Fix issue #41652

[kennytm]

Fix issue #41652. Don't print anything in render_source_line() if no source code is given.

(cc @jonathandturner#34789)

[rust-highfive]

r? @arielb1

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[kennytm]

The error happens because imported modules have no associated source code. As I mentioned in #41652, on stable this leads to a span pointing nowhere 👻

note: candidate #1 is defined in the trait `b::Tr`
--> b/src/lib.rs:11:5
|
11 | | _____^ starting here...
12 | |
| |__________________________^ ...ending here
= help: to disambiguate the method call, write `b::Tr::read(3)` instead

and causes ICE after #41245 because of source_string[0..ann.start_col] (where source_string == "").

In this PR, the error will be printed as

note: candidate #1 is defined in the trait `b::Tr`
--> /full/path/to/b/src/lib.rs:11:5
|
= help: to disambiguate the method call, write `b::Tr::read(3)` instead

Note the line number is still printed, but without any source code following it. While this is certainly better than the current situation, I don't know if it is good enough.

Alternatively we could ensure FileMap actually contains the source code when reporting error, but I don't see an easy way to do so without heavily modifying libsyntax ☺️.

[arielb1]

Could you have a UI test for that?

r? @jonathandturner

[kennytm]

@arielb1 Added (ce92a54). In that case, should I just remove the compile-fail test?

[arielb1]

Yeah. There's no need for duplicate ui + cfail tests. Just a UI test so we can make sure the output is sane.

[arielb1]

@jonathandturner can you review this change?

[sophiajt]

This seems better than what we have, so we may want to approve just based on that. How difficult would it be to have this instead?

note: candidate #1 is defined in the trait `b::Tr`
help: to disambiguate the method call, write `b::Tr::read(3)` instead

Since starting to draw the span but not drawing anything doesn't seem to help the user much.

[kennytm]

@jonathandturner Looks like skipping the whole iteration works. The output is now

error: no method named `f` found fortype `{integer}` in the current scope
--> $DIR/issue_41652.rs:19:11
|
19 | 3.f()
| ^
|
= note: found the following associated functions; to be usedasmethods, functions must have a `self` parameter
note:candidate #1 is defined in the trait `issue_41652_b::Tr`
= help: to disambiguate the method call, write `issue_41652_b::Tr::f(3)` instead
error:aborting due to previous error

[Mark-Simulacrum]

@jonathandturner Just checking in here, looks like this PR is ready for another review.

[sophiajt]

Great!

@bors r+

[bors]

📌 Commit 81bfdc8 has been approved by jonathandturner

[bors]

⌛ Testing commit 81bfdc8 with merge 5b31bf8...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: jonathandturner
Pushing 5b31bf8 to master...