Auto merge of #28900 - cristicbz:typos, r=alexcrichton

I found these automatically, but fixed them manually to ensure the semantics are correct. I know things like these are hardly important, since they only marginally improve clarity. But at least for me typos and simple grammatical errors trigger an---unjustified---sense of unprofessionalism, despite the fact that I make them all the time and I understand that they're the sort of thing that is bound to slip through review.  

Anyway, to find most of these I used:

  * `ag '.*//.*(\b[A-Za-z]{2,}\b) \1\b'` for repeated words

  * `ag '\b(the|this|those|these|a|it) (a|the|this|those|these|it)\b'` to find constructs like 'the this' etc. many false positives, but not too hard to scroll through them to actually find the mistakes.

  * `cat ../../typos.txt | paste -d'|' - - - - - - - - - - - - - - - - - - - - - - | tr '\n' '\0' | xargs -0 -P4 -n1 ag`. Hacky way to find misspellings, but it works ok. I got `typos.txt` from [Wikipedia](https://en.wikipedia.org/wiki/Wikipedia:Lists_of_common_misspellings/For_machines)

* `ag '.*//.* a ([ae][a-z]|(o[^n])|(i[a-rt-z]))'` to find places where 'a' was followed by a vowel (requiring 'an' instead).

I also used a handful more one off regexes that are too boring to reproduce here.

4 files changed, 4 insertions, 5 deletions

diff --git a/src/libsyntax/ext/base.rs b/src/libsyntax/ext/base.rs
index aaaca8bd4d8..7459f8b8f24 100644
--- a/src/libsyntax/ext/base.rs
+++ b/src/libsyntax/ext/base.rs
@@ -600,7 +600,7 @@ impl<'a> ExtCtxt<'a> {
         self.expander().fold_expr(e)
     }
 
-    /// Returns a `Folder` for deeply expanding all macros in a AST node.
+    /// Returns a `Folder` for deeply expanding all macros in an AST node.
     pub fn expander<'b>(&'b mut self) -> expand::MacroExpander<'b, 'a> {
         expand::MacroExpander::new(self)
     }
diff --git a/src/libsyntax/ext/tt/macro_parser.rs b/src/libsyntax/ext/tt/macro_parser.rs
index 01fd4608ecb..8dec9ae1e98 100644
--- a/src/libsyntax/ext/tt/macro_parser.rs
+++ b/src/libsyntax/ext/tt/macro_parser.rs
@@ -32,7 +32,7 @@
 //! As it processes them, it fills up `eof_eis` with items that would be valid if
 //! the macro invocation is now over, `bb_eis` with items that are waiting on
 //! a Rust nonterminal like `$e:expr`, and `next_eis` with items that are waiting
-//! on the a particular token. Most of the logic concerns moving the · through the
+//! on a particular token. Most of the logic concerns moving the · through the
 //! repetitions indicated by Kleene stars. It only advances or calls out to the
 //! real Rust parser when no `cur_eis` items remain
 //!
diff --git a/src/libsyntax/ext/tt/macro_rules.rs b/src/libsyntax/ext/tt/macro_rules.rs
index 0a95f24a721..cce4450b299 100644
--- a/src/libsyntax/ext/tt/macro_rules.rs
+++ b/src/libsyntax/ext/tt/macro_rules.rs
@@ -497,7 +497,7 @@ fn is_in_follow(_: &ExtCtxt, tok: &Token, frag: &str) -> Result<bool, String> {
                 Ok(true)
             },
             "block" => {
-                // anything can follow block, the braces provide a easy boundary to
+                // anything can follow block, the braces provide an easy boundary to
                 // maintain
                 Ok(true)
             },
diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index 18c6d74d62e..39b1b746de8 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -1107,8 +1107,7 @@ pub enum UnstableFeatures {
     /// Errors are bypassed for bootstrapping. This is required any time
     /// during the build that feature-related lints are set to warn or above
     /// because the build turns on warnings-as-errors and uses lots of unstable
-    /// features. As a result, this this is always required for building Rust
-    /// itself.
+    /// features. As a result, this is always required for building Rust itself.
     Cheat
 }