Placing lifetime specifier after &mut emits wall of unhelpful diagnostics

[SNCPlay42]

Playground

fn x<'a>(x: &mut 'a i32){}

Expected result: a diagnostic suggests placing 'a before mut

Actual diagnostic output:

   Compiling playground v0.0.1 (/playground)
error: lifetime in trait object type must be followed by `+`
 --> src/lib.rs:1:18
  |
1 | fn x<'a>(x: &mut 'a i32){}
  |                  ^^

error: expected one of `:`, `@`, or `|`, found `)`
 --> src/lib.rs:1:24
  |
1 | fn x<'a>(x: &mut 'a i32){}
  |                        ^ expected one of `:`, `@`, or `|`
  |
  = note: anonymous parameters are removed in the 2018 edition (see RFC 1685)
help: if this was a parameter name, give it a type
  |
1 | fn x<'a>(x: &mut 'a i32: TypeName){}
  |                     ^^^^^^^^^^^^^
help: if this is a type, explicitly ignore the parameter name
  |
1 | fn x<'a>(x: &mut 'a _: i32){}
  |                     ^^^^^^

error: expected one of `)` or `,`, found `i32`
 --> src/lib.rs:1:21
  |
1 | fn x<'a>(x: &mut 'a i32){}
  |                    -^^^ expected one of `)` or `,`
  |                    |
  |                    help: missing `,`

warning: trait objects without an explicit `dyn` are deprecated
 --> src/lib.rs:1:18
  |
1 | fn x<'a>(x: &mut 'a i32){}
  |                  ^^ help: use `dyn`: `dyn 'a`
  |
  = note: `#[warn(bare_trait_objects)]` on by default

error[E0224]: at least one trait is required for an object type
 --> src/lib.rs:1:18
  |
1 | fn x<'a>(x: &mut 'a i32){}
  |                  ^^

error: aborting due to 4 previous errors; 1 warning emitted

For more information about this error, try `rustc --explain E0224`.
error: could not compile `playground`.

To learn more, run the command again with --verbose.

[SNCPlay42]

Output from rustc 1.43 (godbolt):

error: expected type, found `'a`
 --> <source>:1:18
  |
1 | fn x<'a>(x: &mut 'a i32){}
  |                  ^^ expected type

error: aborting due to previous error

The size of the output appears to be a recent regression.

[SNCPlay42]

cargo-bisect-rustc:

searched nightlies: from nightly-2020-03-09 to nightly-2020-06-21
regressed nightly: nightly-2020-03-12
searched commits: from 1581278 to c20d7ee
regressed commit: c20d7ee

bisected with cargo-bisect-rustc v0.5.2

Host triple: x86_64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc --prompt --start 2020-03-09 

Looks like #69760?

[SNCPlay42]

Further testing confirms #69760, specifically ba3ae46 introduced the large number of diagnostics.

[LeSeulArtichaut]

Assigning P-high as discussed as part of the Prioritization WG procedure.