Lay the foundation for diagnostics in ty lowering, and implement a first diagnostic

The diagnostic implemented is a simple one (E0109). It serves as a test for the new foundation.

This commit only implements diagnostics for type in bodies and body-carrying signatures; the next commit will include diagnostics in the rest of the things.

Also fix one weird bug that was detected when implementing this that caused `Fn::(A, B) -> C` (which is a valid, if bizarre, alternative syntax to `Fn(A, B) -> C` to lower incorrectly.

And also fix a maybe-bug where parentheses were sneaked into a code string needlessly; this was not detected until now because the parentheses were removed (by the make-AST family API), but with a change in this commit they are now inserted. So fix that too.

crates/hir-def/src/path/lower.rs

@@ -17,13 +17,31 @@ use crate::{
     type_ref::{LifetimeRef, TypeBound, TypeRef},
 };
 
+#[cfg(test)]
+thread_local! {
+    /// This is used to test `hir_segment_to_ast_segment()`. It's a hack, but it makes testing much easier.
+    pub(super) static SEGMENT_LOWERING_MAP: std::cell::RefCell<rustc_hash::FxHashMap<ast::PathSegment, usize>> = std::cell::RefCell::default();
+}
+
 /// Converts an `ast::Path` to `Path`. Works with use trees.
 /// It correctly handles `$crate` based path from macro call.
+// If you modify the logic of the lowering, make sure to check if `hir_segment_to_ast_segment()`
+// also needs an update.
 pub(super) fn lower_path(ctx: &mut LowerCtx<'_>, mut path: ast::Path) -> Option<Path> {
     let mut kind = PathKind::Plain;
     let mut type_anchor = None;
     let mut segments = Vec::new();
     let mut generic_args = Vec::new();
+    #[cfg(test)]
+    let mut ast_segments = Vec::new();
+    #[cfg(test)]
+    let mut ast_segments_offset = 0;
+    #[allow(unused_mut)]
+    let mut push_segment = |_segment: &ast::PathSegment, segments: &mut Vec<Name>, name| {
+        #[cfg(test)]
+        ast_segments.push(_segment.clone());
+        segments.push(name);
+    };
     loop {
         let segment = path.segment()?;
 
@@ -34,6 +52,10 @@ pub(super) fn lower_path(ctx: &mut LowerCtx<'_>, mut path: ast::Path) -> Option<
         match segment.kind()? {
             ast::PathSegmentKind::Name(name_ref) => {
                 if name_ref.text() == "$crate" {
+                    if path.qualifier().is_some() {
+                        // FIXME: Report an error.
+                        return None;
+                    }
                     break kind = resolve_crate_root(
                         ctx.db.upcast(),
                         ctx.span_map().span_for_range(name_ref.syntax().text_range()).ctx,
@@ -56,10 +78,10 @@ pub(super) fn lower_path(ctx: &mut LowerCtx<'_>, mut path: ast::Path) -> Option<
                     generic_args.resize(segments.len(), None);
                     generic_args.push(args);
                 }
-                segments.push(name);
+                push_segment(&segment, &mut segments, name);
             }
             ast::PathSegmentKind::SelfTypeKw => {
-                segments.push(Name::new_symbol_root(sym::Self_.clone()));
+                push_segment(&segment, &mut segments, Name::new_symbol_root(sym::Self_.clone()));
             }
             ast::PathSegmentKind::Type { type_ref, trait_ref } => {
                 assert!(path.qualifier().is_none()); // this can only occur at the first segment
@@ -81,6 +103,10 @@ pub(super) fn lower_path(ctx: &mut LowerCtx<'_>, mut path: ast::Path) -> Option<
                         kind = mod_path.kind;
 
                         segments.extend(mod_path.segments().iter().cloned().rev());
+                        #[cfg(test)]
+                        {
+                            ast_segments_offset = mod_path.segments().len();
+                        }
                         if let Some(path_generic_args) = path_generic_args {
                             generic_args.resize(segments.len() - num_segments, None);
                             generic_args.extend(Vec::from(path_generic_args).into_iter().rev());
@@ -112,10 +138,18 @@ pub(super) fn lower_path(ctx: &mut LowerCtx<'_>, mut path: ast::Path) -> Option<
                 }
             }
             ast::PathSegmentKind::CrateKw => {
+                if path.qualifier().is_some() {
+                    // FIXME: Report an error.
+                    return None;
+                }
                 kind = PathKind::Crate;
                 break;
             }
             ast::PathSegmentKind::SelfKw => {
+                if path.qualifier().is_some() {
+                    // FIXME: Report an error.
+                    return None;
+                }
                 // don't break out if `self` is the last segment of a path, this mean we got a
                 // use tree like `foo::{self}` which we want to resolve as `foo`
                 if !segments.is_empty() {
@@ -162,6 +196,13 @@ pub(super) fn lower_path(ctx: &mut LowerCtx<'_>, mut path: ast::Path) -> Option<
         }
     }
 
+    #[cfg(test)]
+    {
+        ast_segments.reverse();
+        SEGMENT_LOWERING_MAP
+            .with_borrow_mut(|map| map.extend(ast_segments.into_iter().zip(ast_segments_offset..)));
+    }
+
     let mod_path = Interned::new(ModPath::from_segments(kind, segments));
     if type_anchor.is_none() && generic_args.is_empty() {
         return Some(Path::BarePath(mod_path));
@@ -181,6 +222,41 @@ pub(super) fn lower_path(ctx: &mut LowerCtx<'_>, mut path: ast::Path) -> Option<
     }
 }
 
+/// This function finds the AST segment that corresponds to the HIR segment
+/// with index `segment_idx` on the path that is lowered from `path`.
+pub fn hir_segment_to_ast_segment(path: &ast::Path, segment_idx: u32) -> Option<ast::PathSegment> {
+    // Too tightly coupled to `lower_path()`, but unfortunately we cannot decouple them,
+    // as keeping source maps for all paths segments will have a severe impact on memory usage.
+
+    let mut segments = path.segments();
+    if let Some(ast::PathSegmentKind::Type { trait_ref: Some(trait_ref), .. }) =
+        segments.clone().next().and_then(|it| it.kind())
+    {
+        segments.next();
+        return find_segment(trait_ref.path()?.segments().chain(segments), segment_idx);
+    }
+    return find_segment(segments, segment_idx);
+
+    fn find_segment(
+        segments: impl Iterator<Item = ast::PathSegment>,
+        segment_idx: u32,
+    ) -> Option<ast::PathSegment> {
+        segments
+            .filter(|segment| match segment.kind() {
+                Some(
+                    ast::PathSegmentKind::CrateKw
+                    | ast::PathSegmentKind::SelfKw
+                    | ast::PathSegmentKind::SuperKw
+                    | ast::PathSegmentKind::Type { .. },
+                )
+                | None => false,
+                Some(ast::PathSegmentKind::Name(name)) => name.text() != "$crate",
+                Some(ast::PathSegmentKind::SelfTypeKw) => true,
+            })
+            .nth(segment_idx as usize)
+    }
+}
+
 pub(super) fn lower_generic_args(
     lower_ctx: &mut LowerCtx<'_>,
     node: ast::GenericArgList,