Diagnostic improvement of macros #97108
Description
Given the following code:
macro_rules! m {
() => {};
( $name:ident; ($tt:tt)* ) => {
m!{ @ $name }
m!{ $($tt)* }
};
(@ $name:ident) => { struct $name; };
}
m! {
A;
B;
}
m! { C; }The current output is:
error: no rules expected the token `B`
--> src/lib.rs:12:5
|
1 | macro_rules! m {
| -------------- when calling this macro
...
12 | B;
| ^ no rules expected this token in macro call
error: unexpected end of macro invocation
--> src/lib.rs:15:8
|
1 | macro_rules! m {
| -------------- when calling this macro
...
15 | m! { C; }
| ^ missing tokens in macro arguments
error: could not compile `playground` due to 2 previous errors
Ideally the output should look like:
error: no rules expected the token `B`
--> src/lib.rs:12:5
|
1 | macro_rules! m {
| -------------- when calling this macro
...
12 | B;
| ^ no rules expected this token in macro call
note: expected `(` but found `B`.
--> src/lib.rs:3:20
|
3 | ( $name:ident; ($tt:tt)* ) => {
| ^ expected token `(`.
...
12 | B;
| ^ But found `B`.
help: do you mean this?
--> src/lib.rs:3:20
|
3 | ( $name:ident; ($tt:tt)* ) => {
+ $($tt:tt)* ) => {
| ^ Add `$` here.
error: unexpected end of macro invocation
--> src/lib.rs:15:8
|
1 | macro_rules! m {
| -------------- when calling this macro
...
15 | m! { C; }
| ^ missing tokens in macro arguments
note: missing `(` but found nothing.
--> src/lib.rs:3:20
|
3 | ( $name:ident; ($tt:tt)* ) => {
| ^ expected token `(`.
...
15 | m! { C; }
| ^ missing tokens in macro arguments
help: do you mean this?
--> src/lib.rs:3:20
|
3 | ( $name:ident; ($tt:tt)* ) => {
+ $($tt:tt)* ) => {
| ^ Add `$` here.
error: could not compile `playground` due to 2 previous errors
The expect error message is written by hand.
The original problem comes from here. It has wasted me a couple of hours to find out the error (a missing $ in the macro rules), so I hope the diagnostic message can be improved, to avoid another one being stuck by this.
If the compiler can not report that I missed a $, I hope at least it can report that the next expected token ( doesn't match the actual arguments.
Activity
est31 commentedon May 17, 2022
Right now,
macro_rulesis a macro itself, and also reuses the same logic for parsing macro declarations that normalmacro_rulesmatchers use. That's probably why the errors are so bad. One could think about turningmacro_rulesinto something "properly" parsed, then it can give better errors.