auto merge of #19027 : nick29581/rust/coercions-4, r=alexcrichton

The forwards compatible parts of #18645, rebased. Converts implicit coercions from `[T, ..n]` to `&[T]` into explicit references.

diff --cc src/libcore/option.rs
Simple merge

diff --cc src/librustc/metadata/tydecode.rs
Simple merge

diff --cc src/librustc/middle/astencode.rs
Simple merge

diff --cc src/librustc/middle/check_match.rs
Simple merge

diff --cc src/librustc/middle/trans/_match.rs
Simple merge

diff --cc src/librustc/middle/trans/base.rs
Simple merge

diff --cc src/librustc/middle/trans/common.rs
Simple merge

diff --cc src/librustc/middle/trans/consts.rs
Simple merge

diff --cc src/librustc/middle/trans/context.rs
Simple merge

diff --cc src/librustc/middle/trans/debuginfo.rs
Simple merge

diff --cc src/librustc/middle/trans/expr.rs
Simple merge

diff --cc src/librustc/middle/trans/meth.rs
Simple merge

diff --cc src/librustc/middle/trans/type_of.rs
Simple merge

diff --cc src/librustc/middle/ty.rs
Simple merge

diff --cc src/librustc/middle/typeck/coherence/mod.rs
Simple merge