Auto merge of #81784 - m-ou-se:rollup-s23fow7, r=m-ou-se

Rollup of 15 pull requests

Successful merges:

 - #79554 (Generic associated types in trait paths)
 - #80726 (relax adt unsizing requirements)
 - #81307 (Handle `Span`s for byte and raw strings and add more detail )
 - #81318 (rustdoc-json: Fix has_body)
 - #81456 (Make remote-test-server easier to use with new targets)
 - #81497 (rustdoc: Move `display_fn` struct inside `display_fn`)
 - #81500 (Remove struct_type from union output)
 - #81542 (Expose correct symlink API on WASI)
 - #81676 (Add more information to the error code for 'crate not found')
 - #81682 (Add additional bitset benchmarks)
 - #81730 (Make `Allocator` object-safe)
 - #81763 (Cleanup rustdoc pass descriptions a bit)
 - #81767 (Update LayoutError/LayoutErr stability attributes)
 - #81771 (Indicate change in RSS from start to end of pass in time-passes output)
 - #81781 (Fix `install-awscli.sh` error in CI)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup

17 files changed, 416 insertions, 187 deletions

diff --git a/compiler/rustc_ast_lowering/src/lib.rs b/compiler/rustc_ast_lowering/src/lib.rs
index b7f227da73e..f076dca5cf5 100644
--- a/compiler/rustc_ast_lowering/src/lib.rs
+++ b/compiler/rustc_ast_lowering/src/lib.rs
@@ -1076,16 +1076,40 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     fn lower_assoc_ty_constraint(
         &mut self,
         constraint: &AssocTyConstraint,
-        itctx: ImplTraitContext<'_, 'hir>,
+        mut itctx: ImplTraitContext<'_, 'hir>,
     ) -> hir::TypeBinding<'hir> {
         debug!("lower_assoc_ty_constraint(constraint={:?}, itctx={:?})", constraint, itctx);
 
-        if let Some(ref gen_args) = constraint.gen_args {
-            self.sess.span_fatal(
-                gen_args.span(),
-                "generic associated types in trait paths are currently not implemented",
-            );
-        }
+        // lower generic arguments of identifier in constraint
+        let gen_args = if let Some(ref gen_args) = constraint.gen_args {
+            let gen_args_ctor = match gen_args {
+                GenericArgs::AngleBracketed(ref data) => {
+                    self.lower_angle_bracketed_parameter_data(
+                        data,
+                        ParamMode::Explicit,
+                        itctx.reborrow(),
+                    )
+                    .0
+                }
+                GenericArgs::Parenthesized(ref data) => {
+                    let mut err = self.sess.struct_span_err(
+                        gen_args.span(),
+                        "parenthesized generic arguments cannot be used in associated type constraints"
+                    );
+                    // FIXME: try to write a suggestion here
+                    err.emit();
+                    self.lower_angle_bracketed_parameter_data(
+                        &data.as_angle_bracketed_args(),
+                        ParamMode::Explicit,
+                        itctx.reborrow(),
+                    )
+                    .0
+                }
+            };
+            self.arena.alloc(gen_args_ctor.into_generic_args(&self.arena))
+        } else {
+            self.arena.alloc(hir::GenericArgs::none())
+        };
 
         let kind = match constraint.kind {
             AssocTyConstraintKind::Equality { ref ty } => {
@@ -1182,6 +1206,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         hir::TypeBinding {
             hir_id: self.lower_node_id(constraint.id),
             ident: constraint.ident,
+            gen_args,
             kind,
             span: constraint.span,
         }
diff --git a/compiler/rustc_ast_lowering/src/path.rs b/compiler/rustc_ast_lowering/src/path.rs
index 9079e26eb50..cb4d5ea6ee6 100644
--- a/compiler/rustc_ast_lowering/src/path.rs
+++ b/compiler/rustc_ast_lowering/src/path.rs
@@ -362,7 +362,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         }
     }
 
-    fn lower_angle_bracketed_parameter_data(
+    pub(crate) fn lower_angle_bracketed_parameter_data(
         &mut self,
         data: &AngleBracketedArgs,
         param_mode: ParamMode,
@@ -426,6 +426,9 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     ) -> hir::TypeBinding<'hir> {
         let ident = Ident::with_dummy_span(hir::FN_OUTPUT_NAME);
         let kind = hir::TypeBindingKind::Equality { ty };
-        hir::TypeBinding { hir_id: self.next_id(), span, ident, kind }
+        let args = arena_vec![self;];
+        let bindings = arena_vec![self;];
+        let gen_args = self.arena.alloc(hir::GenericArgs { args, bindings, parenthesized: false });
+        hir::TypeBinding { hir_id: self.next_id(), gen_args, span, ident, kind }
     }
 }
diff --git a/compiler/rustc_data_structures/src/profiling.rs b/compiler/rustc_data_structures/src/profiling.rs
index 9a85b9d02c9..f0b413c795e 100644
--- a/compiler/rustc_data_structures/src/profiling.rs
+++ b/compiler/rustc_data_structures/src/profiling.rs
@@ -590,24 +590,21 @@ pub fn print_time_passes_entry(
     end_rss: Option<usize>,
 ) {
     let rss_to_mb = |rss| (rss as f64 / 1_000_000.0).round() as usize;
+    let rss_change_to_mb = |rss| (rss as f64 / 1_000_000.0).round() as i128;
 
     let mem_string = match (start_rss, end_rss) {
         (Some(start_rss), Some(end_rss)) => {
-            // It's tempting to add the change in RSS from start to end, but its somewhat confusing
-            // and misleading when looking at time-passes output. Consider two adjacent entries:
-            //
-            // time:  10.000; rss start:  1000MB, end:  1000MB, change:     0MB     pass1
-            // time:   5.000; rss start:  2000MB, end:  2000MB, change:     0MB     pass2
-            //
-            // If you're looking for jumps in RSS based on the change column, you miss the fact
-            // that a 1GB jump happened between pass1 and pass2 (supposing pass1 and pass2 actually
-            // occur sequentially and pass1 isn't just nested within pass2). It's easy to imagine
-            // someone missing this or being confused by the fact that the change is zero.
-
-            format!("; rss: {:>5}MB -> {:>5}MB", rss_to_mb(start_rss), rss_to_mb(end_rss))
+            let change_rss = end_rss as i128 - start_rss as i128;
+
+            format!(
+                "; rss: {:>4}MB -> {:>4}MB ({:>+5}MB)",
+                rss_to_mb(start_rss),
+                rss_to_mb(end_rss),
+                rss_change_to_mb(change_rss),
+            )
         }
-        (Some(start_rss), None) => format!("; rss start: {:>5}MB", rss_to_mb(start_rss)),
-        (None, Some(end_rss)) => format!("; rss end: {:5>}MB", rss_to_mb(end_rss)),
+        (Some(start_rss), None) => format!("; rss start: {:>4}MB", rss_to_mb(start_rss)),
+        (None, Some(end_rss)) => format!("; rss end: {:>4}MB", rss_to_mb(end_rss)),
         (None, None) => String::new(),
     };
 
diff --git a/compiler/rustc_error_codes/src/error_codes/E0463.md b/compiler/rustc_error_codes/src/error_codes/E0463.md
index e46938c607d..d0cd1b1dcb7 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0463.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0463.md
@@ -11,3 +11,24 @@ extern crate cake_is_a_lie; // error: can't find crate for `cake_is_a_lie`
 You need to link your code to the relevant crate in order to be able to use it
 (through Cargo or the `-L` option of rustc example). Plugins are crates as
 well, and you link to them the same way.
+
+## Common causes
+
+- The crate is not present at all. If using Cargo, add it to `[dependencies]`
+  in Cargo.toml.
+- The crate is present, but under a different name. If using Cargo, look for
+  `package = ` under `[dependencies]` in Cargo.toml.
+
+## Common causes for missing `std` or `core`
+
+- You are cross-compiling for a target which doesn't have `std` prepackaged.
+  Consider one of the following:
+  + Adding a pre-compiled version of std with `rustup target add`
+  + Building std from source with `cargo build -Z build-std`
+  + Using `#![no_std]` at the crate root, so you won't need `std` in the first
+    place.
+- You are developing the compiler itself and haven't built libstd from source.
+  You can usually build it with `x.py build library/std`. More information
+  about x.py is available in the [rustc-dev-guide].
+
+[rustc-dev-guide]: https://rustc-dev-guide.rust-lang.org/building/how-to-build-and-run.html#building-the-compiler
diff --git a/compiler/rustc_feature/src/active.rs b/compiler/rustc_feature/src/active.rs
index 818f6c70de0..4f38e060023 100644
--- a/compiler/rustc_feature/src/active.rs
+++ b/compiler/rustc_feature/src/active.rs
@@ -631,6 +631,9 @@ declare_features! (
 
     /// Allows `extern "C-cmse-nonsecure-call" fn()`.
     (active, abi_c_cmse_nonsecure_call, "1.51.0", Some(81391), None),
+
+    /// Lessens the requirements for structs to implement `Unsize`.
+    (active, relaxed_struct_unsize, "1.51.0", Some(1), None),
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------
diff --git a/compiler/rustc_hir/src/hir.rs b/compiler/rustc_hir/src/hir.rs
index 6487b23a6a6..67a15418ea4 100644
--- a/compiler/rustc_hir/src/hir.rs
+++ b/compiler/rustc_hir/src/hir.rs
@@ -2015,6 +2015,7 @@ pub struct TypeBinding<'hir> {
     pub hir_id: HirId,
     #[stable_hasher(project(name))]
     pub ident: Ident,
+    pub gen_args: &'hir GenericArgs<'hir>,
     pub kind: TypeBindingKind<'hir>,
     pub span: Span,
 }
diff --git a/compiler/rustc_hir/src/intravisit.rs b/compiler/rustc_hir/src/intravisit.rs
index 6c1bee2335a..f8b3f0d9b6e 100644
--- a/compiler/rustc_hir/src/intravisit.rs
+++ b/compiler/rustc_hir/src/intravisit.rs
@@ -781,6 +781,7 @@ pub fn walk_assoc_type_binding<'v, V: Visitor<'v>>(
 ) {
     visitor.visit_id(type_binding.hir_id);
     visitor.visit_ident(type_binding.ident);
+    visitor.visit_generic_args(type_binding.span, type_binding.gen_args);
     match type_binding.kind {
         TypeBindingKind::Equality { ref ty } => {
             visitor.visit_ty(ty);
diff --git a/compiler/rustc_hir_pretty/src/lib.rs b/compiler/rustc_hir_pretty/src/lib.rs
index f1c2a6b7e6e..4595855309f 100644
--- a/compiler/rustc_hir_pretty/src/lib.rs
+++ b/compiler/rustc_hir_pretty/src/lib.rs
@@ -1840,6 +1840,7 @@ impl<'a> State<'a> {
             for binding in generic_args.bindings.iter() {
                 start_or_comma(self);
                 self.print_ident(binding.ident);
+                self.print_generic_args(binding.gen_args, false, false);
                 self.s.space();
                 match generic_args.bindings[0].kind {
                     hir::TypeBindingKind::Equality { ref ty } => {
diff --git a/compiler/rustc_index/src/bit_set.rs b/compiler/rustc_index/src/bit_set.rs
index 0b501da7cd9..100824f4b94 100644
--- a/compiler/rustc_index/src/bit_set.rs
+++ b/compiler/rustc_index/src/bit_set.rs
@@ -707,6 +707,18 @@ impl<T: Idx> GrowableBitSet<T> {
         self.bit_set.insert(elem)
     }
 
+    /// Returns `true` if the set has changed.
+    #[inline]
+    pub fn remove(&mut self, elem: T) -> bool {
+        self.ensure(elem.index() + 1);
+        self.bit_set.remove(elem)
+    }
+
+    #[inline]
+    pub fn is_empty(&self) -> bool {
+        self.bit_set.is_empty()
+    }
+
     #[inline]
     pub fn contains(&self, elem: T) -> bool {
         let (word_index, mask) = word_index_and_mask(elem);
diff --git a/compiler/rustc_index/src/bit_set/tests.rs b/compiler/rustc_index/src/bit_set/tests.rs
index 6cc3e9427d1..c11b98e77aa 100644
--- a/compiler/rustc_index/src/bit_set/tests.rs
+++ b/compiler/rustc_index/src/bit_set/tests.rs
@@ -1,6 +1,7 @@
 use super::*;
 
 extern crate test;
+use std::hint::black_box;
 use test::Bencher;
 
 #[test]
@@ -364,3 +365,36 @@ fn union_hybrid_sparse_full_small_domain(b: &mut Bencher) {
         sparse.union(&dense);
     })
 }
+
+#[bench]
+fn bench_insert(b: &mut Bencher) {
+    let mut bs = BitSet::new_filled(99999usize);
+    b.iter(|| {
+        black_box(bs.insert(black_box(100u32)));
+    });
+}
+
+#[bench]
+fn bench_remove(b: &mut Bencher) {
+    let mut bs = BitSet::new_filled(99999usize);
+    b.iter(|| {
+        black_box(bs.remove(black_box(100u32)));
+    });
+}
+
+#[bench]
+fn bench_iter(b: &mut Bencher) {
+    let bs = BitSet::new_filled(99999usize);
+    b.iter(|| {
+        bs.iter().map(|b: usize| black_box(b)).for_each(drop);
+    });
+}
+
+#[bench]
+fn bench_intersect(b: &mut Bencher) {
+    let mut ba: BitSet<u32> = BitSet::new_filled(99999usize);
+    let bb = BitSet::new_filled(99999usize);
+    b.iter(|| {
+        ba.intersect(black_box(&bb));
+    });
+}
diff --git a/compiler/rustc_middle/src/ty/sty.rs b/compiler/rustc_middle/src/ty/sty.rs
index 6b4f08d9f93..c1fa84dcb25 100644
--- a/compiler/rustc_middle/src/ty/sty.rs
+++ b/compiler/rustc_middle/src/ty/sty.rs
@@ -1132,8 +1132,16 @@ impl<'tcx> ProjectionTy<'tcx> {
     /// For example, if this is a projection of `<T as Iterator>::Item`,
     /// then this function would return a `T: Iterator` trait reference.
     pub fn trait_ref(&self, tcx: TyCtxt<'tcx>) -> ty::TraitRef<'tcx> {
+        // FIXME: This method probably shouldn't exist at all, since it's not
+        // clear what this method really intends to do. Be careful when
+        // using this method since the resulting TraitRef additionally
+        // contains the substs for the assoc_item, which strictly speaking
+        // is not correct
         let def_id = tcx.associated_item(self.item_def_id).container.id();
-        ty::TraitRef { def_id, substs: self.substs.truncate_to(tcx, tcx.generics_of(def_id)) }
+        // Include substitutions for generic arguments of associated types
+        let assoc_item = tcx.associated_item(self.item_def_id);
+        let substs_assoc_item = self.substs.truncate_to(tcx, tcx.generics_of(assoc_item.def_id));
+        ty::TraitRef { def_id, substs: substs_assoc_item }
     }
 
     pub fn self_ty(&self) -> Ty<'tcx> {
diff --git a/compiler/rustc_parse/src/lexer/mod.rs b/compiler/rustc_parse/src/lexer/mod.rs
index b5b34c7338d..4a638ec3f80 100644
--- a/compiler/rustc_parse/src/lexer/mod.rs
+++ b/compiler/rustc_parse/src/lexer/mod.rs
@@ -14,7 +14,7 @@ mod tokentrees;
 mod unescape_error_reporting;
 mod unicode_chars;
 
-use unescape_error_reporting::{emit_unescape_error, push_escaped_char};
+use unescape_error_reporting::{emit_unescape_error, escaped_char};
 
 #[derive(Clone, Debug)]
 pub struct UnmatchedBrace {
@@ -122,11 +122,9 @@ impl<'a> StringReader<'a> {
         m: &str,
         c: char,
     ) -> DiagnosticBuilder<'a> {
-        let mut m = m.to_string();
-        m.push_str(": ");
-        push_escaped_char(&mut m, c);
-
-        self.sess.span_diagnostic.struct_span_fatal(self.mk_sp(from_pos, to_pos), &m[..])
+        self.sess
+            .span_diagnostic
+            .struct_span_fatal(self.mk_sp(from_pos, to_pos), &format!("{}: {}", m, escaped_char(c)))
     }
 
     /// Turns simple `rustc_lexer::TokenKind` enum into a rich
@@ -421,7 +419,7 @@ impl<'a> StringReader<'a> {
         let content_start = start + BytePos(prefix_len);
         let content_end = suffix_start - BytePos(postfix_len);
         let id = self.symbol_from_to(content_start, content_end);
-        self.validate_literal_escape(mode, content_start, content_end);
+        self.validate_literal_escape(mode, content_start, content_end, prefix_len, postfix_len);
         (lit_kind, id)
     }
 
@@ -525,17 +523,29 @@ impl<'a> StringReader<'a> {
         .raise();
     }
 
-    fn validate_literal_escape(&self, mode: Mode, content_start: BytePos, content_end: BytePos) {
+    fn validate_literal_escape(
+        &self,
+        mode: Mode,
+        content_start: BytePos,
+        content_end: BytePos,
+        prefix_len: u32,
+        postfix_len: u32,
+    ) {
         let lit_content = self.str_from_to(content_start, content_end);
         unescape::unescape_literal(lit_content, mode, &mut |range, result| {
             // Here we only check for errors. The actual unescaping is done later.
             if let Err(err) = result {
-                let span_with_quotes =
-                    self.mk_sp(content_start - BytePos(1), content_end + BytePos(1));
+                let span_with_quotes = self
+                    .mk_sp(content_start - BytePos(prefix_len), content_end + BytePos(postfix_len));
+                let (start, end) = (range.start as u32, range.end as u32);
+                let lo = content_start + BytePos(start);
+                let hi = lo + BytePos(end - start);
+                let span = self.mk_sp(lo, hi);
                 emit_unescape_error(
                     &self.sess.span_diagnostic,
                     lit_content,
                     span_with_quotes,
+                    span,
                     mode,
                     range,
                     err,
diff --git a/compiler/rustc_parse/src/lexer/unescape_error_reporting.rs b/compiler/rustc_parse/src/lexer/unescape_error_reporting.rs
index 47d317f9188..a580f0c55d0 100644
--- a/compiler/rustc_parse/src/lexer/unescape_error_reporting.rs
+++ b/compiler/rustc_parse/src/lexer/unescape_error_reporting.rs
@@ -13,6 +13,8 @@ pub(crate) fn emit_unescape_error(
     lit: &str,
     // full span of the literal, including quotes
     span_with_quotes: Span,
+    // interior span of the literal, without quotes
+    span: Span,
     mode: Mode,
     // range of the error inside `lit`
     range: Range<usize>,
@@ -26,13 +28,6 @@ pub(crate) fn emit_unescape_error(
         range,
         error
     );
-    let span = {
-        let Range { start, end } = range;
-        let (start, end) = (start as u32, end as u32);
-        let lo = span_with_quotes.lo() + BytePos(start + 1);
-        let hi = lo + BytePos(end - start);
-        span_with_quotes.with_lo(lo).with_hi(hi)
-    };
     let last_char = || {
         let c = lit[range.clone()].chars().rev().next().unwrap();
         let span = span.with_lo(span.hi() - BytePos(c.len_utf8() as u32));
@@ -42,20 +37,22 @@ pub(crate) fn emit_unescape_error(
         EscapeError::LoneSurrogateUnicodeEscape => {
             handler
                 .struct_span_err(span, "invalid unicode character escape")
+                .span_label(span, "invalid escape")
                 .help("unicode escape must not be a surrogate")
                 .emit();
         }
         EscapeError::OutOfRangeUnicodeEscape => {
             handler
                 .struct_span_err(span, "invalid unicode character escape")
+                .span_label(span, "invalid escape")
                 .help("unicode escape must be at most 10FFFF")
                 .emit();
         }
         EscapeError::MoreThanOneChar => {
-            let msg = if mode.is_bytes() {
-                "if you meant to write a byte string literal, use double quotes"
+            let (prefix, msg) = if mode.is_bytes() {
+                ("b", "if you meant to write a byte string literal, use double quotes")
             } else {
-                "if you meant to write a `str` literal, use double quotes"
+                ("", "if you meant to write a `str` literal, use double quotes")
             };
 
             handler
@@ -66,31 +63,44 @@ pub(crate) fn emit_unescape_error(
                 .span_suggestion(
                     span_with_quotes,
                     msg,
-                    format!("\"{}\"", lit),
+                    format!("{}\"{}\"", prefix, lit),
                     Applicability::MachineApplicable,
                 )
                 .emit();
         }
         EscapeError::EscapeOnlyChar => {
-            let (c, _span) = last_char();
+            let (c, char_span) = last_char();
 
-            let mut msg = if mode.is_bytes() {
-                "byte constant must be escaped: "
+            let msg = if mode.is_bytes() {
+                "byte constant must be escaped"
             } else {
-                "character constant must be escaped: "
-            }
-            .to_string();
-            push_escaped_char(&mut msg, c);
-
-            handler.span_err(span, msg.as_str())
+                "character constant must be escaped"
+            };
+            handler
+                .struct_span_err(span, &format!("{}: `{}`", msg, escaped_char(c)))
+                .span_suggestion(
+                    char_span,
+                    "escape the character",
+                    c.escape_default().to_string(),
+                    Applicability::MachineApplicable,
+                )
+                .emit()
         }
         EscapeError::BareCarriageReturn => {
             let msg = if mode.in_double_quotes() {
-                "bare CR not allowed in string, use \\r instead"
+                "bare CR not allowed in string, use `\\r` instead"
             } else {
-                "character constant must be escaped: \\r"
+                "character constant must be escaped: `\\r`"
             };
-            handler.span_err(span, msg);
+            handler
+                .struct_span_err(span, msg)
+                .span_suggestion(
+                    span,
+                    "escape the character",
+                    "\\r".to_string(),
+                    Applicability::MachineApplicable,
+                )
+                .emit();
         }
         EscapeError::BareCarriageReturnInRawString => {
             assert!(mode.in_double_quotes());
@@ -102,21 +112,22 @@ pub(crate) fn emit_unescape_error(
 
             let label =
                 if mode.is_bytes() { "unknown byte escape" } else { "unknown character escape" };
-            let mut msg = label.to_string();
-            msg.push_str(": ");
-            push_escaped_char(&mut msg, c);
-
-            let mut diag = handler.struct_span_err(span, msg.as_str());
+            let ec = escaped_char(c);
+            let mut diag = handler.struct_span_err(span, &format!("{}: `{}`", label, ec));
             diag.span_label(span, label);
             if c == '{' || c == '}' && !mode.is_bytes() {
                 diag.help(
-                    "if used in a formatting string, \
-                           curly braces are escaped with `{{` and `}}`",
+                    "if used in a formatting string, curly braces are escaped with `{{` and `}}`",
                 );
             } else if c == '\r' {
                 diag.help(
-                    "this is an isolated carriage return; \
-                           consider checking your editor and version control settings",
+                    "this is an isolated carriage return; consider checking your editor and \
+                     version control settings",
+                );
+            } else {
+                diag.help(
+                    "for more information, visit \
+                     <https://static.rust-lang.org/doc/master/reference.html#literals>",
                 );
             }
             diag.emit();
@@ -127,45 +138,70 @@ pub(crate) fn emit_unescape_error(
         EscapeError::InvalidCharInHexEscape | EscapeError::InvalidCharInUnicodeEscape => {
             let (c, span) = last_char();
 
-            let mut msg = if error == EscapeError::InvalidCharInHexEscape {
-                "invalid character in numeric character escape: "
+            let msg = if error == EscapeError::InvalidCharInHexEscape {
+                "invalid character in numeric character escape"
             } else {
-                "invalid character in unicode escape: "
-            }
-            .to_string();
-            push_escaped_char(&mut msg, c);
+                "invalid character in unicode escape"
+            };
+            let c = escaped_char(c);
 
-            handler.span_err(span, msg.as_str())
+            handler
+                .struct_span_err(span, &format!("{}: `{}`", msg, c))
+                .span_label(span, msg)
+                .emit();
         }
         EscapeError::NonAsciiCharInByte => {
             assert!(mode.is_bytes());
-            let (_c, span) = last_char();
-            handler.span_err(
-                span,
-                "byte constant must be ASCII. \
-                                    Use a \\xHH escape for a non-ASCII byte",
-            )
+            let (c, span) = last_char();
+            handler
+                .struct_span_err(span, "non-ASCII character in byte constant")
+                .span_label(span, "byte constant must be ASCII")
+                .span_suggestion(
+                    span,
+                    "use a \\xHH escape for a non-ASCII byte",
+                    format!("\\x{:X}", c as u32),
+                    Applicability::MachineApplicable,
+                )
+                .emit();
         }
         EscapeError::NonAsciiCharInByteString => {
             assert!(mode.is_bytes());
             let (_c, span) = last_char();
-            handler.span_err(span, "raw byte string must be ASCII")
+            handler
+                .struct_span_err(span, "raw byte string must be ASCII")
+                .span_label(span, "must be ASCII")
+                .emit();
+        }
+        EscapeError::OutOfRangeHexEscape => {
+            handler
+                .struct_span_err(span, "out of range hex escape")
+                .span_label(span, "must be a character in the range [\\x00-\\x7f]")
+                .emit();
         }
-        EscapeError::OutOfRangeHexEscape => handler.span_err(
-            span,
-            "this form of character escape may only be used \
-                                    with characters in the range [\\x00-\\x7f]",
-        ),
         EscapeError::LeadingUnderscoreUnicodeEscape => {
-            let (_c, span) = last_char();
-            handler.span_err(span, "invalid start of unicode escape")
+            let (c, span) = last_char();
+            let msg = "invalid start of unicode escape";
+            handler
+                .struct_span_err(span, &format!("{}: `{}`", msg, c))
+                .span_label(span, msg)
+                .emit();
         }
         EscapeError::OverlongUnicodeEscape => {
-            handler.span_err(span, "overlong unicode escape (must have at most 6 hex digits)")
-        }
-        EscapeError::UnclosedUnicodeEscape => {
-            handler.span_err(span, "unterminated unicode escape (needed a `}`)")
+            handler
+                .struct_span_err(span, "overlong unicode escape")
+                .span_label(span, "must have at most 6 hex digits")
+                .emit();
         }
+        EscapeError::UnclosedUnicodeEscape => handler
+            .struct_span_err(span, "unterminated unicode escape")
+            .span_label(span, "missing a closing `}`")
+            .span_suggestion_verbose(
+                span.shrink_to_hi(),
+                "terminate the unicode escape",
+                "}".to_string(),
+                Applicability::MaybeIncorrect,
+            )
+            .emit(),
         EscapeError::NoBraceInUnicodeEscape => {
             let msg = "incorrect unicode escape sequence";
             let mut diag = handler.struct_span_err(span, msg);
@@ -195,28 +231,38 @@ pub(crate) fn emit_unescape_error(
 
             diag.emit();
         }
-        EscapeError::UnicodeEscapeInByte => handler.span_err(
-            span,
-            "unicode escape sequences cannot be used \
-                                    as a byte or in a byte string",
-        ),
+        EscapeError::UnicodeEscapeInByte => {
+            let msg = "unicode escape in byte string";
+            handler
+                .struct_span_err(span, msg)
+                .span_label(span, msg)
+                .help("unicode escape sequences cannot be used as a byte or in a byte string")
+                .emit();
+        }
         EscapeError::EmptyUnicodeEscape => {
-            handler.span_err(span, "empty unicode escape (must have at least 1 hex digit)")
+            handler
+                .struct_span_err(span, "empty unicode escape")
+                .span_label(span, "this escape must have at least 1 hex digit")
+                .emit();
+        }
+        EscapeError::ZeroChars => {
+            let msg = "empty character literal";
+            handler.struct_span_err(span, msg).span_label(span, msg).emit()
+        }
+        EscapeError::LoneSlash => {
+            let msg = "invalid trailing slash in literal";
+            handler.struct_span_err(span, msg).span_label(span, msg).emit();
         }
-        EscapeError::ZeroChars => handler.span_err(span, "empty character literal"),
-        EscapeError::LoneSlash => handler.span_err(span, "invalid trailing slash in literal"),
     }
 }
 
 /// Pushes a character to a message string for error reporting
-pub(crate) fn push_escaped_char(msg: &mut String, c: char) {
+pub(crate) fn escaped_char(c: char) -> String {
     match c {
         '\u{20}'..='\u{7e}' => {
             // Don't escape \, ' or " for user-facing messages
-            msg.push(c);
-        }
-        _ => {
-            msg.extend(c.escape_default());
+            c.to_string()
         }
+        _ => c.escape_default().to_string(),
     }
 }
diff --git a/compiler/rustc_span/src/symbol.rs b/compiler/rustc_span/src/symbol.rs
index df23b4006b3..86f8061a24a 100644
--- a/compiler/rustc_span/src/symbol.rs
+++ b/compiler/rustc_span/src/symbol.rs
@@ -907,6 +907,7 @@ symbols! {
         register_attr,
         register_tool,
         relaxed_adts,
+        relaxed_struct_unsize,
         rem,
         rem_assign,
         repr,
diff --git a/compiler/rustc_trait_selection/src/traits/object_safety.rs b/compiler/rustc_trait_selection/src/traits/object_safety.rs
index 3852005ee3f..e155f0366e1 100644
--- a/compiler/rustc_trait_selection/src/traits/object_safety.rs
+++ b/compiler/rustc_trait_selection/src/traits/object_safety.rs
@@ -257,13 +257,11 @@ fn predicates_reference_self(
 }
 
 fn bounds_reference_self(tcx: TyCtxt<'_>, trait_def_id: DefId) -> SmallVec<[Span; 1]> {
-    let trait_ref = ty::Binder::dummy(ty::TraitRef::identity(tcx, trait_def_id));
     tcx.associated_items(trait_def_id)
         .in_definition_order()
         .filter(|item| item.kind == ty::AssocKind::Type)
         .flat_map(|item| tcx.explicit_item_bounds(item.def_id))
-        .map(|&(predicate, sp)| (predicate.subst_supertrait(tcx, &trait_ref), sp))
-        .filter_map(|predicate| predicate_references_self(tcx, predicate))
+        .filter_map(|pred_span| predicate_references_self(tcx, *pred_span))
         .collect()
 }
 
diff --git a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
index 3015188fd44..ed3e117fcfa 100644
--- a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
@@ -823,33 +823,59 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     },
                 };
 
+                // FIXME(eddyb) cache this (including computing `unsizing_params`)
+                // by putting it in a query; it would only need the `DefId` as it
+                // looks at declared field types, not anything substituted.
+
                 // The last field of the structure has to exist and contain type/const parameters.
                 let (tail_field, prefix_fields) =
                     def.non_enum_variant().fields.split_last().ok_or(Unimplemented)?;
                 let tail_field_ty = tcx.type_of(tail_field.did);
 
                 let mut unsizing_params = GrowableBitSet::new_empty();
-                let mut found = false;
-                for arg in tail_field_ty.walk() {
-                    if let Some(i) = maybe_unsizing_param_idx(arg) {
-                        unsizing_params.insert(i);
-                        found = true;
+                if tcx.features().relaxed_struct_unsize {
+                    for arg in tail_field_ty.walk() {
+                        if let Some(i) = maybe_unsizing_param_idx(arg) {
+                            unsizing_params.insert(i);
+                        }
                     }
-                }
-                if !found {
-                    return Err(Unimplemented);
-                }
 
-                // Ensure none of the other fields mention the parameters used
-                // in unsizing.
-                // FIXME(eddyb) cache this (including computing `unsizing_params`)
-                // by putting it in a query; it would only need the `DefId` as it
-                // looks at declared field types, not anything substituted.
-                for field in prefix_fields {
-                    for arg in tcx.type_of(field.did).walk() {
+                    // Ensure none of the other fields mention the parameters used
+                    // in unsizing.
+                    for field in prefix_fields {
+                        for arg in tcx.type_of(field.did).walk() {
+                            if let Some(i) = maybe_unsizing_param_idx(arg) {
+                                unsizing_params.remove(i);
+                            }
+                        }
+                    }
+
+                    if unsizing_params.is_empty() {
+                        return Err(Unimplemented);
+                    }
+                } else {
+                    let mut found = false;
+                    for arg in tail_field_ty.walk() {
                         if let Some(i) = maybe_unsizing_param_idx(arg) {
-                            if unsizing_params.contains(i) {
-                                return Err(Unimplemented);
+                            unsizing_params.insert(i);
+                            found = true;
+                        }
+                    }
+                    if !found {
+                        return Err(Unimplemented);
+                    }
+
+                    // Ensure none of the other fields mention the parameters used
+                    // in unsizing.
+                    // FIXME(eddyb) cache this (including computing `unsizing_params`)
+                    // by putting it in a query; it would only need the `DefId` as it
+                    // looks at declared field types, not anything substituted.
+                    for field in prefix_fields {
+                        for arg in tcx.type_of(field.did).walk() {
+                            if let Some(i) = maybe_unsizing_param_idx(arg) {
+                                if unsizing_params.contains(i) {
+                                    return Err(Unimplemented);
+                                }
                             }
                         }
                     }
diff --git a/compiler/rustc_typeck/src/astconv/mod.rs b/compiler/rustc_typeck/src/astconv/mod.rs
index 437813ea41b..5659345f0ff 100644
--- a/compiler/rustc_typeck/src/astconv/mod.rs
+++ b/compiler/rustc_typeck/src/astconv/mod.rs
@@ -112,12 +112,15 @@ pub enum SizedByDefault {
     No,
 }
 
+#[derive(Debug)]
 struct ConvertedBinding<'a, 'tcx> {
     item_name: Ident,
     kind: ConvertedBindingKind<'a, 'tcx>,
+    gen_args: &'a GenericArgs<'a>,
     span: Span,
 }
 
+#[derive(Debug)]
 enum ConvertedBindingKind<'a, 'tcx> {
     Equality(Ty<'tcx>),
     Constraint(&'a [hir::GenericBound<'a>]),
@@ -323,6 +326,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
 
         let tcx = self.tcx();
         let generics = tcx.generics_of(def_id);
+        debug!("generics: {:?}", generics);
 
         if generics.has_self {
             if generics.parent.is_some() {
@@ -557,7 +561,12 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                         ConvertedBindingKind::Constraint(bounds)
                     }
                 };
-                ConvertedBinding { item_name: binding.ident, kind, span: binding.span }
+                ConvertedBinding {
+                    item_name: binding.ident,
+                    kind,
+                    gen_args: binding.gen_args,
+                    span: binding.span,
+                }
             })
             .collect();
 
@@ -918,60 +927,27 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         dup_bindings: &mut FxHashMap<DefId, Span>,
         path_span: Span,
     ) -> Result<(), ErrorReported> {
-        let tcx = self.tcx();
-
-        if !speculative {
-            // Given something like `U: SomeTrait<T = X>`, we want to produce a
-            // predicate like `<U as SomeTrait>::T = X`. This is somewhat
-            // subtle in the event that `T` is defined in a supertrait of
-            // `SomeTrait`, because in that case we need to upcast.
-            //
-            // That is, consider this case:
-            //
-            // ```
-            // trait SubTrait: SuperTrait<i32> { }
-            // trait SuperTrait<A> { type T; }
-            //
-            // ... B: SubTrait<T = foo> ...
-            // ```
-            //
-            // We want to produce `<B as SuperTrait<i32>>::T == foo`.
-
-            // Find any late-bound regions declared in `ty` that are not
-            // declared in the trait-ref. These are not well-formed.
-            //
-            // Example:
-            //
-            //     for<'a> <T as Iterator>::Item = &'a str // <-- 'a is bad
-            //     for<'a> <T as FnMut<(&'a u32,)>>::Output = &'a str // <-- 'a is ok
-            if let ConvertedBindingKind::Equality(ty) = binding.kind {
-                let late_bound_in_trait_ref =
-                    tcx.collect_constrained_late_bound_regions(&trait_ref);
-                let late_bound_in_ty =
-                    tcx.collect_referenced_late_bound_regions(&ty::Binder::bind(ty));
-                debug!("late_bound_in_trait_ref = {:?}", late_bound_in_trait_ref);
-                debug!("late_bound_in_ty = {:?}", late_bound_in_ty);
+        // Given something like `U: SomeTrait<T = X>`, we want to produce a
+        // predicate like `<U as SomeTrait>::T = X`. This is somewhat
+        // subtle in the event that `T` is defined in a supertrait of
+        // `SomeTrait`, because in that case we need to upcast.
+        //
+        // That is, consider this case:
+        //
+        // ```
+        // trait SubTrait: SuperTrait<i32> { }
+        // trait SuperTrait<A> { type T; }
+        //
+        // ... B: SubTrait<T = foo> ...
+        // ```
+        //
+        // We want to produce `<B as SuperTrait<i32>>::T == foo`.
 
-                // FIXME: point at the type params that don't have appropriate lifetimes:
-                // struct S1<F: for<'a> Fn(&i32, &i32) -> &'a i32>(F);
-                //                         ----  ----     ^^^^^^^
-                self.validate_late_bound_regions(
-                    late_bound_in_trait_ref,
-                    late_bound_in_ty,
-                    |br_name| {
-                        struct_span_err!(
-                            tcx.sess,
-                            binding.span,
-                            E0582,
-                            "binding for associated type `{}` references {}, \
-                             which does not appear in the trait input types",
-                            binding.item_name,
-                            br_name
-                        )
-                    },
-                );
-            }
-        }
+        debug!(
+            "add_predicates_for_ast_type_binding(hir_ref_id {:?}, trait_ref {:?}, binding {:?}, bounds {:?}",
+            hir_ref_id, trait_ref, binding, bounds
+        );
+        let tcx = self.tcx();
 
         let candidate =
             if self.trait_defines_associated_type_named(trait_ref.def_id(), binding.item_name) {
@@ -1030,6 +1006,72 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                 .or_insert(binding.span);
         }
 
+        // Include substitutions for generic parameters of associated types
+        let projection_ty = candidate.map_bound(|trait_ref| {
+            let item_segment = hir::PathSegment {
+                ident: assoc_ty.ident,
+                hir_id: None,
+                res: None,
+                args: Some(binding.gen_args),
+                infer_args: false,
+            };
+
+            let substs_trait_ref_and_assoc_item = self.create_substs_for_associated_item(
+                tcx,
+                path_span,
+                assoc_ty.def_id,
+                &item_segment,
+                trait_ref.substs,
+            );
+
+            debug!(
+                "add_predicates_for_ast_type_binding: substs for trait-ref and assoc_item: {:?}",
+                substs_trait_ref_and_assoc_item
+            );
+
+            ty::ProjectionTy {
+                item_def_id: assoc_ty.def_id,
+                substs: substs_trait_ref_and_assoc_item,
+            }
+        });
+
+        if !speculative {
+            // Find any late-bound regions declared in `ty` that are not
+            // declared in the trait-ref or assoc_ty. These are not well-formed.
+            //
+            // Example:
+            //
+            //     for<'a> <T as Iterator>::Item = &'a str // <-- 'a is bad
+            //     for<'a> <T as FnMut<(&'a u32,)>>::Output = &'a str // <-- 'a is ok
+            if let ConvertedBindingKind::Equality(ty) = binding.kind {
+                let late_bound_in_trait_ref =
+                    tcx.collect_constrained_late_bound_regions(&projection_ty);
+                let late_bound_in_ty =
+                    tcx.collect_referenced_late_bound_regions(&ty::Binder::bind(ty));
+                debug!("late_bound_in_trait_ref = {:?}", late_bound_in_trait_ref);
+                debug!("late_bound_in_ty = {:?}", late_bound_in_ty);
+
+                // FIXME: point at the type params that don't have appropriate lifetimes:
+                // struct S1<F: for<'a> Fn(&i32, &i32) -> &'a i32>(F);
+                //                         ----  ----     ^^^^^^^
+                self.validate_late_bound_regions(
+                    late_bound_in_trait_ref,
+                    late_bound_in_ty,
+                    |br_name| {
+                        struct_span_err!(
+                            tcx.sess,
+                            binding.span,
+                            E0582,
+                            "binding for associated type `{}` references {}, \
+                             which does not appear in the trait input types",
+                            binding.item_name,
+                            br_name
+                        )
+                    },
+                );
+            }
+        }
+
         match binding.kind {
             ConvertedBindingKind::Equality(ref ty) => {
                 // "Desugar" a constraint like `T: Iterator<Item = u32>` this to
@@ -1037,13 +1079,12 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                 //
                 // `<T as Iterator>::Item = u32`
                 bounds.projection_bounds.push((
-                    candidate.map_bound(|trait_ref| ty::ProjectionPredicate {
-                        projection_ty: ty::ProjectionTy::from_ref_and_name(
-                            tcx,
-                            trait_ref,
-                            binding.item_name,
-                        ),
-                        ty,
+                    projection_ty.map_bound(|projection_ty| {
+                        debug!(
+                            "add_predicates_for_ast_type_binding: projection_ty {:?}, substs: {:?}",
+                            projection_ty, projection_ty.substs
+                        );
+                        ty::ProjectionPredicate { projection_ty, ty }
                     }),
                     binding.span,
                 ));
@@ -1055,7 +1096,8 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                 //
                 // Calling `skip_binder` is okay, because `add_bounds` expects the `param_ty`
                 // parameter to have a skipped binder.
-                let param_ty = tcx.mk_projection(assoc_ty.def_id, candidate.skip_binder().substs);
+                let param_ty =
+                    tcx.mk_projection(assoc_ty.def_id, projection_ty.skip_binder().substs);
                 self.add_bounds(param_ty, ast_bounds, bounds);
             }
         }