Fix typo in char's from_digit error message
Improve the error message for missing else clauses in if expressions
Fix issue #17999 (Unused variables inside `for` are not detected)
auto merge of #17876 : ruud-v-a/rust/patch-1, r=alexcrichton

The Windows-specific instruction under 'Quick Start' linked the wiki article on getting started developing Rust itself, but the quick start is just about obtaining a working Rust installation. The actual wiki page with Windows-specific instructions was difficult to find.

The most important thing to note on Windows is that you need mingw-builds, and it is totally not obvious. The only place where I have seen it mentioned is the wiki page, which was difficult to find before.
rollup merge of #17888 : gmfawcett/patch-1
rollup merge of #17927 : alexcrichton/more-const
rollup merge of #17968 : nodakai/really-fix-kw-table
rollup merge of #17970 : nodakai/cleanup-warnings
rollup merge of #17972 : ebfe/rpass-lang-item
rollup merge of #17977 : Behemecoatyl/master
rollup merge of #17979 : jkleint/guide-guess-counter
rollup merge of #17980 : alexcrichton/no-more-exes
rollup merge of #17984 : bkoropoff/issue-17651
rollup merge of #17986 : gamazeps/docissue
rollup merge of #17991 : sfackler/extern-error
rollup merge of #17992 : jkleint/guide-double-borrow
rollup merge of #17993 : redcape/fix-ffi-docs
rollup merge of #18002 : SimonSapin/unicode-version
rollup merge of #18007 : dotdash/arg_lifetime_end
rollup merge of #18012 : pnkfelix/gate-bors-on-building-compiletest