Improve spans for indexing expressions

Indexing is similar to method calls in having an arbitrary
left-hand-side and then something on the right, which is the main part
of the expression. Method calls already have a span for that right part,
but indexing does not. This means that long method chains that use
indexing have really bad spans, especially when the indexing panics and
that span in coverted into a panic location.

This does the same thing as method calls for the AST and HIR, storing an
extra span which is then put into the `fn_span` field in THIR.

1 files changed, 7 insertions, 4 deletions

diff --git a/compiler/rustc_parse/src/parser/expr.rs b/compiler/rustc_parse/src/parser/expr.rs
index dc3b131e7f2..1b45a01f8b3 100644
--- a/compiler/rustc_parse/src/parser/expr.rs
+++ b/compiler/rustc_parse/src/parser/expr.rs
@@ -857,7 +857,7 @@ impl<'a> Parser<'a> {
             let msg = format!(
                 "cast cannot be followed by {}",
                 match with_postfix.kind {
-                    ExprKind::Index(_, _) => "indexing",
+                    ExprKind::Index(..) => "indexing",
                     ExprKind::Try(_) => "`?`",
                     ExprKind::Field(_, _) => "a field access",
                     ExprKind::MethodCall(_) => "a method call",
@@ -1304,7 +1304,10 @@ impl<'a> Parser<'a> {
         let index = self.parse_expr()?;
         self.suggest_missing_semicolon_before_array(prev_span, open_delim_span)?;
         self.expect(&token::CloseDelim(Delimiter::Bracket))?;
-        Ok(self.mk_expr(lo.to(self.prev_token.span), self.mk_index(base, index)))
+        Ok(self.mk_expr(
+            lo.to(self.prev_token.span),
+            self.mk_index(base, index, open_delim_span.to(self.prev_token.span)),
+        ))
     }
 
     /// Assuming we have just parsed `.`, continue parsing into an expression.
@@ -3366,8 +3369,8 @@ impl<'a> Parser<'a> {
         ExprKind::Binary(binop, lhs, rhs)
     }
 
-    fn mk_index(&self, expr: P<Expr>, idx: P<Expr>) -> ExprKind {
-        ExprKind::Index(expr, idx)
+    fn mk_index(&self, expr: P<Expr>, idx: P<Expr>, brackets_span: Span) -> ExprKind {
+        ExprKind::Index(expr, idx, brackets_span)
     }
 
     fn mk_call(&self, f: P<Expr>, args: ThinVec<P<Expr>>) -> ExprKind {