Fix inconsistent symbol mangling with -Zverbose

Always skip arguments that are the defaults of their respective
parameters, to avoid generating inconsistent symbols for builds
with `-Zverbose` flag and without it.

compiler/rustc_middle/src/ty/print/mod.rs

@@ -188,11 +188,6 @@ pub trait Printer<'tcx>: Sized {
             own_params.start = 1;
         }
 
-        // If we're in verbose mode, then print default-equal args too
-        if self.tcx().sess.verbose() {
-            return &substs[own_params];
-        }
-
         // Don't print args that are the defaults of their respective parameters.
         own_params.end -= generics
             .params

src/test/ui/associated-types/substs-ppaux.rs

@@ -25,7 +25,7 @@ fn foo<'z>() where &'z (): Sized {
     let x: () = <i8 as Foo<'static, 'static,  u32>>::bar::<'static, char>;
     //[verbose]~^ ERROR mismatched types
     //[verbose]~| expected unit type `()`
-    //[verbose]~| found fn item `fn() {<i8 as Foo<ReStatic, ReStatic, u32>>::bar::<ReStatic, char>}`
+    //[verbose]~| found fn item `fn() {<i8 as Foo<ReStatic, ReStatic>>::bar::<ReStatic, char>}`
     //[normal]~^^^^ ERROR mismatched types
     //[normal]~| expected unit type `()`
     //[normal]~| found fn item `fn() {<i8 as Foo<'static, 'static>>::bar::<'static, char>}`

src/test/ui/associated-types/substs-ppaux.verbose.stderr

@@ -20,15 +20,15 @@ error[E0308]: mismatched types
   --> $DIR/substs-ppaux.rs:25:17
    |
 LL |     fn bar<'a, T>() where T: 'a {}
-   |     --------------------------- fn() {<i8 as Foo<ReStatic, ReStatic, u32>>::bar::<ReStatic, char>} defined here
+   |     --------------------------- fn() {<i8 as Foo<ReStatic, ReStatic>>::bar::<ReStatic, char>} defined here
 ...
 LL |     let x: () = <i8 as Foo<'static, 'static,  u32>>::bar::<'static, char>;
    |            --   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `()`, found fn item
    |            |
    |            expected due to this
    |
    = note: expected unit type `()`
-                found fn item `fn() {<i8 as Foo<ReStatic, ReStatic, u32>>::bar::<ReStatic, char>}`
+                found fn item `fn() {<i8 as Foo<ReStatic, ReStatic>>::bar::<ReStatic, char>}`
 help: use parentheses to call this function
    |
 LL |     let x: () = <i8 as Foo<'static, 'static,  u32>>::bar::<'static, char>();

src/test/ui/nll/ty-outlives/projection-no-regions-closure.stderr

@@ -6,7 +6,7 @@ LL |     with_signature(x, |mut y| Box::new(y.next()))
    |
    = note: defining type: no_region::<'_#1r, T>::{closure#0} with closure substs [
                i32,
-               extern "rust-call" fn((std::boxed::Box<T, std::alloc::Global>,)) -> std::boxed::Box<(dyn Anything + '_#2r), std::alloc::Global>,
+               extern "rust-call" fn((std::boxed::Box<T>,)) -> std::boxed::Box<(dyn Anything + '_#2r)>,
                (),
            ]
    = note: number of external vids: 3
@@ -42,7 +42,7 @@ LL |     with_signature(x, |mut y| Box::new(y.next()))
    |
    = note: defining type: correct_region::<'_#1r, T>::{closure#0} with closure substs [
                i32,
-               extern "rust-call" fn((std::boxed::Box<T, std::alloc::Global>,)) -> std::boxed::Box<(dyn Anything + '_#2r), std::alloc::Global>,
+               extern "rust-call" fn((std::boxed::Box<T>,)) -> std::boxed::Box<(dyn Anything + '_#2r)>,
                (),
            ]
    = note: number of external vids: 3
@@ -69,7 +69,7 @@ LL |     with_signature(x, |mut y| Box::new(y.next()))
    |
    = note: defining type: wrong_region::<'_#1r, '_#2r, T>::{closure#0} with closure substs [
                i32,
-               extern "rust-call" fn((std::boxed::Box<T, std::alloc::Global>,)) -> std::boxed::Box<(dyn Anything + '_#3r), std::alloc::Global>,
+               extern "rust-call" fn((std::boxed::Box<T>,)) -> std::boxed::Box<(dyn Anything + '_#3r)>,
                (),
            ]
    = note: number of external vids: 4
@@ -105,7 +105,7 @@ LL |     with_signature(x, |mut y| Box::new(y.next()))
    |
    = note: defining type: outlives_region::<'_#1r, '_#2r, T>::{closure#0} with closure substs [
                i32,
-               extern "rust-call" fn((std::boxed::Box<T, std::alloc::Global>,)) -> std::boxed::Box<(dyn Anything + '_#3r), std::alloc::Global>,
+               extern "rust-call" fn((std::boxed::Box<T>,)) -> std::boxed::Box<(dyn Anything + '_#3r)>,
                (),
            ]
    = note: number of external vids: 4

src/test/ui/nll/ty-outlives/ty-param-closure-outlives-from-return-type.stderr

@@ -6,7 +6,7 @@ LL |     with_signature(x, |y| y)
    |
    = note: defining type: no_region::<'_#1r, T>::{closure#0} with closure substs [
                i32,
-               extern "rust-call" fn((std::boxed::Box<T, std::alloc::Global>,)) -> std::boxed::Box<(dyn std::fmt::Debug + '_#2r), std::alloc::Global>,
+               extern "rust-call" fn((std::boxed::Box<T>,)) -> std::boxed::Box<(dyn std::fmt::Debug + '_#2r)>,
                (),
            ]
    = note: number of external vids: 3

src/test/ui/symbol-names/verbose.rs

@@ -0,0 +1,15 @@
+// Regression test for issue #57596, where -Zverbose flag unintentionally
+// affected produced symbols making it impossible to link between crates
+// with a different value of the flag (for symbols involving generic
+// arguments equal to defaults of their respective parameters).
+//
+// build-pass
+// compile-flags: -Zverbose
+
+pub fn error(msg: String) -> Box<dyn std::error::Error> {
+  msg.into()
+}
+
+fn main() {
+  error(String::new());
+}