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Auto merge of #54317 - Centril:feature/dbg_macro, r=SimonSapin
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Implement the dbg!(..) macro

Implements the `dbg!(..)` macro due to #54306.
cc rust-lang/rfcs#2361

r? @alexcrichton
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bors committed Sep 25, 2018
2 parents 2871872 + e5b9331 commit e7416d5
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120 changes: 120 additions & 0 deletions src/libstd/macros.rs
Original file line number Diff line number Diff line change
Expand Up @@ -220,6 +220,126 @@ macro_rules! eprintln {
})
}

/// A macro for quick and dirty debugging with which you can inspect
/// the value of a given expression. An example:
///
/// ```rust
/// #![feature(dbg_macro)]
///
/// let a = 2;
/// let b = dbg!(a * 2) + 1;
/// // ^-- prints: [src/main.rs:4] a * 2 = 4
/// assert_eq!(b, 5);
/// ```
///
/// The macro works by using the `Debug` implementation of the type of
/// the given expression to print the value to [stderr] along with the
/// source location of the macro invocation as well as the source code
/// of the expression.
///
/// Invoking the macro on an expression moves and takes ownership of it
/// before returning the evaluated expression unchanged. If the type
/// of the expression does not implement `Copy` and you don't want
/// to give up ownership, you can instead borrow with `dbg!(&expr)`
/// for some expression `expr`.
///
/// Note that the macro is intended as a debugging tool and therefore you
/// should avoid having uses of it in version control for longer periods.
/// Use cases involving debug output that should be added to version control
/// may be better served by macros such as `debug!` from the `log` crate.
///
/// # Stability
///
/// The exact output printed by this macro should not be relied upon
/// and is subject to future changes.
///
/// # Panics
///
/// Panics if writing to `io::stderr` fails.
///
/// # Further examples
///
/// With a method call:
///
/// ```rust
/// #![feature(dbg_macro)]
///
/// fn foo(n: usize) {
/// if let Some(_) = dbg!(n.checked_sub(4)) {
/// // ...
/// }
/// }
///
/// foo(3)
/// ```
///
/// This prints to [stderr]:
///
/// ```text,ignore
/// [src/main.rs:4] n.checked_sub(4) = None
/// ```
///
/// Naive factorial implementation:
///
/// ```rust
/// #![feature(dbg_macro)]
///
/// fn factorial(n: u32) -> u32 {
/// if dbg!(n <= 1) {
/// dbg!(1)
/// } else {
/// dbg!(n * factorial(n - 1))
/// }
/// }
///
/// dbg!(factorial(4));
/// ```
///
/// This prints to [stderr]:
///
/// ```text,ignore
/// [src/main.rs:3] n <= 1 = false
/// [src/main.rs:3] n <= 1 = false
/// [src/main.rs:3] n <= 1 = false
/// [src/main.rs:3] n <= 1 = true
/// [src/main.rs:4] 1 = 1
/// [src/main.rs:5] n * factorial(n - 1) = 2
/// [src/main.rs:5] n * factorial(n - 1) = 6
/// [src/main.rs:5] n * factorial(n - 1) = 24
/// [src/main.rs:11] factorial(4) = 24
/// ```
///
/// The `dbg!(..)` macro moves the input:
///
/// ```compile_fail
/// #![feature(dbg_macro)]
///
/// /// A wrapper around `usize` which importantly is not Copyable.
/// #[derive(Debug)]
/// struct NoCopy(usize);
///
/// let a = NoCopy(42);
/// let _ = dbg!(a); // <-- `a` is moved here.
/// let _ = dbg!(a); // <-- `a` is moved again; error!
/// ```
///
/// [stderr]: https://en.wikipedia.org/wiki/Standard_streams#Standard_error_(stderr)
#[macro_export]
#[unstable(feature = "dbg_macro", issue = "54306")]
macro_rules! dbg {
($val:expr) => {
// Use of `match` here is intentional because it affects the lifetimes
// of temporaries - https://stackoverflow.com/a/48732525/1063961
match $val {
tmp => {
eprintln!("[{}:{}] {} = {:#?}",
file!(), line!(), stringify!($val), &tmp);
tmp
}
}
}
}

#[macro_export]
#[unstable(feature = "await_macro", issue = "50547")]
#[allow_internal_unstable]
Expand Down
121 changes: 121 additions & 0 deletions src/test/ui/rfc-2361-dbg-macro/dbg-macro-expected-behavior.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,121 @@
// run-pass
// ignore-cloudabi no processes
// ignore-emscripten no processes

// Tests ensuring that `dbg!(expr)` has the expected run-time behavior.
// as well as some compile time properties we expect.

#![feature(dbg_macro)]

#[derive(Copy, Clone, Debug)]
struct Unit;

#[derive(Copy, Clone, Debug, PartialEq)]
struct Point<T> {
x: T,
y: T,
}

#[derive(Debug, PartialEq)]
struct NoCopy(usize);

fn test() {
let a: Unit = dbg!(Unit);
let _: Unit = dbg!(a);
// We can move `a` because it's Copy.
drop(a);

// `Point<T>` will be faithfully formatted according to `{:#?}`.
let a = Point { x: 42, y: 24 };
let b: Point<u8> = dbg!(Point { x: 42, y: 24 }); // test stringify!(..)
let c: Point<u8> = dbg!(b);
// Identity conversion:
assert_eq!(a, b);
assert_eq!(a, c);
// We can move `b` because it's Copy.
drop(b);

// Test that we can borrow and that successive applications is still identity.
let a = NoCopy(1337);
let b: &NoCopy = dbg!(dbg!(&a));
assert_eq!(&a, b);

// Test involving lifetimes of temporaries:
fn f<'a>(x: &'a u8) -> &'a u8 { x }
let a: &u8 = dbg!(f(&42));
assert_eq!(a, &42);

// Test side effects:
let mut foo = 41;
assert_eq!(7331, dbg!({
foo += 1;
eprintln!("before");
7331
}));
assert_eq!(foo, 42);
}

fn validate_stderr(stderr: Vec<String>) {
assert_eq!(stderr, &[
":23] Unit = Unit",

":24] a = Unit",

":30] Point{x: 42, y: 24,} = Point {",
" x: 42,",
" y: 24",
"}",

":31] b = Point {",
" x: 42,",
" y: 24",
"}",

":40] &a = NoCopy(",
" 1337",
")",

":40] dbg!(& a) = NoCopy(",
" 1337",
")",
":45] f(&42) = 42",

"before",
":50] { foo += 1; eprintln!(\"before\"); 7331 } = 7331",
]);
}

fn main() {
// The following is a hack to deal with compiletest's inability
// to check the output (to stdout) of run-pass tests.
use std::env;
use std::process::Command;

let mut args = env::args();
let prog = args.next().unwrap();
let child = args.next();
if let Some("child") = child.as_ref().map(|s| &**s) {
// Only run the test if we've been spawned as 'child'
test()
} else {
// This essentially spawns as 'child' to run the tests
// and then it collects output of stderr and checks the output
// against what we expect.
let out = Command::new(&prog).arg("child").output().unwrap();
assert!(out.status.success());
assert!(out.stdout.is_empty());

let stderr = String::from_utf8(out.stderr).unwrap();
let stderr = stderr.lines().map(|mut s| {
if s.starts_with("[") {
// Strip `[` and file path:
s = s.trim_start_matches("[");
assert!(s.starts_with(file!()));
s = s.trim_start_matches(file!());
}
s.to_owned()
}).collect();

validate_stderr(stderr);
}
}
5 changes: 5 additions & 0 deletions src/test/ui/rfc-2361-dbg-macro/dbg-macro-feature-gate.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,5 @@
// Feature gate test for `dbg!(..)`.

fn main() {
dbg!(1);
}
11 changes: 11 additions & 0 deletions src/test/ui/rfc-2361-dbg-macro/dbg-macro-feature-gate.stderr
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@@ -0,0 +1,11 @@
error[E0658]: macro dbg! is unstable (see issue #54306)
--> $DIR/dbg-macro-feature-gate.rs:4:5
|
LL | dbg!(1);
| ^^^^^^^^
|
= help: add #![feature(dbg_macro)] to the crate attributes to enable

error: aborting due to previous error

For more information about this error, try `rustc --explain E0658`.
14 changes: 14 additions & 0 deletions src/test/ui/rfc-2361-dbg-macro/dbg-macro-move-semantics.nll.stderr
Original file line number Diff line number Diff line change
@@ -0,0 +1,14 @@
error[E0382]: use of moved value: `a`
--> $DIR/dbg-macro-move-semantics.rs:11:18
|
LL | let _ = dbg!(a);
| ------- value moved here
LL | let _ = dbg!(a);
| ^ value used here after move
|
= note: move occurs because `a` has type `NoCopy`, which does not implement the `Copy` trait
= note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

error: aborting due to previous error

For more information about this error, try `rustc --explain E0382`.
12 changes: 12 additions & 0 deletions src/test/ui/rfc-2361-dbg-macro/dbg-macro-move-semantics.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,12 @@
// Test ensuring that `dbg!(expr)` will take ownership of the argument.

#![feature(dbg_macro)]

#[derive(Debug)]
struct NoCopy(usize);

fn main() {
let a = NoCopy(0);
let _ = dbg!(a);
let _ = dbg!(a);
}
25 changes: 25 additions & 0 deletions src/test/ui/rfc-2361-dbg-macro/dbg-macro-move-semantics.stderr
Original file line number Diff line number Diff line change
@@ -0,0 +1,25 @@
error[E0382]: use of moved value: `a`
--> $DIR/dbg-macro-move-semantics.rs:11:18
|
LL | let _ = dbg!(a);
| ------- value moved here
LL | let _ = dbg!(a);
| ^ value used here after move
|
= note: move occurs because `a` has type `NoCopy`, which does not implement the `Copy` trait
= note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

error[E0382]: use of moved value: `a`
--> $DIR/dbg-macro-move-semantics.rs:11:13
|
LL | let _ = dbg!(a);
| ------- value moved here
LL | let _ = dbg!(a);
| ^^^^^^^ value used here after move
|
= note: move occurs because `a` has type `NoCopy`, which does not implement the `Copy` trait
= note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0382`.
9 changes: 9 additions & 0 deletions src/test/ui/rfc-2361-dbg-macro/dbg-macro-requires-debug.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,9 @@
// Test ensuring that `dbg!(expr)` requires the passed type to implement `Debug`.

#![feature(dbg_macro)]

struct NotDebug;

fn main() {
let _: NotDebug = dbg!(NotDebug);
}
15 changes: 15 additions & 0 deletions src/test/ui/rfc-2361-dbg-macro/dbg-macro-requires-debug.stderr
Original file line number Diff line number Diff line change
@@ -0,0 +1,15 @@
error[E0277]: `NotDebug` doesn't implement `std::fmt::Debug`
--> $DIR/dbg-macro-requires-debug.rs:8:23
|
LL | let _: NotDebug = dbg!(NotDebug);
| ^^^^^^^^^^^^^^ `NotDebug` cannot be formatted using `{:?}`
|
= help: the trait `std::fmt::Debug` is not implemented for `NotDebug`
= note: add `#[derive(Debug)]` or manually implement `std::fmt::Debug`
= note: required because of the requirements on the impl of `std::fmt::Debug` for `&NotDebug`
= note: required by `std::fmt::Debug::fmt`
= note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.

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