Add crate name to "main function not found" error message.

[ehuss]

Fixes #44798 and rust-lang/cargo#4948.

I was wondering if it might be cleaner to update the ui tests to add a simple fn main() {} for the unrelated tests. Let me know if you would prefer that.

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[estebank]

In general, yes, I prefer to add an empty main instead.

As for the change, I think there'd be value in having the path of the file we're looking for the method in a note as well, but don't know how hard that'd be to add. What do you think @ehuss?

[ehuss]

I'm not entirely certain which situations local_crate_source_file is None. What do you think about something like this:

--- a/src/librustc/middle/entry.rs
+++ b/src/librustc/middle/entry.rs
@@ -178,6 +178,9 @@ fn configure_main(this: &mut EntryContext, crate_name: &str) {
             err.emit();
             this.session.abort_if_errors();
         } else {
+            if let Some(ref filename) = this.session.local_crate_source_file {
+                err.note(&format!("consider adding a main function to {}", filename.display()));
+            }
             if this.session.teach(&err.get_code().unwrap()) {
                 err.note("If you don't know the basics of Rust, you can go look to the Rust Book \
                           to get started: https://doc.rust-lang.org/book/");

which would render as:

error[E0601]: main function not found in crate bin_no_main
  |
  = note: consider adding a main function to main-not-found/src/bin/bin_no_main.rs

error: aborting due to previous error

If you want more information on this error, try using "rustc --explain E0601"

[pietroalbini]

Ping from triage! Can @estebank or someone from @rust-lang/compiler reply to @ehuss comment?

[estebank]

@ehuss that seems like what I had in mind. Go ahead and add it to the PR. r=me once it's done.

[ehuss]

Thanks @estebank, I have updated it with the new text, updated the tests as discussed, and added a specific test for E0601.

[nikomatsakis]

Nit: can this be using backticks? e.g.,

`main` function to `{}`

[nikomatsakis]

Nit: backticks please :)

`main` function not found in crate `{}`

[nikomatsakis]

@ehuss nice PR =) left a few nits of my own

[ehuss]

Updated, thanks @nikomatsakis!

[estebank]

@bors r+ rollup

[bors]

📌 Commit 2c07c2d has been approved by estebank

[bors]

☔ The latest upstream changes (presumably #48684) made this pull request unmergeable. Please resolve the merge conflicts.

[estebank]

@bors r+

[bors]

📌 Commit 5257275 has been approved by estebank