Provide more information on duplicate lang item error.

[ehuss]

This gives some notes on the location of the files where the lang items were loaded from. Some duplicate lang item errors can be a little confusing, and this might help in diagnosing what has happened.

Here's an example when hitting a bug with Cargo's build-std:

error: duplicate lang item in crate `core` (which `rustc_std_workspace_core` depends on): `try`.
  |
  = note: the lang item is first defined in crate `core` (which `z10` depends on)
  = note: first definition in `core` loaded from /Users/eric/Proj/rust/cargo/scratch/z10/target/target/debug/deps/libcore-a764da499c7385f4.rmeta
  = note: second definition in `core` loaded from /Users/eric/Proj/rust/cargo/scratch/z10/target/target/debug/deps/libcore-5b082675aea34986.rmeta

[rust-highfive]

r? @matthewjasper

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[ehuss]

This is somewhat hacky, as I'm not too familiar with the compiler code (I never know the proper ways to get information from one place to another). I'm not sure if this will actually be helpful to anyone besides myself, so I wanted to open this PR to see if anyone might see some use in it.

cc @alexcrichton, you may or may not have an opinion on the usefulness here.

[alexcrichton]

👍 from a usefulness perspective, seems handy to have!

[matthewjasper]

crate_extern_paths should be a query. I've left a couple of comments to give some explanation on how to do that.
I also have a nit about the error message.

[matthewjasper]

For the local crate the note should be "{} definition in the local crate".

[ehuss]

Ok, tweaked the wording. I kept the crate name, since I find that can be useful at times.

[matthewjasper]

Move this into the provide macro above.

[matthewjasper]

This should be a query: you can declare it be copying this example and updating the signature and description

rust/src/librustc_middle/query/mod.rs

Lines 1035 to 1038 in 7adbc0d

	query extra_filename(_: CrateNum) -> String {
	eval_always
	desc { "looking up the extra filename for a crate" }
	}

[ehuss]

@matthewjasper Thanks for taking a look!

Is there some guidance on what should be a query and what should be a function? I assumed queries were mainly used for caching, and I assumed in this case that it wouldn't be useful since this only appears in an error.

I'm also a bit confused on how to define the query. I'm getting all sorts of errors that I do not understand how to resolve. In particular:

error[E0412]: cannot find type `PathBuf` in this scope
    --> src/librustc_middle/query/mod.rs:1046:54
     |
38   | / rustc_queries! {
39   | |     Other {
40   | |         query trigger_delay_span_bug(key: DefId) -> () {
41   | |             desc { "trigger a delay span bug" }
...    |
1046 | |         query crate_extern_paths(_: CrateNum) -> Vec<PathBuf> {
     | |                                                      ^^^^^^^ not found in this scope
...    |
1446 | |     }
1447 | | }
     | |_- in this expansion of `rustc_query_append!`
     |
    ::: src/librustc_middle/ty/query/mod.rs:104:1
     |
104  |   rustc_query_append! { [define_queries!][<'tcx>] }
     |   ------------------------------------------------- in this macro invocation
     |
help: consider importing this struct
     |
1    | use std::path::PathBuf;
     |

warning: unused import: `std::path::PathBuf`
  --> src/librustc_middle/query/mod.rs:17:5
   |
17 | use std::path::PathBuf;
   |     ^^^^^^^^^^^^^^^^^^
   |
   = note: `#[warn(unused_imports)]` on by default

error[E0283]: type annotations needed
    --> src/librustc_middle/ty/query/plumbing.rs:516:30
     |
237  |  /     macro_rules! define_queries {
238  |  |         (<$tcx:tt> $($category:tt {
239  |  |             $($(#[$attr:meta])* [$($modifiers:tt)*] fn $name:ident: $node:ident($($K:tt)*) -> $V:ty,)*
240  |  |         },)*) => {
241  | /|             define_queries_inner! { <$tcx>
242  | ||                 $($( $(#[$attr])* category<$category> [$($modifiers)*] fn $name: $node($($K)*) -> $V,)*)*
243  | ||             }
     | ||_____________- in this macro invocation (#3)
244  |  |         }
245  |  |     }
     |  |_____- in this expansion of `define_queries!` (#2)
...
252  | /      macro_rules! define_queries_inner {
253  | |          (<$tcx:tt>
254  | |           $($(#[$attr:meta])* category<$category:tt>
255  | |              [$($modifiers:tt)*] fn $name:ident: $node:ident($($K:tt)*) -> $V:ty,)*) => {
...    |
265  | /              define_queries_struct! {
266  |                    tcx: $tcx,
267  |                    input: ($(([$($modifiers)*] [$($attr)*] [$name]))*)
268  | |              }
     | |______________- in this macro invocation (#4)
...
485  | |          }
486  | |      }
     | |______- in this expansion of `define_queries_inner!` (#3)
487  |
488  | /      macro_rules! define_queries_struct {
489  | |          (tcx: $tcx:tt,
490  | |           input: ($(([$($modifiers:tt)*] [$($attr:tt)*] [$name:ident]))*)) => {
491  | |              pub struct Queries<$tcx> {
...    |
516  | |                          $($name: Default::default()),*
     | |                                   ^^^^^^^^^^^^^^^^ cannot infer type
...    |
536  | |          };
537  | |      }
     | |______- in this expansion of `define_queries_struct!` (#4)
     |
    ::: src/librustc_middle/ty/query/mod.rs:104:1
     |
104  |        rustc_query_append! { [define_queries!][<'tcx>] }
     |        ------------------------------------------------- in this macro invocation (#1)
     |
    ::: src/librustc_middle/query/mod.rs:38:1
     |
38   |      / rustc_queries! {
39   |      |     Other {
40   |      |         query trigger_delay_span_bug(key: DefId) -> () {
41   |      |             desc { "trigger a delay span bug" }
...         |
1446 |      |     }
1447 |      | }
     |      | -
     |      | |
     |      |_in this expansion of `rustc_query_append!` (#1)
     |        in this macro invocation (#2)
     |
     = note: cannot satisfy `_: std::default::Default`
     = note: required by `std::default::Default::default`

The first error is very strange, since I am using PathBuf in the same file, I don't know why it can't find it.

The second one is completely opaque to me, perhaps it is a consequence of the first error?

[matthewjasper]

The use needs to be in src/librustc_middle/ty/query/mod.rs, not src/librustc_middle/query/mod.rs.

I couldn't find an explanation of what should be a query. Queries are used both for caching and tracking dependencies for incremental: queries are expected to be depend only on their inputs and the results of any queries that they call. For data that exists prior to the query system, the current approach is to make make the public method to access the data an eval_always query.

[ehuss]

Oh, thanks! In hindsight that was obvious, I'm not sure how I missed that.

[matthewjasper]

@bors r+

[bors]

📌 Commit 1b3ef66 has been approved by matthewjasper