Closes rust-lang#50362

 - Point at format string position inside the formatting string
 - Explain that argument names can't start with an underscore

- Remove the "no nearest common ancestor found" case, because it's never
  hit in practise. (This means `closure_is_enclosed_by` can also be
  removed.)

- Add a comment about why `SmallVec` is used for the "seen" structures.

- Use `&Scope` instead of `Scope` to avoid some `map()` calls.

- Use `any(p)` instead of `position(p).is_some()`.

The grammar defines identifiers in terms of XID_start and XID_continue,
but this is referring to the unstable non_ascii_idents feature.
The documentation implies that non_ascii_idents is forthcoming, but this
is left over from pre-1.0 documentation; in reality, non_ascii_idents
has been without even an RFC for several years now, and will not be
stabilized anytime soon. Furthermore, according to the tracking issue at
rust-lang#28979 , it's highly
questionable whether or not this feature will use XID_start or
XID_continue even when or if non_ascii_idents is stabilized.
This commit fixes this by respecifying identifiers as the usual
[a-zA-Z_][a-zA-Z0-9_]*

The grammar documentation incorrectly says that comments, character literals,
and string literals may not include null.

There is a hot path through `opt_normalize_projection_type`:
- `try_start` does a cache lookup (#1).
- The result is a `NormalizedTy`.
- There are no unresolved type vars, so we call `complete`.
- `complete` does *another* cache lookup (#2), then calls
  `SnapshotMap::insert`.
- `insert` does *another* cache lookup (rust-lang#3), inserting the same value
  that's already in the cache.

This patch optimizes this hot path by introducing `complete_normalized`,
for use when the value is known in advance to be a `NormalizedTy`. It
always avoids lookup #2. Furthermore, if the `NormalizedTy`'s
obligations are empty (the common case), we know that lookup rust-lang#3 would be
a no-op, so we avoid it, while inserting a Noop into the `SnapshotMap`'s
undo log.

This patch changes `opt_normalize_project_type` so it appends
obligations to a given obligations vector, instead of returning a new
obligations vector.

This change avoids lots of allocations. In the most extreme case, for a
clean "Check" build of serde it reduces the total number of allocations
by 20%.

There was [some confusion](rust-lang#49767 (comment)) and I accidentally merged a PR that wasn't ready.

…crichton

Remove leftover tab in libtest outputs

This removes some tabs that were present in the output of libtest.

Related rust-lang#19299
Closes rust-lang#50362

Add Option::xor method

Implements the method requested in rust-lang#50512.

Improve format string errors

Point at format string position inside the formatting string:
```
error: invalid format string: unmatched `}` found
  --> $DIR/format-string-error.rs:21:22
   |
LL |     let _ = format!("}");
   |                      ^ unmatched `}` in format string
```

Explain that argument names can't start with an underscore:
```
error: invalid format string: invalid argument name `_foo`
  --> $DIR/format-string-error.rs:15:23
   |
LL |     let _ = format!("{_foo}", _foo = 6usize);
   |                       ^^^^ invalid argument name in format string
   |
   = note: argument names cannot start with an underscore
```

Fix rust-lang#23476.

The more accurate spans will only be seen when using `format!` directly, when using `println!` the diagnostics machinery makes the span be the entire statement.

…cestor, r=nikomatsakis

Tweak `nearest_common_ancestor()`.

- Remove the "no nearest common ancestor found" case, because it's never
  hit in practise. (This means `closure_is_enclosed_by` can also be
  removed.)

- Add a comment about why `SmallVec` is used for the "seen" structures.

- Use `&Scope` instead of `Scope` to avoid some `map()` calls.

- Use `any(p)` instead of `position(p).is_some()`.

r? @nikomatsakis

Fix grammar documentation wrt Unicode identifiers

The grammar defines identifiers in terms of XID_start and XID_continue,
but this is referring to the unstable non_ascii_idents feature.
The documentation implies that non_ascii_idents is forthcoming, but this
is left over from pre-1.0 documentation; in reality, non_ascii_idents
has been without even an RFC for several years now, and will not be
stabilized anytime soon. Furthermore, according to the tracking issue at
rust-lang#28979 , it's highly
questionable whether or not this feature will use XID_start or
XID_continue even when or if non_ascii_idents is stabilized.
This commit fixes this by respecifying identifiers as the usual
[a-zA-Z_][a-zA-Z0-9_]*

Fix null exclusions in grammar docs

The grammar documentation incorrectly says that comments, character literals,
and string literals may not include null.

Add `bless` x.py subcommand for easy ui test replacement

fixes rust-lang#49815

r? @nikomatsakis

…omatsakis

Speed up `opt_normalize_projection_type`

`opt_normalize_projection_type` is hot in the serde and futures benchmarks in rustc-perf. These two patches speed up the execution of most runs for them by 2--4%.

…isdreavus

Revert rust-lang#49767

There was [some confusion](rust-lang#49767 (comment)) and I accidentally merged a PR that wasn't ready.

Make sure people know the book is free oline

I've used the tutorial a number of times to relearn rust basics.  When i saw this for a moment I was sad thinking it had been taken offline.