REPL: warn when a non-last module-level expression result is discarded (BT-979)

[jamesc]

Summary

• Enable the effect-free statement lint on module.expressions by default, so REPL users see warnings when expression results are silently discarded (e.g., #{#x => 1} #{#y => 2} where the first map literal is discarded)
• Add skip_module_expression_lint flag to CompilerOptions so bootstrap-test compilation can opt out (test fixtures intentionally use top-level expressions with // => assertions)
• Expand REPL warning filter to include Severity::Lint diagnostics

Linear issue: https://linear.app/beamtalk/issue/BT-979

Key Changes

• CompilerOptions gains skip_module_expression_lint: bool (default false)
• check_effect_free_statements now checks module.expressions unless the flag is set
• REPL compiler port surfaces Severity::Lint warnings to users
• Bootstrap-test compilation sets the flag to true to suppress false positives
• 4 new tests, 1 updated test, all 4,838 tests pass

Test plan

• Existing effect-free lint tests pass
• New module-level lint tests verify detection and opt-out
• Bootstrap-test compilation doesn't produce false positives
• Full CI passes (just ci)

🤖 Generated with Claude Code

Summary by CodeRabbit

• Improvements
  • REPL now displays linting hints for effect-free statements, providing enhanced feedback during interactive development.
  • Adjusted compiler linting behavior for module-level expressions in test fixtures.

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between 213f695 and 28bacd6.

📒 Files selected for processing (7)

• crates/beamtalk-cli/src/commands/test_stdlib.rs
• crates/beamtalk-cli/src/main.rs
• crates/beamtalk-compiler-port/src/main.rs
• crates/beamtalk-core/src/lib.rs
• crates/beamtalk-core/src/lint/effect_free_statement.rs
• crates/beamtalk-core/src/semantic_analysis/mod.rs
• crates/beamtalk-core/src/semantic_analysis/validators.rs

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📝 Walkthrough

Walkthrough

Adds a new skip_module_expression_lint boolean field to CompilerOptions to conditionally suppress linting of top-level module expressions. The flag is threaded through the semantic analysis pipeline and set to true during test fixture compilation while defaulting to false elsewhere, allowing effect-free statement checks on module-level expressions to be opted out.

Changes

Cohort / File(s)	Summary
CompilerOptions Struct Definition crates/beamtalk-core/src/lib.rs	Added public field skip_module_expression_lint: bool to control effect-free lint checks on module expressions (defaults to false).
Compilation Options Initialization crates/beamtalk-cli/src/main.rs, crates/beamtalk-compiler-port/src/main.rs	Updated CompilerOptions initialization to use ..Default::default() pattern and wired skip_module_expression_lint flag from options through compilation pipeline.
Effect-Free Statement Linting crates/beamtalk-core/src/semantic_analysis/validators.rs, crates/beamtalk-core/src/lint/effect_free_statement.rs	Updated check_effect_free_statements() signature to accept skip_module_expression_lint boolean parameter; added tests verifying module-level expression linting behavior with and without the flag.
Semantic Analysis Pipeline crates/beamtalk-core/src/semantic_analysis/mod.rs	Threaded skip_module_expression_lint flag through internal analyse_full() function and updated callers (analyse_with_options) to propagate the option from CompilerOptions.
Test Fixture Compilation crates/beamtalk-cli/src/commands/test_stdlib.rs	Set skip_module_expression_lint: true when compiling test fixtures to suppress module-level expression lint checks (BT-979).

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Possibly related PRs

• #990 — Introduces the effect-free-statement lint that this PR extends with conditional skipping of module-expression checks.
• #994 — Refactors module expression traversal in validators.rs, directly affecting the same code paths modified here.
• #992 — Modifies the semantic analysis pipeline entry point (analyse_full) to add a separate lint pass, sharing infrastructure with this PR's analysis-level changes.

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 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 main change: enabling linting to warn about discarded non-last module-level expressions in the REPL.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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✨ Finishing Touches

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• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch worktree-BT-979

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