Add support for 'or' expressions

This particular kind of expression is tricky to implement because of a
special case of backwards compatibility with a rarely used Nixpkgs
function also named `or` (see [parser.y] for more details).

[parser.y]: https://github.com/NixOS/nix/blob/f8b30338ac231262bdf19844f044f0572c460048/src/libexpr/parser.y#L376-L378

Implementing support for this fallback case incurs a slight performance
penalty, as shown in the Criterion benchmark results below:

```
parse example.nix       time:   [681.43 us 683.99 us 686.99 us]
                        change: [-0.9655% +2.6817% +6.4750%] (p = 0.16 > 0.05)
                        No change in performance detected.
Found 13 outliers among 100 measurements (13.00%)
  5 (5.00%) high mild
  8 (8.00%) high severe
```

Also, since the `or` keyword is only ever used in projections and never
on its own, we can safely remove the `ExprOr` type and rely entirely on
the `fallback` argument for `ExprProj::new()` instead.

nix-parser/src/ast.rs

@@ -78,13 +78,11 @@ pub enum Expr {
     Let(ExprLet),
     /// `rec { foo = "bar"; }`
     Rec(ExprRec),
-    /// `x.y`
+    /// `x.y`, `x.y or "true"`
     Proj(Box<ExprProj>),
 
     /// `if true then "success" else "failure"`
     If(Box<ExprIf>),
-    /// `foo.bar or "failed"`
-    Or(Box<ExprOr>),
     /// `assert true != false; true`
     Assert(Box<ExprAssert>),
     /// `with foo; foo.attr`
@@ -122,7 +120,6 @@ impl Display for Expr {
             Expr::Proj(ref e) => write!(fmt, "{}", e),
 
             Expr::If(ref e) => write!(fmt, "{}", e),
-            Expr::Or(ref e) => write!(fmt, "{}", e),
             Expr::Assert(ref e) => write!(fmt, "{}", e),
             Expr::With(ref e) => write!(fmt, "{}", e),
 
@@ -167,7 +164,6 @@ impl HasSpan for Expr {
             Expr::Proj(ref e) => e.span(),
 
             Expr::If(ref e) => e.span(),
-            Expr::Or(ref e) => e.span(),
             Expr::Assert(ref e) => e.span(),
             Expr::With(ref e) => e.span(),
 
@@ -1090,55 +1086,6 @@ impl PartialEq for ExprIf {
     }
 }
 
-#[derive(Clone, Debug)]
-pub struct ExprOr {
-    expr: Expr,
-    fallback: Expr,
-    span: Span,
-}
-
-impl ExprOr {
-    pub fn new(expr: Expr, fallback: Expr, span: Span) -> Self {
-        ExprOr {
-            expr,
-            fallback,
-            span,
-        }
-    }
-
-    pub fn expr(&self) -> &Expr {
-        &self.expr
-    }
-
-    pub fn fallback(&self) -> &Expr {
-        &self.fallback
-    }
-}
-
-impl Display for ExprOr {
-    fn fmt(&self, fmt: &mut Formatter) -> FmtResult {
-        write!(fmt, "{} or {}", self.expr, self.fallback)
-    }
-}
-
-impl HasSpan for ExprOr {
-    fn span(&self) -> Span {
-        self.span
-    }
-}
-
-impl From<ExprOr> for Expr {
-    fn from(e: ExprOr) -> Expr {
-        Expr::Or(Box::new(e))
-    }
-}
-
-impl PartialEq for ExprOr {
-    fn eq(&self, other: &Self) -> bool {
-        self.expr == other.expr && self.fallback == other.fallback
-    }
-}
-
 #[derive(Clone, Debug)]
 pub struct ExprAssert {
     cond: Expr,

nix-parser/src/parser/expr.rs

@@ -8,9 +8,10 @@ use nom::multi::many0;
 use nom::sequence::{pair, preceded};
 
 use super::partial::{
-    expect_terminated, map_partial, map_partial_spanned, pair_partial, verify_full, Partial,
+    expect_terminated, map_partial, map_partial_spanned, opt_partial, pair_partial, Partial,
 };
 use super::{tokens, IResult};
+use crate::ast::tokens::Ident;
 use crate::ast::{BinaryOp, Expr, ExprBinary, ExprFnApp, ExprIf, ExprProj, ExprUnary, UnaryOp};
 use crate::error::{Errors, UnexpectedError};
 use crate::lexer::{Token, Tokens};
@@ -213,15 +214,35 @@ fn fn_app(input: Tokens) -> IResult<Partial<Expr>> {
 }
 
 fn project(input: Tokens) -> IResult<Partial<Expr>> {
-    let path = preceded(tokens::dot, verify_full(attr::attr_path));
-    let expr = pair(atomic, opt(path));
-    map(expr, |(base, path)| match path {
-        None => base,
-        Some(path) => base.map(|base| {
-            let span = Span::merge(base.span(), path.span());
-            Expr::Proj(Box::new(ExprProj::new(base, path, None, span)))
-        }),
-    })(input)
+    let (input, atomic) = atomic(input)?;
+
+    if let Ok((remaining, _)) = tokens::dot(input) {
+        let default = alt((project, error, util::error_expr_if(tokens::eof, "<eof>")));
+        let or_default = opt_partial(preceded(tokens::keyword_or, default));
+
+        let (remaining, path) = pair_partial(attr::attr_path, or_default)(remaining)?;
+        let proj = atomic.flat_map(|atomic| {
+            path.map(|(path, default)| {
+                let span = Span::merge(atomic.span(), path.span());
+                match default {
+                    Some(expr) => Expr::from(ExprProj::new(atomic, path, Some(expr), span)),
+                    None => Expr::from(ExprProj::new(atomic, path, None, span)),
+                }
+            })
+        });
+
+        Ok((remaining, proj))
+    } else if let Ok((remaining, or_span)) = tokens::keyword_or(input) {
+        let expr = atomic.map(|atomic| {
+            let arg = Expr::Ident(Ident::from(("or", or_span)));
+            let span = Span::merge(atomic.span(), or_span);
+            Expr::from(ExprFnApp::new(atomic, arg, span))
+        });
+
+        Ok((remaining, expr))
+    } else {
+        Ok((input, atomic))
+    }
 }
 
 fn atomic(input: Tokens) -> IResult<Partial<Expr>> {