Auto merge of #85427 - ehuss:fix-use-placement, r=jackh726

Fix use placement for suggestions near main.

This fixes an edge case for the suggestion to add a `use`. When running with `--test`, the `main` function will be annotated with an `#[allow(dead_code)]` attribute. The `UsePlacementFinder` would end up using the dummy span of that synthetic attribute. If there are top-level inner attributes, this would place the `use` in the wrong position. The solution here is to ignore attributes with dummy spans.

In the process of working on this, I discovered that the `use_suggestion_placement` test was broken. `UsePlacementFinder` is unaware of active attributes. Attributes like `#[derive]` don't exist in the AST since they are removed. Fixing that is difficult, since the AST does not retain enough information. I considered trying to place the `use` towards the top of the module after any `extern crate` items, but I couldn't find a way to get a span for the start of a module block (the `mod` span starts at the `mod` keyword, and it seems tricky to find the spot just after the opening bracket and past inner attributes). For now, I just put some comments about the issue. This appears to have been a known issue in #44215 where the test for it was introduced, and the fix seemed to be deferred to later.

1 files changed, 8 insertions, 8 deletions

diff --git a/compiler/rustc_resolve/src/lib.rs b/compiler/rustc_resolve/src/lib.rs
index 00f0fe4a288..83e904eb16b 100644
--- a/compiler/rustc_resolve/src/lib.rs
+++ b/compiler/rustc_resolve/src/lib.rs
@@ -335,15 +335,15 @@ impl UsePlacementFinder {
                     if self.span.map_or(true, |span| item.span < span)
                         && !item.span.from_expansion()
                     {
+                        self.span = Some(item.span.shrink_to_lo());
                         // don't insert between attributes and an item
-                        if item.attrs.is_empty() {
-                            self.span = Some(item.span.shrink_to_lo());
-                        } else {
-                            // find the first attribute on the item
-                            for attr in &item.attrs {
-                                if self.span.map_or(true, |span| attr.span < span) {
-                                    self.span = Some(attr.span.shrink_to_lo());
-                                }
+                        // find the first attribute on the item
+                        // FIXME: This is broken for active attributes.
+                        for attr in &item.attrs {
+                            if !attr.span.is_dummy()
+                                && self.span.map_or(true, |span| attr.span < span)
+                            {
+                                self.span = Some(attr.span.shrink_to_lo());
                             }
                         }
                     }