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f-string interpolation calls `str` by name, so a user binding named `str` changes every f-string

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
68/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
c
Domain
compilers, testing-qa

Research direction

Start in the f-string block of src/lexer.c and inspect the lowering used by the VM, then compare the ouroboros frontend/AOT path. Run the two reproductions with EIGS_JIT_OFF=1 and use tests/test_fstrings.eigs as the regression-test entry point. Done means the global, defined, and function-local str cases produce the specified result or fail loudly, with both lowering paths aligned.

Written by the indexing model from the issue text.

Description

area:runtime-vm bug kind:silent-wrong

The lexer lowers f"a{e}b" to ("a" + (str of (e)) + "b") using an ordinary TOK_IDENT "str" (src/lexer.c, f-string block). That identifier is resolved like any other name at run time. A program that binds str therefore changes, or breaks, every f-string evaluated in that scope. Nothing in the source of the f-string mentions str.

Reproduce (origin/main 94c4c5b, release build)

define str(x) as:
    return "HIJACK"
print of f"<{5}>"

Actual: exit 0, stdout <HIJACK>. Expected: <5> (or a loud error, if rebinding str is to be forbidden).

str is 3
print of f"<{str}>"

Actual: exit 1, Error line 2: cannot call num. The failing call is one the user never wrote.

The first case is a silent wrong answer (PRINCIPLES.md §2). A str local inside a function body does the same thing to every f-string in that function.

Done when

  • f-string interpolation converts values with the builtin conversion no matter what str is bound to in the enclosing scopes (for example, a dedicated opcode or a reserved internal name the user cannot bind). The alternative is a decision, recorded here, that rebinding str is an error.
  • Both programs above print <5> and <3> (or fail loudly at the rebinding, if that is the decision), under the VM and with EIGS_JIT_OFF=1.
  • A regression in tests/test_fstrings.eigs covers a global str, a define str, and a function-local str.
  • The ouroboros frontend/AOT mirror whatever lowering is chosen (it lowers f-strings the same way).

Found while fixing #1244, where the synthetic str token is now marked as having no source span so LSP rename never edits it; this issue is about the runtime meaning of that token.

Dominant language
C
Stars
3
Forks
7
Avg merge
4h 15m
Merged PRs (30d)
106

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