Use try_fold in Vec::extend_desugared

[el-ev]

Try to revive #68046.
Resolves #63340.

Vec::extend_desugared is the fallback path for collect&extend from non-TrustedLen iterators. It previously used while let Some(element) = iterator.next(), which makes adapters like Chain to execute extra code on every call to next().

This PR switches to try_fold, which lets adapters use their specialized fold implementations, avoiding overhead.

[rustbot]

r? @Mark-Simulacrum

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[el-ev]

Tested locally:

bench_extend_chain_filter                                                                                                                                                    
                            main (ns)   HEAD (ns)   change                                                                                                                     
    0000_0010                  189.88      158.91     -16%                                                                                                                     
    0000_0100                  724.40      339.66     -53%                                                                                                                     
    0000_1000                 5106.20     1248.02     -76%                                                                                                                     
    0010_0010                  186.01      128.01     -31%                                                                                                                     
    0100_0100                  602.00      205.02     -66%                                                                                                                     
    1000_1000                 5251.13     1167.29     -78%
                                                                                                                                                                               
bench_collect_chain_filter                                      
                            main (ns)   HEAD (ns)   change                                                                                                                     
    0010                       142.93      137.68      -4%        
    0100                       512.12      305.18     -40%
    1000                      3004.60     1267.32     -58%                                                                                                                     
   
bench_extend_flatten                                                                                                                                                         
                            main (ns)   HEAD (ns)   change        
    0001_0010                  108.39      115.08      +6%
    0010_0100                  838.90      534.83     -36%                                                                                                                     
    0100_0100                 6745.96     1984.14     -71%
    1000_0010                 6196.60     1655.14     -73%                                                                                                                     

[the8472]

Vec code is very hot and highly responsive to perturbations, let's see what perf says

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: dc931d4 (dc931d446f7059c1a67b165a4c202d6107484ba5, parent: 64a965e9013a9d14e83c4d370af26f6be6bf96fb)

[rust-timer]

Error occured while categorizing benchmark run:

could not find start commit for bound Commit("64a965e9013a9d14e83c4d370af26f6be6bf96fb")