COBOL to Rust Migration - Automated Converter Tool
Convert COBOL programs to Rust using a full compiler pipeline with semantic analysis. Desktop tool for Windows, macOS, and Linux, with professional migration services for safety-critical and performance-sensitive systems.
Rust is an increasingly popular target for COBOL migration when memory safety and performance both matter. My COBOL to Rust migration tool uses a full compiler pipeline to convert COBOL source into Rust with structs for group items, sized integer types (i16, i32, i64), match expressions for EVALUATE, and structured function decomposition from COBOL paragraphs and sections. The result is compilable Rust code that runs on any platform Rust supports.
Challenges With COBOL to Rust Migration
Ownership Model Is Fundamentally Different
COBOL uses flat WORKING-STORAGE with implicit access to all data items. Rust enforces strict ownership and borrowing rules. A proper conversion must map COBOL's data model to Rust structs without fighting the borrow checker.
Decimal Precision Needs Attention
COBOL's PIC 9 clauses define exact decimal precision for financial calculations. The Rust generator uses f64 for decimal fields, which may not preserve exact precision for all financial use cases. The Migration Report flags these fields so you can evaluate whether rust_decimal or similar crates are needed.
Control Flow Translation
COBOL's PERFORM ranges, paragraph-level control flow, and GO TO statements have no direct Rust equivalent. Correct conversion requires semantic analysis to produce structured Rust functions and match expressions instead of unsafe workarounds.
What Makes This COBOL to Rust Converter Different
Full Compiler Pipeline
A multi-stage compiler (Lexer, Parser, Semantic Analyzer, Code Generator) builds a complete AST with 36 statement types and 220+ keywords. The Rust generator works from the semantic model, producing structurally correct Rust.
Idiomatic Rust Output
Generated code uses Rust structs for group items, sized integers (i16, i32, i64), String for alphanumeric fields, match expressions for EVALUATE, and structured functions. Reserved words are handled with raw identifiers (r# prefix).
Migration Diagnostics
Every conversion produces a Migration Report flagging EXEC SQL blocks, EXEC CICS interactions, dynamic CALL targets, and decimal precision considerations so you know exactly what needs manual work.
COPY and REPLACE Resolution
The preprocessor resolves all COPY and REPLACE directives before parsing, including nested copybooks up to 10 levels with pseudo-text substitution. No manual copybook flattening needed.
Dual-Pane Editor
COBOL source on the left, generated Rust on the right. Syntax highlighting for both languages, dark and light themes, and instant one-click conversion.
Handles All COBOL Formats
Auto-detects fixed-format (COBOL-85) and free-format source. Column 7 indicators, continuation lines, debug lines, and the >>SOURCE FORMAT FREE directive are all supported.
How COBOL to Rust Migration Works
Load Your COBOL Source
Open any COBOL program in the dual-pane editor. The tool auto-detects the source format and resolves all COPY/REPLACE directives.
Select Rust as Target
Choose Rust from the target language selector. The code generator configures for Rust output with struct definitions and sized type mapping.
Run the Conversion
The compiler pipeline processes your source through lexing, parsing, semantic analysis, and Rust code generation. Output appears instantly in the right pane.
Review the Migration Report
Check flagged items: EXEC SQL blocks that need a Rust database library (sqlx, diesel, rusqlite), decimal precision notes for f64 fields, dynamic CALL targets, and areas requiring manual review.
Compile, Test, and Integrate
Compile the generated Rust with cargo. Address flagged items, implement the data access layer, and add the crates your project needs. For full-service projects, I handle this phase entirely.
What the Rust Output Includes
Rust Source Files
Compilable Rust with struct-based data structures, sized integer types, String for alphanumeric fields, and structured function decomposition from COBOL paragraphs and sections.
Migration Report
Per-program diagnostics: EXEC SQL/CICS/DLI flags, decimal precision warnings for f64 fields, dynamic CALL targets, and all items requiring manual review.
Data Type Mapping
Every COBOL PIC clause mapped to the appropriate Rust type: String for alphanumeric, i16/i32/i64 for numeric, f64 for decimal, and struct for group items.
Program Structure
COBOL SECTIONs and PARAGRAPHs decomposed into Rust functions. PERFORM ranges converted to function calls. EVALUATE statements converted to match expressions.
Resolved Copybooks
All COPY/REPLACE directives resolved and included in the output, with a dependency report showing which copybooks each program uses.
Validation Guidance
Approach for comparing Rust output against original COBOL results, with recommendations for testing critical business logic and decimal-sensitive calculations.
Screenshots
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Frequently Asked Questions About COBOL to Rust Migration
How does the tool handle COBOL packed decimal fields in Rust?
COMP-3 and other decimal fields are converted to f64 in Rust. The Migration Report flags these fields because f64 uses IEEE 754 double precision, which may introduce rounding for some financial calculations. For applications that require exact decimal arithmetic, you can replace f64 with the rust_decimal crate after conversion. The report tells you exactly which fields need attention.
How are COBOL reserved word conflicts handled in Rust?
Rust has its own reserved words (let, match, type, etc.) that may collide with transformed COBOL identifiers. The code generator uses Rust’s raw identifier syntax (r# prefix) to handle these collisions. For example, a COBOL field named TYPE becomes r#type in the Rust output. This is idiomatic Rust and compiles without issues.
Can the tool convert COBOL with embedded SQL to Rust with database access?
The tool detects and flags all EXEC SQL blocks in the Migration Report. Automated SQL-to-Rust database library conversion is not included because the correct approach (sqlx, diesel, rusqlite, or another crate) depends on your target database and architecture. For full-service engagements, I design and implement the Rust data access layer as part of the migration.
Why choose Rust over C++ or Java for COBOL migration?
Choose Rust when you need both memory safety and high performance without a garbage collector. Rust’s ownership model eliminates entire classes of bugs (buffer overflows, use-after-free, data races) at compile time, which is valuable for replacing safety-critical COBOL systems. If your team is more experienced with C++, consider COBOL to C++. If developer ecosystem size and enterprise library support matter more, Java or C# may be better fits. The migration tool supports all six languages.
What COBOL constructs does the Rust generator support?
The Rust generator handles all 36 statement types supported by the compiler pipeline: arithmetic (ADD, SUBTRACT, MULTIPLY, DIVIDE, COMPUTE), control flow (IF/ELSE, EVALUATE/WHEN, PERFORM, GO TO), string operations (STRING, UNSTRING, INSPECT), file I/O (OPEN, CLOSE, READ, WRITE, REWRITE, DELETE, START), and data manipulation (MOVE, INITIALIZE, SET). EVALUATE statements map to Rust match expressions. PERFORM ranges become function calls. See the full list on the COBOL modernization overview page.
Need Full-Service COBOL to Rust Migration?
For large codebases or safety-critical systems, I offer complete migration services including assessment, conversion, data access layer implementation, and output parity testing.
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