lambdas in `alt` text have unexpected and likely incorrect behavior

Problem

If I try to include a lambda in the alt text of labs, the plot will throw an error.

> ggplot2::ggplot(mtcars, ggplot2::aes(x = wt, y = mpg)) +
+   ggplot2::geom_point() +
+   ggplot2::labs(
+     alt = ~ sprintf(
+       "Scatter plot showing %s on x-axis and %s on y-axis",
+       .@labels$x,
+       .@labels$y
+     )
+   )
Error in `.@labels`:
! no applicable method for `@` applied to an object of class "character"
Hide Traceback
    ▆
 1. ├─base (local) `<fn>`(x)
 2. └─ggplot2 (local) `print.ggplot2::ggplot`(x)
 3.   ├─ggplot2::ggplot_build(x)
 4.   └─ggplot2 (local) `ggplot_build.ggplot2::ggplot`(x)
 5.     └─ggplot2:::setup_plot_labels(plot, layers, data)
 6.       └─base::lapply(...)
 7.         └─ggplot2 (local) FUN(X[[i]], ...)
 8.           └─global label(labels[[nm]] %||% "")
 9.             └─base::sprintf(...)

However, ggplot2::get_alt_text does work as expected.

> p <- ggplot2::ggplot(mtcars, ggplot2::aes(x = wt, y = mpg)) +
+   ggplot2::geom_point() +
+   ggplot2::labs(
+     alt = ~ sprintf(
+       "Scatter plot showing %s on x-axis and %s on y-axis",
+       .@labels$x,
+       .@labels$y
+     )
+   )
> ggplot2::get_alt_text(p)
[1] "Scatter plot showing wt on x-axis and mpg on y-axis"

Solution

I will open a small PR to fix this. I believe the solution is as simple as changing this lapply on line labels.R#L108 to match the call on labels.R#L336

Start in R/labels.R at the lapply on line 108 and compare it with the call around line 336. Reproduce the mtcars example with labs(alt = ~ sprintf(...)) and verify that printing the plot no longer errors while get_alt_text(p) still returns the expected axis-label text.