies with any arm type, so the arm type-checks without special-casing, while the returned value is still checked against the function's declared return type (identical typing to the block form `=> { return x }`). Statement-position `return` is unchanged (still a `RETURN_STMT`); the expression path only fires when `return` is reached through expression parsing, so no existing snapshots move. The formatter wraps a braceless arm body into a block (as it already does for `throw`) and appends the `;` a block-position `return` needs, so the output round-trips (idempotent) for both catch and match arms. Tests: - ns_catch_arm_return: executable catch/match-arm cases (runtime behavior) - compiles/catch_arm_return: clean compile + pins the formatter wrap - diagnostic_errors/catch_arm_braceless_return: a wrong-typed braceless `return` now yields a single clear `type mismatch`, not the cascade

vercel · 2026-06-28T23:53:26Z

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coderabbitai · 2026-06-28T23:53:42Z

No actionable comments were generated in the recent review. 🎉

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Run ID: 95178e45-87cf-4dfc-a144-ffc1b9325a13

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Reviewing files that changed from the base of the PR and between 1824fcd and 5719ccc.

⛔ Files ignored due to path filters (6)

baml_language/crates/baml_tests/snapshots/diagnostic_errors/catch_arm_braceless_return/baml_tests__diagnostic_errors__catch_arm_braceless_return__01_lexer__repro.snap is excluded by !**/*.snap
baml_language/crates/baml_tests/snapshots/diagnostic_errors/catch_arm_braceless_return/baml_tests__diagnostic_errors__catch_arm_braceless_return__02_parser__repro.snap is excluded by !**/*.snap
baml_language/crates/baml_tests/snapshots/diagnostic_errors/catch_arm_braceless_return/baml_tests__diagnostic_errors__catch_arm_braceless_return__03_hir.snap is excluded by !**/*.snap
baml_language/crates/baml_tests/snapshots/diagnostic_errors/catch_arm_braceless_return/baml_tests__diagnostic_errors__catch_arm_braceless_return__04_tir.snap is excluded by !**/*.snap
baml_language/crates/baml_tests/snapshots/diagnostic_errors/catch_arm_braceless_return/baml_tests__diagnostic_errors__catch_arm_braceless_return__05_diagnostics.snap is excluded by !**/*.snap
baml_language/crates/baml_tests/snapshots/diagnostic_errors/catch_arm_braceless_return/baml_tests__diagnostic_errors__catch_arm_braceless_return__10_formatter__repro.snap is excluded by !**/*.snap

📒 Files selected for processing (1)

baml_language/crates/baml_tests/projects/diagnostic_errors/catch_arm_braceless_return/repro.baml

📝 Walkthrough

Walkthrough

Adds return as a diverging expression usable in catch and match arm values. The parser, AST, lowering, analysis, formatter, tests, diagnostics, and tooling output are updated to recognize and preserve expression-position returns.

Changes

Expression-position return in catch/match arms

Layer / File(s)	Summary
Syntax and parser `baml_language/crates/baml_compiler_syntax/src/syntax_kind.rs`, `baml_language/crates/baml_compiler_syntax/src/ast.rs`, `baml_language/crates/baml_compiler_parser/src/parser.rs`	`RETURN_EXPR` and `ReturnExpr` are added, along with parser support for `return` in expression position.
AST lowering `baml_language/crates/baml_compiler2_ast/src/ast.rs`, `baml_language/crates/baml_compiler2_ast/src/lower_expr_body.rs`	`Expr::Return { value }` is added and `RETURN_EXPR` lowers through shared optional-value extraction into expression-position return nodes.
HIR and MIR flow `baml_language/crates/baml_compiler2_hir/src/builder.rs`, `baml_language/crates/baml_compiler2_mir/src/lower.rs`	Return values are walked in HIR, and MIR lowering stores the value in `_0`, replays defers, unwinds watched locals, and jumps to the function exit.
TIR typing and analysis `baml_language/crates/baml_compiler2_tir/src/builder.rs`, `baml_language/crates/baml_compiler2_tir/src/throws_analysis.rs`, `baml_language/crates/baml_compiler2_visualization/src/control_flow/from_ast.rs`	`Expr::Return` is handled in forward-reference collection, typing, throw-fact collection, throws analysis, and CFG callee-name collection.
Formatter support `baml_language/crates/baml_fmt/src/ast/expressions.rs`	`RETURN_EXPR` maps to `Expression::Return`, printing preserves source ranges, never uses single-line formatting, and inserts semicolons when wrapping braceless return into arm blocks.
Tests, diagnostics, and renderers `baml_language/crates/baml_tests/baml_src/ns_catch_arm_return/catch_arm_return.baml`, `baml_language/crates/baml_tests/projects/compiles/catch_arm_return/catch_arm_return.baml`, `baml_language/crates/baml_tests/projects/diagnostic_errors/catch_arm_braceless_return/repro.baml`, `baml_language/crates/baml_tests/src/compiler2_tir/mod.rs`, `baml_language/crates/tools_onionskin/src/compiler.rs`	Runtime and compile fixtures, a diagnostic repro, and expression renderers are updated for braceless return.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Possibly related PRs

BoundaryML/baml#3719: Shares formatter work in baml_fmt/src/ast/expressions.rs around catch-arm rendering.
BoundaryML/baml#3785: Shares TIR defer control-flow escape handling in baml_compiler2_tir/src/builder.rs.

Poem

I nibble on syntax, hop by hop,
A braceless return that won’t stop.
Through catch and match I dart away,
Then land in _0 and fade to gray.
The formatter tucks me in with care,
🐇 return now leaps through every layer.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the core change: enabling `return` as a diverging expression in catch/match arms.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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Commit unit tests in branch antonio/catch-arm-return-error

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github-actions · 2026-06-28T23:53:57Z

⏭️ Performance benchmarks were skipped

Perf benchmarks (CodSpeed) are opt-in on pull requests — they no longer run on every push. They always run automatically after merge to canary/main.

To run them on this PR, do any of the following, then push a commit (or re-run CI):

Add RUN_CODSPEED=1 to the PR description, or
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coderabbitai

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In
`@baml_language/crates/baml_tests/projects/diagnostic_errors/catch_arm_braceless_return/repro.baml`:
- Around line 23-26: The repro in caller() is not exhaustive, so it can trigger
an extra non-exhaustive/throws diagnostic in addition to the intended
return-type mismatch on return e. Update the catch arm handling in caller() by
either adding a fallback arm in the catch expression or narrowing callee() so it
throws only Errors.AuthenticationError, ensuring the fixture isolates the
return-type mismatch alone.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

Push a commit to this branch (recommended)
Create a new PR with the fixes

ℹ️ Review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: 4813c0ad-b63c-46cd-8b18-477812b47f11

📥 Commits

Reviewing files that changed from the base of the PR and between 6b7d7bd and 1824fcd.

⛔ Files ignored due to path filters (16)

baml_language/crates/baml_tests/snapshots/baml_src/_root.snap is excluded by !**/*.snap
baml_language/crates/baml_tests/snapshots/baml_src/catch_arm_return.snap is excluded by !**/*.snap
baml_language/crates/baml_tests/snapshots/compiles/catch_arm_return/baml_tests__compiles__catch_arm_return__01_lexer__catch_arm_return.snap is excluded by !**/*.snap
baml_language/crates/baml_tests/snapshots/compiles/catch_arm_return/baml_tests__compiles__catch_arm_return__02_parser__catch_arm_return.snap is excluded by !**/*.snap
baml_language/crates/baml_tests/snapshots/compiles/catch_arm_return/baml_tests__compiles__catch_arm_return__03_hir.snap is excluded by !**/*.snap
baml_language/crates/baml_tests/snapshots/compiles/catch_arm_return/baml_tests__compiles__catch_arm_return__04_5_mir.snap is excluded by !**/*.snap
baml_language/crates/baml_tests/snapshots/compiles/catch_arm_return/baml_tests__compiles__catch_arm_return__04_tir.snap is excluded by !**/*.snap
baml_language/crates/baml_tests/snapshots/compiles/catch_arm_return/baml_tests__compiles__catch_arm_return__05_diagnostics.snap is excluded by !**/*.snap
baml_language/crates/baml_tests/snapshots/compiles/catch_arm_return/baml_tests__compiles__catch_arm_return__06_codegen.snap is excluded by !**/*.snap
baml_language/crates/baml_tests/snapshots/compiles/catch_arm_return/baml_tests__compiles__catch_arm_return__10_formatter__catch_arm_return.snap is excluded by !**/*.snap
baml_language/crates/baml_tests/snapshots/diagnostic_errors/catch_arm_braceless_return/baml_tests__diagnostic_errors__catch_arm_braceless_return__01_lexer__repro.snap is excluded by !**/*.snap
baml_language/crates/baml_tests/snapshots/diagnostic_errors/catch_arm_braceless_return/baml_tests__diagnostic_errors__catch_arm_braceless_return__02_parser__repro.snap is excluded by !**/*.snap
baml_language/crates/baml_tests/snapshots/diagnostic_errors/catch_arm_braceless_return/baml_tests__diagnostic_errors__catch_arm_braceless_return__03_hir.snap is excluded by !**/*.snap
baml_language/crates/baml_tests/snapshots/diagnostic_errors/catch_arm_braceless_return/baml_tests__diagnostic_errors__catch_arm_braceless_return__04_tir.snap is excluded by !**/*.snap
baml_language/crates/baml_tests/snapshots/diagnostic_errors/catch_arm_braceless_return/baml_tests__diagnostic_errors__catch_arm_braceless_return__05_diagnostics.snap is excluded by !**/*.snap
baml_language/crates/baml_tests/snapshots/diagnostic_errors/catch_arm_braceless_return/baml_tests__diagnostic_errors__catch_arm_braceless_return__10_formatter__repro.snap is excluded by !**/*.snap

📒 Files selected for processing (16)

baml_language/crates/baml_compiler2_ast/src/ast.rs
baml_language/crates/baml_compiler2_ast/src/lower_expr_body.rs
baml_language/crates/baml_compiler2_hir/src/builder.rs
baml_language/crates/baml_compiler2_mir/src/lower.rs
baml_language/crates/baml_compiler2_tir/src/builder.rs
baml_language/crates/baml_compiler2_tir/src/throws_analysis.rs
baml_language/crates/baml_compiler2_visualization/src/control_flow/from_ast.rs
baml_language/crates/baml_compiler_parser/src/parser.rs
baml_language/crates/baml_compiler_syntax/src/ast.rs
baml_language/crates/baml_compiler_syntax/src/syntax_kind.rs
baml_language/crates/baml_fmt/src/ast/expressions.rs
baml_language/crates/baml_tests/baml_src/ns_catch_arm_return/catch_arm_return.baml
baml_language/crates/baml_tests/projects/compiles/catch_arm_return/catch_arm_return.baml
baml_language/crates/baml_tests/projects/diagnostic_errors/catch_arm_braceless_return/repro.baml
baml_language/crates/baml_tests/src/compiler2_tir/mod.rs
baml_language/crates/tools_onionskin/src/compiler.rs

github-actions · 2026-06-29T00:02:44Z

Binary size checks passed

✅ 7 passed

	Artifact	Platform	File	Gzip	Gated on	Baseline	Delta	Status
✅	`baml-cli`	Linux	🔒 21.5 MB	9.2 MB	file	21.5 MB	+8.0 KB (+0.0%)	OK
✅	`packed-program`	Linux	🔒 15.6 MB	6.6 MB	file	15.6 MB	+6.9 KB (+0.0%)	OK
✅	`baml-cli`	macOS	🔒 16.6 MB	8.0 MB	file	16.6 MB	+0 B (+0.0%)	OK
✅	`packed-program`	macOS	🔒 12.1 MB	5.7 MB	file	12.1 MB	+16 B (+0.0%)	OK
✅	`baml-cli`	Windows	🔒 18.1 MB	8.2 MB	file	18.1 MB	+6.7 KB (+0.0%)	OK
✅	`packed-program`	Windows	🔒 13.0 MB	5.8 MB	file	13.0 MB	+4.8 KB (+0.0%)	OK
✅	`bridge_wasm`	WASM	14.3 MB	🔒 4.1 MB	gzip	4.1 MB	+4.6 KB (+0.1%)	OK

🔒 = the size this artifact is GATED on (ceiling + delta). Binaries gate on file size (installed binary); WASM gates on gzip (download size). The other size is shown for information only.

Generated by cargo size-gate · workflow run

CodeRabbit: the `caller()` repro caught only one of `Errors`' four variants, so the fixture emitted an `extraneous throws` warning alongside the intended return-type mismatch. Make the catch arm exhaustive (`throws string` + a `string =>` arm) so the fixture isolates exactly one diagnostic — the `type mismatch: expected bool, got string` on `return e` — which is the point the regression is meant to pin (no cascade, single clear error).

Latest canary (catch/match-arm `return` support, #3881). Clean merge with no conflicts; workspace compiles and 750 affected-crate tests pass.

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antoniosarosi added this pull request to the merge queue Jun 29, 2026

Merged via the queue into canary with commit 8305061 Jun 29, 2026
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antoniosarosi deleted the antonio/catch-arm-return-error branch June 29, 2026 01:01

codeshaunted added a commit that referenced this pull request Jun 29, 2026

Merge origin/canary into avery/sem-tok-cleanup

5e6a089

Latest canary (catch/match-arm `return` support, #3881). Clean merge with no conflicts; workspace compiles and 750 affected-crate tests pass.

This was referenced Jul 6, 2026

Allow break and continue in match-arm expression position (B-619) #3921

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Preserve trailing line comment on defer statements in baml fmt (B-629) #3919

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fix(lang): support `return` as a diverging expression in catch/match arms#3881

fix(lang): support `return` as a diverging expression in catch/match arms#3881
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