diff --git a/tests/ui/asm/bad-template.aarch64.stderr b/tests/ui/asm/bad-template.aarch64.stderr
index b18946d7c6d..5023cf317d7 100644
--- a/tests/ui/asm/bad-template.aarch64.stderr
+++ b/tests/ui/asm/bad-template.aarch64.stderr
@@ -194,8 +194,8 @@ warning: formatting may not be suitable for sub-register argument
 LL |         asm!("{:foo}", in(reg) foo);
    |               ^^^^^^           --- for this argument
    |
-   = help: use `{0:w}` to have the register formatted as `w0`
-   = help: or use `{0:x}` to keep the default formatting of `x0`
+   = help: use `{0:w}` to have the register formatted as `w0` (for 32-bit values)
+   = help: or use `{0:x}` to keep the default formatting of `x0` (for 64-bit values)
    = note: `#[warn(asm_sub_register)]` on by default
 
 error: aborting due to 21 previous errors; 1 warning emitted
diff --git a/tests/ui/asm/bad-template.x86_64.stderr b/tests/ui/asm/bad-template.x86_64.stderr
index 2f584c30a32..1b9775636f5 100644
--- a/tests/ui/asm/bad-template.x86_64.stderr
+++ b/tests/ui/asm/bad-template.x86_64.stderr
@@ -194,8 +194,8 @@ warning: formatting may not be suitable for sub-register argument
 LL |         asm!("{:foo}", in(reg) foo);
    |               ^^^^^^           --- for this argument
    |
-   = help: use `{0:e}` to have the register formatted as `eax`
-   = help: or use `{0:r}` to keep the default formatting of `rax`
+   = help: use `{0:e}` to have the register formatted as `eax` (for 32-bit values)
+   = help: or use `{0:r}` to keep the default formatting of `rax` (for 64-bit values)
    = note: `#[warn(asm_sub_register)]` on by default
 
 error: aborting due to 21 previous errors; 1 warning emitted
diff --git a/tests/ui/asm/x86_64/type-check-3.stderr b/tests/ui/asm/x86_64/type-check-3.stderr
index 1baf50ff6e0..34bfcd71cac 100644
--- a/tests/ui/asm/x86_64/type-check-3.stderr
+++ b/tests/ui/asm/x86_64/type-check-3.stderr
@@ -44,8 +44,8 @@ warning: formatting may not be suitable for sub-register argument
 LL |         asm!("{0} {0}", in(reg) 0i16);
    |               ^^^ ^^^           ---- for this argument
    |
-   = help: use `{0:x}` to have the register formatted as `ax`
-   = help: or use `{0:r}` to keep the default formatting of `rax`
+   = help: use `{0:x}` to have the register formatted as `ax` (for 16-bit values)
+   = help: or use `{0:r}` to keep the default formatting of `rax` (for 64-bit values)
    = note: `#[warn(asm_sub_register)]` on by default
 
 warning: formatting may not be suitable for sub-register argument
@@ -54,8 +54,8 @@ warning: formatting may not be suitable for sub-register argument
 LL |         asm!("{0} {0:x}", in(reg) 0i16);
    |               ^^^                 ---- for this argument
    |
-   = help: use `{0:x}` to have the register formatted as `ax`
-   = help: or use `{0:r}` to keep the default formatting of `rax`
+   = help: use `{0:x}` to have the register formatted as `ax` (for 16-bit values)
+   = help: or use `{0:r}` to keep the default formatting of `rax` (for 64-bit values)
 
 warning: formatting may not be suitable for sub-register argument
   --> $DIR/type-check-3.rs:38:15
@@ -63,8 +63,8 @@ warning: formatting may not be suitable for sub-register argument
 LL |         asm!("{}", in(reg) 0i32);
    |               ^^           ---- for this argument
    |
-   = help: use `{0:e}` to have the register formatted as `eax`
-   = help: or use `{0:r}` to keep the default formatting of `rax`
+   = help: use `{0:e}` to have the register formatted as `eax` (for 32-bit values)
+   = help: or use `{0:r}` to keep the default formatting of `rax` (for 64-bit values)
 
 warning: formatting may not be suitable for sub-register argument
   --> $DIR/type-check-3.rs:41:15
@@ -72,8 +72,8 @@ warning: formatting may not be suitable for sub-register argument
 LL |         asm!("{}", in(ymm_reg) 0i64);
    |               ^^               ---- for this argument
    |
-   = help: use `{0:x}` to have the register formatted as `xmm0`
-   = help: or use `{0:y}` to keep the default formatting of `ymm0`
+   = help: use `{0:x}` to have the register formatted as `xmm0` (for 128-bit values)
+   = help: or use `{0:y}` to keep the default formatting of `ymm0` (for 256-bit values)
 
 error: type `i8` cannot be used with this register class
   --> $DIR/type-check-3.rs:52:28