Overhaul `MacArgs::Eq`.

The value in `MacArgs::Eq` is currently represented as a `Token`.
Because of `TokenKind::Interpolated`, `Token` can be either a token or
an arbitrary AST fragment. In practice, a `MacArgs::Eq` starts out as a
literal or macro call AST fragment, and then is later lowered to a
literal token. But this is very non-obvious. `Token` is a much more
general type than what is needed.

This commit restricts things, by introducing a new type `MacArgsEqKind`
that is either an AST expression (pre-lowering) or an AST literal
(post-lowering). The downside is that the code is a bit more verbose in
a few places. The benefit is that makes it much clearer what the
possibilities are (though also shorter in some other places). Also, it
removes one use of `TokenKind::Interpolated`, taking us a step closer to
removing that variant, which will let us make `Token` impl `Copy` and
remove many "handle Interpolated" code paths in the parser.

Things to note:
- Error messages have improved. Messages like this:
  ```
  unexpected token: `"bug" + "found"`
  ```
  now say "unexpected expression", which makes more sense. Although
  arbitrary expressions can exist within tokens thanks to
  `TokenKind::Interpolated`, that's not obvious to anyone who doesn't
  know compiler internals.
- In `parse_mac_args_common`, we no longer need to collect tokens for
  the value expression.

8 files changed, 151 insertions, 66 deletions

diff --git a/compiler/rustc_ast/src/ast.rs b/compiler/rustc_ast/src/ast.rs
index a310828b9fb..bea0482bf73 100644
--- a/compiler/rustc_ast/src/ast.rs
+++ b/compiler/rustc_ast/src/ast.rs
@@ -23,7 +23,7 @@ pub use GenericArgs::*;
 pub use UnsafeSource::*;
 
 use crate::ptr::P;
-use crate::token::{self, CommentKind, Delimiter, Token};
+use crate::token::{self, CommentKind, Delimiter, Token, TokenKind};
 use crate::tokenstream::{DelimSpan, LazyTokenStream, TokenStream, TokenTree};
 
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
@@ -1526,7 +1526,7 @@ impl MacCall {
 }
 
 /// Arguments passed to an attribute or a function-like macro.
-#[derive(Clone, Encodable, Decodable, Debug, HashStable_Generic)]
+#[derive(Clone, Encodable, Decodable, Debug)]
 pub enum MacArgs {
     /// No arguments - `#[attr]`.
     Empty,
@@ -1536,11 +1536,20 @@ pub enum MacArgs {
     Eq(
         /// Span of the `=` token.
         Span,
-        /// "value" as a nonterminal token.
-        Token,
+        /// The "value".
+        MacArgsEq,
     ),
 }
 
+// The RHS of a `MacArgs::Eq` starts out as an expression. Once macro expansion
+// is completed, all cases end up either as a literal, which is the form used
+// after lowering to HIR, or as an error.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub enum MacArgsEq {
+    Ast(P<Expr>),
+    Hir(Lit),
+}
+
 impl MacArgs {
     pub fn delim(&self) -> Option<Delimiter> {
         match self {
@@ -1553,7 +1562,10 @@ impl MacArgs {
         match self {
             MacArgs::Empty => None,
             MacArgs::Delimited(dspan, ..) => Some(dspan.entire()),
-            MacArgs::Eq(eq_span, token) => Some(eq_span.to(token.span)),
+            MacArgs::Eq(eq_span, MacArgsEq::Ast(expr)) => Some(eq_span.to(expr.span)),
+            MacArgs::Eq(_, MacArgsEq::Hir(lit)) => {
+                unreachable!("in literal form when getting span: {:?}", lit);
+            }
         }
     }
 
@@ -1563,7 +1575,23 @@ impl MacArgs {
         match self {
             MacArgs::Empty => TokenStream::default(),
             MacArgs::Delimited(.., tokens) => tokens.clone(),
-            MacArgs::Eq(.., token) => TokenTree::Token(token.clone()).into(),
+            MacArgs::Eq(_, MacArgsEq::Ast(expr)) => {
+                // Currently only literals are allowed here. If more complex expression kinds are
+                // allowed in the future, then `nt_to_tokenstream` should be used to extract the
+                // token stream. This will require some cleverness, perhaps with a function
+                // pointer, because `nt_to_tokenstream` is not directly usable from this crate.
+                // It will also require changing the `parse_expr` call in `parse_mac_args_common`
+                // to `parse_expr_force_collect`.
+                if let ExprKind::Lit(lit) = &expr.kind {
+                    let token = Token::new(TokenKind::Literal(lit.token), lit.span);
+                    TokenTree::Token(token).into()
+                } else {
+                    unreachable!("couldn't extract literal when getting inner tokens: {:?}", expr)
+                }
+            }
+            MacArgs::Eq(_, MacArgsEq::Hir(lit)) => {
+                unreachable!("in literal form when getting inner tokens: {:?}", lit)
+            }
         }
     }
 
@@ -1574,6 +1602,30 @@ impl MacArgs {
     }
 }
 
+impl<CTX> HashStable<CTX> for MacArgs
+where
+    CTX: crate::HashStableContext,
+{
+    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
+        mem::discriminant(self).hash_stable(ctx, hasher);
+        match self {
+            MacArgs::Empty => {}
+            MacArgs::Delimited(dspan, delim, tokens) => {
+                dspan.hash_stable(ctx, hasher);
+                delim.hash_stable(ctx, hasher);
+                tokens.hash_stable(ctx, hasher);
+            }
+            MacArgs::Eq(_eq_span, MacArgsEq::Ast(expr)) => {
+                unreachable!("hash_stable {:?}", expr);
+            }
+            MacArgs::Eq(eq_span, MacArgsEq::Hir(lit)) => {
+                eq_span.hash_stable(ctx, hasher);
+                lit.hash_stable(ctx, hasher);
+            }
+        }
+    }
+}
+
 #[derive(Copy, Clone, PartialEq, Eq, Encodable, Decodable, Debug, HashStable_Generic)]
 pub enum MacDelimiter {
     Parenthesis,
diff --git a/compiler/rustc_ast/src/attr/mod.rs b/compiler/rustc_ast/src/attr/mod.rs
index b14367aa1c2..84654a9f737 100644
--- a/compiler/rustc_ast/src/attr/mod.rs
+++ b/compiler/rustc_ast/src/attr/mod.rs
@@ -3,14 +3,16 @@
 use crate::ast;
 use crate::ast::{AttrId, AttrItem, AttrKind, AttrStyle, Attribute};
 use crate::ast::{Lit, LitKind};
-use crate::ast::{MacArgs, MacDelimiter, MetaItem, MetaItemKind, NestedMetaItem};
+use crate::ast::{MacArgs, MacArgsEq, MacDelimiter, MetaItem, MetaItemKind, NestedMetaItem};
 use crate::ast::{Path, PathSegment};
+use crate::ptr::P;
 use crate::token::{self, CommentKind, Delimiter, Token};
 use crate::tokenstream::{AttrAnnotatedTokenStream, AttrAnnotatedTokenTree};
 use crate::tokenstream::{DelimSpan, Spacing, TokenTree, TreeAndSpacing};
 use crate::tokenstream::{LazyTokenStream, TokenStream};
 use crate::util::comments;
 
+use rustc_data_structures::thin_vec::ThinVec;
 use rustc_index::bit_set::GrowableBitSet;
 use rustc_span::source_map::BytePos;
 use rustc_span::symbol::{sym, Ident, Symbol};
@@ -475,7 +477,16 @@ impl MetaItemKind {
     pub fn mac_args(&self, span: Span) -> MacArgs {
         match self {
             MetaItemKind::Word => MacArgs::Empty,
-            MetaItemKind::NameValue(lit) => MacArgs::Eq(span, lit.to_token()),
+            MetaItemKind::NameValue(lit) => {
+                let expr = P(ast::Expr {
+                    id: ast::DUMMY_NODE_ID,
+                    kind: ast::ExprKind::Lit(lit.clone()),
+                    span: lit.span,
+                    attrs: ThinVec::new(),
+                    tokens: None,
+                });
+                MacArgs::Eq(span, MacArgsEq::Ast(expr))
+            }
             MetaItemKind::List(list) => {
                 let mut tts = Vec::new();
                 for (i, item) in list.iter().enumerate() {
@@ -552,12 +563,16 @@ impl MetaItemKind {
 
     fn from_mac_args(args: &MacArgs) -> Option<MetaItemKind> {
         match args {
+            MacArgs::Empty => Some(MetaItemKind::Word),
             MacArgs::Delimited(_, MacDelimiter::Parenthesis, tokens) => {
                 MetaItemKind::list_from_tokens(tokens.clone())
             }
             MacArgs::Delimited(..) => None,
-            MacArgs::Eq(_, token) => Lit::from_token(token).ok().map(MetaItemKind::NameValue),
-            MacArgs::Empty => Some(MetaItemKind::Word),
+            MacArgs::Eq(_, MacArgsEq::Ast(expr)) => match &expr.kind {
+                ast::ExprKind::Lit(lit) => Some(MetaItemKind::NameValue(lit.clone())),
+                _ => None,
+            },
+            MacArgs::Eq(_, MacArgsEq::Hir(lit)) => Some(MetaItemKind::NameValue(lit.clone())),
         }
     }
 
diff --git a/compiler/rustc_ast/src/mut_visit.rs b/compiler/rustc_ast/src/mut_visit.rs
index c85b763a820..333ffce774d 100644
--- a/compiler/rustc_ast/src/mut_visit.rs
+++ b/compiler/rustc_ast/src/mut_visit.rs
@@ -370,17 +370,12 @@ pub fn visit_mac_args<T: MutVisitor>(args: &mut MacArgs, vis: &mut T) {
             visit_delim_span(dspan, vis);
             visit_tts(tokens, vis);
         }
-        MacArgs::Eq(eq_span, token) => {
+        MacArgs::Eq(eq_span, MacArgsEq::Ast(expr)) => {
             vis.visit_span(eq_span);
-            // The value in `#[key = VALUE]` must be visited as an expression for backward
-            // compatibility, so that macros can be expanded in that position.
-            match &mut token.kind {
-                token::Interpolated(nt) => match Lrc::make_mut(nt) {
-                    token::NtExpr(expr) => vis.visit_expr(expr),
-                    t => panic!("unexpected token in key-value attribute: {:?}", t),
-                },
-                t => panic!("unexpected token in key-value attribute: {:?}", t),
-            }
+            vis.visit_expr(expr);
+        }
+        MacArgs::Eq(_, MacArgsEq::Hir(lit)) => {
+            unreachable!("in literal form when visiting mac args eq: {:?}", lit)
         }
     }
 }
diff --git a/compiler/rustc_ast/src/visit.rs b/compiler/rustc_ast/src/visit.rs
index e08ba73e0ae..3a2d957194c 100644
--- a/compiler/rustc_ast/src/visit.rs
+++ b/compiler/rustc_ast/src/visit.rs
@@ -14,7 +14,6 @@
 //! those that are created by the expansion of a macro.
 
 use crate::ast::*;
-use crate::token;
 
 use rustc_span::symbol::{Ident, Symbol};
 use rustc_span::Span;
@@ -937,14 +936,9 @@ pub fn walk_mac_args<'a, V: Visitor<'a>>(visitor: &mut V, args: &'a MacArgs) {
     match args {
         MacArgs::Empty => {}
         MacArgs::Delimited(_dspan, _delim, _tokens) => {}
-        // The value in `#[key = VALUE]` must be visited as an expression for backward
-        // compatibility, so that macros can be expanded in that position.
-        MacArgs::Eq(_eq_span, token) => match &token.kind {
-            token::Interpolated(nt) => match &**nt {
-                token::NtExpr(expr) => visitor.visit_expr(expr),
-                t => panic!("unexpected token in key-value attribute: {:?}", t),
-            },
-            t => panic!("unexpected token in key-value attribute: {:?}", t),
-        },
+        MacArgs::Eq(_eq_span, MacArgsEq::Ast(expr)) => visitor.visit_expr(expr),
+        MacArgs::Eq(_, MacArgsEq::Hir(lit)) => {
+            unreachable!("in literal form when walking mac args eq: {:?}", lit)
+        }
     }
 }
diff --git a/compiler/rustc_ast_lowering/src/lib.rs b/compiler/rustc_ast_lowering/src/lib.rs
index 526679e5315..d6bbd23cdce 100644
--- a/compiler/rustc_ast_lowering/src/lib.rs
+++ b/compiler/rustc_ast_lowering/src/lib.rs
@@ -38,7 +38,6 @@
 #![recursion_limit = "256"]
 #![allow(rustc::potential_query_instability)]
 
-use rustc_ast::token::{self, Token, TokenKind};
 use rustc_ast::tokenstream::{CanSynthesizeMissingTokens, TokenStream};
 use rustc_ast::visit;
 use rustc_ast::{self as ast, *};
@@ -874,23 +873,24 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 )
             }
             // This is an inert key-value attribute - it will never be visible to macros
-            // after it gets lowered to HIR. Therefore, we can synthesize tokens with fake
-            // spans to handle nonterminals in `#[doc]` (e.g. `#[doc = $e]`).
-            MacArgs::Eq(eq_span, ref token) => {
-                // In valid code the value is always representable as a single literal token.
-                // Otherwise, a dummy token suffices because the error is handled elsewhere.
-                let token = if let token::Interpolated(nt) = &token.kind
-                    && let token::NtExpr(expr) = &**nt
-                {
-                    if let ExprKind::Lit(Lit { token, span, .. }) = expr.kind {
-                        Token::new(TokenKind::Literal(token), span)
-                    } else {
-                        Token::dummy()
-                    }
+            // after it gets lowered to HIR. Therefore, we can extract literals to handle
+            // nonterminals in `#[doc]` (e.g. `#[doc = $e]`).
+            MacArgs::Eq(eq_span, MacArgsEq::Ast(ref expr)) => {
+                // In valid code the value always ends up as a single literal. Otherwise, a dummy
+                // literal suffices because the error is handled elsewhere.
+                let lit = if let ExprKind::Lit(lit) = &expr.kind {
+                    lit.clone()
                 } else {
-                    unreachable!()
+                    Lit {
+                        token: token::Lit::new(token::LitKind::Err, kw::Empty, None),
+                        kind: LitKind::Err(kw::Empty),
+                        span: DUMMY_SP,
+                    }
                 };
-                MacArgs::Eq(eq_span, token)
+                MacArgs::Eq(eq_span, MacArgsEq::Hir(lit))
+            }
+            MacArgs::Eq(_, MacArgsEq::Hir(ref lit)) => {
+                unreachable!("in literal form when lowering mac args eq: {:?}", lit)
             }
         }
     }
diff --git a/compiler/rustc_ast_pretty/src/pprust/state.rs b/compiler/rustc_ast_pretty/src/pprust/state.rs
index c41de17212d..e79f4f0a095 100644
--- a/compiler/rustc_ast_pretty/src/pprust/state.rs
+++ b/compiler/rustc_ast_pretty/src/pprust/state.rs
@@ -13,7 +13,7 @@ use rustc_ast::util::comments::{gather_comments, Comment, CommentStyle};
 use rustc_ast::util::parser;
 use rustc_ast::{self as ast, BlockCheckMode, PatKind, RangeEnd, RangeSyntax};
 use rustc_ast::{attr, Term};
-use rustc_ast::{GenericArg, MacArgs};
+use rustc_ast::{GenericArg, MacArgs, MacArgsEq};
 use rustc_ast::{GenericBound, SelfKind, TraitBoundModifier};
 use rustc_ast::{InlineAsmOperand, InlineAsmRegOrRegClass};
 use rustc_ast::{InlineAsmOptions, InlineAsmTemplatePiece};
@@ -472,11 +472,18 @@ pub trait PrintState<'a>: std::ops::Deref<Target = pp::Printer> + std::ops::Dere
             MacArgs::Empty => {
                 self.print_path(&item.path, false, 0);
             }
-            MacArgs::Eq(_, token) => {
+            MacArgs::Eq(_, MacArgsEq::Ast(expr)) => {
                 self.print_path(&item.path, false, 0);
                 self.space();
                 self.word_space("=");
-                let token_str = self.token_to_string_ext(token, true);
+                let token_str = self.expr_to_string(expr);
+                self.word(token_str);
+            }
+            MacArgs::Eq(_, MacArgsEq::Hir(lit)) => {
+                self.print_path(&item.path, false, 0);
+                self.space();
+                self.word_space("=");
+                let token_str = self.literal_to_string(lit);
                 self.word(token_str);
             }
         }
@@ -818,6 +825,10 @@ pub trait PrintState<'a>: std::ops::Deref<Target = pp::Printer> + std::ops::Dere
         Self::to_string(|s| s.print_expr(e))
     }
 
+    fn literal_to_string(&self, lit: &ast::Lit) -> String {
+        Self::to_string(|s| s.print_literal(lit))
+    }
+
     fn tt_to_string(&self, tt: &TokenTree) -> String {
         Self::to_string(|s| s.print_tt(tt, false))
     }
diff --git a/compiler/rustc_parse/src/parser/mod.rs b/compiler/rustc_parse/src/parser/mod.rs
index 96cca68257e..c4a26359f51 100644
--- a/compiler/rustc_parse/src/parser/mod.rs
+++ b/compiler/rustc_parse/src/parser/mod.rs
@@ -26,11 +26,10 @@ use rustc_ast::tokenstream::{TokenStream, TokenTree};
 use rustc_ast::AttrId;
 use rustc_ast::DUMMY_NODE_ID;
 use rustc_ast::{self as ast, AnonConst, AstLike, AttrStyle, AttrVec, Const, CrateSugar, Extern};
-use rustc_ast::{Async, Expr, ExprKind, MacArgs, MacDelimiter, Mutability, StrLit, Unsafe};
-use rustc_ast::{Visibility, VisibilityKind};
+use rustc_ast::{Async, Expr, ExprKind, MacArgs, MacArgsEq, MacDelimiter, Mutability, StrLit};
+use rustc_ast::{Unsafe, Visibility, VisibilityKind};
 use rustc_ast_pretty::pprust;
 use rustc_data_structures::fx::FxHashMap;
-use rustc_data_structures::sync::Lrc;
 use rustc_errors::PResult;
 use rustc_errors::{
     struct_span_err, Applicability, DiagnosticBuilder, ErrorGuaranteed, FatalError, MultiSpan,
@@ -1157,13 +1156,7 @@ impl<'a> Parser<'a> {
             } else if !delimited_only {
                 if self.eat(&token::Eq) {
                     let eq_span = self.prev_token.span;
-
-                    // Collect tokens because they are used during lowering to HIR.
-                    let expr = self.parse_expr_force_collect()?;
-                    let span = expr.span;
-
-                    let token_kind = token::Interpolated(Lrc::new(token::NtExpr(expr)));
-                    MacArgs::Eq(eq_span, Token::new(token_kind, span))
+                    MacArgs::Eq(eq_span, MacArgsEq::Ast(self.parse_expr_force_collect()?))
                 } else {
                     MacArgs::Empty
                 }
diff --git a/compiler/rustc_parse/src/validate_attr.rs b/compiler/rustc_parse/src/validate_attr.rs
index 4781813ee8e..47477898b24 100644
--- a/compiler/rustc_parse/src/validate_attr.rs
+++ b/compiler/rustc_parse/src/validate_attr.rs
@@ -2,8 +2,9 @@
 
 use crate::parse_in;
 
-use rustc_ast::tokenstream::{DelimSpan, TokenTree};
-use rustc_ast::{self as ast, Attribute, MacArgs, MacDelimiter, MetaItem, MetaItemKind};
+use rustc_ast::tokenstream::DelimSpan;
+use rustc_ast::{self as ast, Attribute, MacArgs, MacArgsEq, MacDelimiter, MetaItem, MetaItemKind};
+use rustc_ast_pretty::pprust;
 use rustc_errors::{Applicability, FatalError, PResult};
 use rustc_feature::{AttributeTemplate, BuiltinAttribute, BUILTIN_ATTRIBUTE_MAP};
 use rustc_session::lint::builtin::ILL_FORMED_ATTRIBUTE_INPUT;
@@ -42,16 +43,40 @@ pub fn parse_meta<'a>(sess: &'a ParseSess, attr: &Attribute) -> PResult<'a, Meta
         path: item.path.clone(),
         kind: match &item.args {
             MacArgs::Empty => MetaItemKind::Word,
-            MacArgs::Eq(_, t) => {
-                let t = TokenTree::Token(t.clone()).into();
-                let v = parse_in(sess, t, "name value", |p| p.parse_unsuffixed_lit())?;
-                MetaItemKind::NameValue(v)
-            }
             MacArgs::Delimited(dspan, delim, t) => {
                 check_meta_bad_delim(sess, *dspan, *delim, "wrong meta list delimiters");
                 let nmis = parse_in(sess, t.clone(), "meta list", |p| p.parse_meta_seq_top())?;
                 MetaItemKind::List(nmis)
             }
+            MacArgs::Eq(_, MacArgsEq::Ast(expr)) => {
+                if let ast::ExprKind::Lit(lit) = &expr.kind {
+                    if !lit.kind.is_unsuffixed() {
+                        let mut err = sess.span_diagnostic.struct_span_err(
+                            lit.span,
+                            "suffixed literals are not allowed in attributes",
+                        );
+                        err.help(
+                            "instead of using a suffixed literal (`1u8`, `1.0f32`, etc.), \
+                            use an unsuffixed version (`1`, `1.0`, etc.)",
+                        );
+                        return Err(err);
+                    } else {
+                        MetaItemKind::NameValue(lit.clone())
+                    }
+                } else {
+                    // The non-error case can happen with e.g. `#[foo = 1+1]`. The error case can
+                    // happen with e.g. `#[foo = include_str!("non-existent-file.rs")]`; in that
+                    // case we delay the error because an earlier error will have already been
+                    // reported.
+                    let msg = format!("unexpected expression: `{}`", pprust::expr_to_string(expr));
+                    let mut err = sess.span_diagnostic.struct_span_err(expr.span, msg);
+                    if let ast::ExprKind::Err = expr.kind {
+                        err.downgrade_to_delayed_bug();
+                    }
+                    return Err(err);
+                }
+            }
+            MacArgs::Eq(_, MacArgsEq::Hir(lit)) => MetaItemKind::NameValue(lit.clone()),
         },
     })
 }