Fix stray notes when the source code is not available

[FabianWolff]

Fixes #87060. To reproduce it with a local build of rustc, you have to copy the compiler (e.g. build/x86_64-unknown-linux-gnu/stage1/) somewhere and then rename the compiler source directory (maybe there is a smarter way as well). Then, rustc won't find the standard library sources and report stray notes such as

note: deref defined here

with no location for "here". Another example I've found is this:

use std::ops::Add;

fn foo<T: Add<Output=()>>(x: T) {
    x + x;
}

fn main() {}

error[E0382]: use of moved value: `x`
  --> binop.rs:4:9
   |
3  | fn foo<T: Add<Output=()>>(x: T) {
   |                           - move occurs because `x` has type `T`, which does not implement the `Copy` trait
4  |     x + x;
   |     ----^
   |     |   |
   |     |   value used here after move
   |     `x` moved due to usage in operator
   |
note: calling this operator moves the left-hand side
help: consider further restricting this bound
   |
3  | fn foo<T: Add<Output=()> + Copy>(x: T) {
   |                          ^^^^^^

error: aborting due to previous error

where, again, the note is supposed to point somewhere but doesn't. I have fixed this by checking whether the corresponding source code is actually available before emitting the note.

[rust-highfive]

r? @estebank

(rust-highfive has picked a reviewer for you, use r? to override)

[bors]

☔ The latest upstream changes (presumably #80522) made this pull request unmergeable. Please resolve the merge conflicts.

[jackh726]

Any way to write a test for this?

[estebank]

r=me after rebase

[estebank]

It would be nice if we could add something akin to span_note_with_alternative so that we could provide a fallback span.

[jackh726]

@bors r=estebank

[bors]

📌 Commit 57fcb2e has been approved by estebank

[bors]

🌲 The tree is currently closed for pull requests below priority 100. This pull request will be tested once the tree is reopened.

[estebank]

Any way to write a test for this?

It would likely require a Makefile test that uses extern crates and mucks around with its path after building.