Rollup merge of rust-lang#157025 - Dnreikronos:fix/crate-root-path-help-in-attr-style-diagnostic, r=JonathanBrouwer

Show crate root path in misplaced crate-level attribute diagnostic

Fixes rust-lang#157001

If you put `#![feature(...)]` inside a non-root module, the diagnostic tells you it can only be used at the crate root, but never says where the crate root actually is. Not great if you're new to Rust or working in a big workspace with nested modules.

Now the diagnostic includes a help line with the crate root file path when the misplaced attribute is an inner attribute. Outer attributes already get a span note pointing at what they're applied to, so they don't need this. Path goes through `RemapPathScopeComponents::DIAGNOSTICS` so `--remap-path-prefix` is respected.

```
warning: the `#![feature]` attribute can only be used at the crate root
  --> src/foo.rs:1:1
   |
LL | #![feature(test)]
   | ^^^^^^^^^^^^^^^^^
   |
   = help: the crate root is at `src/main.rs`
```

Author: matthiaskrgr

compiler/rustc_attr_parsing/src/errors.rs

@@ -165,6 +165,9 @@ pub(crate) struct InvalidAttrStyle {
     #[note("this attribute does not have an `!`, which means it is applied to this {$target}")]
     pub target_span: Option<Span>,
     pub target: &'static str,
+    pub crate_root_path: String,
+    #[help("the crate root is at `{$crate_root_path}`")]
+    pub show_crate_root_help: bool,
 }
 
 #[derive(Diagnostic)]

compiler/rustc_attr_parsing/src/target_checking.rs

@@ -5,7 +5,7 @@ use rustc_errors::{DiagArgValue, Diagnostic, MultiSpan, StashKey};
 use rustc_feature::Features;
 use rustc_hir::attrs::AttributeKind;
 use rustc_hir::{AttrItem, Attribute, MethodKind, Target};
-use rustc_span::{BytePos, Span, Symbol, sym};
+use rustc_span::{BytePos, FileName, RemapPathScopeComponents, Span, Symbol, sym};
 
 use crate::AttributeParser;
 use crate::context::AcceptContext;
@@ -186,13 +186,29 @@ impl<'sess> AttributeParser<'sess> {
         let target_span = cx.target_span;
         let attr_span = cx.attr_span;
 
+        let (show_crate_root_help, crate_root_path) = is_used_as_inner
+            .then(|| cx.cx.sess.local_crate_source_file())
+            .flatten()
+            .filter(|src| {
+                !matches!(
+                    cx.cx.sess.source_map().span_to_filename(attr_span),
+                    FileName::Real(ref name) if name == src
+                )
+            })
+            .map(|src| {
+                (true, src.path(RemapPathScopeComponents::DIAGNOSTICS).display().to_string())
+            })
+            .unwrap_or_default();
+
         cx.emit_lint(
             rustc_session::lint::builtin::UNUSED_ATTRIBUTES,
             crate::errors::InvalidAttrStyle {
                 name,
                 is_used_as_inner,
                 target_span: (!is_used_as_inner).then_some(target_span),
                 target: target.name(),
+                crate_root_path,
+                show_crate_root_help,
             },
             attr_span,
         );

tests/ui/attributes/auxiliary/submod.rs

@@ -0,0 +1 @@
+#![crate_name = "bar"]

tests/ui/attributes/crate-root-path-in-different-file.rs

@@ -0,0 +1,8 @@
+#![deny(unused_attributes)]
+
+#[path = "auxiliary/submod.rs"]
+mod submod;
+
+fn main() {}
+
+//~? ERROR the `#![crate_name]` attribute can only be used at the crate root

tests/ui/attributes/crate-root-path-in-different-file.stderr

@@ -0,0 +1,15 @@
+error: the `#![crate_name]` attribute can only be used at the crate root
+  --> $DIR/auxiliary/submod.rs:1:1
+   |
+LL | #![crate_name = "bar"]
+   | ^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: the crate root is at `$DIR/crate-root-path-in-different-file.rs`
+note: the lint level is defined here
+  --> $DIR/crate-root-path-in-different-file.rs:1:9
+   |
+LL | #![deny(unused_attributes)]
+   |         ^^^^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+