Implement RFC-2011 (Nicer assert! messages)

compiler/rustc_builtin_macros/src/assert.rs

@@ -1,11 +1,13 @@
+mod context;
+
 use crate::edition_panic::use_panic_2021;
 use rustc_ast::ptr::P;
 use rustc_ast::token;
 use rustc_ast::tokenstream::{DelimSpan, TokenStream};
-use rustc_ast::{self as ast, *};
+use rustc_ast::{Expr, ExprKind, MacArgs, MacCall, MacDelimiter, Path, PathSegment, UnOp};
 use rustc_ast_pretty::pprust;
 use rustc_errors::{Applicability, PResult};
-use rustc_expand::base::*;
+use rustc_expand::base::{DummyResult, ExtCtxt, MacEager, MacResult};
 use rustc_parse::parser::Parser;
 use rustc_span::symbol::{sym, Ident, Symbol};
 use rustc_span::{Span, DUMMY_SP};
@@ -15,7 +17,7 @@ pub fn expand_assert<'cx>(
     span: Span,
     tts: TokenStream,
 ) -> Box<dyn MacResult + 'cx> {
-    let Assert { cond_expr, custom_message } = match parse_assert(cx, span, tts) {
+    let Assert { cond_expr, custom_message } = match Assert::parse(cx, span, tts) {
         Ok(assert) => assert,
         Err(mut err) => {
             err.emit();
@@ -25,13 +27,13 @@ pub fn expand_assert<'cx>(
 
     // `core::panic` and `std::panic` are different macros, so we use call-site
     // context to pick up whichever is currently in scope.
-    let sp = cx.with_call_site_ctxt(span);
+    let call_site_span = cx.with_call_site_ctxt(span);
 
-    let panic_call = if let Some(tokens) = custom_message {
-        let path = if use_panic_2021(span) {
+    let panic_path = || {
+        if use_panic_2021(span) {
             // On edition 2021, we always call `$crate::panic::panic_2021!()`.
             Path {
-                span: sp,
+                span: call_site_span,
                 segments: cx
                     .std_path(&[sym::panic, sym::panic_2021])
                     .into_iter()
@@ -42,27 +44,39 @@ pub fn expand_assert<'cx>(
         } else {
             // Before edition 2021, we call `panic!()` unqualified,
             // such that it calls either `std::panic!()` or `core::panic!()`.
-            Path::from_ident(Ident::new(sym::panic, sp))
-        };
-        // Pass the custom message to panic!().
-        cx.expr(
-            sp,
+            Path::from_ident(Ident::new(sym::panic, call_site_span))
+        }
+    };
+
+    // Simply uses the user provided message instead of generating custom outputs
+    let expr = if let Some(tokens) = custom_message {
+        let then = cx.expr(
+            call_site_span,
             ExprKind::MacCall(MacCall {
-                path,
+                path: panic_path(),
                 args: P(MacArgs::Delimited(
-                    DelimSpan::from_single(sp),
+                    DelimSpan::from_single(call_site_span),
                     MacDelimiter::Parenthesis,
                     tokens,
                 )),
                 prior_type_ascription: None,
             }),
-        )
-    } else {
+        );
+        expr_if_not(cx, call_site_span, cond_expr, then, None)
+    }
+    // If `generic_assert` is enabled, generates rich captured outputs
+    //
+    // FIXME(c410-f3r) See https://github.com/rust-lang/rust/issues/96949
+    else if let Some(features) = cx.ecfg.features && features.generic_assert {
+        context::Context::new(cx, call_site_span).build(cond_expr, panic_path())
+    }
+    // If `generic_assert` is not enabled, outputs "assertion failed: ..."
+    else {
         // Pass our own message directly to $crate::panicking::panic(),
         // because it might contain `{` and `}` that should always be
         // passed literally.
-        cx.expr_call_global(
-            sp,
+        let then = cx.expr_call_global(
+            call_site_span,
             cx.std_path(&[sym::panicking, sym::panic]),
             vec![cx.expr_str(
                 DUMMY_SP,
@@ -71,82 +85,96 @@ pub fn expand_assert<'cx>(
                     pprust::expr_to_string(&cond_expr).escape_debug()
                 )),
             )],
-        )
+        );
+        expr_if_not(cx, call_site_span, cond_expr, then, None)
     };
-    let if_expr =
-        cx.expr_if(sp, cx.expr(sp, ExprKind::Unary(UnOp::Not, cond_expr)), panic_call, None);
-    MacEager::expr(if_expr)
+
+    MacEager::expr(expr)
 }
 
 struct Assert {
-    cond_expr: P<ast::Expr>,
+    cond_expr: P<Expr>,
     custom_message: Option<TokenStream>,
 }
 
-fn parse_assert<'a>(cx: &mut ExtCtxt<'a>, sp: Span, stream: TokenStream) -> PResult<'a, Assert> {
-    let mut parser = cx.new_parser_from_tts(stream);
+impl Assert {
+    fn parse<'a>(cx: &mut ExtCtxt<'a>, sp: Span, stream: TokenStream) -> PResult<'a, Assert> {
+        let mut parser = cx.new_parser_from_tts(stream);
 
-    if parser.token == token::Eof {
-        let mut err = cx.struct_span_err(sp, "macro requires a boolean expression as an argument");
-        err.span_label(sp, "boolean expression required");
-        return Err(err);
-    }
-
-    let cond_expr = parser.parse_expr()?;
-
-    // Some crates use the `assert!` macro in the following form (note extra semicolon):
-    //
-    // assert!(
-    //     my_function();
-    // );
-    //
-    // Emit an error about semicolon and suggest removing it.
-    if parser.token == token::Semi {
-        let mut err = cx.struct_span_err(sp, "macro requires an expression as an argument");
-        err.span_suggestion(
-            parser.token.span,
-            "try removing semicolon",
-            String::new(),
-            Applicability::MaybeIncorrect,
-        );
-        err.emit();
+        if parser.token == token::Eof {
+            let mut err =
+                cx.struct_span_err(sp, "macro requires a boolean expression as an argument");
+            err.span_label(sp, "boolean expression required");
+            return Err(err);
+        }
 
-        parser.bump();
-    }
+        let cond_expr = parser.parse_expr()?;
 
-    // Some crates use the `assert!` macro in the following form (note missing comma before
-    // message):
-    //
-    // assert!(true "error message");
-    //
-    // Emit an error and suggest inserting a comma.
-    let custom_message =
-        if let token::Literal(token::Lit { kind: token::Str, .. }) = parser.token.kind {
-            let mut err = cx.struct_span_err(parser.token.span, "unexpected string literal");
-            let comma_span = parser.prev_token.span.shrink_to_hi();
-            err.span_suggestion_short(
-                comma_span,
-                "try adding a comma",
-                ", ".to_string(),
+        // Some crates use the `assert!` macro in the following form (note extra semicolon):
+        //
+        // assert!(
+        //     my_function();
+        // );
+        //
+        // Emit an error about semicolon and suggest removing it.
+        if parser.token == token::Semi {
+            let mut err = cx.struct_span_err(sp, "macro requires an expression as an argument");
+            err.span_suggestion(
+                parser.token.span,
+                "try removing semicolon",
+                String::new(),
                 Applicability::MaybeIncorrect,
             );
             err.emit();
 
-            parse_custom_message(&mut parser)
-        } else if parser.eat(&token::Comma) {
-            parse_custom_message(&mut parser)
-        } else {
-            None
-        };
+            parser.bump();
+        }
+
+        // Some crates use the `assert!` macro in the following form (note missing comma before
+        // message):
+        //
+        // assert!(true "error message");
+        //
+        // Emit an error and suggest inserting a comma.
+        let custom_message =
+            if let token::Literal(token::Lit { kind: token::Str, .. }) = parser.token.kind {
+                let mut err = cx.struct_span_err(parser.token.span, "unexpected string literal");
+                let comma_span = parser.prev_token.span.shrink_to_hi();
+                err.span_suggestion_short(
+                    comma_span,
+                    "try adding a comma",
+                    ", ".to_string(),
+                    Applicability::MaybeIncorrect,
+                );
+                err.emit();
 
-    if parser.token != token::Eof {
-        return parser.unexpected();
+                Self::parse_custom_message(&mut parser)
+            } else if parser.eat(&token::Comma) {
+                Self::parse_custom_message(&mut parser)
+            } else {
+                None
+            };
+
+        if parser.token != token::Eof {
+            return parser.unexpected();
+        }
+
+        Ok(Assert { cond_expr, custom_message })
     }
 
-    Ok(Assert { cond_expr, custom_message })
+    fn parse_custom_message(parser: &mut Parser<'_>) -> Option<TokenStream> {
+        let ts = parser.parse_tokens();
+        if !ts.is_empty() { Some(ts) } else { None }
+    }
 }
 
-fn parse_custom_message(parser: &mut Parser<'_>) -> Option<TokenStream> {
-    let ts = parser.parse_tokens();
-    if !ts.is_empty() { Some(ts) } else { None }
+// if !{ ... } { ... } else { ... }
+fn expr_if_not(
+    cx: &ExtCtxt<'_>,
+    span: Span,
+    cond: P<Expr>,
+    then: P<Expr>,
+    els: Option<P<Expr>>,
+) -> P<Expr> {
+    cx.expr_if(span, cx.expr(span, ExprKind::Unary(UnOp::Not, cond)), then, els)
 }

compiler/rustc_builtin_macros/src/lib.rs

@@ -1,17 +1,18 @@
 //! This crate contains implementations of built-in macros and other code generating facilities
 //! injecting code into the crate before it is lowered to HIR.
 
+#![allow(rustc::potential_query_instability)]
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![feature(array_windows)]
 #![feature(box_patterns)]
 #![feature(decl_macro)]
 #![feature(is_sorted)]
-#![feature(nll)]
+#![feature(let_chains)]
 #![feature(let_else)]
+#![feature(nll)]
 #![feature(proc_macro_internals)]
 #![feature(proc_macro_quote)]
 #![recursion_limit = "256"]
-#![allow(rustc::potential_query_instability)]
 
 extern crate proc_macro;
 

compiler/rustc_expand/src/build.rs

@@ -160,7 +160,7 @@ impl<'a> ExtCtxt<'a> {
             attrs: AttrVec::new(),
             tokens: None,
         });
-        ast::Stmt { id: ast::DUMMY_NODE_ID, kind: ast::StmtKind::Local(local), span: sp }
+        self.stmt_local(local, sp)
     }
 
     // Generates `let _: Type;`, which is usually used for type assertions.
@@ -174,6 +174,10 @@ impl<'a> ExtCtxt<'a> {
             attrs: AttrVec::new(),
             tokens: None,
         });
+        self.stmt_local(local, span)
+    }
+
+    pub fn stmt_local(&self, local: P<ast::Local>, span: Span) -> ast::Stmt {
         ast::Stmt { id: ast::DUMMY_NODE_ID, kind: ast::StmtKind::Local(local), span }
     }
 

compiler/rustc_feature/src/active.rs

@@ -150,6 +150,8 @@ declare_features! (
     (active, allow_internal_unstable, "1.0.0", None, None),
     /// Allows identifying the `compiler_builtins` crate.
     (active, compiler_builtins, "1.13.0", None, None),
+    /// Outputs useful `assert!` messages
+    (active, generic_assert, "1.63.0", None, None),
     /// Allows using the `rust-intrinsic`'s "ABI".
     (active, intrinsics, "1.0.0", None, None),
     /// Allows using `#[lang = ".."]` attribute for linking items to special compiler logic.

compiler/rustc_span/src/symbol.rs

@@ -156,6 +156,7 @@ symbols! {
         C,
         CStr,
         CString,
+        Capture,
         Center,
         Clone,
         Continue,
@@ -262,6 +263,8 @@ symbols! {
         ToOwned,
         ToString,
         Try,
+        TryCaptureGeneric,
+        TryCapturePrintable,
         TryFrom,
         TryInto,
         Ty,
@@ -271,6 +274,7 @@ symbols! {
         UnsafeArg,
         Vec,
         VecDeque,
+        Wrapper,
         Yield,
         _DECLS,
         _Self,
@@ -353,6 +357,7 @@ symbols! {
         assert_receiver_is_total_eq,
         assert_uninit_valid,
         assert_zero_valid,
+        asserting,
         associated_const_equality,
         associated_consts,
         associated_type_bounds,
@@ -730,6 +735,7 @@ symbols! {
         generator_state,
         generators,
         generic_arg_infer,
+        generic_assert,
         generic_associated_types,
         generic_associated_types_extended,
         generic_const_exprs,
@@ -1426,6 +1432,7 @@ symbols! {
         truncf32,
         truncf64,
         try_blocks,
+        try_capture,
         try_from,
         try_into,
         try_trait_v2,
@@ -1488,6 +1495,7 @@ symbols! {
         unsized_tuple_coercion,
         unstable,
         untagged_unions,
+        unused_imports,
         unused_qualifications,
         unwind,
         unwind_attributes,

src/test/ui/macros/assert-trailing-junk.rs

@@ -1,3 +1,6 @@
+// revisions: with-generic-asset without-generic-asset
+// [with-generic-asset] compile-flags: --cfg feature="generic_assert"
+
 // Ensure assert macro does not ignore trailing garbage.
 //
 // See https://github.com/rust-lang/rust/issues/60024 for details.

src/test/ui/macros/assert-trailing-junk.stderr → src/test/ui/macros/assert-trailing-junk.with-generic-asset.stderr

@@ -1,49 +1,49 @@
 error: expected one of `,`, `.`, `?`, or an operator, found `some`
-  --> $DIR/assert-trailing-junk.rs:6:18
+  --> $DIR/assert-trailing-junk.rs:9:18
    |
 LL |     assert!(true some extra junk, "whatever");
    |                  ^^^^ expected one of `,`, `.`, `?`, or an operator
 
 error: expected one of `,`, `.`, `?`, or an operator, found `some`
-  --> $DIR/assert-trailing-junk.rs:9:18
+  --> $DIR/assert-trailing-junk.rs:12:18
    |
 LL |     assert!(true some extra junk);
    |                  ^^^^ expected one of `,`, `.`, `?`, or an operator
 
 error: no rules expected the token `blah`
-  --> $DIR/assert-trailing-junk.rs:12:30
+  --> $DIR/assert-trailing-junk.rs:15:30
    |
 LL |     assert!(true, "whatever" blah);
    |                             -^^^^ no rules expected this token in macro call
    |                             |
    |                             help: missing comma here
 
 error: unexpected string literal
-  --> $DIR/assert-trailing-junk.rs:15:18
+  --> $DIR/assert-trailing-junk.rs:18:18
    |
 LL |     assert!(true "whatever" blah);
    |                 -^^^^^^^^^^
    |                 |
    |                 help: try adding a comma
 
 error: no rules expected the token `blah`
-  --> $DIR/assert-trailing-junk.rs:15:29
+  --> $DIR/assert-trailing-junk.rs:18:29
    |
 LL |     assert!(true "whatever" blah);
    |                            -^^^^ no rules expected this token in macro call
    |                            |
    |                            help: missing comma here
 
 error: macro requires an expression as an argument
-  --> $DIR/assert-trailing-junk.rs:19:5
+  --> $DIR/assert-trailing-junk.rs:22:5
    |
 LL |     assert!(true;);
    |     ^^^^^^^^^^^^-^
    |                 |
    |                 help: try removing semicolon
 
 error: unexpected string literal
-  --> $DIR/assert-trailing-junk.rs:22:27
+  --> $DIR/assert-trailing-junk.rs:25:27
    |
 LL |     assert!(false || true "error message");
    |                          -^^^^^^^^^^^^^^^

src/test/ui/macros/assert-trailing-junk.without-generic-asset.stderr

@@ -0,0 +1,54 @@
+error: expected one of `,`, `.`, `?`, or an operator, found `some`
+  --> $DIR/assert-trailing-junk.rs:9:18
+   |
+LL |     assert!(true some extra junk, "whatever");
+   |                  ^^^^ expected one of `,`, `.`, `?`, or an operator
+
+error: expected one of `,`, `.`, `?`, or an operator, found `some`
+  --> $DIR/assert-trailing-junk.rs:12:18
+   |
+LL |     assert!(true some extra junk);
+   |                  ^^^^ expected one of `,`, `.`, `?`, or an operator
+
+error: no rules expected the token `blah`
+  --> $DIR/assert-trailing-junk.rs:15:30
+   |
+LL |     assert!(true, "whatever" blah);
+   |                             -^^^^ no rules expected this token in macro call
+   |                             |
+   |                             help: missing comma here
+
+error: unexpected string literal
+  --> $DIR/assert-trailing-junk.rs:18:18
+   |
+LL |     assert!(true "whatever" blah);
+   |                 -^^^^^^^^^^
+   |                 |
+   |                 help: try adding a comma
+
+error: no rules expected the token `blah`
+  --> $DIR/assert-trailing-junk.rs:18:29
+   |
+LL |     assert!(true "whatever" blah);
+   |                            -^^^^ no rules expected this token in macro call
+   |                            |
+   |                            help: missing comma here
+
+error: macro requires an expression as an argument
+  --> $DIR/assert-trailing-junk.rs:22:5
+   |
+LL |     assert!(true;);
+   |     ^^^^^^^^^^^^-^
+   |                 |
+   |                 help: try removing semicolon
+
+error: unexpected string literal
+  --> $DIR/assert-trailing-junk.rs:25:27
+   |
+LL |     assert!(false || true "error message");
+   |                          -^^^^^^^^^^^^^^^
+   |                          |
+   |                          help: try adding a comma
+
+error: aborting due to 7 previous errors
+

src/test/ui/macros/assert.rs

@@ -1,3 +1,6 @@
+// revisions: with-generic-asset without-generic-asset
+// [with-generic-asset] compile-flags: --cfg feature="generic_assert"
+
 fn main() {
     assert!();  //~ ERROR requires a boolean expression
     assert!(struct); //~ ERROR expected expression

src/test/ui/macros/assert.stderr → src/test/ui/macros/assert.with-generic-asset.stderr

@@ -1,25 +1,25 @@
 error: macro requires a boolean expression as an argument
-  --> $DIR/assert.rs:2:5
+  --> $DIR/assert.rs:5:5
    |
 LL |     assert!();
    |     ^^^^^^^^^ boolean expression required
 
 error: expected expression, found keyword `struct`
-  --> $DIR/assert.rs:3:13
+  --> $DIR/assert.rs:6:13
    |
 LL |     assert!(struct);
    |             ^^^^^^ expected expression
 
 error: macro requires a boolean expression as an argument
-  --> $DIR/assert.rs:4:5
+  --> $DIR/assert.rs:7:5
    |
 LL |     debug_assert!();
    |     ^^^^^^^^^^^^^^^ boolean expression required
    |
    = note: this error originates in the macro `debug_assert` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: expected expression, found keyword `struct`
-  --> $DIR/assert.rs:5:19
+  --> $DIR/assert.rs:8:19
    |
 LL |     debug_assert!(struct);
    |                   ^^^^^^ expected expression

src/test/ui/macros/assert.without-generic-asset.stderr

@@ -0,0 +1,28 @@
+error: macro requires a boolean expression as an argument
+  --> $DIR/assert.rs:5:5
+   |
+LL |     assert!();
+   |     ^^^^^^^^^ boolean expression required
+
+error: expected expression, found keyword `struct`
+  --> $DIR/assert.rs:6:13
+   |
+LL |     assert!(struct);
+   |             ^^^^^^ expected expression
+
+error: macro requires a boolean expression as an argument
+  --> $DIR/assert.rs:7:5
+   |
+LL |     debug_assert!();
+   |     ^^^^^^^^^^^^^^^ boolean expression required
+   |
+   = note: this error originates in the macro `debug_assert` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: expected expression, found keyword `struct`
+  --> $DIR/assert.rs:8:19
+   |
+LL |     debug_assert!(struct);
+   |                   ^^^^^^ expected expression
+
+error: aborting due to 4 previous errors
+

src/test/ui/rfc-2011-nicer-assert-messages/all-expr-kinds.rs

@@ -0,0 +1,188 @@
+// aux-build:common.rs
+// edition:2021
+// ignore-tidy-linelength
+// run-pass
+
+#![allow(path_statements, unused_allocation)]
+#![feature(box_syntax, core_intrinsics, generic_assert, generic_assert_internals)]
+
+extern crate common;
+
+// Use common::test once https://github.com/rust-lang/rust/issues/96997 is fixed
+macro_rules! test {
+  (
+    let mut $elem_ident:ident = $elem_expr:expr;
+    [ $($assert:tt)* ] => $msg:literal
+  ) => {
+    {
+      #[allow(unused_assignments, unused_mut, unused_variables)]
+      let rslt = std::panic::catch_unwind(|| {
+        let mut $elem_ident = $elem_expr;
+        assert!($($assert)*);
+      });
+      let err = rslt.unwrap_err();
+      if let Some(elem) = err.downcast_ref::<String>() {
+        assert_eq!(elem, &$msg);
+      }
+      else if let Some(elem) = err.downcast_ref::<&str>() {
+        assert_eq!(elem, &$msg);
+      }
+      else {
+        panic!("assert!( ... ) should return a string");
+      }
+    }
+  }
+}
+
+macro_rules! tests {
+  (
+    let mut $elem_ident:ident = $elem_expr:expr;
+
+    $(
+      [ $($elem_assert:tt)* ] => $elem_msg:literal
+    )+
+  ) => {
+    $(
+      test!(
+        let mut $elem_ident = $elem_expr;
+        [ $($elem_assert)* ] => $elem_msg
+      );
+    )+
+  }
+}
+
+const FOO: Foo = Foo { bar: 1 };
+
+#[derive(Clone, Copy, Debug, PartialEq)]
+struct Foo {
+  bar: i32
+}
+
+impl Foo {
+  fn add(&self, a: i32, b: i32) -> i32 { a + b }
+}
+
+fn add(a: i32, b: i32) -> i32 { a + b }
+
+fn main() {
+  // ***** Allowed *****
+
+  tests!(
+    let mut elem = 1i32;
+
+    // addr of
+    [ &elem == &3 ] => "Assertion failed: &elem == &3\nWith captures:\n  elem = 1\n"
+
+    // array
+    [ [elem][0] == 3 ] => "Assertion failed: [elem][0] == 3\nWith captures:\n  elem = 1\n"
+
+    // binary
+    [ elem + 1 == 3 ] => "Assertion failed: elem + 1 == 3\nWith captures:\n  elem = 1\n"
+
+    // call
+    [ add(elem, elem) == 3 ] => "Assertion failed: add(elem, elem) == 3\nWith captures:\n  elem = 1\n"
+
+    // cast
+    [ elem as i32 == 3 ] => "Assertion failed: elem as i32 == 3\nWith captures:\n  elem = 1\n"
+
+    // index
+    [ [1i32, 1][elem as usize] == 3 ] => "Assertion failed: [1i32, 1][elem as usize] == 3\nWith captures:\n  elem = 1\n"
+
+    // method call
+    [ FOO.add(elem, elem) == 3 ] => "Assertion failed: FOO.add(elem, elem) == 3\nWith captures:\n  elem = 1\n"
+
+    // paren
+    [ (elem) == 3 ] => "Assertion failed: (elem) == 3\nWith captures:\n  elem = 1\n"
+
+    // range
+    [ (0..elem) == (0..3) ] => "Assertion failed: (0..elem) == (0..3)\nWith captures:\n  elem = 1\n"
+
+    // repeat
+    [ [elem; 1] == [3; 1] ] => "Assertion failed: [elem; 1] == [3; 1]\nWith captures:\n  elem = 1\n"
+
+    // struct
+    [ Foo { bar: elem } == Foo { bar: 3 } ] => "Assertion failed: Foo { bar: elem } == Foo { bar: 3 }\nWith captures:\n  elem = 1\n"
+
+    // tuple
+    [ (elem, 1) == (3, 3) ] => "Assertion failed: (elem, 1) == (3, 3)\nWith captures:\n  elem = 1\n"
+
+    // unary
+    [ -elem == -3 ] => "Assertion failed: -elem == -3\nWith captures:\n  elem = 1\n"
+  );
+
+  // ***** Disallowed *****
+
+  tests!(
+    let mut elem = 1i32;
+
+    // assign
+    [ { let local = elem; local } == 3 ] => "Assertion failed: { let local = elem; local } == 3"
+
+    // assign op
+    [ { elem += 1; elem } == 3 ] => "Assertion failed: { elem += 1; elem } == 3"
+
+    // async
+    [ { let _ = async { elem }; elem } == 3 ] => "Assertion failed: { let _ = async { elem }; elem } == 3"
+
+    // await
+
+    // block
+    [ { elem } == 3 ] => "Assertion failed: { elem } == 3"
+
+    // box
+    [ box elem == box 3 ] => "Assertion failed: box elem == box 3"
+
+    // break
+    [ loop { break elem; } ==  3 ] => "Assertion failed: loop { break elem; } == 3"
+
+    // closure
+    [(|| elem)() ==  3 ] => "Assertion failed: (|| elem)() == 3"
+
+    // const block
+
+    // continue
+
+    // err
+
+    // field
+    [ FOO.bar ==  3 ] => "Assertion failed: FOO.bar == 3"
+
+    // for loop
+    [ { for _ in 0..elem { elem; } elem } ==  3 ] => "Assertion failed: { for _ in 0..elem { elem; } elem } == 3"
+
+    // if
+    [ if true { elem } else { elem } == 3 ] => "Assertion failed: if true { elem } else { elem } == 3"
+
+    // inline asm
+
+    // let
+    [ if let true = true { elem } else { elem } == 3 ] => "Assertion failed: if let true = true { elem } else { elem } == 3"
+
+    // lit
+
+    // loop
+    [ loop { elem; break elem; } == 3 ] => "Assertion failed: loop { elem; break elem; } == 3"
+
+    // mac call
+
+    // match
+    [ match elem { _ => elem } == 3 ] => "Assertion failed: match elem { _ => elem, } == 3"
+
+    // ret
+    [ (|| { return elem; })() == 3 ] => "Assertion failed: (|| { return elem; })() == 3"
+
+    // try
+    [ (|| { Some(Some(elem)?) })() == Some(3) ] => "Assertion failed: (|| { Some(Some(elem)?) })() == Some(3)"
+
+    // try block
+
+    // underscore
+
+    // while
+    [ { while false { elem; break; } elem } == 3 ] => "Assertion failed: { while false { elem; break; } elem } == 3"
+
+    // yeet
+
+    // yield
+  );
+}

src/test/ui/rfc-2011-nicer-assert-messages/all-not-available-cases.rs

@@ -0,0 +1,42 @@
+// aux-build:common.rs
+// ignore-tidy-linelength
+// run-pass
+
+#![feature(core_intrinsics, generic_assert, generic_assert_internals)]
+
+extern crate common;
+
+#[derive(Clone, Copy, PartialEq)]
+struct CopyNoDebug(i32);
+
+#[derive(Debug, PartialEq)]
+struct NoCopyDebug(i32);
+
+#[derive(PartialEq)]
+struct NoCopyNoDebug(i32);
+
+fn main() {
+  // Has Copy but does not have Debug
+  common::test!(
+    let mut copy_no_debug = CopyNoDebug(1);
+    [ copy_no_debug == CopyNoDebug(3) ] => "Assertion failed: copy_no_debug == CopyNoDebug(3)\nWith captures:\n  copy_no_debug = N/A\n"
+  );
+
+  // Does not have Copy but has Debug
+  common::test!(
+    let mut no_copy_debug = NoCopyDebug(1);
+    [ no_copy_debug == NoCopyDebug(3) ] => "Assertion failed: no_copy_debug == NoCopyDebug(3)\nWith captures:\n  no_copy_debug = N/A\n"
+  );
+
+  // Does not have Copy and does not have Debug
+  common::test!(
+    let mut no_copy_no_debug = NoCopyNoDebug(1);
+    [ no_copy_no_debug == NoCopyNoDebug(3) ] => "Assertion failed: no_copy_no_debug == NoCopyNoDebug(3)\nWith captures:\n  no_copy_no_debug = N/A\n"
+  );
+
+  // Unevaluated (Expression short-circuited)
+  common::test!(
+    let mut elem = true;
+    [ false && elem ] => "Assertion failed: false && elem\nWith captures:\n  elem = N/A\n"
+  );
+}

src/test/ui/rfc-2011-nicer-assert-messages/assert-with-custom-errors-does-not-create-unnecessary-code.rs

@@ -0,0 +1,13 @@
+// compile-flags: --test
+// run-pass
+
+#![feature(core_intrinsics, generic_assert, generic_assert_internals)]
+
+#[should_panic(expected = "OMG!")]
+#[test]
+fn test() {
+  assert!(1 == 3, "OMG!");
+}
+
+fn main() {
+}

src/test/ui/rfc-2011-nicer-assert-messages/assert-without-captures-does-not-create-unnecessary-code.rs

@@ -0,0 +1,13 @@
+// aux-build:common.rs
+// run-pass
+
+#![feature(core_intrinsics, generic_assert, generic_assert_internals)]
+
+extern crate common;
+
+fn main() {
+  common::test!(
+    let mut _nothing = ();
+    [ 1 == 3 ] => "Assertion failed: 1 == 3"
+  );
+}

src/test/ui/rfc-2011-nicer-assert-messages/auxiliary/common.rs

@@ -0,0 +1,25 @@
+#[macro_export]
+macro_rules! test {
+  (
+    let mut $elem_ident:ident = $elem_expr:expr;
+    [ $($assert:tt)* ] => $msg:literal
+  ) => {
+    {
+      #[allow(unused_assignments, unused_mut, unused_variables)]
+      let rslt = std::panic::catch_unwind(|| {
+        let mut $elem_ident = $elem_expr;
+        assert!($($assert)*);
+      });
+      let err = rslt.unwrap_err();
+      if let Some(elem) = err.downcast_ref::<String>() {
+        assert_eq!(elem, &$msg);
+      }
+      else if let Some(elem) = err.downcast_ref::<&str>() {
+        assert_eq!(elem, &$msg);
+      }
+      else {
+        panic!("assert!( ... ) should return a string");
+      }
+    }
+  }
+}

src/test/ui/rfc-2011-nicer-assert-messages/feature-gate-generic_assert.rs

@@ -0,0 +1,26 @@
+// compile-flags: --test
+// ignore-tidy-linelength
+// run-pass
+
+#![feature(core_intrinsics, generic_assert, generic_assert_internals)]
+
+use std::fmt::{Debug, Formatter};
+
+#[derive(Clone, Copy, PartialEq)]
+struct CopyDebug(i32);
+
+impl Debug for CopyDebug {
+  fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), std::fmt::Error> {
+    f.write_str("With great power comes great electricity bills")
+  }
+}
+
+#[should_panic(expected = "Assertion failed: copy_debug == CopyDebug(3)\nWith captures:\n  copy_debug = With great power comes great electricity bills\n")]
+#[test]
+fn test() {
+  let copy_debug = CopyDebug(1);
+  assert!(copy_debug == CopyDebug(3));
+}
+
+fn main() {
+}

src/test/ui/rfc-2011-nicer-assert-messages/non-consuming-methods-have-optimized-codegen.rs

@@ -0,0 +1,32 @@
+// check-pass
+// compile-flags: -Z unpretty=expanded
+
+#![feature(core_intrinsics, generic_assert, generic_assert_internals)]
+
+fn arbitrary_consuming_method_for_demonstration_purposes() {
+    let elem = 1i32;
+    assert!(elem as usize);
+}
+
+fn addr_of() {
+    let elem = 1i32;
+    assert!(&elem);
+}
+
+fn binary() {
+    let elem = 1i32;
+    assert!(elem == 1);
+    assert!(elem >= 1);
+    assert!(elem > 0);
+    assert!(elem < 3);
+    assert!(elem <= 3);
+    assert!(elem != 3);
+}
+
+fn unary() {
+    let elem = &1i32;
+    assert!(*elem);
+}
+
+fn main() {
+}

src/test/ui/rfc-2011-nicer-assert-messages/non-consuming-methods-have-optimized-codegen.stdout

@@ -0,0 +1,147 @@
+#![feature(prelude_import)]
+#![no_std]
+// check-pass
+// compile-flags: -Z unpretty=expanded
+
+#![feature(core_intrinsics, generic_assert, generic_assert_internals)]
+#[prelude_import]
+use ::std::prelude::rust_2015::*;
+#[macro_use]
+extern crate std;
+
+fn arbitrary_consuming_method_for_demonstration_purposes() {
+    let elem = 1i32;
+    {
+        #[allow(unused_imports)]
+        use ::core::asserting::{TryCaptureGeneric, TryCapturePrintable};
+        let mut __capture0 = ::core::asserting::Capture::new();
+        let __local_bind0 = &elem;
+        if ::core::intrinsics::unlikely(!(*{
+                                    (&::core::asserting::Wrapper(__local_bind0)).try_capture(&mut __capture0);
+                                    __local_bind0
+                                } as usize)) {
+
+
+
+
+                {
+                    ::std::rt::panic_fmt(::core::fmt::Arguments::new_v1(&["Assertion failed: elem as usize\nWith captures:\n  elem = ",
+                                        "\n"], &[::core::fmt::ArgumentV1::new_debug(&__capture0)]))
+                }
+            }
+    };
+}
+fn addr_of() {
+    let elem = 1i32;
+    {
+        #[allow(unused_imports)]
+        use ::core::asserting::{TryCaptureGeneric, TryCapturePrintable};
+        let mut __capture0 = ::core::asserting::Capture::new();
+        let __local_bind0 = &elem;
+        if ::core::intrinsics::unlikely(!&*__local_bind0) {
+                (&::core::asserting::Wrapper(__local_bind0)).try_capture(&mut __capture0);
+                {
+                    ::std::rt::panic_fmt(::core::fmt::Arguments::new_v1(&["Assertion failed: &elem\nWith captures:\n  elem = ",
+                                        "\n"], &[::core::fmt::ArgumentV1::new_debug(&__capture0)]))
+                }
+            }
+    };
+}
+fn binary() {
+    let elem = 1i32;
+    {
+        #[allow(unused_imports)]
+        use ::core::asserting::{TryCaptureGeneric, TryCapturePrintable};
+        let mut __capture0 = ::core::asserting::Capture::new();
+        let __local_bind0 = &elem;
+        if ::core::intrinsics::unlikely(!(*__local_bind0 == 1)) {
+                (&::core::asserting::Wrapper(__local_bind0)).try_capture(&mut __capture0);
+                {
+                    ::std::rt::panic_fmt(::core::fmt::Arguments::new_v1(&["Assertion failed: elem == 1\nWith captures:\n  elem = ",
+                                        "\n"], &[::core::fmt::ArgumentV1::new_debug(&__capture0)]))
+                }
+            }
+    };
+    {
+        #[allow(unused_imports)]
+        use ::core::asserting::{TryCaptureGeneric, TryCapturePrintable};
+        let mut __capture0 = ::core::asserting::Capture::new();
+        let __local_bind0 = &elem;
+        if ::core::intrinsics::unlikely(!(*__local_bind0 >= 1)) {
+                (&::core::asserting::Wrapper(__local_bind0)).try_capture(&mut __capture0);
+                {
+                    ::std::rt::panic_fmt(::core::fmt::Arguments::new_v1(&["Assertion failed: elem >= 1\nWith captures:\n  elem = ",
+                                        "\n"], &[::core::fmt::ArgumentV1::new_debug(&__capture0)]))
+                }
+            }
+    };
+    {
+        #[allow(unused_imports)]
+        use ::core::asserting::{TryCaptureGeneric, TryCapturePrintable};
+        let mut __capture0 = ::core::asserting::Capture::new();
+        let __local_bind0 = &elem;
+        if ::core::intrinsics::unlikely(!(*__local_bind0 > 0)) {
+                (&::core::asserting::Wrapper(__local_bind0)).try_capture(&mut __capture0);
+                {
+                    ::std::rt::panic_fmt(::core::fmt::Arguments::new_v1(&["Assertion failed: elem > 0\nWith captures:\n  elem = ",
+                                        "\n"], &[::core::fmt::ArgumentV1::new_debug(&__capture0)]))
+                }
+            }
+    };
+    {
+        #[allow(unused_imports)]
+        use ::core::asserting::{TryCaptureGeneric, TryCapturePrintable};
+        let mut __capture0 = ::core::asserting::Capture::new();
+        let __local_bind0 = &elem;
+        if ::core::intrinsics::unlikely(!(*__local_bind0 < 3)) {
+                (&::core::asserting::Wrapper(__local_bind0)).try_capture(&mut __capture0);
+                {
+                    ::std::rt::panic_fmt(::core::fmt::Arguments::new_v1(&["Assertion failed: elem < 3\nWith captures:\n  elem = ",
+                                        "\n"], &[::core::fmt::ArgumentV1::new_debug(&__capture0)]))
+                }
+            }
+    };
+    {
+        #[allow(unused_imports)]
+        use ::core::asserting::{TryCaptureGeneric, TryCapturePrintable};
+        let mut __capture0 = ::core::asserting::Capture::new();
+        let __local_bind0 = &elem;
+        if ::core::intrinsics::unlikely(!(*__local_bind0 <= 3)) {
+                (&::core::asserting::Wrapper(__local_bind0)).try_capture(&mut __capture0);
+                {
+                    ::std::rt::panic_fmt(::core::fmt::Arguments::new_v1(&["Assertion failed: elem <= 3\nWith captures:\n  elem = ",
+                                        "\n"], &[::core::fmt::ArgumentV1::new_debug(&__capture0)]))
+                }
+            }
+    };
+    {
+        #[allow(unused_imports)]
+        use ::core::asserting::{TryCaptureGeneric, TryCapturePrintable};
+        let mut __capture0 = ::core::asserting::Capture::new();
+        let __local_bind0 = &elem;
+        if ::core::intrinsics::unlikely(!(*__local_bind0 != 3)) {
+                (&::core::asserting::Wrapper(__local_bind0)).try_capture(&mut __capture0);
+                {
+                    ::std::rt::panic_fmt(::core::fmt::Arguments::new_v1(&["Assertion failed: elem != 3\nWith captures:\n  elem = ",
+                                        "\n"], &[::core::fmt::ArgumentV1::new_debug(&__capture0)]))
+                }
+            }
+    };
+}
+fn unary() {
+    let elem = &1i32;
+    {
+        #[allow(unused_imports)]
+        use ::core::asserting::{TryCaptureGeneric, TryCapturePrintable};
+        let mut __capture0 = ::core::asserting::Capture::new();
+        let __local_bind0 = &elem;
+        if ::core::intrinsics::unlikely(!**__local_bind0) {
+                (&::core::asserting::Wrapper(__local_bind0)).try_capture(&mut __capture0);
+                {
+                    ::std::rt::panic_fmt(::core::fmt::Arguments::new_v1(&["Assertion failed: *elem\nWith captures:\n  elem = ",
+                                        "\n"], &[::core::fmt::ArgumentV1::new_debug(&__capture0)]))
+                }
+            }
+    };
+}
+fn main() {}