pretty: trim paths of unique symbols

If a symbol name can only be imported from one place for a type, and
as long as it was not glob-imported anywhere in the current crate, we
can trim its printed path and print only the name.

This has wide implications on error messages with types, for example,
shortening `std::vec::Vec` to just `Vec`, as long as there is no other
`Vec` importable anywhere.

This adds a new '-Z trim-diagnostic-paths=false' option to control this
feature.

On the good path, with no diagnosis printed, we should try to avoid
issuing this query, so we need to prevent trimmed_def_paths query on
several cases.

This change also relies on a previous commit that differentiates
between `Debug` and `Display` on various rustc types, where the latter
is trimmed and presented to the user and the former is not.

7 files changed, 79 insertions, 81 deletions

diff --git a/src/test/ui/iterators/array-of-ranges.stderr b/src/test/ui/iterators/array-of-ranges.stderr
index 3dbed9a1065..6271d8107bc 100644
--- a/src/test/ui/iterators/array-of-ranges.stderr
+++ b/src/test/ui/iterators/array-of-ranges.stderr
@@ -4,49 +4,49 @@ error[E0277]: `[std::ops::Range<{integer}>; 1]` is not an iterator
 LL |     for _ in [0..1] {}
    |              ^^^^^^ if you meant to iterate between two values, remove the square brackets
    |
-   = help: the trait `std::iter::Iterator` is not implemented for `[std::ops::Range<{integer}>; 1]`
+   = help: the trait `Iterator` is not implemented for `[std::ops::Range<{integer}>; 1]`
    = note: `[start..end]` is an array of one `Range`; you might have meant to have a `Range` without the brackets: `start..end`
-   = note: required by `std::iter::IntoIterator::into_iter`
+   = note: required by `into_iter`
 
-error[E0277]: `[std::ops::RangeInclusive<{integer}>; 1]` is not an iterator
+error[E0277]: `[RangeInclusive<{integer}>; 1]` is not an iterator
   --> $DIR/array-of-ranges.rs:4:14
    |
 LL |     for _ in [0..=1] {}
-   |              ^^^^^^^ if you meant to iterate between two values, remove the square brackets
+   |              ^^^^^^^ borrow the array with `&` or call `.iter()` on it to iterate over it
    |
-   = help: the trait `std::iter::Iterator` is not implemented for `[std::ops::RangeInclusive<{integer}>; 1]`
-   = note: `[start..=end]` is an array of one `RangeInclusive`; you might have meant to have a `RangeInclusive` without the brackets: `start..=end`
-   = note: required by `std::iter::IntoIterator::into_iter`
+   = help: the trait `Iterator` is not implemented for `[RangeInclusive<{integer}>; 1]`
+   = note: arrays are not iterators, but slices like the following are: `&[1, 2, 3]`
+   = note: required by `into_iter`
 
-error[E0277]: `[std::ops::RangeFrom<{integer}>; 1]` is not an iterator
+error[E0277]: `[RangeFrom<{integer}>; 1]` is not an iterator
   --> $DIR/array-of-ranges.rs:6:14
    |
 LL |     for _ in [0..] {}
-   |              ^^^^^ if you meant to iterate from a value onwards, remove the square brackets
+   |              ^^^^^ borrow the array with `&` or call `.iter()` on it to iterate over it
    |
-   = help: the trait `std::iter::Iterator` is not implemented for `[std::ops::RangeFrom<{integer}>; 1]`
-   = note: `[start..]` is an array of one `RangeFrom`; you might have meant to have a `RangeFrom` without the brackets: `start..`, keeping in mind that iterating over an unbounded iterator will run forever unless you `break` or `return` from within the loop
-   = note: required by `std::iter::IntoIterator::into_iter`
+   = help: the trait `Iterator` is not implemented for `[RangeFrom<{integer}>; 1]`
+   = note: arrays are not iterators, but slices like the following are: `&[1, 2, 3]`
+   = note: required by `into_iter`
 
-error[E0277]: `[std::ops::RangeTo<{integer}>; 1]` is not an iterator
+error[E0277]: `[RangeTo<{integer}>; 1]` is not an iterator
   --> $DIR/array-of-ranges.rs:8:14
    |
 LL |     for _ in [..1] {}
-   |              ^^^^^ if you meant to iterate until a value, remove the square brackets and add a starting value
+   |              ^^^^^ borrow the array with `&` or call `.iter()` on it to iterate over it
    |
-   = help: the trait `std::iter::Iterator` is not implemented for `[std::ops::RangeTo<{integer}>; 1]`
-   = note: `[..end]` is an array of one `RangeTo`; you might have meant to have a bounded `Range` without the brackets: `0..end`
-   = note: required by `std::iter::IntoIterator::into_iter`
+   = help: the trait `Iterator` is not implemented for `[RangeTo<{integer}>; 1]`
+   = note: arrays are not iterators, but slices like the following are: `&[1, 2, 3]`
+   = note: required by `into_iter`
 
-error[E0277]: `[std::ops::RangeToInclusive<{integer}>; 1]` is not an iterator
+error[E0277]: `[RangeToInclusive<{integer}>; 1]` is not an iterator
   --> $DIR/array-of-ranges.rs:10:14
    |
 LL |     for _ in [..=1] {}
-   |              ^^^^^^ if you meant to iterate until a value (including it), remove the square brackets and add a starting value
+   |              ^^^^^^ borrow the array with `&` or call `.iter()` on it to iterate over it
    |
-   = help: the trait `std::iter::Iterator` is not implemented for `[std::ops::RangeToInclusive<{integer}>; 1]`
-   = note: `[..=end]` is an array of one `RangeToInclusive`; you might have meant to have a bounded `RangeInclusive` without the brackets: `0..=end`
-   = note: required by `std::iter::IntoIterator::into_iter`
+   = help: the trait `Iterator` is not implemented for `[RangeToInclusive<{integer}>; 1]`
+   = note: arrays are not iterators, but slices like the following are: `&[1, 2, 3]`
+   = note: required by `into_iter`
 
 error[E0277]: `[std::ops::Range<{integer}>; 1]` is not an iterator
   --> $DIR/array-of-ranges.rs:14:14
@@ -54,9 +54,9 @@ error[E0277]: `[std::ops::Range<{integer}>; 1]` is not an iterator
 LL |     for _ in [start..end] {}
    |              ^^^^^^^^^^^^ if you meant to iterate between two values, remove the square brackets
    |
-   = help: the trait `std::iter::Iterator` is not implemented for `[std::ops::Range<{integer}>; 1]`
+   = help: the trait `Iterator` is not implemented for `[std::ops::Range<{integer}>; 1]`
    = note: `[start..end]` is an array of one `Range`; you might have meant to have a `Range` without the brackets: `start..end`
-   = note: required by `std::iter::IntoIterator::into_iter`
+   = note: required by `into_iter`
 
 error[E0277]: `[std::ops::Range<{integer}>; 1]` is not an iterator
   --> $DIR/array-of-ranges.rs:17:14
@@ -64,9 +64,9 @@ error[E0277]: `[std::ops::Range<{integer}>; 1]` is not an iterator
 LL |     for _ in array_of_range {}
    |              ^^^^^^^^^^^^^^ if you meant to iterate between two values, remove the square brackets
    |
-   = help: the trait `std::iter::Iterator` is not implemented for `[std::ops::Range<{integer}>; 1]`
+   = help: the trait `Iterator` is not implemented for `[std::ops::Range<{integer}>; 1]`
    = note: `[start..end]` is an array of one `Range`; you might have meant to have a `Range` without the brackets: `start..end`
-   = note: required by `std::iter::IntoIterator::into_iter`
+   = note: required by `into_iter`
 
 error[E0277]: `[std::ops::Range<{integer}>; 2]` is not an iterator
   --> $DIR/array-of-ranges.rs:19:14
@@ -74,19 +74,19 @@ error[E0277]: `[std::ops::Range<{integer}>; 2]` is not an iterator
 LL |     for _ in [0..1, 2..3] {}
    |              ^^^^^^^^^^^^ borrow the array with `&` or call `.iter()` on it to iterate over it
    |
-   = help: the trait `std::iter::Iterator` is not implemented for `[std::ops::Range<{integer}>; 2]`
+   = help: the trait `Iterator` is not implemented for `[std::ops::Range<{integer}>; 2]`
    = note: arrays are not iterators, but slices like the following are: `&[1, 2, 3]`
-   = note: required by `std::iter::IntoIterator::into_iter`
+   = note: required by `into_iter`
 
-error[E0277]: `[std::ops::RangeInclusive<{integer}>; 1]` is not an iterator
+error[E0277]: `[RangeInclusive<{integer}>; 1]` is not an iterator
   --> $DIR/array-of-ranges.rs:21:14
    |
 LL |     for _ in [0..=1] {}
-   |              ^^^^^^^ if you meant to iterate between two values, remove the square brackets
+   |              ^^^^^^^ borrow the array with `&` or call `.iter()` on it to iterate over it
    |
-   = help: the trait `std::iter::Iterator` is not implemented for `[std::ops::RangeInclusive<{integer}>; 1]`
-   = note: `[start..=end]` is an array of one `RangeInclusive`; you might have meant to have a `RangeInclusive` without the brackets: `start..=end`
-   = note: required by `std::iter::IntoIterator::into_iter`
+   = help: the trait `Iterator` is not implemented for `[RangeInclusive<{integer}>; 1]`
+   = note: arrays are not iterators, but slices like the following are: `&[1, 2, 3]`
+   = note: required by `into_iter`
 
 error: aborting due to 9 previous errors
 
diff --git a/src/test/ui/iterators/array.stderr b/src/test/ui/iterators/array.stderr
index 94731f1c745..f86c82e4917 100644
--- a/src/test/ui/iterators/array.stderr
+++ b/src/test/ui/iterators/array.stderr
@@ -4,9 +4,9 @@ error[E0277]: `[{integer}; 2]` is not an iterator
 LL |     for _ in [1, 2] {}
    |              ^^^^^^ borrow the array with `&` or call `.iter()` on it to iterate over it
    |
-   = help: the trait `std::iter::Iterator` is not implemented for `[{integer}; 2]`
+   = help: the trait `Iterator` is not implemented for `[{integer}; 2]`
    = note: arrays are not iterators, but slices like the following are: `&[1, 2, 3]`
-   = note: required by `std::iter::IntoIterator::into_iter`
+   = note: required by `into_iter`
 
 error[E0277]: `[{integer}; 2]` is not an iterator
   --> $DIR/array.rs:5:14
@@ -14,9 +14,9 @@ error[E0277]: `[{integer}; 2]` is not an iterator
 LL |     for _ in x {}
    |              ^ borrow the array with `&` or call `.iter()` on it to iterate over it
    |
-   = help: the trait `std::iter::Iterator` is not implemented for `[{integer}; 2]`
+   = help: the trait `Iterator` is not implemented for `[{integer}; 2]`
    = note: arrays are not iterators, but slices like the following are: `&[1, 2, 3]`
-   = note: required by `std::iter::IntoIterator::into_iter`
+   = note: required by `into_iter`
 
 error[E0277]: `[{float}; 2]` is not an iterator
   --> $DIR/array.rs:7:14
@@ -24,9 +24,9 @@ error[E0277]: `[{float}; 2]` is not an iterator
 LL |     for _ in [1.0, 2.0] {}
    |              ^^^^^^^^^^ borrow the array with `&` or call `.iter()` on it to iterate over it
    |
-   = help: the trait `std::iter::Iterator` is not implemented for `[{float}; 2]`
+   = help: the trait `Iterator` is not implemented for `[{float}; 2]`
    = note: arrays are not iterators, but slices like the following are: `&[1, 2, 3]`
-   = note: required by `std::iter::IntoIterator::into_iter`
+   = note: required by `into_iter`
 
 error: aborting due to 3 previous errors
 
diff --git a/src/test/ui/iterators/bound.stderr b/src/test/ui/iterators/bound.stderr
index 1a5aad6c36d..eaf2e66d0f8 100644
--- a/src/test/ui/iterators/bound.stderr
+++ b/src/test/ui/iterators/bound.stderr
@@ -6,7 +6,7 @@ LL | struct S<I: Iterator>(I);
 LL | struct T(S<u8>);
    |          ^^^^^ `u8` is not an iterator
    |
-   = help: the trait `std::iter::Iterator` is not implemented for `u8`
+   = help: the trait `Iterator` is not implemented for `u8`
    = note: if you want to iterate between `start` until a value `end`, use the exclusive range syntax `start..end` or the inclusive range syntax `start..=end`
 
 error: aborting due to previous error
diff --git a/src/test/ui/iterators/integral.stderr b/src/test/ui/iterators/integral.stderr
index 71e1e81e5af..c4c46412611 100644
--- a/src/test/ui/iterators/integral.stderr
+++ b/src/test/ui/iterators/integral.stderr
@@ -4,9 +4,9 @@ error[E0277]: `{integer}` is not an iterator
 LL |     for _ in 42 {}
    |              ^^ `{integer}` is not an iterator
    |
-   = help: the trait `std::iter::Iterator` is not implemented for `{integer}`
+   = help: the trait `Iterator` is not implemented for `{integer}`
    = note: if you want to iterate between `start` until a value `end`, use the exclusive range syntax `start..end` or the inclusive range syntax `start..=end`
-   = note: required by `std::iter::IntoIterator::into_iter`
+   = note: required by `into_iter`
 
 error[E0277]: `u8` is not an iterator
   --> $DIR/integral.rs:4:14
@@ -14,9 +14,9 @@ error[E0277]: `u8` is not an iterator
 LL |     for _ in 42 as u8 {}
    |              ^^^^^^^^ `u8` is not an iterator
    |
-   = help: the trait `std::iter::Iterator` is not implemented for `u8`
+   = help: the trait `Iterator` is not implemented for `u8`
    = note: if you want to iterate between `start` until a value `end`, use the exclusive range syntax `start..end` or the inclusive range syntax `start..=end`
-   = note: required by `std::iter::IntoIterator::into_iter`
+   = note: required by `into_iter`
 
 error[E0277]: `i8` is not an iterator
   --> $DIR/integral.rs:6:14
@@ -24,9 +24,9 @@ error[E0277]: `i8` is not an iterator
 LL |     for _ in 42 as i8 {}
    |              ^^^^^^^^ `i8` is not an iterator
    |
-   = help: the trait `std::iter::Iterator` is not implemented for `i8`
+   = help: the trait `Iterator` is not implemented for `i8`
    = note: if you want to iterate between `start` until a value `end`, use the exclusive range syntax `start..end` or the inclusive range syntax `start..=end`
-   = note: required by `std::iter::IntoIterator::into_iter`
+   = note: required by `into_iter`
 
 error[E0277]: `u16` is not an iterator
   --> $DIR/integral.rs:8:14
@@ -34,9 +34,9 @@ error[E0277]: `u16` is not an iterator
 LL |     for _ in 42 as u16 {}
    |              ^^^^^^^^^ `u16` is not an iterator
    |
-   = help: the trait `std::iter::Iterator` is not implemented for `u16`
+   = help: the trait `Iterator` is not implemented for `u16`
    = note: if you want to iterate between `start` until a value `end`, use the exclusive range syntax `start..end` or the inclusive range syntax `start..=end`
-   = note: required by `std::iter::IntoIterator::into_iter`
+   = note: required by `into_iter`
 
 error[E0277]: `i16` is not an iterator
   --> $DIR/integral.rs:10:14
@@ -44,9 +44,9 @@ error[E0277]: `i16` is not an iterator
 LL |     for _ in 42 as i16 {}
    |              ^^^^^^^^^ `i16` is not an iterator
    |
-   = help: the trait `std::iter::Iterator` is not implemented for `i16`
+   = help: the trait `Iterator` is not implemented for `i16`
    = note: if you want to iterate between `start` until a value `end`, use the exclusive range syntax `start..end` or the inclusive range syntax `start..=end`
-   = note: required by `std::iter::IntoIterator::into_iter`
+   = note: required by `into_iter`
 
 error[E0277]: `u32` is not an iterator
   --> $DIR/integral.rs:12:14
@@ -54,9 +54,9 @@ error[E0277]: `u32` is not an iterator
 LL |     for _ in 42 as u32 {}
    |              ^^^^^^^^^ `u32` is not an iterator
    |
-   = help: the trait `std::iter::Iterator` is not implemented for `u32`
+   = help: the trait `Iterator` is not implemented for `u32`
    = note: if you want to iterate between `start` until a value `end`, use the exclusive range syntax `start..end` or the inclusive range syntax `start..=end`
-   = note: required by `std::iter::IntoIterator::into_iter`
+   = note: required by `into_iter`
 
 error[E0277]: `i32` is not an iterator
   --> $DIR/integral.rs:14:14
@@ -64,9 +64,9 @@ error[E0277]: `i32` is not an iterator
 LL |     for _ in 42 as i32 {}
    |              ^^^^^^^^^ `i32` is not an iterator
    |
-   = help: the trait `std::iter::Iterator` is not implemented for `i32`
+   = help: the trait `Iterator` is not implemented for `i32`
    = note: if you want to iterate between `start` until a value `end`, use the exclusive range syntax `start..end` or the inclusive range syntax `start..=end`
-   = note: required by `std::iter::IntoIterator::into_iter`
+   = note: required by `into_iter`
 
 error[E0277]: `u64` is not an iterator
   --> $DIR/integral.rs:16:14
@@ -74,9 +74,9 @@ error[E0277]: `u64` is not an iterator
 LL |     for _ in 42 as u64 {}
    |              ^^^^^^^^^ `u64` is not an iterator
    |
-   = help: the trait `std::iter::Iterator` is not implemented for `u64`
+   = help: the trait `Iterator` is not implemented for `u64`
    = note: if you want to iterate between `start` until a value `end`, use the exclusive range syntax `start..end` or the inclusive range syntax `start..=end`
-   = note: required by `std::iter::IntoIterator::into_iter`
+   = note: required by `into_iter`
 
 error[E0277]: `i64` is not an iterator
   --> $DIR/integral.rs:18:14
@@ -84,9 +84,9 @@ error[E0277]: `i64` is not an iterator
 LL |     for _ in 42 as i64 {}
    |              ^^^^^^^^^ `i64` is not an iterator
    |
-   = help: the trait `std::iter::Iterator` is not implemented for `i64`
+   = help: the trait `Iterator` is not implemented for `i64`
    = note: if you want to iterate between `start` until a value `end`, use the exclusive range syntax `start..end` or the inclusive range syntax `start..=end`
-   = note: required by `std::iter::IntoIterator::into_iter`
+   = note: required by `into_iter`
 
 error[E0277]: `usize` is not an iterator
   --> $DIR/integral.rs:20:14
@@ -94,9 +94,9 @@ error[E0277]: `usize` is not an iterator
 LL |     for _ in 42 as usize {}
    |              ^^^^^^^^^^^ `usize` is not an iterator
    |
-   = help: the trait `std::iter::Iterator` is not implemented for `usize`
+   = help: the trait `Iterator` is not implemented for `usize`
    = note: if you want to iterate between `start` until a value `end`, use the exclusive range syntax `start..end` or the inclusive range syntax `start..=end`
-   = note: required by `std::iter::IntoIterator::into_iter`
+   = note: required by `into_iter`
 
 error[E0277]: `isize` is not an iterator
   --> $DIR/integral.rs:22:14
@@ -104,9 +104,9 @@ error[E0277]: `isize` is not an iterator
 LL |     for _ in 42 as isize {}
    |              ^^^^^^^^^^^ `isize` is not an iterator
    |
-   = help: the trait `std::iter::Iterator` is not implemented for `isize`
+   = help: the trait `Iterator` is not implemented for `isize`
    = note: if you want to iterate between `start` until a value `end`, use the exclusive range syntax `start..end` or the inclusive range syntax `start..=end`
-   = note: required by `std::iter::IntoIterator::into_iter`
+   = note: required by `into_iter`
 
 error[E0277]: `{float}` is not an iterator
   --> $DIR/integral.rs:24:14
@@ -114,8 +114,8 @@ error[E0277]: `{float}` is not an iterator
 LL |     for _ in 42.0 {}
    |              ^^^^ `{float}` is not an iterator
    |
-   = help: the trait `std::iter::Iterator` is not implemented for `{float}`
-   = note: required by `std::iter::IntoIterator::into_iter`
+   = help: the trait `Iterator` is not implemented for `{float}`
+   = note: required by `into_iter`
 
 error: aborting due to 12 previous errors
 
diff --git a/src/test/ui/iterators/ranges.stderr b/src/test/ui/iterators/ranges.stderr
index e5e2d87879d..0324d5f1a92 100644
--- a/src/test/ui/iterators/ranges.stderr
+++ b/src/test/ui/iterators/ranges.stderr
@@ -1,22 +1,20 @@
-error[E0277]: `std::ops::RangeTo<{integer}>` is not an iterator
+error[E0277]: `RangeTo<{integer}>` is not an iterator
   --> $DIR/ranges.rs:2:14
    |
 LL |     for _ in ..10 {}
-   |              ^^^^ if you meant to iterate until a value, add a starting value
+   |              ^^^^ `RangeTo<{integer}>` is not an iterator
    |
-   = help: the trait `std::iter::Iterator` is not implemented for `std::ops::RangeTo<{integer}>`
-   = note: `..end` is a `RangeTo`, which cannot be iterated on; you might have meant to have a bounded `Range`: `0..end`
-   = note: required by `std::iter::IntoIterator::into_iter`
+   = help: the trait `Iterator` is not implemented for `RangeTo<{integer}>`
+   = note: required by `into_iter`
 
-error[E0277]: `std::ops::RangeToInclusive<{integer}>` is not an iterator
+error[E0277]: `RangeToInclusive<{integer}>` is not an iterator
   --> $DIR/ranges.rs:4:14
    |
 LL |     for _ in ..=10 {}
-   |              ^^^^^ if you meant to iterate until a value (including it), add a starting value
+   |              ^^^^^ `RangeToInclusive<{integer}>` is not an iterator
    |
-   = help: the trait `std::iter::Iterator` is not implemented for `std::ops::RangeToInclusive<{integer}>`
-   = note: `..=end` is a `RangeToInclusive`, which cannot be iterated on; you might have meant to have a bounded `RangeInclusive`: `0..=end`
-   = note: required by `std::iter::IntoIterator::into_iter`
+   = help: the trait `Iterator` is not implemented for `RangeToInclusive<{integer}>`
+   = note: required by `into_iter`
 
 error: aborting due to 2 previous errors
 
diff --git a/src/test/ui/iterators/string.rs b/src/test/ui/iterators/string.rs
index 4373dcaabe5..ad58a463e9b 100644
--- a/src/test/ui/iterators/string.rs
+++ b/src/test/ui/iterators/string.rs
@@ -1,6 +1,6 @@
 fn main() {
     for _ in "".to_owned() {}
-    //~^ ERROR `std::string::String` is not an iterator
+    //~^ ERROR `String` is not an iterator
     for _ in "" {}
     //~^ ERROR `&str` is not an iterator
 }
diff --git a/src/test/ui/iterators/string.stderr b/src/test/ui/iterators/string.stderr
index 927de952cc8..fecdbd1785f 100644
--- a/src/test/ui/iterators/string.stderr
+++ b/src/test/ui/iterators/string.stderr
@@ -1,11 +1,11 @@
-error[E0277]: `std::string::String` is not an iterator
+error[E0277]: `String` is not an iterator
   --> $DIR/string.rs:2:14
    |
 LL |     for _ in "".to_owned() {}
-   |              ^^^^^^^^^^^^^ `std::string::String` is not an iterator; try calling `.chars()` or `.bytes()`
+   |              ^^^^^^^^^^^^^ `String` is not an iterator
    |
-   = help: the trait `std::iter::Iterator` is not implemented for `std::string::String`
-   = note: required by `std::iter::IntoIterator::into_iter`
+   = help: the trait `Iterator` is not implemented for `String`
+   = note: required by `into_iter`
 
 error[E0277]: `&str` is not an iterator
   --> $DIR/string.rs:4:14
@@ -13,8 +13,8 @@ error[E0277]: `&str` is not an iterator
 LL |     for _ in "" {}
    |              ^^ `&str` is not an iterator; try calling `.chars()` or `.bytes()`
    |
-   = help: the trait `std::iter::Iterator` is not implemented for `&str`
-   = note: required by `std::iter::IntoIterator::into_iter`
+   = help: the trait `Iterator` is not implemented for `&str`
+   = note: required by `into_iter`
 
 error: aborting due to 2 previous errors