lang_tester

This crate provides a simple language testing framework designed to help when you are testing things like compilers and virtual machines. It allows users to embed simple tests for process success/failure and for stderr/stdout inside a source file. It is loosely based on the compiletest_rs crate, but is much simpler (and hence sometimes less powerful), and designed to be used for testing non-Rust languages too.

For example, a Rust language tester, loosely in the spirit of compiletest_rs, looks as follows:

use std::{path::PathBuf, process::Command};

use lang_tester::LangTester;
use tempdir::TempDir;

fn main() {
    // We use rustc to compile files into a binary: we store those binary files
    // into `tempdir`. This may not be necessary for other languages.
    let tempdir = TempDir::new("rust_lang_tester").unwrap();
    LangTester::new()
        .test_dir("examples/rust_lang_tester/lang_tests")
        // Only use files named `*.rs` as test files.
        .test_file_filter(|p| p.extension().unwrap().to_str().unwrap() == "rs")
        // Extract the first sequence of commented line(s) as the tests.
        .test_extract(|s| {
            Some(
                s.lines()
                    // Skip non-commented lines at the start of the file.
                    .skip_while(|l| !l.starts_with("//"))
                    // Extract consecutive commented lines.
                    .take_while(|l| l.starts_with("//"))
                    .map(|l| &l[2..])
                    .collect::<Vec<_>>()
                    .join("\n"),
            )
        })
        // We have two test commands:
        //   * `Compiler`: runs rustc.
        //   * `Run-time`: if rustc does not error, and the `Compiler` tests
        //     succeed, then the output binary is run.
        .test_cmds(move |p| {
            // Test command 1: Compile `x.rs` into `tempdir/x`.
            let mut exe = PathBuf::new();
            exe.push(&tempdir);
            exe.push(p.file_stem().unwrap());
            let mut compiler = Command::new("rustc");
            compiler.args(&["-o", exe.to_str().unwrap(), p.to_str().unwrap()]);
            // Test command 2: run `tempdir/x`.
            let runtime = Command::new(exe);
            vec![("Compiler", compiler), ("Run-time", runtime)]
        })
        .run();
}

This defines a lang tester that uses all *.rs files in a given directory as test files, running two test commands against them: Compiler (i.e. rustc); and Run-time (the compiled binary).

Users can then write test files such as the following:

// Compiler:
//   stderr:
//     warning: unused variable: `x`
//       ...unused_var.rs:12:9
//       ...
//
// Run-time:
//   stdout: Hello world
fn main() {
    let x = 0;
    println!("Hello world");
}

Test data is specified with a two-level indentation syntax: the outer most level of indentation defines a test command (multiple command names can be specified, as in the above); the inner most level of indentation defines alterations to the general command or sub-tests. Each test command must define at least one sub-test:

• status: <success|failure|<int>>, where success and failure map to platform specific notions of a command completing successfully or unsuccessfully respectively and <int> is a signed integer checking for a specific exit code on platforms that support it. If not specified, defaults to success.
• stderr: [<string>], stdout: [<string>] are matched strings against a command's stderr or stdout. The special string ... can be used as a simple wildcard: if a line consists solely of ..., it means "match zero or more lines"; if a line begins with ..., it means "match the remainder of the line only"; if a line ends with ..., it means "match the start of the line only". A line may start and end with .... Note that stderr/stdout matches ignore leading/trailing whitespace and newlines, but are case sensitive. If not specified, defaults to ... (i.e. match anything). Note that the empty string matches only the empty string so e.g. stderr: on its own means that a command's stderr muct not contain any output.

Test commands can alter the general command by specifying zero or more of the following:

• extra-args: <arg 1> [... <arg n>], where each space separated argument will be appended, in order, to those arguments specified as part of the test_cmds function.

The above file thus contains 4 meaningful tests, two specified by the user and two implied by defaults: the Compiler should succeed (e.g. return a 0 exit code when run on Unix), and its stderr output should warn about an unused variable on line 12; and the resulting binary should succeed produce Hello world on stdout.

lang_tester's output is deliberately similar to Rust's normal testing output. Running the example rust_lang_tester in this crate produces the following output:

$ cargo run --example=rust_lang_tester
   Compiling lang_tester v0.1.0 (/home/ltratt/scratch/softdev/lang_tester)
    Finished dev [unoptimized + debuginfo] target(s) in 3.49s
     Running `target/debug/examples/rust_lang_tester`

running 4 tests
test lang_tests::no_main ... ok
test lang_tests::unknown_var ... ok
test lang_tests::unused_var ... ok
test lang_tests::exit_code ... ok

test result: ok. 4 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

If you want to run a subset of tests, you can specify simple filters which use substring match to run a subset of tests:

$ cargo run --example=rust_lang_tester var
   Compiling lang_tester v0.1.0 (/home/ltratt/scratch/softdev/lang_tester)
    Finished dev [unoptimized + debuginfo] target(s) in 3.37s
     Running `target/debug/examples/rust_lang_tester var`

running 2 tests
test lang_tests::unknown_var ... ok
test lang_tests::unused_var ... ok

test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 2 filtered out

Integration with Cargo.

Tests created with lang_tester can be used as part of an existing test suite and can be run with the cargo test command. For example, if the Rust source file that runs your lang tests is lang_tests/run.rs then add the following to your Cargo.toml:

[[test]]
name = "lang_tests"
path = "lang_tests/run.rs"
harness = false