Add e substitution flag#413

[PranavRJoshi]

This PR introduces e substitution mentioned in #404 and supersedes #413.

What this does

Implements the e substitute flag, which executes the result of the substitution as a shell command and replaces the pattern space with stdout (minus trailing newline).

$ echo a | ./target/release/sed  's/.*/echo hi/e'
hi

$ echo a | ./target/release/sed --posix 's/.*/echo hi/e'
sed: <script argument 1>:1:14: error: the 'e' substitute flag is not allowed with --posix or --sandbox

Changes

• src/sed/command.rs: new execute: bool field on Substitution
• src/sed/compiler.rs: 'e' arm in compile_subst_flags; rejected at compile time under --posix or --sandbox.
• src/sed/processor.rs: shell execution via /bin/sh -c after substitution

Tests

• Three new unit tests for the flag parser covering the intended path, --posix rejection, and --sandbox rejection.
• Full cargo test run passes.
• GNU testsuite tests still in progress.

[codecov]

Codecov Report

❌ Patch coverage is 58.90411% with 30 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.87%. Comparing base (f083fa0) to head (18dd718).

Files with missing lines	Patch %	Lines
src/sed/processor.rs	0.00%	30 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #424      +/-   ##
==========================================
- Coverage   82.20%   81.87%   -0.34%     
==========================================
  Files          13       13              
  Lines        5542     5604      +62     
  Branches      310      314       +4     
==========================================
+ Hits         4556     4588      +32     
- Misses        983     1013      +30     
  Partials        3        3              

Flag	Coverage Δ
macos_latest	82.61% <63.23%> (-0.29%)	⬇️
ubuntu_latest	82.72% <63.23%> (-0.28%)	⬇️
windows_latest	0.00% <0.00%> (ø)

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[codspeed-hq]

Merging this PR will improve performance by 3.1%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 1 improved benchmark
✅ 10 untouched benchmarks

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	genome_subst	215.5 ms	209 ms	+3.1%

Tip

Curious why this is faster? Comment @codspeedbot explain why this is faster on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing PranavRJoshi:feat/execute-flag (18dd718) with main (f083fa0)}

[PranavRJoshi]

Was looking into GNU's testsuite and the test files that was mentioned in the issue. The file subst-options.sh does not seem to have dedicated e flag tests. I did manage to find some tests for e flag in execute-tests.sh file.