Parser: Fix spans of explicit self arg idents
enable parallel codegen by default

Enable parallel codegen (2 units) by default when --opt-level is 0 or 1.  This
gives a minor speedup on large crates (~10%), with only a tiny slowdown (~2%)
for small ones (which usually build in under a second regardless).  The current
default (no parallelization) is used when the user requests optimization
(--opt-level 2 or 3), and when the user has enabled LTO (which is incompatible
with parallel codegen).

This commit also changes the rust build system to use parallel codegen
when appropriate.  This means codegen-units=4 for stage0 always, and
also for stage1 and stage2 when configured with --disable-optimize.
(Other settings use codegen-units=1 for stage1 and stage2, to get
maximum performance for release binaries.)  The build system also sets
codegen-units=1 for compiletest tests (compiletest does its own
parallelization) and uses the same setting as stage2 for crate tests.
auto merge of #18213 : pcwalton/rust/pcg-default, r=aturon

Enable parallel codegen (2 units) by default when --opt-level is 0 or 1.  This
gives a minor speedup on large crates (~10%), with only a tiny slowdown (~2%)
for small ones (which usually build in under a second regardless).  The current
default (no parallelization) is used when the user requests optimization
(--opt-level 2 or 3), and when the user has enabled LTO (which is incompatible
with parallel codegen).

This commit also changes the rust build system to use parallel codegen
when appropriate.  This means codegen-units=4 for stage0 always, and
also for stage1 and stage2 when configured with --disable-optimize.
(Other settings use codegen-units=1 for stage1 and stage2, to get
maximum performance for release binaries.)  The build system also sets
codegen-units=1 for compiletest tests (compiletest does its own
parallelization) and uses the same setting as stage2 for crate tests.

r? @aturon
auto merge of #18141 : phildawes/rust/master, r=brson

Hello! I noticed spans are wrong for the PatIdents of self args. (I use spans a lot in racer)
Add Span and separate open/close delims to TTDelim

This came up when working [on the gl-rs generator extension](https://github.com/bjz/gl-rs/blob/990383de801bd2e233159d5be07c9b5622827620/src/gl_generator/lib.rs#L135-L146).

The new definition of  `TTDelim` adds an associated `Span` that covers the whole token tree and enforces the invariant that a delimetered sequence of token trees must have an opening and closing delimeter.

A `get_span` method has also been added to `TokenTree` type to make it easier to implement better error messages for syntax extensions.