Auto merge of rust-lang#52602 - scottmcm:tryblock-expr, r=nikomatsakis

Implement try block expressions

I noticed that `try` wasn't a keyword yet in Rust 2018, so...

~~Fix​es rust-lang#52604 That was fixed by PR rust-lang#53135
cc rust-lang#31436 rust-lang#50412

Author: bors

src/doc/unstable-book/src/language-features/catch-expr.md renamed to src/doc/unstable-book/src/language-features/try-blocks.md

@@ -1,27 +1,29 @@
-# `catch_expr`
+# `try_blocks`
 
 The tracking issue for this feature is: [#31436]
 
 [#31436]: https://github.com/rust-lang/rust/issues/31436
 
 ------------------------
 
-The `catch_expr` feature adds support for a `catch` expression. The `catch`
-expression creates a new scope one can use the `?` operator in.
+The `try_blocks` feature adds support for `try` blocks. A `try`
+block creates a new scope one can use the `?` operator in.
 
-```rust
-#![feature(catch_expr)]
+```rust,ignore
+// This code needs the 2018 edition
+
+#![feature(try_blocks)]
 
 use std::num::ParseIntError;
 
-let result: Result<i32, ParseIntError> = do catch {
+let result: Result<i32, ParseIntError> = try {
     "1".parse::<i32>()?
         + "2".parse::<i32>()?
         + "3".parse::<i32>()?
 };
 assert_eq!(result, Ok(6));
 
-let result: Result<i32, ParseIntError> = do catch {
+let result: Result<i32, ParseIntError> = try {
     "1".parse::<i32>()?
         + "foo".parse::<i32>()?
         + "3".parse::<i32>()?

src/librustc/hir/lowering.rs

@@ -3613,10 +3613,10 @@ impl<'a> LoweringContext<'a> {
                     hir::LoopSource::Loop,
                 )
             }),
-            ExprKind::Catch(ref body) => {
+            ExprKind::TryBlock(ref body) => {
                 self.with_catch_scope(body.id, |this| {
                     let unstable_span =
-                        this.allow_internal_unstable(CompilerDesugaringKind::Catch, body.span);
+                        this.allow_internal_unstable(CompilerDesugaringKind::TryBlock, body.span);
                     let mut block = this.lower_block(body, true).into_inner();
                     let tail = block.expr.take().map_or_else(
                         || {

src/librustc/ich/impls_syntax.rs

@@ -412,7 +412,7 @@ impl_stable_hash_for!(enum ::syntax_pos::hygiene::CompilerDesugaringKind {
     QuestionMark,
     ExistentialReturnType,
     ForLoop,
-    Catch
+    TryBlock
 });
 
 impl_stable_hash_for!(enum ::syntax_pos::FileName {

src/librustc/lib.rs

@@ -65,7 +65,6 @@
 #![feature(trace_macros)]
 #![feature(trusted_len)]
 #![feature(vec_remove_item)]
-#![feature(catch_expr)]
 #![feature(step_trait)]
 #![feature(integer_atomics)]
 #![feature(test)]

src/librustc_mir/borrow_check/nll/mod.rs

@@ -312,14 +312,14 @@ fn dump_mir_results<'a, 'gcx, 'tcx>(
     );
 
     // Also dump the inference graph constraints as a graphviz file.
-    let _: io::Result<()> = do catch {
+    let _: io::Result<()> = try_block! {
         let mut file =
             pretty::create_dump_file(infcx.tcx, "regioncx.all.dot", None, "nll", &0, source)?;
         regioncx.dump_graphviz_raw_constraints(&mut file)?;
     };
 
     // Also dump the inference graph constraints as a graphviz file.
-    let _: io::Result<()> = do catch {
+    let _: io::Result<()> = try_block! {
         let mut file =
             pretty::create_dump_file(infcx.tcx, "regioncx.scc.dot", None, "nll", &0, source)?;
         regioncx.dump_graphviz_scc_constraints(&mut file)?;

src/librustc_mir/lib.rs

@@ -21,7 +21,6 @@ Rust MIR: a lowered representation of Rust. Also: an experiment!
 #![feature(slice_sort_by_cached_key)]
 #![feature(box_patterns)]
 #![feature(box_syntax)]
-#![feature(catch_expr)]
 #![feature(crate_visibility_modifier)]
 #![feature(const_fn)]
 #![feature(core_intrinsics)]
@@ -63,6 +62,14 @@ extern crate rustc_apfloat;
 extern crate byteorder;
 extern crate core;
 
+// Once we can use edition 2018 in the compiler,
+// replace this with real try blocks.
+macro_rules! try_block {
+    ($($inside:tt)*) => (
+        (||{ ::std::ops::Try::from_ok({ $($inside)* }) })()
+    )
+}
+
 mod diagnostics;
 
 mod borrow_check;

src/librustc_mir/util/pretty.rs

@@ -140,7 +140,7 @@ fn dump_matched_mir_node<'a, 'gcx, 'tcx, F>(
 ) where
     F: FnMut(PassWhere, &mut dyn Write) -> io::Result<()>,
 {
-    let _: io::Result<()> = do catch {
+    let _: io::Result<()> = try_block! {
         let mut file = create_dump_file(tcx, "mir", pass_num, pass_name, disambiguator, source)?;
         writeln!(file, "// MIR for `{}`", node_path)?;
         writeln!(file, "// source = {:?}", source)?;
@@ -156,7 +156,7 @@ fn dump_matched_mir_node<'a, 'gcx, 'tcx, F>(
     };
 
     if tcx.sess.opts.debugging_opts.dump_mir_graphviz {
-        let _: io::Result<()> = do catch {
+        let _: io::Result<()> = try_block! {
             let mut file =
                 create_dump_file(tcx, "dot", pass_num, pass_name, disambiguator, source)?;
             write_mir_fn_graphviz(tcx, source.def_id, mir, &mut file)?;

src/librustc_typeck/check/mod.rs

@@ -4471,7 +4471,7 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
         // In some cases, blocks have just one exit, but other blocks
         // can be targeted by multiple breaks. This can happen both
         // with labeled blocks as well as when we desugar
-        // a `do catch { ... }` expression.
+        // a `try { ... }` expression.
         //
         // Example 1:
         //

src/libsyntax/ast.rs

@@ -987,7 +987,7 @@ impl Expr {
             ExprKind::Match(..) => ExprPrecedence::Match,
             ExprKind::Closure(..) => ExprPrecedence::Closure,
             ExprKind::Block(..) => ExprPrecedence::Block,
-            ExprKind::Catch(..) => ExprPrecedence::Catch,
+            ExprKind::TryBlock(..) => ExprPrecedence::TryBlock,
             ExprKind::Async(..) => ExprPrecedence::Async,
             ExprKind::Assign(..) => ExprPrecedence::Assign,
             ExprKind::AssignOp(..) => ExprPrecedence::AssignOp,
@@ -1108,8 +1108,8 @@ pub enum ExprKind {
     /// created during lowering cannot be made the parent of any other
     /// preexisting defs.
     Async(CaptureBy, NodeId, P<Block>),
-    /// A catch block (`catch { ... }`)
-    Catch(P<Block>),
+    /// A try block (`try { ... }`)
+    TryBlock(P<Block>),
 
     /// An assignment (`a = foo()`)
     Assign(P<Expr>, P<Expr>),

src/libsyntax/feature_gate.rs

@@ -333,8 +333,8 @@ declare_features! (
     // `extern "x86-interrupt" fn()`
     (active, abi_x86_interrupt, "1.17.0", Some(40180), None),
 
-    // Allows the `catch {...}` expression
-    (active, catch_expr, "1.17.0", Some(31436), None),
+    // Allows the `try {...}` expression
+    (active, try_blocks, "1.29.0", Some(31436), None),
 
     // Used to preserve symbols (see llvm.used)
     (active, used, "1.18.0", Some(40289), None),
@@ -1734,8 +1734,8 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                                   e.span,
                                   "yield syntax is experimental");
             }
-            ast::ExprKind::Catch(_) => {
-                gate_feature_post!(&self, catch_expr, e.span, "`catch` expression is experimental");
+            ast::ExprKind::TryBlock(_) => {
+                gate_feature_post!(&self, try_blocks, e.span, "`try` expression is experimental");
             }
             ast::ExprKind::IfLet(ref pats, ..) | ast::ExprKind::WhileLet(ref pats, ..) => {
                 if pats.len() > 1 {

src/libsyntax/fold.rs

@@ -1351,7 +1351,7 @@ pub fn noop_fold_expr<T: Folder>(Expr {id, node, span, attrs}: Expr, folder: &mu
             }
             ExprKind::Yield(ex) => ExprKind::Yield(ex.map(|x| folder.fold_expr(x))),
             ExprKind::Try(ex) => ExprKind::Try(folder.fold_expr(ex)),
-            ExprKind::Catch(body) => ExprKind::Catch(folder.fold_block(body)),
+            ExprKind::TryBlock(body) => ExprKind::TryBlock(folder.fold_block(body)),
         },
         id: folder.new_id(id),
         span: folder.new_span(span),

src/libsyntax/lib.rs

@@ -26,8 +26,8 @@
 #![feature(rustc_diagnostic_macros)]
 #![feature(slice_sort_by_cached_key)]
 #![feature(str_escape)]
+#![feature(try_trait)]
 #![feature(unicode_internals)]
-#![feature(catch_expr)]
 
 #![recursion_limit="256"]
 

src/libsyntax/parse/classify.rs

@@ -31,7 +31,7 @@ pub fn expr_requires_semi_to_be_stmt(e: &ast::Expr) -> bool {
         ast::ExprKind::WhileLet(..) |
         ast::ExprKind::Loop(..) |
         ast::ExprKind::ForLoop(..) |
-        ast::ExprKind::Catch(..) => false,
+        ast::ExprKind::TryBlock(..) => false,
         _ => true,
     }
 }

src/libsyntax/parse/parser.rs

@@ -1757,9 +1757,17 @@ impl<'a> Parser<'a> {
 
             let parser_snapshot_before_pat = self.clone();
 
+            // Once we can use edition 2018 in the compiler,
+            // replace this with real try blocks.
+            macro_rules! try_block {
+                ($($inside:tt)*) => (
+                    (||{ ::std::ops::Try::from_ok({ $($inside)* }) })()
+                )
+            }
+
             // We're going to try parsing the argument as a pattern (even though it's not
             // allowed). This way we can provide better errors to the user.
-            let pat_arg: PResult<'a, _> = do catch {
+            let pat_arg: PResult<'a, _> = try_block! {
                 let pat = self.parse_pat()?;
                 self.expect(&token::Colon)?;
                 (pat, self.parse_ty()?)
@@ -2387,11 +2395,15 @@ impl<'a> Parser<'a> {
                         BlockCheckMode::Unsafe(ast::UserProvided),
                         attrs);
                 }
-                if self.is_catch_expr() {
+                if self.is_do_catch_block() {
+                    let mut db = self.fatal("found removed `do catch` syntax");
+                    db.help("Following RFC #2388, the new non-placeholder syntax is `try`");
+                    return Err(db);
+                }
+                if self.is_try_block() {
                     let lo = self.span;
-                    assert!(self.eat_keyword(keywords::Do));
-                    assert!(self.eat_keyword(keywords::Catch));
-                    return self.parse_catch_expr(lo, attrs);
+                    assert!(self.eat_keyword(keywords::Try));
+                    return self.parse_try_block(lo, attrs);
                 }
                 if self.eat_keyword(keywords::Return) {
                     if self.token.can_begin_expr() {
@@ -3453,13 +3465,13 @@ impl<'a> Parser<'a> {
             ExprKind::Async(capture_clause, ast::DUMMY_NODE_ID, body), attrs))
     }
 
-    /// Parse a `do catch {...}` expression (`do catch` token already eaten)
-    fn parse_catch_expr(&mut self, span_lo: Span, mut attrs: ThinVec<Attribute>)
+    /// Parse a `try {...}` expression (`try` token already eaten)
+    fn parse_try_block(&mut self, span_lo: Span, mut attrs: ThinVec<Attribute>)
         -> PResult<'a, P<Expr>>
     {
         let (iattrs, body) = self.parse_inner_attrs_and_block()?;
         attrs.extend(iattrs);
-        Ok(self.mk_expr(span_lo.to(body.span), ExprKind::Catch(body), attrs))
+        Ok(self.mk_expr(span_lo.to(body.span), ExprKind::TryBlock(body), attrs))
     }
 
     // `match` token already eaten
@@ -4408,12 +4420,20 @@ impl<'a> Parser<'a> {
         )
     }
 
-    fn is_catch_expr(&mut self) -> bool {
+    fn is_do_catch_block(&mut self) -> bool {
         self.token.is_keyword(keywords::Do) &&
         self.look_ahead(1, |t| t.is_keyword(keywords::Catch)) &&
         self.look_ahead(2, |t| *t == token::OpenDelim(token::Brace)) &&
+        !self.restrictions.contains(Restrictions::NO_STRUCT_LITERAL)
+    }
+
+    fn is_try_block(&mut self) -> bool {
+        self.token.is_keyword(keywords::Try) &&
+        self.look_ahead(1, |t| *t == token::OpenDelim(token::Brace)) &&
+
+        self.span.edition() >= Edition::Edition2018 &&
 
-        // prevent `while catch {} {}`, `if catch {} {} else {}`, etc.
+        // prevent `while try {} {}`, `if try {} {} else {}`, etc.
         !self.restrictions.contains(Restrictions::NO_STRUCT_LITERAL)
     }
 

src/libsyntax/print/pprust.rs

@@ -2379,8 +2379,8 @@ impl<'a> State<'a> {
                 self.print_expr_maybe_paren(e, parser::PREC_POSTFIX)?;
                 self.s.word("?")?
             }
-            ast::ExprKind::Catch(ref blk) => {
-                self.head("do catch")?;
+            ast::ExprKind::TryBlock(ref blk) => {
+                self.head("try")?;
                 self.s.space()?;
                 self.print_block_with_attrs(blk, attrs)?
             }

src/libsyntax/util/parser.rs

@@ -273,7 +273,7 @@ pub enum ExprPrecedence {
     Loop,
     Match,
     Block,
-    Catch,
+    TryBlock,
     Struct,
     Async,
 }
@@ -332,7 +332,7 @@ impl ExprPrecedence {
             ExprPrecedence::Loop |
             ExprPrecedence::Match |
             ExprPrecedence::Block |
-            ExprPrecedence::Catch |
+            ExprPrecedence::TryBlock |
             ExprPrecedence::Async |
             ExprPrecedence::Struct => PREC_PAREN,
         }

src/libsyntax/visit.rs

@@ -809,7 +809,7 @@ pub fn walk_expr<'a, V: Visitor<'a>>(visitor: &mut V, expression: &'a Expr) {
         ExprKind::Try(ref subexpression) => {
             visitor.visit_expr(subexpression)
         }
-        ExprKind::Catch(ref body) => {
+        ExprKind::TryBlock(ref body) => {
             visitor.visit_block(body)
         }
     }

src/libsyntax_pos/hygiene.rs

@@ -595,7 +595,7 @@ impl ExpnFormat {
 #[derive(Clone, Copy, Hash, Debug, PartialEq, Eq, RustcEncodable, RustcDecodable)]
 pub enum CompilerDesugaringKind {
     QuestionMark,
-    Catch,
+    TryBlock,
     /// Desugaring of an `impl Trait` in return type position
     /// to an `existential type Foo: Trait;` + replacing the
     /// `impl Trait` with `Foo`.
@@ -609,7 +609,7 @@ impl CompilerDesugaringKind {
         Symbol::intern(match self {
             CompilerDesugaringKind::Async => "async",
             CompilerDesugaringKind::QuestionMark => "?",
-            CompilerDesugaringKind::Catch => "do catch",
+            CompilerDesugaringKind::TryBlock => "try block",
             CompilerDesugaringKind::ExistentialReturnType => "existential type",
             CompilerDesugaringKind::ForLoop => "for loop",
         })

src/libsyntax_pos/symbol.rs

@@ -415,23 +415,25 @@ declare_keywords! {
 
     // Edition-specific keywords reserved for future use.
     (51, Async,              "async") // >= 2018 Edition Only
+    (52, Try,                "try") // >= 2018 Edition Only
 
     // Special lifetime names
-    (52, UnderscoreLifetime, "'_")
-    (53, StaticLifetime,     "'static")
+    (53, UnderscoreLifetime, "'_")
+    (54, StaticLifetime,     "'static")
 
     // Weak keywords, have special meaning only in specific contexts.
-    (54, Auto,               "auto")
-    (55, Catch,              "catch")
-    (56, Default,            "default")
-    (57, Dyn,                "dyn")
-    (58, Union,              "union")
-    (59, Existential,        "existential")
+    (55, Auto,               "auto")
+    (56, Catch,              "catch")
+    (57, Default,            "default")
+    (58, Dyn,                "dyn")
+    (59, Union,              "union")
+    (60, Existential,        "existential")
 }
 
 impl Symbol {
     fn is_unused_keyword_2018(self) -> bool {
-        self == keywords::Async.name()
+        self >= keywords::Async.name() &&
+        self <= keywords::Try.name()
     }
 }
 

src/test/parse-fail/do-catch-suggests-try.rs

@@ -0,0 +1,17 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// compile-flags: -Z parse-only
+
+fn main() {
+    let _: Option<()> = do catch {};
+    //~^ ERROR found removed `do catch` syntax
+    //~^^ HELP Following RFC #2388, the new non-placeholder syntax is `try`
+}